A national laboratory of the U.S. Department of Energy

Office of Energy Efficiency & Renewable Energy

National Renewable Energy Laboratory 

Innovation for Our Energy Future 

Studies of Basic Electronic 

Properties of CdTe-Based Solar 

Cells and Their Evolution During 

Processing and Stress 

Final Technical Report 

16 October 2001—31 August 2005 

V.I. Kaydanov and T.R. Ohno 

Colorado School of Mines 

Golden, Colorado 

Subcontract Report 

NREL/SR-520-41129 

February 2007 

NREL is operated by Midwest Research Institute ● Battelle Contract No. DE-AC36-99-GO10337 
Studies of Basic Electronic 

Properties of CdTe-Based Solar 

Cells and Their Evolution During 

Processing and Stress 

Final Technical Report 

16 October 2001—31 August 2005 

V.I. Kaydanov and T.R. Ohno 

Colorado School of Mines 

Golden, Colorado 

NREL Technical Monitor: Harin S. Ullal 

Prepared under Subcontract No. ADJ-2-30630-05 

Subcontract Report 

NREL/SR-520-41129 

February 2007 

National Renewable Energy Laboratory

1617 Cole Boulevard, Golden, Colorado 80401-3393 

303-275-3000 • www.nrel.gov 

Operated for the U.S. Department of Energy 

Office of Energy Efficiency and Renewable Energy 

by Midwest Research Institute • Battelle 

Contract No. DE-AC36-99-GO10337
 

This publication was reproduced from the best available copy 

Submitted by the subcontractor and received no editorial review at NREL

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PREFACE 

This project, “Studies of Basic Electronic Properties of CdTe-Based Solar Cells & Their 

Evolution during Processing & Stressing”, which has been performed at the Colorado School of 

Mines (CSM), is a part of the NREL Thin-Film Partnership Program. There is a lack of 

knowledge and understanding of basic issues behind the CdTe/CdS cells performance and 

stability, such as the nature and electronic properties of impurities and defects that control the 

majority carrier concentration, mechanisms of the dopant compensation, recombination 

processes, their nature and properties, migration and transformation of defects under various 

processing, stress, and operating conditions, etc. We believe that better basic understanding of 

the specific influence of grain boundaries, especially for fine-grain materials like those making 

up CdTe-based cells, is now one of the most important issues we must address. We need to 

clarify the role of grain boundaries (GB) in forming the film electronic properties as well as 

those of the p-n junction. This report presents studies relevant to the problems formulated above 

that were carried out at CSM during this Project according to the Statement of Work and Tasks 

of the Subcontract. 

Section 1 presents the results of our studies of electroluminescence (EL) that demonstrate 

the high sensitivity of EL measurements to the cell processing/composition and stressing 

conditions. EL obtained in conjunction with photoluminescence (PL) and J-V characterization 

often showed that nonuniformity in transport through the cell back and front contacts are major 

sources of nonuniformity in injection level and EL intensity. This concept was confirmed 

experimentally by the study of cells with intentionally introduced lateral nonuniformity. New 

equipment allowed high spatial resolution two-dimensional imaging of spectrally integrated EL 

and PL intensity as well as one-dimensional scans of the spectrally-resolved luminescence. EL 

and PL images are obtained from the same area of a cell. Comparison of EL and PL spectra and 

spatial intensity distribution along with the LBIC images of the same cells obtained at CSU, 

provided new opportunities to study mechanisms of nonuniformity. In particular, luminescence 

spectra demonstrate several defect-related optical transitions lower than the band gap energy. 

The spatial distribution of intensity is energy dependent and is effected by nonuniform doping. 

Electroluminescence studies provided an independent proof of a strong photoconductivity in the 

CdS layer. They also revealed strong long-lasting (hours to days) changes in the EL pattern after 

laser irradiation. 

Section 2 presents analysis of admittance spectroscopy (AS) and admittance transient 

measurements for deep electronic states (DES) detection and studying. Important results were: 

(a) high concentrations of detected traps in thin film CdTe cells fabricated at different facilities, 

exceeding the “doping level” commonly determined with C-V profiling; (b) Changes in 

processing that diminish cell efficiency often are accompanied with reduced trap concentration. 

We discuss errors of common approaches to AS and DLTS data analysis due to high trap 

concentration. In particular, trap concentration is underestimated while the doping level derived 

from C-V measurements can be strongly overestimated. Other problems related to detection and 

studying deep states, especially very slow states, are discussed along with some new (at least for 

CdTe) approaches to their solution. A new measurement system was developed that allow us to 

measure frequency- and bias-dependent admittance in a wide temperature range. In addition, we 

can make measurements of the long transients (up to tens of hours) at a fixed temperature. We 

conclude with a brief description of the parameters and options provided by this new 

iii
measurement system, the first results of measurements, focused mostly on transients and 

information on deep/slow states already derived from them. 

The methods analyzed in Sec. 2 were applied to studying deep traps and their evolution at 

various combinations of common processing steps: CdCl2 treatment and doping with Cu. These 

studies are presented in Sec.3. Rather “fast” traps were detected and studied with AS in the 

temperature range of -190 OC to above room temperature. “Slow” traps were detected and 

studied with the capacitance transient measurements. These combined methods revealed traps for 

majority carrier holes, as well as for minority electrons with activation energy varying form 0.13 

eV to ~0.8 eV and apparent capture cross sections varying by four orders of magnitude. It was 

shown that Cu interstitial defects reduce the VCd density, but not greatly effect the concentration 

of the VCd-ClTe A-centers. The DES with Ea=0.47eV well pronounced in the cells subjected to 

N-P etch, become undetectable after Cu-doping. The deepest states of E~0.8 eV were detectable 

and demonstrated a density above 1015cm-3 only in cells treated both with CdCl2 and Cu. 

Preliminary, non-systematic stress experiments revealed influence of the treatments on behavior 

of different states under stress. 

Studies of CdTe/ZnTe interface with scanning tunneling microscopy (STM) are presented 

in Sec.4. We have significantly modified techniques that provided new opportunities in mapping 

and studying of electrical properties of the back contact. A novel scanning technique, charge 

injection spectroscopy (CIS) provides detailed maps of carrier transport near the interface. The 

lateral resolution below 20 nm is about two orders of magnitude below the CdTe grain size. It 

has been shown that charge transfer between p+

-ZnTe and p-CdTe is highly nonuniform, 

demonstrating large areas with roughly constant high contact resistance coexistent with 

nanoscale high conductance pathways. A majority of the cell current flows along these pathways, 

which are often localized at grain boundaries. This may indicate a possible role of grain 

boundaries as efficient transport channels, at least for the ZnTe:Cu/metal back contact. 

Conducting paths found within a few of the grains suggest a specific intragrain defect also may 

cause lowering of the contact resistance. 

We have also worked upon further development of the near-field scanning optical 

microscopy (NSOM) (Sec. 5). High-resolution maps of the photocurrent collection in plan-view 

were obtained from substrate configuration CdTe/CdS cells prepared by the University of 

Toledo. Considerable contrast is seen between the grain bulk and grain boundary regions. 

Photocurrent collection was higher in the intergrain regions than in the grain bulk indicated some 

possible beneficial effect of grain boundaries on cell performance. In addition to NSOM 

measurements in the photocurrent mode, the technique of NSOM mapping of photoluminescence 

(PL) was developed. High spatially resolved PL images were both in plan-view and crosssectional. The PL pattern in the former obviously correlate with the topography image that 

allows distinguish the grain boundary and grain bulk regions. 

Sec.6 presents results of development and/or improvement of the electrodeposition (ED) 

and gas jet deposition (GJD systems and CdTe deposition procedures. With a new ED system the 

CdTe growth rate was increased by an order of magnitude. The GJD technique provided 

deposition rate up to 20 μm/min, although typically we used a rate of ~ 3-5 μm/min. 

Comparative studies were performed on the cells grown on different types of single-layer 

TCO/glass and bi-layer TCO/glass substrates, and also differently CdCl2-treatments. The best 

results with an efficiency of 12.4% were obtained for Corning glass/bi-layer substrate with 

increased duration of the CdCl2 treatment. These studies indicated that there is a room for further 

optimization of processing procedures and significant increase of the cell efficiency. 

iv
TABLE OF CONTENTS 

PREFACE................................................................................................................................ iii 

TABLE OF CONTENTS ..........................................................................................................v 

LIST OF FIGURES .............................................................................................................. viii 

LIST OF TABLES.................................................................................................................... x 

1. SPATIALLY AND SPECTRALLY RESOLVED ELECTRO- AND PHOTO- 

 LUMINECENCE IN CdTe SOLAR CELLS ......................................................................1 

 1.1 Introduction ...................................................................................................................1 

 1.2 Development and Application of Electroluminescence Imaging ................................2 

 1.2.1 Experimental setup and data analysis .................................................................2 

 1.2.2 Influence of processing and stressing on EL intensity and pattern......................4 

 1.2.3 Summary ............................................................................................................8 

 1.3 Effects of Cu Studied with Patterned Doping and Spatially Resolved..........................8 

 Luminescence Techniques.............................................................................................8 

1.3.1 Experiment ..........................................................................................................9 

1.3.2 Results and Discussion .....................................................................................10 

1.3.3 Summary ..........................................................................................................13 

1.4 Expanded Experimental Space for Luminescence Studies..........................................13 

1.4.1 Technical ..........................................................................................................13 

1.4.2 Results and discussion ......................................................................................14 

1.4.3 Summary ..........................................................................................................19 

1.5 Interactions between Cu and CdCl2 ............................................................................19 

 1.5.1. Experiment.........................................................................................................20 

 1.5.2 Results and Discussion .....................................................................................20 

 1.5.3 Summary ..........................................................................................................24 

1.6 CdCl2 Effect Studied with Non-uniform Treatment and Spatially Resolved 

 Luminescence Techniques ..........................................................................................24 

1.6.1 Experiment ........................................................................................................24 

1.6.2 Results and Discussion .....................................................................................26 

1.6.3 Summary ..........................................................................................................28 

1.7 Non-Uniformity Mitigation: The Effects of Window Buffer Layers ..........................28 

1.7.1 Experiment ........................................................................................................29 

1.7.2 Results and discussion ......................................................................................29 

1.7.3 Summary ...........................................................................................................32 

1.8 Long-Term Photoconductivity Transients in CdS .......................................................33 

1.8.1 Experiment .......................................................................................................33 

1.8.2 Results and discussion ......................................................................................33 

1.8.3 Summary ..........................................................................................................36 

1.9 Conclusions .................................................................................................................36

v
2. ADMITTANCE SPECTROSCOPY AND CAPACITANCE TRANSIENT 

 MEASUREMENTS AS METHODS FOR STUDYING DEEP ELECTRONIC 

 STATES IN CdTe SOLAR CELLS ..................................................................................37 

2.1 Introduction .................................................................................................................37 

2.2 Admittance Spectroscopy of Deep States in CdTe Based Cells .................................39 

2.2.1 Admittance spectroscopy (AS) basics ..............................................................39 

2.2.2 Modifications of admittance spectroscopy .......................................................41 

2.2.3 Some results of the AS application to CdTe cells ............................................43 

2.2.4 Summary ..........................................................................................................46 

2.3 Some Problems and Possible Approaches to Their Solution.......................................46 

2.3.1 Effect of high trap concentration ......................................................................46 

2.3.2 Technical limitations on the characteristic time of traps 

 for detection and studies ...................................................................................50 

2.3.3 Capture cross section. Very slow traps .............................................................51 

2.3.4 Transients .........................................................................................................55 

2.4 New System for Measuring Admittance in a Wide Temperature Range.....................56 

2.5 Preliminary Experimental Results Obtained with the New System ...........................58 

2.5.1 Bias induced transient ..................................................................................... 58t 

2.5.2 Light induced transients ...................................................................................67 

2.6 Summary .....................................................................................................................69 

3. EFFECT OF CdCl2, DOPING WITH Cu, AND STRESSING ON DEEP 

 ELECTRONIC STATES (DES) .......................................................................................70 

3.1 Samples........................................................................................................................70 

3.2 Admittance Spectroscopy Measurements and Analysis ..............................................72 

 3.2.1 AS spectra and their analysis. Trap signatures .................................................72 

 3.2.2 Attribution of detected traps..............................................................................76 

3.3 Deep States Studied with Capacitance Transients.......................................................77 

 3.3.1 Trap signatures. Arrhenius plots ......................................................................77 

 3.3.2 Relative densities of different traps...................................................................79 

 3.4 Effect of Stress.............................................................................................................79 

 3.5 Summary .....................................................................................................................83 

4. SCANNING TUNNELING MICROSCOPY AND LOCAL SPECTROSCOPY 

OF THE BACK CONTACT INTERFACE.......................................................................85

4.1 High Resolution Current Mapping at p+

-ZnTe/p-CdTe Back Contact Interface ........85 

4.2 New Experimental Capabilities ..................................................................................89 

4.2.1 High-resolution conductance mapping .............................................................89 

4.2.2 Measurements in high- and ultrahigh vacuum...................................................90 

5. NEAR FIELD SCANNING OPTICAL MICROSCOPY OF PHOTO-CURRENT AND 

 PHOTOLUMINESCENCE IN CdTe SOLAR CELLS.....................................................91 

 5.1 Plan-View Imaging of Photocurrent ............................................................................92 

5.1.1 CdTe/CdS cell ...................................................................................................92 

5.1.2 CIGS/CdS cell....................................................................................................93 

5.2 NSOM Studies of Photoluminescence on CdTe/CdS Cells.........................................94 

vi
6. CELL PROCESSING ......................................................................................................100 

6.1 Electrodeposition .......................................................................................................100 

 6.2 Gas Jet Deposition System and the Cell Fabrication.................................................101 

 6.3 Characteristics of Recently Fabricated GJD Cells ....................................................103 

7. SUMMARY.....................................................................................................................106 

8. REFERENCES ................................................................................................................109 

9. ACKNOWLEDGEMENTS.............................................................................................115 

 

10. APPENDICES .................................................................................................................116 

10.1 Personnel.................................................................................................................116 

 10.2 Laboratory Improvements.......................................................................................117 

 10.3 Publications.............................................................................................................118 

 10.4 Presentations ...........................................................................................................120 

vii
LIST OF FIGURES 

Figure 1.1 EL experimental setup.............................................................................................2

Figure 1.2 EL changes with current .........................................................................................3 

Figure 1.3 Composite, low magnification EL images .............................................................3 

Figure 1.4 High magnification image of UT cell shown in Fig. 1.1 above .............................3 

Figure 1.5 Selected histograms ................................................................................................4 

Figure 1.6 Effect of CdCl2 treatment on EL. ...........................................................................5 

Figure 1.7 Effect of CdCl2 treatment on EL II .......................................................................5 

Figure 1.8 EL from stressed VTD cell .....................................................................................6 

Figure 1.9 EL from a CdTe sub-module .................................................................................7 

Figure 1.10 Line-scan of EL intensity and modeled current for sub-module ............................8 

Figure 1.11 Schematic representation of patterned Cu doping ..................................................8 

Figure 1.12 EL images of Cu dot cells as a function of temperature ......................................10 

Figure 1.13 EL and PL scans....................................................................................................11 

Figure 1.14 Spectra from Cu and non-Cu regions....................................................................12 

Figure 1.15 EL non-uniformity and mean intensity versus light bias .....................................14 

Figure 1.16 EL images with no light bias versus 3.1.eV light bias ..........................................15 

Figure 1.17 EL image (a) and EL and PL spectra (b) taken from three different points 

on a cell .................................................................................................................15 

Figure 1.18 Normalized EL (a) and PL (b) spectra .................................................................16 

Figure 1.19 Integrated PL as a function of applied bias ..........................................................17 

Figure 1.20 PL spectrum versus bias .......................................................................................18 

Figure 1.21 Modeled depth distribution of EL versus PL intensities ......................................18 

Figure 1.22 EL from a Cu dot cell with the wet, impure CdCl2 treatment. Average current 

 density=25 mA/cm2 ..............................................................................................21 

Figure 1.23 EL from a Cu dot cell with the wet, impure CdCl2 treatment. Average current 

density = 125 mA/cm2 ..........................................................................................21 

Figure 1.24 EL inverting Cu dot cell with the vapor, impure CdCl2 treatment .......................22 

Figure 1.25 EL inverting Cu dot cell with the vapor, pure CdCl2 treatment ...........................22 

Figure 1.26 EL non-inverting Cu dot cell with the vapor, impure CdCl2 treatment.................23 

Figure 1.27 EL non-inverting Cu dot cell with the vapor, pure CdCl2 treatment.....................23 

Figure 1.28 Experimental setup for processing CdCl2 dots ....................................................25 

Figure 1.29 EL images versus temperature for CdCl2 dot cells ..............................................26 

Figure 1.30 EL spectra from CdCl2 dots and untreated regions ..............................................27 

Figure 1.31 EL spectra from a CdCl2 region at different injection intensities ........................28 

Figure 1.32 Electroluminescence from cell produced at CSM and NREL with and without 

an HRT layer ........................................................................................................30 

Figure 1.33 Light-beam induced current measurements on CSM cells with and without 

an HRT layer ........................................................................................................30 

Figure 1.34 Mapping the CdS with optical transmission and AFM measurements ...............31 

Figure 1.35 A proposed explanation of why high spots in the AFM images of CdS-coated 

plates could correspond to thin spots in the CdS .................................................32 

Figure 1.36 Photoconductive CdS effect in EL image of CU dot cell .....................................33 

Figure 1.37 Photoconductive CdS effect as a function of excitation energy ...........................34 

viii
Figure 1.38 Photoconductive CdS effect on EL bright spots with the CSM cell`....................35 

Figure 2.1 Hole concentration vs. temperature in CdTe with = 0.35 eV, Ea

= 10 NA

18 cm-3, and varying compensation degree D NNC A = / ....................38 

Figure 2.2 Capacitance and conductance of a VTD/CSM cell with the Cu/Au back 

contact measured at room temperature..................................................................43 

Figure 2.3 dfdG )/( − f plot with characteristic times of 1.6 and 0.35 μs ........................43 

Figure 2.4. AS measurements on cells with different postdeposition treatments....................45 

Figure 2.5 Correction factor F tb ),( in Eq. 2.13 ..................................................................47 

Figure 2.6 Apparent doping level versus real one for the two-trap-level model....................49 

Figure 2.7 Capacitance transients at T= 230C for the samples listed in table 2.3 ................59 

Figure 2.8 Capacitance transients at T=230C for cell D220B in a wide time range ..............60 

Figure 2.9 Capacitance transients at reduced temperature for cell D220B ...........................61 

Figure 2.10 DLTS function for the capacitance depemdence on time after a voltage switch 

from -1 V to 0 V, in the temperature range of 100C to -250C. Cell D220.............62 

Figure 2.11 Capacitance transients at elevated temperatures for cell D217C .........................63 

Figure 2.12 DLTS function for the data presented in Fig. 2.11................................................64 

Figure 2.13 Variation of the C-f dependence with varying temperature in the 

elevated temperature rang. Cell D220B ................................................................64 

Figure 2.14 Variation of the C-V dependence with varying temperature in the 

elevated temperature range. Cell D220B ............................................................65 

Figure 2.15 Capacitance transients measured after switching bias from 0 V to V=+0.5V 

and from 0 V to -1 V. Cell D220B, T≈220C, f=100kHz ......................................66 

Figure 2.16 Light induced capacitance transients for 0.1 mW/cm2 lighting intensity 

 and wavelengths of 680 nm, 1064 nm, and 1300 nm ...........................................68 

Figure 2.17 Capacitance transients induced by the sub-bandgap (1064 nm) light of 

 various intensities..................................................................................................68 

Figure 3.1 Light JV curves for the four CdTe solar cells .......................................................71 

Figure 3.2 AC conductance versus temperature lines shown for AC frequencies 

ranging from 1kHz (bottom line) to 1MHz (top line) ..........................................73 

Figure 3.3 Temperature dependence of capacitance at frequencies ranging from 1 kHz 

(top line) to 1 MHz (bottom line) ........................................................................75 

Figure 3.4 Arrhenius plots for traps detected by AS (H1, H2, H3) and those 

detected by C-Tr (H4, H5, H6, E2, E3). H1to H6 are traps for holes; 

E2 and E3 are traps for minority carrier electrons ................................................78 

Figure 3.5 Current-voltage characteristics of [No Cu – Yes CdCl2] and 

[Yes Cu – Yes CdCl2] cells. (a) before stress, (b) after stress ..............................81 

Figure 3.6. Arrhenius plots for cells [Yes Cu – Yes CdCl2] and [Yes Cu – Yes CdCl2] 

before (empty marks) and after stress (filled marks). ...........................................82

Figure 4.1 XTEM cross-section of a p-CdTe/n-CdS solar cell, showing the cell structure 

and contact configuration used in our STM measurements ..................................86 

Figure 4.2 Identification of the contrast mechanism in STM current maps 

on CdTe/CdS cells ................................................................................................87 

Figure 4.3 (a) Topographic image obtained on a ZnTe capped CdTe/CdS cell; 

(b) Current map of the same region at 2V tip-collector bias;(c) Overlay of 

topographic data (color) with contours of the 2V current map ............................88 

ix
Figure 4.4 Conductance spectra obtained at a grain boundary and in intragrain 

 Material .................................................................................................................89 

Figure 4.5 Simultaneously measured topography and conductance maps 

(a) dI/dV at V = +1 V; (b) Topography; (c) dI/dV at V = - 2.3V .....................90 

Figure 4.6 Compact UHV STM system for imaging and spectroscopy on 

solar cell materials ................................................................................................91 

Figure 5.1 Correlation between topography and photocurrent plan-view images 

on CdTe cell of substrate configuration fabricated at the U. of Toledo................92 

Figure 5.2 Correlation between topography and photocurrent on CIGS cell ........................93 

Figure 5.3 Difference in PCC for wavelengths of 632.8 nm (on the left) 

and 457nm (on the right) ......................................................................................93 

Figure 5.4 Topography (a) and photocurrent (b) images for the CIGS cell ...........................94 

Figure 5.5 Near field PL spectrum obtained on a cell of substrate configuration 

λexcit. = 488nm . Nominal temperature T~250

C................................................95 

Figure 5.6 Correlation between the topography and PL NSOM plan-view images. 

The PL images are taken at λ~750, and λ~850.....................................................96 

Figure 5.7 Effect of bias on the NSOM PL images................................................................97 

Figure 5.8 Spatial distribution of the NSOM PL intensity at the various biases 

PL excited by the light of λ=750 and λ=850 nm ..................................................97 

Figure 5.9 Topography and PL NSOM images from the cross section of a cell ...................98 

Figure 6.1 Nominal temperatures of a source and substrate versus time 

for a typical growth process ...............................................................................102 

Figure 6.2 Characteristics of cells grown using GJ-deposition of CdTe 

on different substrates with different CdCl2 treatments ......................................104

LIST OF TABLES 

Table 2.1 Characteristics of cells with different postdeposition procedures ............................. 45 

Table 2.2 Dependence of the characteristic time on temperature: τ τ KT )300(/)( ................... 51 

Table 2.3 Variations in processing specifications and front contact structure 

 in the cells studied...................................................................................................... 58 

Table 3.1 Activation energy, Ea, apparent cross section, σ, and trap contribution 

 to capacitance, Ct, for traps detected with AS in cells with different 

 combinations of Cu and CdCl2 ................................................................................. 74 

Table 3.2 Traps detected with the capacitance transient (C-Tr) studies ................................... 78 

Table 3.3 Transient capacitance Ct for various types of traps ................................................... 79 

Table 3.4 Cell parameters before and after stress ...................................................................... 80 

Table 3.5 Transient capacitance Ct for various types of traps before and after stress ............... 82 

x
1. SPATIALLY AND SPECTRALLY RESOLVED ELECTRO- AND PHOTO-

 LUMINECENCE IN CdTe SOLAR CELLS 

1.1. Introduction 

Our previous studies [1] indicated that CdTe/CdS cells emitted light under forward bias 

in the photon energy range in the vicinity of the CdTe bandgap. Electroluminescence (EL) was 

observed at room temperature as well as at T~15 K. Though the excitation mechanism is 

different, EL is similar to the more commonly employed photoluminescence (PL). Instead of 

optical excitation (PL), excess electron-hole pairs are injected electrically in EL measurements. 

Their recombination produces luminescence. The intensity of EL emission is roughly 

proportional to the product of the electron and hole concentrations. If n-CdS is doped heavier 

than the p-CdTe absorber layer, the × pn product reaches a maximum in the CdTe near the 

junction and this is the region where EL originates. Our previous spectral studies of EL are 

consistent with this analysis [1]. Furthermore, EL spectra have been found to be similar to PL 

spectra in CdTe [1] and also for many other materials The same was found previously for many 

other materials such as SrS:Cu and SrS:Ag, Cu, Ga thin films [2], Si+

-implanted SiO2 [3], CdS 

nanocrystals [4], and Si1-xGex alloys [5]. However, few studies of EL in CdTe/CdS cells are 

known whereas studies of PL are numerous. Our intensive EL studies are not intended to replace 

PL. The similarity of the spectra observed along with difference in the mechanisms and 

techniques of excitation, lends further credence to the usefulness of EL and PL as 

complementary techniques and the ability to use information from one to explain effects in the 

other. 

One inherent property solar cells based on polycrystalline thin films is nonuniformity of 

the photo-electronic properties in various spatial scales. It is obvious that macro-nonuniformities 

reduce the efficiency of solar modules. Recently it was shown (Karpov, Shvidka, Compaan, et.al, 

[6]) that a single micro-nonuniformity, e.g. small area region of reduced open circuit voltage, can 

significantly affect the performance of the whole cell. Standard characterization techniques such 

as current-voltage, capacitance-voltage, and quantum efficiency measurements average 

properties over the area of a solar cell and do not detect micro-nonuniformities. Contrasting 

these methods is a new wave of spatially resolved characterization tools such as 

cathodoluminescence (CL) [7], electron beam induced current (EBIC) [8,9], laser beam induced 

current (LBIC) [10], near-field scanning optical microscopy (NSOM) [11], PL mapping [12], 

and, finally, the spatially resolved electroluminescence. 

There are several advantages making EL attractive as a characterization tool. Though our 

EL technique does not have the highest spatial resolution among the techniques listed above, the 

spatial dynamic range of resolved EL data may be the greatest, with a linear resolution of several 

microns to scans of over a centimeter. Also, unlike other methods that usually involve high 

injection rates, the minimum degree of excitation required to induce measurable EL could be on 

the order of or even less. This moderate excitation level increases confidence that the 

observed EL patterns can be related to cell performance in normal operation. Low injection 

ensures that characterization will not degrade device performance, as verified by I-V data taken 

before and after EL imaging. EL gives insight into material quality because recombination 

centers degrade EL emission and cell efficiency alike. As a final point, EL is technically simple 

to perform as it can be observed at room temperature, and data acquisition is relatively quick. 

sc J

1
Because it is simple to implement, EL may find use as a standard production monitoring tool in 

an industrial application. 

1.2 Development and Application of Electroluminescence Imaging 

1.2.1 Experimental setup and data analysis 

` 

Figure 1.1. EL experimental setup. 

The main experimental setup used in these studies is shown in Fig. 1.1. Biasing to induce 

EL was provided by a Keithley 2120 SourceMeter. This device also allowed I-V measurements 

without removing the cell from the EL apparatus. EL images of cells were acquired with a Santa 

Barbara Instrument Group ST-5C cooled CCD camera that had a 320 × 240 array of 10 by 10 

μm

2

 square pixels. The cell was mounted on a micrometer-controlled stage and imaged through 

either a 1.3x or 10x microscope objective. 

The 10x objective allowed for resolution on the order of the optical limit for the infrared 

EL, namely several microns. The 1.3x lens imaged a 1.6 × 1.2 mm2 rectangular area of a cell. 

Multiple images could then be stitched together graphically to form a composite of the entire 

cell. Cells to be compared were imaged at constant average current density as noted below, and 

standard CCD acquisition time is 60 seconds. 

 For large-area EL measurements of sub-modules, a Sony SSCM183 black and white 

security camera was employed. The camera had a manually adjustable 3.5-8 mm focal length 

wide-angle lens. These optics allowed for imaging a large area of the cell with the camera in 

close proximity. The large solid angle image increased sensitivity of the camera at the expense of 

some aberration of the image. 

Often the EL emission is concentrated in small bright spots. The brightness and the 

number of spots increases with increasing injection current levels (Fig. 1.2). Irreversible changes 

in EL were observed with current density in excess of 1A/cm2

. Currents below this threshold 

applied for periods less than an hour did not appear to change the EL images. 

2
 

 J = 20 mA/cm2 J = 90 mA/cm2 J = 190 mA/cm2

Fig. 1.2. EL changes with current 

Fig. 1.3 shows stitched EL images of cells from a vapor transport deposition cell (VTD), 

a University of South Florida cell (USF), and a University of Toledo cell (UT), manufactured 

with different deposition techniques (vapor transport, close-space sublimation, and sputtering, 

respectively) and back contacts (proprietary, Ag paste, and Au, respectively). 

FS USF

4.4 mm

UT

Figure 1.3 Composite, low magnification EL images. All have the same scale in dimensions 

but not brightness. Differences in total EL intensity did not exceed ~30 %. Note, the VTD cell 

was scribed from a circle into a sector to approximate the area of the UT cell while still showing 

the edge of the original cell. Current density is 33mA/cm2 for all images. 

Though mean emission intensity was comparable, pattern and degree of non-uniformity 

varied greatly from institution to institution. To quantify nonuniformity we introduced a 

parameter called the “nonuniformity” (NU) and defined it as the standard deviation of EL 

intensity normalized by mean EL intensity. The calculated NU values for the cells were VTD: 

0.22, USF: 0.15, and UT: 0.11. 

Comparison of the low-resolution image of the UT cell in Fig.1.3 to the highly magnified 

image of the same cell in Fig. 1.4 reveals non-uniformity on different scales. 100 



m 

Figure 1.4. High magnification image of UT cell shown in Fig. 1.3. Current density is 33 

mA/cm2

3
Selected histograms in Fig.1.5 make differences in EL non-uniformity more visible. 

Note how the low magnification UT image has the narrowest histogram (indicating that it is the 

most uniform), whereas the FS histogram is significantly wider. The VTD histogram has a peak 

with a shoulder, indicating two separate populations. 

Figure 1.5. Selected histograms 

The differences between the low- and high-magnification image histograms for the UT 

cell indicate that there coexist several characteristic non-uniformity spatial scales. To detect and 

study them we have used variogram analysis which is a geostatistical technique used to quantify 

the spatial correlation of data [13]. The extent of this spatial correlation helps to quantify the 

scale and possible anisotropy in the non-uniformity structures underlying electroluminescence. 

For example, correlation feature sizes of ~10 μm and ~60 μm were found in the UT cells. 

1.2.2 Influence of processing and stressing on EL intensity and pattern 

Cadmium chloride treatment 

A study was conducted using cells with VTD material deposited elsewhere whose back 

contacts (ZnTe:Cu/Au) were processed at CSM. EL from cells that underwent CdCl2 treatment 

was compared to EL from nominally identical cells without CdCl2 treatment. Both sets of cells 

showed isolated bright spots less than 20 microns in size with emission between the bright spots 

that was indistinguishable with the dark counts (Fig. 1.6). With this form of EL emission, which 

likely resulted from non-ideal back contact processing, non-uniformity was un-quantifiable 

because of the high contrast. The cells with CdCl2 had more bright spots, and spot intensity was 

greater. Furthermore, the rate of increase of EL intensity with current was lower in the cells 

without CdCl2. Drive level capacitance profiling (DLCP) was also used to estimate the deep 

level density in these cells [14]. It was found that the cells subjected to the CdCl2 treatment 

possessed more traps than the cells without CdCl2 treatment. This result was unexpected, as a 

greater trap concentration would imply less luminescent recombination, but is important 

nonetheless because it proves that effects of CdCl2 treatment (a crucial step for producing 

efficient CdTe devices) can be observed with EL. 

4
Figure 1.6. Effects of CdCl2 treatment on EL. Current density = 50 mA/cm2

. 

 This experiment was later repeated with a set of cells that exhibited a more-uniform EL 

pattern in order to quantify non-uniformity (Fig. 1.7). Again, the CdCl2 treated cell exhibited 

greatly enhanced emission with respect to the untreated cell. The untreated cell required an 

extreme injection rate of 1.5A/cm2 in order to show measurable EL, as compared to a more 

modest 75 mA/cm2 for the CdCl2 treated cell. The NU was 0.22 for the treated cell and 0.63 for 

the untreated cell. Though the injection conditions varied greatly, non-uniformity generally 

decreases with increased injection, thus supporting the conclusion that the CdCl2 improves EL 

uniformity. Also relevant is the pattern of the non-uniformity in the treated cell. The scale of the 

features is on the order of hundreds of microns. This pattern likely results from droplets present 

in a wet CdCl2 treatment. 

Figure 1.7. Effects of CdCl2 treatment on EL II. Current density = 75 mA/cm2 for the CdCl2

treated cell and 1,500 mA/cm2 for the untreated cell. 

Doping with Cu

 EL in cells from Colorado State University (CSU) was found to be profoundly influenced 

by the concentration of Cu in the cell (not shown). There was a noticeable change in EL (both in 

intensity and pattern) due to the presence of Cu. The cells were processed with 0, ~0.75, ~1.5, 

5
and ~6 monolayers (ML) of Cu through vapor deposition. Cell performance improved with 

increased Cu content. The most brightly emitting cells were those with the 0.75 and 1.5 ML of 

Cu. These cells also demonstrated the least-uniform EL intensity. Compared to these two 

processing conditions, 6 ML of Cu lead to a small decrease in emission intensity (less than a 

factor of 2) as well as an increase in uniformity. The cells with no Cu showed a ~4x decrease in 

emission intensity and slightly increased uniformity compared to those cells with Cu. Though 

spectrally resolved EL emission of these cells might yield information on the cause of the 

observed change in the intensity and non-uniformity with respect to Cu content, EL spectra could 

not be obtained due to low signal strength. 

Response to stress 

 In general, the observed response of EL to stress has been a dramatic drop in EL intensity 

with amounts of stress that did not degrade efficiency much at all. Compaan et al. have also 

observed a similar effect with EL and electrically biased PL [15]. This drop in emission intensity 

is usually accompanied by an increase in non-uniformity. These huge changes suggest that EL 

can perhaps be used as an early warning system for detecting cells that are likely to degrade 

quickly. 

 A more dramatic stress effect has been seen on VTD cells: the change from a mottled, but 

relatively uniform EL emission pattern to emission coming only from isolated spots (Fig. 1.8). 

This effect occurred in cells light soaked at open-circuit voltage (Voc) for 56 days as well as in 

cells stressed in light for only 7 days at either Voc or –2 V reverse bias. Comparing I-V curves 

(not shown), one can see a large series resistance in the light curves as well as a high resistance 

at forward bias in the dark (the condition in which EL is measured). These high resistances 

suggest a degraded back contact [16]. Consistent with the high resistance is a model in which the 

back contact has degraded non-uniformly, such that some small areas still have low resistances 

through the cell. Since the metallized back contact is an equipotential surface, current would 

stream through these small areas, inducing EL only in isolated dots. The fact that the dots are 

mostly round in shape supports this model because current streaming through a microscopically 

small low resistance region of the back contact would spread in a radial fashion before it reached 

the main junction, resulting in circular EL emission. 

Figure 1.8. EL from stressed VTD cell. The dashed line in the stressed image indicates the edges 

of the cell. Current density = 33 mA/cm2 for both. 

6
EL measured on the CdTe/CdS sub-module 

 EL was also studied on the sub-module scale. Data was taken from CdTe sub-modules 

made from VTD material produced with a moving substrate. The sub-modules consisted of eight 

1 × 10 cm2

 cells defined by scribes that were connected in series through an integrated process. 

A total current of 1 A, that corresponds to an average current density of ~125 mA/cm2 per cell, 

was used to induce EL in these cells. An image is shown in Fig. 1.9. Two features of note exist in 

the EL image. First, there is a subtle variation in intensity parallel to the direction of the scribe 

lines. Moving from right to left, the EL changes from brighter to dimmer to brighter again. This 

variation occurs perpendicular to the direction in which the substrate is moved during deposition 

and therefore likely results from a systematic non-uniformity in the deposition process. 

Figure 1.9. EL from a CdTe sub-module. Average current density = 125 mA/cm2

. 

Secondly, the EL is concentrated in bright strips that fade in the direction perpendicular to 

the scribe lines. This feature dominated the EL emission and resulted from the non-negligible 

sheet resistance of the TCO front contact. Portions of the cells closest to the forward-biased 

scribe line exhibited greater EL intensity because they received the greatest current. This 

hypothesis was confirmed with modeling by using a program [17] that simulated the voltage 

drops and current through a series of micro-diodes and resistors (representing the main junction 

of the cell, back contact and TCO distributed resistance). EL intensity has been found to be 

proportional to current density, which varies between scribes. Fig. 1.10 shows a line-scan taken 

from the EL image data in comparison to the current flowing through each microdiode in the 

model. A good qualitative correlation between the model and the EL line scan corroborates the 

analysis above. 

7
Figure 1.10 Line-scan of EL intensity and modeled current for sub-module. 

1.2.3. Summary 

EL images have been obtained of CdTe solar cells at high (several μm) spatial resolution 

with low current injection levels (on the order of short-circuit current densities). Non-uniform 

patterns have been shown to occur at different scales on differently grown cells, suggesting that 

perhaps different physical models should be used when interpreting the data for different 

materials. EL is also affected by changes in important processing steps, namely CdCl2 treatment 

and Cu doping before or along with application of the back contact. Accordingly, EL may be 

able to help unravel the effects of CdCl2 treatment and may aid in finding how to stop Cu from 

leaving the back contact. Also, even gently stressed cells have shown drastically affected EL 

intensity. EL may prove useful in an industrial setting because of its possible ability to predict 

long-term stability based upon short term stress data. This non-destructive testing technique has 

met some success with GaP LEDs [18]. Changes in EL in some cells due to stressing imply 

highly non-uniform back contact degradation. Finally, EL was used to identify non-uniformities 

resulting specifically from the processing of sub-modules, suggesting that it may aid in scale-up 

from the laboratory to an industrial setting. 

1.3. Effects of Cu Studied with Patterned Doping and Spatially Resolved 

 Luminescence Techniques 

 The application of Cu to improve CdTe performance, particularly to reduce back contact 

series resistance (Rs), is crucial to obtaining high efficiency devices as shown by its presence in 

record CdTe solar cells. At the same time, Cu is suspected of having some detrimental effects on 

stability in CdTe/CdS devices. 

 The defects Cu can form include Cu substitutions onto a Cd site (CuCd), Cu interstitials 

(Cui), and various complexes with both native and non-native defects formed during processing. 

Previously, we have tried to isolate Cu effects by fabricating nominally identical devices with 

and without intentionally adding Cu. However, the interpretation of these experiments is 

complicated by the fact that Cu is present in most CdTe thin films, even when it is not 

intentionally added [19]. Furthermore, identically processed cells, even nearest neighbors from 

the same substrate, suffer from non-uniformity issues [6], resulting in different characteristics, 

8
making it even more difficult to isolate the effects of Cu. In this study, we avoid many of these 

problems by creating a well-defined array of Cu doped and undoped regions in a single cell. 

Spatially resolved electro-optical characterization techniques then allow Cu doped and undoped 

regions of the exact same cell to be studied. 

1.3.1. Experiment 

 A VTD material was used as the basis for the devices studied. Post-processing and back 

contact application was conducted at the CSM. The material was first etched for 30 seconds with 

a 0.015% Br: Methanol solution. In a standard process, 30 Å of Cu is then evaporated over the 

entire surface of the substrate. After annealing in nitrogen at 200 °C for 7 minutes, excess Cu 

was removed with a 30s dip in a 0.005% Br: Methanol solution. Finally, 500 Å of Au is 

evaporated to complete the back contact. The Au pad, 1.6mm in diameter, defines the cell size. 

 To introduce a well-defined array of Cu rich regions into a cell for this study, one 

modification was made. Fig. 1.11 schematically demonstrates deposition of this array. Instead of 

depositing Cu over the entire back surface of the CdTe, Cu was evaporated through a shadow 

mask with circular holes ~150 μm in diameter. The result is a cell with an Au back contact and 

with many circular Cu rich regions (dots) underneath, occupying ~15% of the area of the cell. 

Figure 1.11 Schematic representation of patterned Cu doping. 

 Electroluminescence (EL) and photoluminescence (PL) were employed as probes of 

material properties. In the case of PL, carriers are injected optically (1.96 eV helium-neon laser 

was used) and are to first order unaffected by contact properties under the open circuit conditions 

used here. The absorption coefficient for CdTe for the light used is α = 4.8 ×104

 cm-1. All EL and 

PL measurements were performed from the glass side of the cell and under the Au back contact 

except for the lower right corner of the EL images. 

 To obtain spatial images of EL, the cell was forward-biased to obtain an average current 

density of 25 mA/cm2

 and the resultant EL was focused onto the entrance slit of an Acton 300i 

spectrometer with a Princeton Instruments Spec-10:100BR liquid nitrogen cooled CCD array 

detector. The spectrometer had two gratings and a mirror mounted on a rotating turret. Using 

the mirror, and opening the slit to 2 mm, spatial images of EL as in Fig. 1.12 were obtained. For 

spectral resolution, a grating is rotated into position and the slit width is reduced. The signal that 

passes through the spectrometer is dispersed onto the CCD detector. Because the detector is a 2-

9
dimensional array, spectrographic information is obtained along the horizontal axis of the CCD, 

while spatial information is obtained along the vertical axis. With PL, the laser beam was 

focused with a cylindrical lens such that the beam was roughly uniform (within a factor of 3) 

over the area of the cell imaged through the slit. The average laser power was ~20 mW/cm2 (~0.2 

suns), similar to normal operating conditions. PL data was normalized to the laser power. 

 

Figure 1.12 EL Images of Cu dot cell as a function of temperature. Average injection level is 

25 mA/cm2. 

1.3.2. Results and discussion 

EL emission correlated well with the location of the Cu dots as shown by the images in Fig. 

1.12. From room temperature down to T~100 K, EL intensity was up to an order of magnitude 

greater on the Cu dots. This result is consistent with Cu reducing the back contact resistance. 

Below 100 K, the EL pattern inverted, with the intensity from non-Cu (undoped) regions several 

times that of Cu-doped regions. This inversion occurred within a temperature range of 

approximately 25 K, as shown in Fig. 1.12. A possible explanation for this inversion involves 

changes in the nature of the defects responsible for transport. Assuming that Cu incorporates 

itself primarily as CuCd for these processing conditions, Cu regions should contain fewer VCd

defects as Cu occupies the vacant Cd sites. Theoretical work by Wei and Zang places CuCd as a 

deep acceptor at Ev + 0.22 eV, whereas VCd is shallower at Ev + 0.13 eV [20]. (Numerous 

experimental studies provided E(CuCd) ≈Ev+ 0.3 eV see, e.g., [21], but it was pointed out in [20] 

that the accuracy of calculated levels is ~0.1 eV). As the temperature drops below 100 K, the 

deeper CuCd levels freeze out, creating a higher Rs in Cu doped regions than in undoped regions 

where shallower defects (such as VCd) are still electrically active. Undoped regions then have the 

lower Rs, draw more current, and therefore show brighter EL emission. Because the signal is 

greater at lower temperatures, all comparisons between EL and PL were made with the data 

taken at 70 K. It is important to note that cells processed with a different CdCl2 treatment 

(namely very high purity CdCl2 with vapor deposition) suppressed the inversion, suggesting that 

10
CdCl2 treatment can affect the defect structure of the material as well (as discussed in Sec. 3). 

However, inversion was present in all cells used in this study. 

 The data in Fig. 1.13 were obtained with a slit width of 100 μm and a 300 groove/mm 

grating resulting in both spatial and spectral information. First, data were integrated over all 

photon energies to produce the line scans shown. Though EL shows changes in intensity 

correlated spatially with the Cu dots, integrated PL intensity shows no correlation with Cu. In 

other words, EL emission is dominated by Cu doping while integrated PL (to lowest order) is 

unaffected by Cu. Because EL relies upon electrical injection whereas PL uses direct optical 

injection, one can conclude that Cu greatly affects conduction through the back contact to the 

main diode. At room temperature Cu increases conduction, while at low temperature Cu 

suppresses conduction, as discussed above. 

Figure 1.13. EL and PL line scans. The scans were taken in the region of the dashed line in 

T=70 K EL image to the right. The top scans are integrated over photon energy where EL is 

correlated with the location of Cu whereas PL is not. A line scan taken at 1.43 eV shows EL and 

PL patterns that are spatially correlated. A line scan taken at 1.31 eV shows EL and PL patterns 

that are anti-correlated. 

 When observing line scans from individual energies, one can see an effect of Cu on PL. 

At 1.43 eV, in the range of a shallow defect possibly related to VCd (Eg in CdTe is slightly above 

1.6eV at this temperature), PL intensity is suppressed in the Cu doped regions, correlating with 

the EL pattern. Probing a deeper feature at 1.31 eV (possibly related to CuCd) results in enhanced 

11
PL and suppressed EL over the Cu dot; the PL and EL patterns are anti-correlated. However, the 

EL is suppressed by the Cu less at this energy than at 1.43 eV. This result indicates that the EL 

pattern is being dominated by transport, but is still influenced by the variations in recombination 

that cause the enhanced PL signal in the Cu region. 

 Although emission lines tend to be broad in polycrystalline materials, the spectra 

presented in Fig. 1.14 show visible structure. 

Figure 1.14. Spectra from Cu and non-Cu regions. Note separate arbitrary intensity scales for EL 

and PL. EL intensity from the Cu region was multiplied by 3. PL data is to scale. Features are 

tentatively identified as band edge excitonic structure (X), Cd vacancy (VCd), donor-acceptorpair (DAP), and emission related to Cu substitutional defects (CuCd). Slight differences between 

Cu regions and non-Cu regions are visible only because of the techniques used here. T = 70 K. 

 In contrast to the spectrally integrated data above, subtle differences exist in PL between 

Cu doped and undoped regions. These features are similar in EL and PL and are tentatively 

identified in the figure based on prior studies, noting that their attribution is not unambiguous. 

Excitonic structure (X) is observed near the band-edge. The most intense feature is a donoracceptor pair (DAP) band that has been widely reported in other studies [7, 22, 23]. Both these 

features exist in spectra taken from both Cu and non-Cu (undoped) regions. The Cu regions of 

the cell show increased low-energy emission (towards 1.3 eV) in the donor-acceptor pairs (DAP) 

band, labeled as CuCd related.[7, 23]. The presence of some Cu related emission from the 

undoped region of the cell is not unexpected due to Cu impurities in the CdTe source material 

and CdCl2 treatment. Comparison of the spectral region above the DAP band shows more PL 

near 1.5 eV in the undoped regions, labeled as VCd related [23]. These small but real differences 

12
between the Cu and non-Cu spectra could only be confidently identified using the Cu dot 

technique along with spatially resolved characterization methods. Seemingly, the presence of Cu 

affects the concentration of another defect (such as VCd or related complex) in order to 

incorporate itself into the lattice (as opposed to Cui, which does not involve replacing another 

defect), agreeing with the identification of the deep and shallow features as CuCd and VCd related, 

respectively. 

1.3.3 Summary 

By Cu doping CdTe cells in a patterned array, we were able to confirm through a 

combination of spatially resolved EL and PL measurements that Cu improves performance at 

least partially by increasing carrier injection (lowering Rs) through the back of the cell. The EL 

pattern was found to invert at low temperature suggesting that the presence of Cu must modify 

another defect in the material that is responsible for transport. Also, our technique allowed us to 

detect subtle differences in PL and EL spectra between Cu doped and non-Cu doped regions. The 

spectral differences along with the EL inversion at low temperature suggest that, for this process, 

electrically active Cu incorporates itself into the lattice in the form of CuCd, displacing VCd or a 

related complex. 

1.4. Expanded Experimental Space for Luminescence Studies 

 In this section, we measure EL and PL with simultaneous spatial and spectral resolution. 

We utilize a large range of experimental variation including injection level, temperature, bias 

(light for EL, electrical for PL), and laser energy for PL. 

 The electrical injection of excess carriers makes EL very sensitive to the electron 

transport specifics of the material whereas PL induced by direct optical injection is less sensitive 

to electrical transport. Combining of the two allows one to isolate effects due to transport. In 

order to observe EL, one needs a complete device whereas PL is also applicable to films and 

single crystals. Since the PL measurements on CdTe crystals or films are not necessarily 

analogous to measurements of CdTe within a device because of additional processing steps such 

as CdCl2 treatment and back contact application, the study of both EL and PL on the same 

complete device are more reasonable. Measurements of this kind are presented in this section. 

 In previous section, using intentionally engineered non-uniformities we have shown that 

inhomogeneity in EL intensity results mostly from variations in transport, rather than variations 

in recombination [24]. Indeed, PL intensity varied little with location. Subtle spectral changes 

were noted between Cu-rich and Cu-poor regions in both EL and PL. Not addressed was the 

difference in spectral structure between EL and PL. This difference is the focus of this section. 

1.4.1 Technical 

 Light biased EL measurements were performed at room temperature. A VTD solar cell 

was forward biased to obtain an average current density of ~125 mA/cm2

. Monochromatic light 

bias was provided with a halogen lamp and narrow band-pass filters. A 1.91 eV low-pass filter 

placed in front of the objective allowed EL to pass while blocking reflected bias light. 

 The Acton 300i spectrometer and Spec-10:100BR CCD array detector was used for 

temperature variation, simultaneous spatial/spectral resolution, and combined EL/PL 

13
measurements. The specifics of this setup can be found in Sec. 1.3.2. With PL, the path of the 

laser beam to the sample was several meters, resulting in a wide beam that was roughly uniform 

over the area of the cell imaged through the slit. Except where indicated, a 1.96 eV HeNe laser 

was used to induce PL. Apart from bias-dependent PL, PL was measured under open-circuit 

conditions (Voc). 

1.4.2 Results and discussion 

 Fig. 1.15 shows that mean EL intensity increases with light bias while non-uniformity 

decreases (NU = the standard deviation of EL intensity divided by the average EL intensity). 

Keeping light power approximately constant, the photon energy used for the bias had the greatest 

effect in the 2 – 2.25 eV range, the highest that is not absorbed by the CdS, indicating that the 

effects seen were mostly due to changes in the CdTe with light (possibly photoconductivity of 

the CdTe). A no-current image was subtracted from the EL image to isolate EL from any PL that 

may have been generated by the bias light, fluorescence of the glass, or reflected light that was 

not completely filtered out. Fig.1.16 shows an example of EL images with and without light bias. 

Figure 1.15. EL non-uniformity and mean intensity versus light bias. EL non-uniformity (stand. 

dev. intensity/ avg. intensity) is indicated by the left axis and solid lines and relative EL intensity 

is indicated by the dashed lines. T = 300 K. 

 Both EL and PL spectra were found to have the greatest intensity and most amount of 

structure at lower temperatures, thus most of our effort was concentrated on low temperature 

measurements. As mentioned above, EL and PL spectra can differ significantly from each other. 

One way in which this effect manifests itself is a spectral variation by location that is present in 

EL but not in PL as can be seen in Fig. 1.17. 

14
Figure 1.16. EL images with no light bias versus 3.1 eV light bias. T = 300 K. 

Figure 1.17 EL image (a) and EL and PL spectra (b) taken from three different points on a 

cell. The EL spectrum varies by location but the PL spectrum does not. T = 83 K. EL injection 

level is of 0.13 mA/cm2. PL injection is of 25 mW/cm2 

15
Injection level variation revealed some key differences between EL and PL, shown in 

Fig. 1.18. 

Figure 1.18. Normalized EL (a) and PL (b) spectra. EL shows changes in structure and increase 

in band-edge emission with decreasing injection. In contrast, PL shows no change in structure 

and an increase in band edge emission (detailed in the inset figure) with injection. T = 83 K. 

 As injection level increased, EL and PL intensity both increased linearly. We note that 

this observation is in contrast to Compaan et al. who have observed a super-quadratic increase in 

EL intensity with current density [15]. We believe this discrepancy may have resulted from the 

use of very low injection conditions and/or a detector with a cut-off at 1.1eV here as opposed to 

1.4 eV in [15]. In EL, there are two major trends in the spectra as injection level increases: 

structure becomes less prevalent and the band edge features diminish. By contrast, PL spectra 

show no change in structure with varying injection and show increased band edge emission with 

increased injection. 

 To find the origin of these dissimilarities, the PL conditions were varied to better 

resemble the conditions under which EL is measured. These results are summarized in Fig.1.19. 

16
Figure 1.19 Integrated PL as a function of applied bias. There is non-monotonic behavior of 

PL intensity with bias at temperatures of ~100-150 K. Temperatures of 175 K and above show 

monotonic behavior. Injection level = 25 mW/cm2

. 

 One difference between EL and PL is the applied bias. Though some forward bias will 

result from the laser in PL measured at Voc, EL is under much higher forward bias for even 

moderate injection. Furthermore, current is allowed to flow out of the cell with EL and non-Voc

PL. At higher temperatures (175 K), forward bias (FB) increased PL intensity whereas reverse 

bias (RB) suppressed it with little change in spectrum. FB decreases the size of the depletion 

region, allowing injected carriers more time to recombine while RB increases the depletion 

region, sweeping carriers away before they can recombine. As the temperature is decreased to 

150 K, the RB behavior remained qualitatively the same whereas FB behavior changed. +1 V FB 

greatly increased PL intensity with respect to Voc but +2 V FB yielded decreased emission with 

respect to +1 V (though still slightly greater than Voc). Again, the spectrum changed little with 

applied bias. This non-monotonic dependence of PL intensity on voltage can possibly be 

explained by considering effects of the back contact Schottky barrier. When the size of the main 

junction is attenuated under FB, the Schottky barrier is under RB and therefore grows in size. 

There is no reason to expect the temperature dependence of the main junction and Schottky 

barrier to be identical. Perhaps at 150 K, the Schottky barrier width grows faster with bias than 

the main junction. At +2 V FB, the back barrier may be large enough to separate light-induced 

electron-hole pairs and suppress PL emission. 

 Below 100 K, PL intensity was nearly constant with applied bias. However, the spectrum 

changed. With greater than +1 V FB, the band edge emission was suppressed (Fig. 1.20), much 

like EL under higher injection conditions. Still, no change was observed in spectral structure of 

the main peak. Therefore, the opposite trend of the EL and PL band edge as injection level is 

varied can be attributed to the difference in bias conditions and/or current transport. Note, PL 

intensity was much greater than EL intensity at the biases used. Accordingly, EL contribution to 

the biased PL signal was negligible. 

17
Figure 1.20 PL spectrum versus bias. Note the decreased band edge emission in forward biased 

PL at temperatures below 100 K. The inset shows a detail of the band edge. Injection level = 25 

mW/cm2

. 

 Besides voltage, EL measurement conditions could be dissimilar from PL because of the 

location of recombination. In addition to lateral variation as shown in Fig. 1.17, EL and PL could 

originate from different depths. PL should mainly originate from approximately the same depth 

at which the laser is absorbed, i.e. 99% within the first micron of CdTe for photons of 1.96 eV. 

The depth of EL recombination can be approximated by taking the product of free electron and 

hole concentrations, n × p [25]. As shown in Fig. 1,21, modeling with AMPS-1D places the peak 

in free carrier concentration, (and therefore EL emission) at the main junction, as in PL, but 

penetrating deeper into the CdTe than the PL. 

 Figure 1.21 Modeled depth distributions of EL versus PL intensities. The n × p product 

distribution indicates the depth of EL emission and the distribution of absorption of different 

laser wavelengths indicates the depth of PL emission. 

18
 By changing to a lower energy laser, however, the laser absorption depth (and therefore 

the PL emission) can be changed to more closely resemble the depth profile of the EL emission. 

A tunable Ti-sapphire laser was used for this purpose, providing a range of laser energies down 

to 1.57 eV. This energy was very close to the CdTe band gap at 83 K and thus should provide 

electron-hole pairs throughout the depth of the device. No change in the PL spectrum was 

observed with the change in laser energy indicating that the difference in EL and PL spectra is 

not due to a difference in the depth of recombination. 

 We could not achieve an increase in spectral structure by changing the bias conditions of 

PL or the laser excitation energy (and hence sampling depth of PL). We speculate that the 

increased structure in EL spectra likely results from recombination that is laterally separated 

from that of PL. This hypothesis is further supported by the EL spectral variations observed by 

altering location. The fact that PL spectra do not vary by location implies that the variations 

occur on a scale smaller than can be resolved with our equipment (less than 10 microns). This 

scale is approximately that of CdTe grains. There is much evidence for increased conduction 

along grain boundaries and/or certain grains. These results include increased current injected 

with a scanning tunneling microscope along grain boundaries and certain grains [26], depletion 

of grain boundaries as measured by scanning capacitance measurements [27], and increased 

photocurrent collection along grain boundaries [11, 28]. Because EL is selective of areas of 

lowest resistance, perhaps under low injection conditions it originates primarily from grain 

boundaries and/or certain grains. If these areas are more defective than the bulk, this reasoning 

explains the increased spectral structure EL possesses at small currents. The lack of spectral 

structure of PL is also explained because the injection is uniform across grain boundaries and the 

bulk. This large sample area results in spectra that are averaged over a larger region (many grains 

and grain boundaries). 

1.4.3. Summary 

The differences between EL and PL spectra were explored at wide variation of conditions 

available for EL and PL measurements. The strongest spectral structure was obtained with low 

injection EL at low temperature. Band edge features in EL become weaker with increased 

injection while for PL the opposite behavior was observed. The enhanced spectral structure of 

EL may be attributed to the sensitivity of EL to current transport that may preferentially select 

defective areas of the device. For this reason, EL may have advantages relative to PL for defect 

studies of CdTe solar cells. 

1.5 Interactions between Cu and CdCl2 

 Two key processes that allow for production of high efficiency CdTe solar cells are the 

use of Cu doping at the back contact and employing an anneal in the presence of CdCl2

following CdTe deposition. Except for special laboratory test cells, these processes are used by 

all known current producers of CdTe based devices. The effects of Cu were isolated in Sec.1.3 

with the use of Cu dot cells. It was noted that some features attributed to Cu were visible even in 

non-Cu doped regions. The explanation of this observation was that some amount of Cu was 

present on the entirety of the cell due to Cu contamination of the CdTe source material and 

CdCl2. Here, different CdCl2 treatments (including the use of 99.999% pure CdCl2) were 

19
employed in hopes of enhancing the contrast between intentionally Cu doped and non-Cu 

regions. However, it was found that Cu related features were still present in all samples but that 

the type of CdCl2 treatment profoundly influenced the spectral structure of EL emission. 

1.5.1 Experiment 

 Cu-dot cells were processed using VTD material as described in Sec.1.3. The CdCl2

treatment, however, was varied. In addition to the standard CdCl2 treatment used for VTD 

material (the “wet” treatment), an anneal in the presence of CdCl2 was conducted using a dry, 

vapor process. Two CdCl2 bead sources were used: a standard grade of CdCl2 (“impure, vapor”), 

as well as 99.999% pure CdCl2, with 0 ppm Cu (“pure, vapor”). The CdCl2 anneal was 

performed at NREL. The CdTe substrate was suspended above the CdCl2 beads. The chamber 

was evacuated and the CdCl2 heated to 200 oC with a quartz lamp to outgas the CdCl2. The 

chamber was then filled with 100 Torr O2 and 400 Torr He. Both the CdTe and the CdCl2 were 

radiantly heated to 400 oC for 10 minutes, and then allowed to cool. 

 Both EL and PL were measured on the cells. The measurement temperature was in the 

range of 37-55 K as noted in the figures below. EL injection conditions were 25 mA/cm2

, except 

where noted. PL measurements were found to be largely insensitive to the type of CdCl2

treatment, and showed little variation between Cu doped and non-Cu regions. As such, the data 

presented here will focus on EL measurements. The lack of contrast between different PL 

measurements as compared to EL measurements was discussed in Sec. 1.4. 

1.5.2. Results and discussion 

 Cells processed with the wet, impure CdCl2 treatment showed consistent inversion of the 

EL pattern below 100 K. The EL spectra of these cells varied somewhat from cell to cell, but the 

general shape remained the same and they all resembled the spectra presented in Fig.1.13. In 

particular, the DAP peak remained the dominant feature. Depending upon temperature and 

injection conditions, Cu related features showed enhanced intensity relative to the main peak in 

Cu doped regions, consistent with previous experiments (Figs. 1.22 and 1.23). Excitonic features 

near the band edge were more prominent in cells with the wet treatment. 

 Inversion of the EL pattern as temperature decreased occurred inconsistently in cells with 

the vapor treatment. This result implies a different set of electrical transport dominating defects 

than CuCd and VCd that were described in Sec.1.3. Four cells treated with the pure, vapor CdCl2

and three cells treated with the impure, vapor CdCl2 were characterized. In cells in which the 

inversion did occur (type A behavior), spectra were similar to that of the cells with the impure, 

wet CdCl2 treatments (Figs 1.24 and 1.25). 

20
Figure 1.22 EL from a Cu dot cell with the wet, impure CdCl2 treatment. Average current 

density is of 25 mA/cm2. 

 Figure 1.23 EL from a Cu dot cell with the wet, impure CdCl2 treatment. Average current 

density is of 125 mA/cm2

. 

21
Figure 1.24 EL from an inverting Cu dot cell with the vapor, impure CdCl2 treatment. 

Figure 1.25 EL from an inverting Cu dot cell with the vapor, pure CdCl2 treatment. 

Two of the “vapor, pure” cells, and one “vapor, impure” cell exhibited type A behavior. 

Where inversion did not occur (type B behavior), spectra from non-Cu regions resembled 

previous data, but spectra from Cu doped regions departed from previous measurements (Figs. 

1.26 and 1.27). The spectral differences in type B cells were more pronounced in the cells treated 

with the pure CdCl2. In both pure and impure CdCl2 treated type B cells, the Cu regions showed 

greatly enhanced emission around 1.30 and 1.25 eV, attributed to CuCd, in Sec1.3. Though a 

similar effect was seen in type A samples, the relative intensities of these features were much 

greater in type B cells. 

22
Figure 1.26 EL from a non-inverting Cu dot cell with the vapor, impure CdCl2 treatment. 

Figure 1.27 EL from a non-inverting Cu dot cell with the vapor, pure CdCl2 treatment. 

 In the type B, pure, vapor CdCl2 treated cells, the intensities of these low energy peaks 

was equal to, or exceeded that of the DAP band, centered near 1.4 eV. It should be noted, 

however, that the exact peak location of the lower energy features in Figs. 1.26 and 1.27 is 

obscured because the detector loses sensitivity as the energy approaches its band gap of 1.1 eV. 

The Cu regions of the type B, pure, vapor treated cells also showed a very sharp feature at 1.45 

eV. This feature is sometimes visible as a small shoulder on inverting cells, but not as a distinct 

peak. Due to the non-inverting nature of the type B EL pattern, it was considered that these 

differences in spectra were injection related. (That is, Cu doped regions in non-inverting, type B 

cells were receiving a disproportionately high amount of current compared to Cu dots in 

inverting, type A cells.) However, varying the average current density such that there were EL 

spectra of comparable intensity from both Cu doped and non-Cu regions resulted in qualitatively 

the same behavior, showing that injection level was not the only factor affecting the spectra in 

23
type B cells. The large spectral differences in type B cells support the hypothesis that the 

electrically active defects vary from those of type A cells. 

In summary, (a) all cells that exhibited an inverting EL pattern at low temperature (bright 

Cu dots → dim Cu dots), regardless of CdCl2 treatment consistently showed similar spectral 

features; (b) inversion was always observed in cells with the impure, wet treatment; (c) when cell 

remained un-inverted at low temperature (Cu dots remained bright), a different spectrum was 

observed in Cu-doped regions. 

1.5.3. Summary 

 It is shown that the variability of the CdCl2 treatment is most prevalent in Cu-doped 

regions of the cell, which suggests that the incorporation of Cu into the CdTe is profoundly 

influenced by the CdCl2 treatment. When studying CdTe devices, it may be impossible to totally 

decouple the effects of Cu doping from those of the CdCl2 treatment. This would be consistent 

with the general observation that changing one step in device processing often necessitates reoptimization of many or all other steps. 

 Further defect studies involving variations of the CdCl2 treatment and its interaction with 

Cu would help to clarify the results. Identification of the defects responsible for the peaks 

observed in the cell PL and EL spectra is not unambiguous. However, there were still patterns 

that can be identified within the inconsistencies: 

• all cells showed similar spectra in non-Cu regions 

• only cells treated with vapor CdCl2 showed non-inverting behavior 

• only non-inverting, pure, vapor treated cells showed spectral structure that greatly departed 

from that of cells measured previously, and only in Cu doped regions 

• different spectral structure was consistent amongst the non-inverting, pure, vapor treated 

cells measured. 

• the wet CdCl2 treatment was probably more uniform than the vapor one because those cells 

produced consistent, inverting EL behavior at low temperatures. 

• variations among cells processed with vapor, pure CdCl2 treatment indicate greater process 

sensitivity to high purity CdCl2 than to lower purity CdCl2. 

1.6 CdCl2 Effects Studied with Non-Uniform Treatment and Spatially Resolved 

 Luminescence Techniques 

 The intentionally non-uniformly processed cells (the approach used in Sec.1.3 for 

studying Cu), are used in this section for investigation of the effects of CdCl2 treatment. As was 

the case with Cu the use of patterned CdCl2 allows for built-in experiment and control conditions 

within each solar cell, eliminating cell-to-cell variations that contaminate data in studies that try 

to isolate a single processing condition (e.g., see Sec.1.5). 

1.6.1. Experiment 

 Structures used for this experiment were of VTD CdS/CdTe material not treated with 

CdCl2. Post CdTe deposition processing and contacting were performed at CSM. A modified 

CSM CdCl2 misting process, followed by annealing, was used to apply CdCl2 to the CdTe 

surface. Prior to application of CdCl2, the cell was placed in a special mount shown in Fig. 1.28. 

24
A stainless steel frame clamped a stainless steel shadow mask with 150 μm diameter holes and 

the cell down to the aluminum base. It was necessary to keep the shadow mask pressed to the 

cell during anneal to ensure that masked regions would remain untouched by CdCl2 mist. Despite 

this precaution, some contamination of masked regions likely occurred. Use of the metal base 

and shadow mask was necessary to withstand the standard 400 oC anneal temperature. An 

alternative might be to use photolithography to pattern the cell. However, photoresist, 

polymerizes and is difficult to remove at the high temperatures of the CdCl2 treatment. A 1 M

solution of CdCl2 was atomized and misted onto the CdTe surface for 2 minutes. This method 

produced some cells with well-defined regions of brighter EL emission (and therefore welldefined CdCl2 treated regions) but other cells showed vague non-uniformities. A superior 

method of producing CdCl2 dots for the future might be to use the shadow mask with a vapor 

transport type CdCl2 system, rather than the wet treatment used here. The back contacts were 

applied with the standard Cu/Au recipe as described in Sec. 1.3.1. 

Figure 1.28 Experimental setup for processing CdCl2 dots. 

 To find CdCl2 regions of the cell, the same type of technique that was employed with the 

Cu dots was used. EL images showed enhanced emission from CdCl2 treated regions as detected 

with the spectroscopy system in imaging mode. However, this contrast between CdCl2 treated 

regions and untreated regions disappeared at temperatures below 200 K, leaving an EL image 

with no regular pattern. Thus, to locate CdCl2 regions at low temperature, images at higher 

temperatures were relied upon. This method was not without its faults, as the position of the cell 

would change slightly at lower temperatures due to thermal contraction of the cryostat arm. To 

mitigate this shift, the edge of the solar cell could be used as a reference point. Once the CdCl2

regions were located with EL imaging at higher temperatures, the temperature was lowered to 

25
increase spectral resolution. Spectra were obtained by closing the entrance slits to 100 μm and 

rotating a diffraction grating into position within the spectrometer. 

1.6.2. Results and discussion 

Fig. 1.29 shows a series of EL images acquired at different temperatures with an injection 

intensity of 125 mA/cm2 to increase the EL contrast. The CdCl2 treated regions (some of which 

are indicated with red arrows) are visible as bright dots at higher temperatures, but disappear 

below 200 K. The edge of the cell can be seen in the lower right corner of each image. Image 

brightness was set for each image individually to allow for maximum contrast. Mean EL 

intensity increased with decreasing temperature. Indeed, the image obtained at 50 K has several 

bright spots where the pixels are saturated. 

Figure 1.29 EL images versus temperature for CdCl2 dot cells. Average injection level is of 

125 mA/cm2

. The red arrows indicate the locations of selected CdCl2 treated regions. 

 The increased EL emission from CdCl2 treated regions at higher temperatures implies 

increased conduction, likely resulting from doping of the CdTe during the CdCl2 treatment, 

leading to reduced series resistance of the bulk CdTe and possibly reduced back contact 

resistance. One possible doping mechanism is the formation of the so-called “A-center,” a 

complex consisting of VCd and ClTe. This shallow acceptor has been identified with electrical 

transient measurements to exist at EV + 0.12 eV [29]. The A-center may help to dope the CdTe 

by replacing the VCd acceptors otherwise compensated with Cl, Cui, or other donor defects. 

 The disappearance of the CdCl2 dots below 200 K might be attributed to the freeze-out 

effect as it was for Cu-dot cells. The fact that this freeze-out occurs at a relatively high 

temperature indicates the involvement of a deeper defect state than the A-center. Because the EL 

image transitions from bright CdCl2 dots to a “flat” image, the CdCl2-induced defect in question 

is likely not replacing another defect that aids conduction (albeit to a lesser extent than the 

CdCl2-induced defect), as was the case with the CuCd versus VCd in the inverting Cu dot cells

 Because of the aforementioned difficulty with consistent CdCl2 processing, EL spectral 

data fluctuated from cell-to-cell. By comparison, PL spectra varied little, both by location and 

between cells, as observed in Sections 1.4 and 1.5, respectively. Accordingly, EL spectra are 

focused on here. 

26
Figure 1.30 EL spectra from CdCl2 dots and untreated regions. The arrow indicates emission 

from CdCl2 treated regions that could involve the A-center. 

 In order to ensure consistent results from areas that could be confidently identified as 

being CdCl2 treated or not, spectra were acquired from regions of the cell that produced multiple 

areas of both CdCl2 dots and untreated regions. Fig. 1.30 shows spectra from such a zone, two 

each from CdCl2 dots and untreated regions. A sharp feature at 1.51 eV consistently appears in 

the CdCl2 dots but does not appear in the untreated regions. A spectrum that corresponds well to 

that of the CdCl2 dot spectra from Fig. 1.30 has been observed before in Ref.22, and was labeled 

as an “unidentified spectrum”, thus not attributed to the A-center. 

 It should be noted that a clearly resolved spectrum in which the suspected the peak under 

discussion could be seen was only obtained from one cell. However, we can be confident in this 

data because the feature consistently appeared in two different CdCl2 treated regions and did not 

appear in two separate untreated regions. In order to see clearly this tiny feature, a very specific 

injection level was required (Fig.1.31). Too low a current density and it was not excited. Too 

high a current density and the shallow defect was washed out from much more intense emission 

resulting from the main DAP band. It was only through the “fortuitous” choice of injection 

current that the tiny feature could be clearly resolved. Probably this is the reason why we did not 

see this peak in spectra presented in previous sections. The current density required to see such a 

subtle feature would logically change from cell-to-cell along with other varying properties such 

as J-V behavior. 

27
Figure 1.31 EL spectra from a CdCl2 treated region at different injection intensities. The arrow 

indicates emission from CdCl2 treated regions that only visible at 12.5 mA/cm2

. 

1.6.3 Summary 

 Main results of preliminary investigation of the effects of CdCl2 through the use of nonuniform CdCl2 treatment are as follows: 

• CdCl2 treated regions provide higher current densities causing higher EL intensity than the 

untreated regions at T>200K. 

• CdCl2 treatment apparently increase doping level at T>200K through creating not too deep 

acceptor levels, supposedly VCd-ClTe complexes (A-centers) 

• Mechanism of the observed inversion of the El brightness around T=200K is still unclear. It 

might be that the major defect responsible for higher conductance at higher temperature is 

not A-center, but deeper acceptor, perhaps partially compensated which makes temperature 

dependence of carrier concentration sharper. 

• It is important to widely vary conditions of measurements in purpose not to miss 

manifestations of some defect related electronic states in the EL and PL spectra. This may 

help making our knowledge of the defect related states more complete as well as 

identification of these states more reliable. 

1.7. Non-Uniformity Mitigation: the Effects of Window Buffer layers 

 The addition of the high-resistance TCO (HRT) buffer layers is known to increase 

efficiency, and its value is underscored by its presence in record CdTe and CIGS devices. 

Though HRT’s are a potential boon to growers of high-efficiency solar cells, the additional 

processing step that is required to deposit the HRT is not ideal for large-scale production. Thus, 

determination of the exact role of the HRT could provide manufacturers with an alternative to 

adding a processing step, decreasing production costs. 

 Through numerical modeling and indirect, one-dimensional measurements, Rau and 

Schmidt attribute the role of the HRT intrinsic ZnO layer to prevention of electrical 

28
inhomogeneities in CIGS cells [30]. Karpov et al. further show how, in general, non-uniformity 

can be detrimental to solar cell performance [6, 31]. 

Here, we directly observe the non-uniformity mitigation effects of the HRT in CdTe cells 

through spatially resolved electroluminescence (EL) and light-beam induced current (LBIC) 

measurements. This decrease in non-uniformity has been measured in CdTe cells prepared at 

both NREL and CSM, both with and without HRT layers. Finally, we attempt to identify the 

cause of the non-uniformity in question, which appears to be related to areas of thin CdS. 

1.7.1 Experiment 

 Cells were manufactured using standard procedures at NREL [32], and CSM [33, 34], 

detailed elsewhere. Addition of an HRT layer was the only intentional variation made at each 

institution. 

EL is stimulated by injecting excess carriers through forward bias. Radiative 

recombination of excess carriers is detected with a CCD array detector. Though influenced by 

recombination effects at the main junction, EL is most highly sensitive to current distribution 

[24] and therefore provides a map of electrical transport through the device. Complimenting EL 

is LBIC. These measurements were performed by Tim Nagle at Colorado State University. In 

LBIC, light is locally injected into a solar cell with an electronically modulated, focused, 638 nm 

laser beam. The light-generated current is detected with a lock-in amplifier as the sample is 

scanned under the laser, giving a spatially resolved quantum efficiency (QE) measurement. For 

non-uniformity studies of layers of incomplete devices, an atomic force microscope (AFM) 

along with optical transmission measurements were performed on the same area of the substrate. 

1.7.2. Results and discussion 

 Cells from both NREL and CSM with a HRT layer were found to be more efficient than 

cells without the HRT. The greatest efficiency boost came from fill factor (FF), with open 

circuit-voltage (Voc) and short-circuit current (Jsc) being relatively constant between the two 

processing conditions. 

 Fig. 1.32 graphically shows a decrease in EL non-uniformity associated with the HRT in 

CdTe cells. Without the HRT, EL originates from isolated spots on the solar cell, with little to no 

detectable emission between bright spots. Though non-uniformity is not completely quenched 

with the addition of the HRT, it is greatly mitigated. Bright spots still exist, but they are much 

less intense with respect to the average intensity, and measurable emission is seen on nearly the 

entire solar cell. The HRT decreased the NU parameter in the CSM cells from 1.93 to 0.48 and in 

the NREL cells from 0.64 to 0.21. 

29
Figure 1.32. Electroluminescence from cells produced at CSM and NREL with and without an 

HRT layer. The non-uniformity visibly decreases with the addition of the HRT. 

 A similar decrease in non-uniformity with the addition of the HRT layer was observed in 

LBIC measurements of CSM cells, as shown in Fig. 7.2. 

Figure 1.33. Light-beam induced current measurements on CSM cells with and without an HRT 

layer. There is a much lower level of contrast with LBIC than with the EL images of GI. 7.1. 

Quantum efficiency (QE) is indicated by the scale at the right. There is a decrease in nonuniformity as well as the increase in average QE with the addition of the HRT. 

 In addition to nearly eliminating isolated spots of low QE, the HRT also increases the 

average QE. NU decreased by a factor of ~2 (from 0.0036 to 0.0017) with the addition of the 

HRT. The NU measured with LBIC was much smaller than the NU measured with EL. We have 

made this observation when comparing EL and LBIC data with many cells in the past, noting 

that EL is more sensitive than LBIC to variations in device properties. 

 A correlation was found between the number of bright spots in EL and the number of 

areas of low QE in LBIC. Namely, both techniques yielded a number density of 105

-106/cm2 in 

30
cells without the HRT. Because the HRT is deposited between the highly doped TCO and the 

CdS, we suspected the CdS could be responsible for the non-uniformity. 

 Optical transmission and AFM measurements were performed on CdS grown by 

chemical bath deposition (CBD) at CSM. These measurements (see Fig. 1.34) were performed 

on the exact same spot of the CdS. Bright spots in the transmission measurements correspond to 

high spots in the AFM measurement. The fact that the high spots seen under AFM measurements 

has not been more widely reported is probably due to the fact that their spacing (~10 μm apart) is 

large enough to be outside of normal scan ranges (a composite of four adjacent AFM scans at 

full range was used for this study). The number density of these spots is ~106/cm2, suggesting 

these inhomogeneities in the CdS could be responsible for the observed non-uniformity in EL 

and LBIC measurements. 

Figure 1.34. Mapping the CdS with optical transmission and AFM measurements. Lighter areas 

in the AFM image correspond to higher spots. The AFM is a composite of four scans. The 

circled bright spot in the transmission measurement corresponds to the circled high spot in the 

AFM measurement. 

 The increase in optical transmission coinciding with increased height warrants further 

discussion. Greater transmission implies decreased CdS thickness, but increased height implies 

increased thickness. However, the CdS is not grown on a completely flat surface, thus the AFM 

measures height of the CdS plus the TCO and any contaminants in the layers. High spots could at 

least partially be due to areas of increased TCO thickness. Indeed, AFM measurements of TCO 

films (not shown) reveal a surface roughness ~60% that of the CdS surface. In addition to 

increases in TCO thickness, some of the surface roughness of the CdS is likely due to transparent 

particulates (such as CdO, organic compounds, or even gas bubbles) formed during deposition. 

Thus, these rises are not necessarily increases in CdS thickness and could possibly leave a 

thinner CdS layer on top (Fig1.35). In this self-consistent explanation, areas of high TCO and/or 

contaminants would account for the high spots observed with the AFM while the thinner CdS 

atop the mounds would account for the increased transmission. 

31
Figure 1.35. (not to scale) A proposed explanation of why high spots in the AFM images of 

CdS-coated plates could correspond to thin spots in the CdS. The high spots could be caused by 

either high spots in the TCO or particles incorporated into the CdS film. Non-conformal CdS 

deposition could result in thin CdS on high spots and result in regions with high topography but 

reduced film thickness. 

 Though we have shown direct evidence of the non-uniformity mitigating properties of the 

HRT, the exact mechanism has not been discussed. Visoly-Fisher et al. propose that the HRT 

serves to add additional thickness to the n-type layer that is necessary for high Voc [35]. 

Accordingly, the full benefits of the HRT are realized when the CdS is intentionally thinned to 

increase photocurrent (a variation not performed here). Not fully discussed in their study are 

spatial variations within the plane of the solar cell, the main effect discussed in this section. A 

solar cell with an average CdS thickness sufficient to support a high Voc can still have local areas 

of thin CdS exhibiting reduced Voc. These regions could be one source of “weak diodes”. 

Therefore, the use of an HRT can improve cell efficiencies even without thinning the CdS, as 

was the case with the cells studied here. It must be emphasized that thin CdS is not the only 

possible type of weak diode; they could be generated from multiple sources. Though the most 

significant change in overall device performance was in increased FF and not Voc, simple 

modeling of a weak diode in parallel with a standard diode (both having some I-V roll-over) can 

produce a large drop in FF without much change in Voc. 

 The weak diode explanation is corroborated by the EL and LBIC results. As the weak 

diode has a lower Voc, it will pass significantly more current at a given forward bias than the 

standard diodes surrounding it. This high current produces the bright spots observed in EL. In 

addition, the origin of the low Voc weak diode is a thin CdS layer, which would result in less 

band bending. Less band bending would result in less photocurrent, as was observed with LBIC. 

1.7.3. Summary 

• Presence of HRT buffer layer obviously decreases non-uniformity of electric properties of 

 a cell which was demonstrated by comparison of the EL images of cells with and without 

 buffer layers 

• The CdS layer could be a strong contributor into efficiency-robbing nonuniformity in the cell 

electrical properties. It was concluded based on EL mapping of completed cells combined 

with AFM and optical measurements of TCO/CBD CDS structures. Observed microscopic 

areas of thinned CdS may constitute one form of a “weak diode”. 

• HRT layer between TCO and CdS layer probably blocks weak diodes although mechanism 

of blocking is still unclear 

32
1.8 Long-Term Photoconductivity Transients in CdS 

 CdS photoconductivity has been widely reported, and has been used to explain effects 

such the dependence of apparent quantum efficiency on electrical bias [38,39]. However, CdS 

effects are sometimes confused with CdTe photoconductivity when explaining effects such as IV crossover (i.e., non-superposition of the light and dark J-V curves) [40]. Here, evidence of 

photoconductivity in CdS was observed with electroluminescence (EL) measurements of 

CdS/CdTe solar cells. 

1.8.1. Experiment 

 EL was induced under constant current conditions of 25 mA/cm2. The EL was imaged 

with the same system as in Sec.1.2. To induce a photoconductive response in the CdS, an Ar-ion 

laser was employed. This laser can support several discrete excitation energies. Laser energies 

used were 1.94, 2.41, 2.54, and 2.71 eV. The laser beam was focused into an elliptical shape 

using a cylindrical lens. After taking an EL image in the dark, laser light was shined on the 

device. An image of the reflected laser light was recorded. Following this exposure, a second EL 

image was acquired in the dark. 

1.8.2. Results and discussion 

 The photoconductive CdS effect is most easily observed visually with a cell with a highcontrast EL pattern. A Cu dot cell (as described in Sec.1.3) serves this purpose well, as shown in 

Fig.1.36. The laser energy used was 2.71 eV, with a power density of approximately 1.7 W/cm2

. 

The cell was exposed to the laser beam for 3 minutes. Before laser exposure, EL is seen from Cu 

dots with a relatively uniform distribution throughout the cell. After laser exposure, the cell’s EL 

pattern changes; bright areas of EL emission are only seen on Cu dot areas that were exposed to 

laser light. 

Figure 1.36. Photoconductive CdS effect in EL image of Cu dot cell. (a) EL image before laser 

exposure, the edge of the cell can be seen in the lower right corner. (b) Laser pattern, acquired 

with reflected image. (c) EL image after 3 minutes laser exposure. 

33
 The EL pattern is generally dominated by electrical transport effects [24]. In particular, 

areas of low resistance, such as the Cu dots described in Sec.1.3, show increased EL because 

they are preferential current pathways (Fig.1.36a). The image in Fig.1.36c presents the EL image 

from the portion of the cell exposed to laser. Now the strong EL comes from the area exposed to 

the laser beam. Within this area it is stronger at the Cu-dot location. Beyond the exposed area 

even Cu dots look much darker than before. The EL image transformation can be explained by 

the persistent photoconductivity induced by laser in the exposed area. Current density (and EL 

intensity) redistributes in favor of now low resistance region in expense of higher resistance 

unexposed area. The observed effect should be ascribed specifically to CdS by based on 

experiments with varying laser excitation energy. The band gap of CdS is 2.42 eV [41]. If this 

effect is to be attributed solely to CdS, then no photoconductive effect should be observed with a 

laser of insufficient energy to excite the CdS (≤ 2.41 eV). 

 The processing details of the CSM gas jet deposition cells (GJD) used in the following 

experiment can be found in Refs. 33 and 34 as well as in Sec. 5 of this report. These cells were 

made without any intentional non-uniformity (e.g., Cu dots). The results are summarized in Fig. 

1.37. An initial EL image was acquired of a cell. Then, the cell was exposed to 1.91 eV laser 

light of power density ~ 170 mW/cm2

 for 5 minutes. A second EL image was acquired. No 

significant changes were observed in the EL pattern. Next, the cell was exposed to 2.41 eV laser 

light (just below the band gap of CdS) under nominally the same conditions. 

Figure 1.37. Photoconductive CdS effect as a function of excitation energy. The line scans 

were produced by integrating along the horizontal axis of the detector. EL images were acquired 

in sequence with laser exposures. The data towards the top of the graph were acquired earlier. 

 

 The peak in the EL near 600 μm was due to a localized bright spot. The two peaks in the 

1.94 eV laser reflection line are not important. The EL pattern was unaffected by the laser 

exposure until exposed to 2.54 eV light, at which point the EL line scan mimicked that of the 

laser. 

34
 Minor changes were observed with EL, such as diminished bright spot intensity, but the 

pattern remained largely the same. After exposure to 2.54 eV, laser light (exceeding the CdS 

band gap) under the same nominal conditions, the EL pattern changed significantly. The cell 

exhibited enhanced EL over regions that were exposed to the laser light, and suppressed EL in 

unexposed regions. Both effects are a result of localized enhanced conductivity that was induced 

by the laser. The EL intensity is diminished in the unexposed regions because of the constant 

current excitation conditions: more current in one area must result in less current in others. 

Because these effects were only observed with optical excitation energies exceeding the CdS 

band gap, we can be confident that they are due to CdS photoconductivity. 

The fact that the change in the EL pattern observable after the laser was turned off, shows 

that the photoconductive effect persisted after light exposure. Again, a cell was exposed to laser 

light of 2.71 eV and 1.7 W/cm2 for 30 minutes (not shown). A laser-induced EL pattern change 

was observed again. After being allowed to “recover” for 21 hours under short circuit conditions 

in the dark, another EL image was acquired. The cell still showed enhanced emission in the laser 

exposed region and diminished emission in unexposed regions. However, some decay of the 

effect was observed, as the contrast between regions was smaller after 21 hours than immediately 

following laser exposure. 

The general trend of greater EL in laser-exposed regions and lesser EL in unexposed 

regions was used above to show the basic effect of laser-induced, localized CdS 

photoconductivity. Another observed change in the EL pattern was diminished intensity of bright 

spots relative to the “background” EL. Bright spot intensity was diminished in both laserexposed and unexposed regions. A striking example of this effect is shown in Fig. 1.38. A one 

minute laser exposure of 2.71 eV and 1 W/cm2 was used. Bright spots in the entirety of the 

image show greatly diminished intensity. The fact that these bright spots are so influenced by the 

conductivity of the CdS (as modulated by the laser exposure) could be considered as supporting 

the conclusion in Sec. 1.7 that CdS layer could be a strong contributor into the electrical nonuniformity of a cell. 

Figure 1.38. Photoconductive CdS effect on EL bright spots with CSM cell. (a) EL image before 

laser exposure. The circled bright spot has an intensity of ~14,000 counts. (b) Laser pattern, 

acquired with reflected image. (c) EL image after 1 minute laser exposure. The circled bright 

spot now has an intensity of only ~4,000 counts. 

 The extremely long lasting effects of the photoconductivity of CdS were easily observed 

because of extreme sensitivity of EL to spatial variations in cell properties. Previously, the 

effects of Cu were studied by adding an intentional non-uniformity in the device during 

processing in the form of Cu dots [24]. Here, a focused laser was used to induce non-uniformity 

35
into the device post-processing. In this way, the CdS was forced into a dominant role in 

determining cell uniformity with respect to electrical transport and therefore EL pattern. The 

effect of laser exposure appeared to be a long-term transient. The decay of the photoconductivity 

could be studied more in-depth by carefully controlling the laser spot and then comparing EL 

intensities of laser-exposed and unexposed regions over time. Monitoring this decay at different 

temperatures might allow the extraction of an activation energy for the decay process. Using this 

technique with completed cells allows one to specifically target CdS related effects, such as 

variations in CdS process or the use of TCO buffer layers. 

1.8.3 Summary 

• Irradiation of a cell with a laser of photon energy above the CdS bandgap causes significant 

changes in the EL pattern that could be explained by the induced photoconductivity of CdS. 

• Irradiation makes the electrical current density distribution more uniform, which indicates 

diminishing of nonuniformity in electrical properties of the CdS layer detrimental for the cell 

performance. 

• The effect of laser exposure on EL is long-lasting (hours-days. Analysis of observed long 

term transients along with similar studies at varying conditions of irradiation, injection level 

and temperature, can be useful for investigation of the photoconductivity mechanisms. 

Identification of defects involved (point defects, grain boundaries) and their transformation 

under illumination is an important goal for these studies. 

1.9 Conclusions 

• Spatially resolved measurements of electroluminescence (EL) on scales of several microns to 

centimeters is easy to perform on CdTe cells at various temperatures. These measurements 

demand only current densities of the order of 101

A/cm2. EL mapping is a very useful and 

informative method for revealing and studying lateral nonuniformities in solar cells/modules.

• EL mean intensity and pattern (bright spots on darker background) depends strongly on 

processing technology and cell stressing. EL is so sensitive to cell degradation that could be used 

for early predictions of the cell/module lifetime. 

• Simultaneous studies of spatially resolved EL and PL from the same cell area allows 

distinguish nonuniformities in carrier transport from those in recombination rate as sources of 

nonuniformity in EL. 

• Patterned doping with Cu and patterned CdCl2 treatment demonstrated possibility to reveal 

nonuniformity in defect distribution and also strong influence of CdCl2 on accommodation and 

manifestation of Cu atoms. Cu signatures are easy to detect with EL spectra. Cu distribution 

could be determined with EL scans. 

• EL measurements on cells with the single- and bi-layer structure of front contact have shown 

that both back and front contacts influence non-uniformity in electron transport through the cell. 

Bi-layer front contact significantly reduces the nonuniformity. 

• EL studies have provided and independent evidence of photoconductivity in the CdS layer 

and revealed long-lasting (hours-days) effect of exposure to a laser with the photon energy above 

the CdS bandgap. This provides a new method for investigations of the CdS photoconductivity 

mechanisms, e.g., identification of defects responsible for the photoeffect, their generation, 

transformation and migration under illumination. 

36
2. ADMITTANCE SPECTROSCOPY AND CAPACITANCE TRANSIENT 

MEASURE-MENTS AS METHODS FOR STUDYING DEEP ELECTRONIC 

STATES IN CdTe/CdS SOLAR CELLS 

2.1 Introduction. 

Deep electronic states are a common concern for the developers and manufacturers of 

solar cells. First, these states may act as recombination centers, reducing the lifetime of photogenerated carriers, hence photocurrent collection and cell efficiency. This effect competes with 

the inevitable band-to-band recombination and is especially detrimental for the indirect gap 

semiconductors like Si, where the lifetime controlled by radiative band-to-band transitions is 

rather high. Direct gap semiconductors like CdTe are much less sensitive to recombination 

centers because their band-to-band lifetime is in the range of 10-8 s. Therefore the restrictions on 

the impurity content and structural defects concentration are less severe than for Si. However, in 

the case of thin film polycrystalline CdTe the material contains many structural defects. Indeed, 

the fine grain structure of the film (grain size of ~ 1μm) inevitably introduces a very high 

concentration of deep electronic states in the gap that could act as recombination centers. 

Another aspect of the deep states problem arises when the concentration of traps, , is 

comparable with or higher than the density of shallow levels that control the majority carrier 

concentration (“doping level”). In this case traps can significantly contribute to the space charge 

density in the depletion region, influencing the width of the region where the built-in electric 

field effectively separates photogenerated electrons and holes, providing higher photocurrent. 

Recent studies of thin film CdTe cells detected trap densities ( τ ) higher than the 

“doping level” determined from C-V profiling, [14, 42-45]. Also high trap concentration 

can lead to the significantly overestimated doping level [46]. For CdTe the analysis and the 

estimates of possible errors were presented in [47]. 

Nt

Nt

315 10 − cm

N −VC

Deep states of high density may considerably influence free hole concentration (Fermi 

level position) and its temperature dependence. We have already discussed the problem with 

respect to Cu doping effect in Sec. 1.3. SIMS measurements in CdTe:Cu based cells always 

indicate high Cu concentration, up to 1018 cm-3. It is believed that Cu occupies mostly Cd sites, 

providing an acceptor state. However, the net “doping level” determined with C-V profiling is 

usually of the order of at room temperature. Various models were proposed to explain 

this effect, such as Cu accumulation by grain boundaries while the concentration in the grain 

bulk is small or strong compensation of Cu acceptor states with the comparable concentration of 

donors. Another explanation may be that the Cu concentration in the grain bulk is high, but the 

resultant free hole concentration is much lower due to only partial ionization of these acceptors. 

Below we present results of modeling based on the assumption that the energy ionization of the 

level is (see e.g., [21, 48]) 

314 10 − cm

EV + 35.0 eV

37
p(T) [Ea=0.35eV, N=10^18]

1000000000

10000000000

1E+11

1E+12

1E+13

1E+14

1E+15

1E+16

1E+17

1E+18

0 100 200 300 400 500 600

T, K

p, cm^-3

C=0.99

C=0.9

C=0.5

C=0.2

C=0

Figure 2.1. Hole concentration vs. temperature in CdTe for eVE , a = 35.0 318 10 − NA = cm , and 

varying compensation degree = D / NNC A

Important conclusions can be made based on the curves in the figure: 

• For the assumed energy of the level, hole concentration at room temperature can be much 

lower than the acceptor density. The difference decreases at elevated temperature and rapidly 

increases with cooling. 

• Even a moderate compensation considerably decreases ratio , especially at lower 

temperatures. 

Np A /

• Even a slight variation of the compensation degree adjusts the modeled hole concentration to 

that measured one. 

 We have briefly discussed some aspects of the deep states problem to support the need to 

continue and further develop studies of deep states in CdTe thin film solar cells. However, our 

own experience and analysis of other investigations revealed numerous problems we have to 

solve to be successful. This urged us to review our previous results, revise experimental methods 

and approaches to the data interpretation, develop new instrumentation, measurement and data 

analysis procedures. 

We begin (Sec. 2.2) with a description and discussion of the admittance spectroscopy 

(AS) method and modifications we have used for studying deep states in CdTe cells. Then we 

briefly discuss some results of our AS studies, mostly those that revealed the specific problems 

to be discussed in Sec. 2.3. The latter is devoted to the analysis of the problems we face when 

applying widely used AS methods or deep level transient spectroscopy, their technical 

limitations, and shortcomings of common theory used for the data interpretation. We discuss 

alternative approaches that can help, at least partially, to overcome the problems revealed by 

previous studies. In particular, we are focusing on measurements of transient effects, detecting 

and studying very slow, metastable states. Sec. 2.4 contains description of our recent 

developments in the technique of measurements aimed at realization of the alternative 

38
approaches. Finally, in Sec. 2.5, we present and preliminary results obtained with a new 

measuring system. 

2.2 Admittance Spectroscopy of Deep States in CdTe Based Cells. 

2.2.1 Admittance spectroscopy (AS) basics 

AS is based on the measurement and analysis of frequency dependent admittance/ 

impedance. Trap recharging (capture/emission of free carriers) caused by application of AC 

voltage to the diode (cell) leads to the appearance of AC current that has both out-of-phase and inphase components providing correspondingly the imaginary and real parts of admittance, Y , and 

impedance, Z . LCR meters usually display the data in the “parallel, p ,” and “series, s,” modes 

as YRG )Re(/1 , , and pp ≡= Re(ZR ) s ≡ p ≡ YC /)Im( ω , [ ] 1 Im − Cs ω ⋅≡ Z . The equivalent 

electrical circuit of a cell could be more or less complicated, therefore it is important to be 

cautious when extracting the trap related parts of capacitance, , and conductance, , 

from the raw data provided by LCR meter. Frequeny dependence of and arises from the 

finite rates of capture/emission processes. Each deep state (trap) has its own characteristic 

frequency (inverse characteristic time: ) usually close to the emission rate. For a 

simple circuit: main diode + single level traps in the depletion region, the frequency dependencies 

are described by Eqs. (2.1-2.3) and illustrated by the following schematic figures 

(ω) Ct (ω) Gt

Ct Gt

ωt tt = /1 τω

 C

 ( )2 /1

)(

t

o

t

d

C CC

ωω

ω

+

+= (2.1) d C o Ct

 lnω

 

 G

( )

( )2

2

/1

/ )(

t

t ot

d

G GG

ωω

ωω

ω

+

+= (2.2) Gd o Gt

 lnω 

 (2.3) t o

t ot = CG ω

 Here and are the maximum and values, while and are the 

parameters characterizing the same diode without traps. These four parameters can be 

determined by comparison the frequency dependences in the low frequency (LF) and high 

frequency (HF) limits, respectively at and : 

o Ct o Gt (ω) Ct (ω) Gt Cd Gd

<< ωω t >>ωω t

 HF )( ; ; ; (2.4) d = CC ω ( ) d = GG ωLF HF )()( LF o

t −= CCC ωω )()( HF LF ot GG ω −= G ω

39
The trap characteristic frequency can be determined from any of the four frequency dependent 

functions: );( 1 ωφ

ω

=− d

dC );( 2 ωφ

ω= d

dG );( 3 ωφ

ω

ω =⋅− d

dC )(/ G ω =φ4 ω . Each has a peak whose 

position is determined by the trap characteristic frequency: 

3/ =ωω tpeak forφ1 and φ2 ; ω =ωtpeak for and φ3 φ4 (2.5) 

For φ4 it is so if , which is usually the case for our cells. << GG tD

Which of the functions is more suitable to utilize depends on the properties of the main 

diode, trap characteristics, conditions of measurements, e.g. bias, light/dark conditions, etc. 

When the LF or HF limits are not available for the instruments in use, but the peaks in the 

dependencies are within the available frequency range, then the trap related parameters 

can be determined as and or, by similar analysis of 

ωφ )(i

(2 ) 3 pt

o Ct ⋅= ωφω 4 pt )(2 o Gt ⋅= ωφω φ1

and φ2 . 

The position of the trap energy level in the bandgap is usually derived from the 

temperature dependence of the characteristic frequency based on the equation (specified for the 

hole trap): 

⎟

⎠

⎞ ⎜

⎝

⎛ − ⋅⋅〉〈⋅= kT

EE Nv vt

vthtt 2σω exp (2.6) 

where is the trap level energy with respect to the valence band edge, is the 

average thermal velocity, is the effective density of states in the valence band and 

is the trap capture cross section. If the Arrhenius plot vs. provides a linear 

graph, then the activation energy is derived from the slope. The intercept of the 

extrapolated straight line with the axis provides the value of the product where 

− EE vt

2/1 Tvth ∝〉〈

2/3 v ∝ TN

σ t )/ln( −2 ωt T /1 T

− EE vt

/1 T σγ t ⋅

122 * 21

3

*2

2 (103 ) 16 − ⋅⋅⋅⋅≈ ⋅⋅ =〉〈⋅ = sKcm

m

m

h

mk

T

vN

o

thv π

γ . Thus the only parameter we need to 

know is the effective mass. There is some uncertainty in the effective mass value we must use in 

calculations because of the complicated valence band structure in CdTe (degenerate heavy and 

light hole bands) as well as the spread in published values for both. However, if we use 

, the possible error should not exceed a few tens %. Other factors can dominate the 

error, e.g., nonlinear graph, errors in the extrapolation, etc. Uncertainty in the estimate of 

within one order of magnitude could be considered acceptable. 

≈ 8.0/ ∗ mm o

σ t

 To estimate the trap concentration from AS data, the simple theory for a single trap level, 

and uniform distribution of “doping level” N and trap concentration , provides an equation: Nt

 ( ) )/(/1

/1

dtt

t dt

d

o

t

xxNN

xx

N

N

C

C

⋅+

− = (2.7) 

40
where is the depletion width and is the “crossing point”, the distance from the interface 

where the deep level crosses the Fermi level, . For a small ratio, e.g. at 

forward bias, . Thus, if one observes , that means that the trap 

density is higher than the “doping level”. Indeed, sometimes we have observed a two- to 

threefold decrease in capacitance value with increasing frequency. It is hard to make more 

definite quantitative estimates because of the assumptions of the concentrations uniformity. In 

the case of small trap concentration ( ), Eq. 2.7 takes a simpler form: 

dx t x

= ExE Ftt ( ) dt / xx

td NNCCot ≈ // ≥ 1/ d ot CC

t << NN

 ⎟

⎟

⎠

⎞

⎜

⎜

⎝

⎛ −=

d

tt

d

o

t

x

x

N

N

C

C 1

2/1

⎟

⎠

⎞ ⎜

⎝

⎛ = U

U

N

N tt (2.7a) 

where is the total band bending and . − += VVU biasib −≡ Ftt /)( eEEU

2.2.2 Modifications of admittance spectroscopy

We have also used two modifications of AS for studying trap density of states in CdTe 

cells. As formulated, both methods can be applied not only to detecting and studying single 

levels or isolated trap levels, but also to study and quantitatively estimate continuous distribution 

of trap levels over energy. 

The first modification developed by Walter et al. [49] proposes the following equation 

for deriving the trap density of states, , (DOS), from the capacitance frequency dependence: Dt

d kT

dC

t xEUAED dF

ω

ω

ω −= ),,()( ⋅⋅ (2.8) 

Here A is a coefficient that depends on the total band bending, U, Fermi level position in the 

quasi-neutral region, EF , and depletion width, . The energy is calculated as dx Eω

Eω = ( /2ln ων ) o kT ⋅ (2.9) 

where is the “attempt-to-escape” frequency. Nv vthto σν ⋅〉〈⋅=

For CdTe at T = 300 K, , e.g., for . 

is the demarcation energy between trap states that can recharge under the AC voltage of 

frequency 

)(10)( 261 2 ν o s ⋅≈ σ t cm − 112 10 − ≈ s ν o 214

t 10 cm − σ = Eω

ω and those that are not able to follow oscillations of testing voltage. With increasing 

testing frequency shifts toward the valence band. Eω

In the case of small trap concentration ( NN ) and uniform doping level t <<

 ,,( ) xEUA dF 2/1

2/1

2

ω

εε U

U

e

N

o

⎟

⎟

⎠

⎞

⎜

⎜

⎝

⎛ = (2.10) 

where 

e

EE U − F = ω

ω . 

41
The DOS function can be also derived from the conductance dependence on frequency: 

 kT

G

d

d

t xEUAED dF

2

2 ),,()( ω

ω ω

ω ⋅ ⎟

⎟

⎠

⎞

⎜

⎜

⎝

⎛ −= ⋅ (2.11) 

Thus, this method, contrary to the standard AS, can provide information not only on single trap 

levels that manifest themselves as peaks in the DOS function, but also on trap level bands and 

continuous distribution of deep states in the bandgap. It should be mentioned that single levels 

and bands could not be distinguished if the width of the latter does not exceed a few kT. 

Since , measurements at different temperatures widen the range of energies for 

which DOS can be determined. Integrating the measured DOS function over energy provides the 

concentration of various traps. Note that only defects located in the vicinity of the crossing point 

(trap level crosses Fermi level) contribute to the measured capacitance. With increasing trap 

energy, the crossing point moves toward the interface. Thus, is actually determined at 

different locations. If the DOS function is spatially uniform, this method can provide exact and 

unambiguous information. 

ω ∝ TE

(ED ω) t

The second modification, drive level capacitance profiling (DLCP) method [50], is also 

aimed at determination of the trap DOS function. The method is based on measuring capacitance 

at a fixed frequency and makes use of the dependence of the measured capacitance on the 

amplitude of the AC testing voltage. As the amplitude δV (“driving level”) increases we obtain 

higher order on δV corrections to the capacitance. At relatively small δV , and 

the density of trap states can be determined as 

o +≈ 1δVCCC

 2/()( ) (2.12) 1

3 2

t

′ ω −= εε o ⋅CAeCEN

Here has the same meaning as before and is determined by Eq. 2.9. is the 

concentration of traps of energy below at the distance from the interface where the Fermi 

level crosses the level . Varying the applied DC bias voltage can change the crossing 

point position. By measuring 

Eω (EN ω) t

′

Eω xtω

Eω Vbias

δVC )( at different biases we can obtain information on the spatial 

distribution of . Since depends on testing frequency and temperature, measurements 

of the same kind at varying frequency and temperature provide broader and more detailed 

information on the DOS function and its spatial variation. Again we must note that due to 

variation of hence with frequency, the shape of the DOS dependence on energy could be 

determined correctly only if the DOS does not depend on distance. If it does depend, we face 

also the problem of uncertainty in location at which 

(EN ω) t

′ Eω

Eω xtω

(EN ω) t

′ is measured. Measurements at 

varying bias, due to moving crossing point can provide at most the trends in spatial 

dependence of DOS. 

xtω

2.2.3 Some results of the AS application to CdTe cells 

Admittance spectroscopy measurements at CSM were performed on cells fabricated with 

various processing techniques at various facilities (VTD, CSS - NREL, sputtering – UT). 

42
Specifically, the effects of back contact, CdCl2 treatment and etching of CdTe surface were 

studied with the cells based on the VTD material with all postdeposition processing steps 

provided at CSM. In nearly all cells a high trap concentration was found with non-uniform spatial 

distribution. 

Fig. 2.2 shows typical frequency dependencies of capacitance and conductance measured 

on the FS/CSM cells with Cu/Au back contact. Measurements were done using two LCR meters; 

one for the frequency range of 20 Hz to 1 MHz and the other for 75 kHz to 30 MHz. Switching 

from one to the other sometimes introduced a small shift in displayed capacitance like that seen in 

the left figure. Compensating the shift provides a continuous graph with smooth variation of the 

slope. The shift is insignificant in the fG )( dependence. 

Cu/Au back contact:

1.E-08

1.E-07

1.E-06

1.E-05

1.E-04

1.E-03

1.E-02

1.E-01

0.00001 0.0001 0.001 0.01 0.1 1 10 100

Frequency (MHz)

Conductance (Ω−1)

Cu/Au Back Contact:

1.0E-10

2.0E-10

3.0E-10

4.0E-10

5.0E-10

6.0E-10

7.0E-10

0.00001 0.0001 0.001 0.01 0.1 1 10 100

Frequency (MHz)

Ca

pacitance (Farad)

Figure 2.2. Capacitance and conductance of a VTD/CSM cell with the Cu/Au back contact 

measured at room temperature 

The shape of both dependencies correlate with the trap concept and equations presented 

in Sec. 2.2.1. The capacitance value gradually decreases while the frequency increase several 

orders of magnitude. This suggests the presence of several types of traps or a continuous 

distribution of levels and characteristic times. The two-fold decrease in C between 20 Hz and 

100 KHz means that the total density of traps with characteristic times in the range of 10~ msto 

5.1~ μs , exceeds for this cell measured at zero bias. Reduction of 

capacitance without saturation at higher frequencies indicates the presence of faster traps. The 

 dependence does not manifest any tendency to saturation at low frequency indicating the 

presence of slower traps (

314 103 − − N VC ⋅≈ cm

fC )(

τ > 8ms). 

To examine the slower traps we studied capacitance changes after switching the applied 

bias, which revealed long-term transients with characteristic times varying from seconds to 

minutes to hours. This may be at least partially attributed to changes in the space charge density 

caused by slow reoccupation of traps by carriers. Based on the theory of transients [46] we 

43
estimated the concentration of slow traps responsible for the observed effects 

was τ . slow Nt

315 10 − cm

 The vs. f plot in Fig.2.3 demonstrates two peaks that could be attributed to 

single levels or relatively narrow trap bands of enhanced trap DOS. 

dfdG )/(

C u/Au B ack C ontact

2.0E-11

2.2E-10

4.2E-10

6.2E-10

1E-05 1E-04 0.001 0.01 0.1 1 10 100

Frequency (M Hz)

dG/d

ω (Ω Hz)-1

Figure 2.3. )/( − fdfdG plot has two peaks with characteristic times of 1.6 ms and 0.35 μs 

The characteristic times derived from the peak frequencies are ~1.6 ms and ~0.35 μs. The 

trap concentrations estimated from the peak magnitudes are comparable to the “doping level” for 

both peaks. Application of the Walter/Herberholz method [49] (Sec. 2.2.2) showed the presence 

of the narrow band centered at the energy Eω ≈(0.32-0.37) eV (depending on temperature). The 

maximum DOS value was ~ . We estimated the total concentration of traps in 

the band as ~ . 

3116 102 −− ⋅ cmeV

315 105.1 − ⋅ cm

Comparison of the results of AS for Cu/Au and Au-contacted cells demonstrates 

significant effects of Cu dopant. In particular, the high frequency peak 

(ΔE ±= )02.035.0( eV ) that is well pronounced in Cu-doped cells disappears in the undoped 

cells. Instead, a small peak at 100 kHz is seen. The decrease in magnitude and some shift in peak 

position was observed for the low-frequency peak (~ 100 Hz). Significant differences were noted 

in transients. Changes caused by stressing are also more significant for cells doped with Cu. 

Effect of postdeposition treatment, specifically CdCl2 treatment and etching of CdTe 

surface, on deep states in CdTe bandgap was studied by using a standard AS and DLCP (Sec. 

2.2.2). The VTD/CSM cells were studied. Postdeposition treatments and ZnTe:Cu/Au back 

contact were applied at CSM. Averaged characteristics of differently treated cells are presented 

in Table 1. Results of AS measurements for typical cell of each type are shown in Fig. 2.4. 

The table shows that a significant trap density belongs to the energy range between 0.4 and 0.6 

eV; the deeper trap concentration is high at all depths. For all types of cells the estimated trap 

density is greater than the “C-V doping level”. Cells A and B with the highest efficiency both 

44
have high trap concentration, while the least efficient cells D (without CdCl2 and etch) have a 

much smaller trap concentration. 

Table 2.1. Characteristics of cells with different postdeposition procedures 

Density of traps (1014cm-3) for E t<Eω**)

x, μm ~2.3 ~1 ~0.15 

Cell 

type 

CdCl2

treated 

Etch Eff., 

% 

Ncv *) 

1014cm-3

Eω***)eV 0.6 0.4 0.6 0.4 0.6 0.4 

A Yes Yes 10.8 2.3 20 10 25 7 35 9 

B Yes No 10.5 3.1 25 10 20 8 20 8 

C No Yes 8.6 1.9 30 8 20 5 25 5 

D No No 6.2 0.75 10 6 7 3 8 3 

 *) Doping level at the depth x=1.5μm (from C-V profile at f=100 KHz) 

 **) Obtained with drive level capacitance profiling (DLCP) 

 ***)

 Approximate values 

 (a) (b) 

Figure 2.4 Results of AS measurements on cells with different postdeposition treatments 

(see A, B, C, D in Table 2.1) 

The results of the standard AS measurements confirm the DLCP results. A significant (about two 

times) decrease in capacitance (Fig. 2.4.a) means that the total trap concentration in the 

characteristic time range from 10 ms to 1 μs exceeds the doping level. Peaks in Fig. 2.4.b reveal 

the existence of several distinct trapping levels or bands with widely varying characteristic 

frequencies (ωpeak = ωt). Assuming the ”attempt-to-escape” frequency value of the 

trap energy was estimated: for the lower frequency peaks (ω

ν o

112 10 −s

peak~5x102

s

-1) Et-Ev ~ 0.6eV; for the 

higher frequency peaks (ωpeak~106

 s-1), Et-Ev ~ 0.4eV. The CdCl2 treatment (cells A and B) 

increases concentration of states in both bands. For cells C and D (without CdCl2) the peaks at 

(Gtot-Gd)/ω

Log-Log Plot

1.E-11

1.E-10

1.E+2 1.E+4 1.E+6 1.E+8

Angular Frequency (rad/sec)

(Gtot-Gd)/ω 

(Ωrad/sec)-1

A

B

C

D

C(ω) 

Log-Log Plot

1.E-10

1.E-9

1.E+2 1.E+4 1.E+6 1.E+8

Angular Frequency (rad/sec)

Capacitance 

(Farad)

A

B

C

D

45
ωpeak~8x105 s

-1 are not clearly visible. Instead, peaks appear close to 2x106

 s-1. The influence of 

etching on the conductance spectra is well seen for these two samples. 

 Differences in trap concentrations and depth profiles were also revealed by DLCP 

measurements on traps prepared with different CdTe deposition techniques (VTD, NREL, UT). 

For all cells, trap density was minimal in the vicinity of the depletion edge and increased toward 

the interface and back contact. The highest trap density was found in the UT cell at 

reverse bias. The VTD cell maximum density was around . For the NREL cell it was 

in the range of . 

316 101 − ⋅ cm

315 101 − ⋅ cm

315 10)64( − ⋅− cm

2.2.4. Summary 

The standard admittance spectroscopy and its modifications (Walter et al. [49] and DLCP 

[50]) applied to studies of thin film CdTe cells provided mutually complementary and consistent 

information on deep electronic states in CdTe. The main results are: 

• In all cells manufactured at different facilities, a high trap concentration was found, 

sometimes several times greater than the “C-V doping level”. 

• At least two narrow bands of deep states were detected in the energy range of − 4.033.0 eV

and . There was evidence of a significant amount of deeper/slower traps 

(

− 63.057.0 eV

τ >10ms) and much slower traps that are not detected with our LCR meters.

• Concentration of traps and their spatial distribution depend on CdTe deposition technique 

and postdeposition treatment. Significant changes were caused by doping with Cu and 

stressing.

• No evidence of detrimental effect on cell performance due to enhanced trap concentration 

was found for the traps detected in these studies (τ < ms,10 − δ EE Vt 6.0 eV ). 

2.3. Some Problems and Possible Approaches to Their Solution 

2.3.1. Effect of high trap concentration 

The analysis of results of AS and DLTS measurements usually is based on equations 

valid for small trap concentration: . As shown in the previous section, CdTe thin film 

cells have rather high trap concentration. In particular, both narrow bands detected by AS contain 

density of states exceeding the “C-V doping level”. To illustrate complications caused by high 

trap concentrations, let us consider a simple model of a single trap level while .

For , the value can be determined from the 

t << NN

t )( = constxN

t << NN Nt C ω)( dependence, if N (hence EF ) and 

the trap level are already known, by using an equation from Sec. 2.2.1: Et

d

o

t

C

C 2/1

⎟

⎠

⎞ ⎜

⎝

⎛ = U

U

N

N tt (2.7a) 

where is the total band bending and . − += VVU biasib −≡ Ftt /)( eEEU

The equation for an arbitrary trap concentration, Eq. 2.7, can be presented in a similar form 

46
 

d

o

t

C

C 2.1

),( ⎟

⎠

⎞ ⎜

⎝

⎛ ⋅= U

U

N

N btF tt (2.13) 

with the correction factor 

 b

bt tbF )1(1 ),( −+ = (2.14) 

where UUb and . t ≡ / NNt t ≡ /

F(b,t)

1

10

100

0.01 0.1 1

b

F

t=100

t=10

t=1

t=0.01

Figure 2.5 Correction factor tbF ),( in Eq. 2.13 

It is seen from the figure that commonly used equation (2.7a) can lead to underestimation of trap 

concentration, especially significant if the Fermi level is located close to the trap level (small b ). 

The error increases with the ratio growth. It should be noted that the Eqs. 2.8 and 2.10 

used for the determination of trap density of states when applying modified AS, also should be 

corrected in a similar way for a high trap concentration. 

t / NN

C-V Profiling in the presence of high trap concentration 

We also face another problem when using common theory for estimating trap density 

from AS measurements. Equations 2.7 and 2.10, corrected or not, allow us to estimate not the 

trap concentration itself but the ratio . Thus to estimate trap concentration or DOS 

function we need to first determine the “doping level” which is usually derived from C-V 

measurements. But in the case of high trap concentration the common C-V data treatment 

procedure can provide incorrect N profiles with an overestimated doping level. 

t / NN

 C-V profiling is usually performed with the testing signal frequency as high as possible to 

exclude the trapped charge oscillations contribution in the measured capacitance. It can be done 

if the testing frequency is higher than the characteristic frequency of all traps so that the trapped 

47
charge cannot follow the voltage oscillations. If this condition is satisfied, measured capacitance 

is determined by the depletion width. Specific capacitance (per unit area of a cell) is 

 

d

o

x

C εε

= (2.15) 

The common approach to the C-V profiling assumes that a change in space charge in the 

depletion region δQ due to small change in bias δU is due only to the shift of the depletion 

edge , hence d δx

 dU

dC

dU

dx

dx

dC d

d

= = dU

dx

x

d

d

o

2

εε

− (2.16) 

The space charge density at the depletion edge is , therefore the change in 

voltage across the depletion region is 

()( ) d d ρ −= xeNx

δU = ddd

o

x

o o

xxxN e dxxx

d

δ

εε

ρ

εε

δ )( )( 1 =

⎪⎭

⎪

⎬

⎫

⎪⎩

⎪

⎨

⎧ − ∫ 

and 

 

dd

d o

xxNedU

dx

⋅⋅ = )(

εε (2.17) 

The well known equation to derive the doping profile from C-V measurements is 

 

1 3 1 2 2 )(− − −

⎟

⎟

⎠

⎞

⎜

⎜

⎝

⎛ ⎟ = ⎠

⎞ ⎜

⎝

⎛ −= dU

dC

edU

dC

e

C xN

o o

d

εε εε

 (2.18) 

Eq. 2.17 is approximately correct only for a low trap concentration. However, small change in 

the DC bias applied to a cell not only shifts the depletion edge, but shifts the crossing points of 

all trap levels as well, . If the trap is able to follow the bias changes, this leads to 

changes in trapped electrical charge. As pointed by Kimmerling [51], the total time of C-V 

profiling, as well as the time spent for applying the bias increment and measurements at a new 

bias, can be much longer than the characteristic time of a trap. In other words, the characteristic 

time of the bias change, 

dti = δδ xx

θ , can be (and usually is) much longer than and . This 

means that, if the trapped charge does not oscillate, it however follows the changes in DC bias. 

The effect under discussion has been confirmed by numerous observations of the hysteresis 

loops in C-V profiles, and correspondingly in profiles, usually more significant in the 

latter. 

−1 = ωτ tt −1 ω

xN )(

Changes in space charge density within the depletion width influence the value 

as illustrated by Eq. 2.19 derived for the multi-level model: 

dUdxd /

 

1

)()(

−

⎭

⎬

⎫

⎩

⎨

⎧ = + ∑

i

tititidd

od xNxxNx

eU

x εε

δ

δ (2.19) 

48
where summation is over all traps with . By replacing Eq. 2.17 with 2.19 we obtain the 

equation for the “apparent” doping level, 

<θτ t

N′, derived from C-V measurements with the common 

procedure: 

= ⎟

⎟

⎠

⎞

⎜

⎜

⎝

⎛ ′ =

− − 1 2 2 )( dU

dC

e

xN

o

d

εε ⎭

⎬

⎫

⎩

⎨

⎧

+ ∑

i

titi

d

ti

d N

xN

x

x xN )( 1)( (2.20) 

To evaluate possible effect of traps on the “C-V doping profile” we have calculated the apparent 

doping level for a two-trap-level model with trap energy and , close to 

those detected in CdTe using AS (see Sec. 2.2.3). The characteristic times determined for these 

two traps, and , are shorter than the “delay time” provided by our LCR 

meters, which means that the trapped charge follows DC bias changes. The model was simplified 

by the assumption that the real doping level and trap concentrations are spatially uniform. The 

results are shown in Fig. 2.6 for the real doping level varying in the range 

, and two sets of trap concentrations, consistent with the 

concentrations estimated in Sec. 2.2.3: (1) , and 

(2) , . 

t1 = 4.0 eVE t2 = 6.0 eVE

s 6

1 10~ − τ s 32 10~ − τ

313 314 10 103 − − Ncm ⋅≤≤ cm

314

1 103 − Nt ⋅= cm

315

2 101 − Nt ⋅= cm

315

1 101 − Nt ⋅= cm

315

2 103 − Nt ⋅= cm

App. Dop. Level vs. Real One

1

10

100

0.1 1 10

N, 10^14 cm^-3

N', 10^14cm^-3

Nt1=10^15;

Nt2=3x10^15

Nt1=3x10^14;

Nt2=10^15

Figure 2.6 Apparent doping level versus the real one for the two-trap-level model. 

The main features of the apparent doping level, N′ , seen from the figure are: 

• ' N could be significantly higher than the real doping level N for high trap concentration 

• varies much less than . In our calculations a 30x increase in is accompanied with 

only a two- to threefold increase in 

N′ N N

N′ . 

49
• The apparent doping level, calculated for trap density not exceeding the values determined 

experimentally, is in the range of , which is close to what is thought to 

be the carrier concentration/doping level in thin film CdTe cells. However, we can say 

nothing certain about the real doping level/free hole concentration based only on C-V 

measurements. 

14 315 101103 − ⋅−⋅ cm

To cope with the problems discussed above we have been working in two directions: 

1. Experimentally obtaining more detailed and reliable information on trap energy level/bands, 

characteristic times, estimated concentration (or DOS function). To do this we have 

developed a new system (see Sec. 2.5) and significantly broadened conditions of 

measurements, such as temperature range, time of monitoring, bias, light intensity and 

spectrum, etc. AS, C-V profiling and J-V measurements are accompanied by measurements 

of transients caused by bias switch and illumination in the time range from seconds to hours 

and days. 

2. Modeling cell characteristics, including J-V, C-V, C(f), and transients. The more detailed 

information we get from experiment and the greater the number of characteristics we model, 

the more reliable estimates we can make of the trap properties. 

2.3.2 Technical limitations on the characteristic time of traps available for detection and 

studies. 

The LCR meters in use limit the frequency range of admittance spectroscopy. In 

particular, our LCR meters nominally provide measurements in the range of 20Hz to 30MHz, but 

in reality the range is narrower due to the impact of parasitic effects and noises in the vicinity of 

upper and lower frequency limits. Another limitation on high frequency range comes from the 

cell series resistance, which in combination with the cell capacitance provides features in the 

admittance-frequency dependence similar to those due to traps. [52]. For a well developed back 

contact Schottky diode this feature’s frequency is temperature-dependent with an activation 

energy in the range of ~hundreds of meV. 

Thus, the AS capabilities to detect and study deep traps are confined to traps with 

characteristic times in the range of to ~ . The major problem is to extend the 

higher

2 10~ − τ s 6 10−

τ limit up to seconds, minutes and hours because there is evidence of a significant amount 

of very slow traps in our cells. The progress in this direction is strongly limited by electronic 

devices capabilities and principal difficulties of electrical measurements at very low frequencies. 

For example, accurate measurement of admittance at a single frequency such as 0.01 Hz will 

take at least tens of minutes, and measurements of frequency dependence in this range – many 

hours. More feasible solution of the problem is to use the temperature dependence of the trap 

characteristic time and bring τ into the range available for LCR meter. Indeed, for deep levels 

characteristic time is thermally activated and exponentially depends on temperature: 

)( )/exp( . The ratio 2 ⋅∝ t kTETT − τ τ τ KT )300(/)( presented in Table 2.2 shows that if the 

characteristic time is too short at room temperature for a deep level with , it could be 

increased ~100 times with cooling to T=220K and much more by further cooling. Conversely, 

very long 

t = 3.0 eVE

τ K)300( for deeper levels like eVE can be made ~100 times faster at T=350K t = 8.0

50
Another approach to study slow traps, especially very slow ones, is to study and analyze 

admittance transients with characteristic times in the range of seconds, minutes, hours, days. The 

most unambiguous information on trap characteristic time and density can be derived from 

measurements and analysis of capacitance transients. Traps with the characteristic time in the 

range 10 ms δτ δ 10s at room temperature, not detectable by AS and not suitable for the transient 

studies, could be brought to the desired range by cooling. Since traps of this kind demonstrate 

high activation energy, we do not need too deep cooling (Table 2.2). Thus, combining AS and 

transient measurements with temperature variation allows the trap detection and studies in the 

whole reasonable range of characteristic times. 

Table 2.2. Dependence of the characteristic time on temperature: τ τ KT )300(/)(

τ τ KT )300(/)(

T, K t = 3.0 eVE t = 4.0 eVE t = 6.0 eVE t = 8.0 eVE

350 0.14 0.08 0.027 8.8x10-3

300 1.0 1.0 1.0 1.00 

260 7.7 14 50 1.6x102

220 1.3x102 5.3x102 9.1x103 1.5x105

185 3.6x103 4x104 5.0x106 ---- 

160 9.1x104 2.7x106 ---- ---- 

140 2.7x106 ---- ---- ---- 

2.3.3 Capture cross section. Very slow traps. 

Analysis of (T) data based on Eq. and the Arrhenius plot, vs. , is ωt 6.2 )/ln( −2 ωt T /1 T

aimed at determination of the trap level distance from the band edge, , and the capture cross 

section value, . The treatment of the data is based on the assumption that the capture cross 

section is temperature independent, which sometimes is not true. Even if the Arrhenius plot 

provides a straight line graph, the activation energy derived from the slope is not necessarily 

equal to . Capture cross section can itself depend on temperature exponentially 

Et

σ t

Et

 ⎟

⎠

⎞ ⎜

⎝

⎛ ∞ −⋅= kT

E T σ ( σσ exp) (2.21) 

 In this case the activation energy is the sum of two energies ( ) and is used only as 

a “signature” of a specific trap but does not provide an estimate of the trap level position. At the 

same time, the common procedure to derive from the y-intercept actually estimates the value 

of

ta += EEE σ

σ t

σ ∞ σ T ∞→≡ )( , but not the real cross section at finite temperature. An approach to determine 

σ T)( and separate and from DLTS measurements by varying the filling pulse time is 

presented in [46]. For example, application of this approach to InGaAsN alloys [53] suggested 

that 

Eσ Et

σ (T) follows Eq. 2.21 with the capture rate activation energy σ ≈ 4.0 eVE . The real capture 

51
cross section turned out orders of magnitude higher than that determined from the y-intercept. 

This kind of σ T)( dependence is usually explained based on the model of a Coulomb-repulsive 

center having a potential barrier. 

Very slow traps

The deeper the level the higher the trap characteristic time. For example, at T=300K, 

τ ≈ 5.1 s for and . But the capture cross section is another factor that 

can make a trap slow or even “very slow”. We will discuss here two possible mechanisms for the 

extremely slow capture rate, hence very low capture cross section. Both are based on the 

essentially inhomogeneous distribution of traps, namely traps gathered in clusters or traps 

belonging to the grain boundaries. 

t = 7.0 eVE 214 10 cm − σ =

Cluster model

The cluster model proposed in [54] for α − Si was recently used for very slow 

capacitance transients in CuInGaSe2/CdS cells [55]. It is common to determineσ from DLTS by 

measuring the density of trapped charge as a function of trap-filling pulse duration, . 

Common theory predicts the dependence 

N f pt

 

⎥

⎥

⎦

⎤

⎢

⎢

⎣

⎡

⎟

⎟

⎠

⎞

⎜

⎜

⎝

⎛

−−=

t

p

tpf

t

NtN

τ

( exp1) (2.22) 

with saturation on the level when all traps are filled. In [55] unusually long pulses were used, 

up to ~ . Measured change in capacitance indicated trapping of majority carriers and 

demonstrated a logarithmic growth with pulse length in the five-decade range of 

. An apparent saturation of the signal occurred only at . The 

explanation, as in [54], is that the potential barrier for free carriers created by the collective 

action of closely spaced charged traps is much higher than that for a single trap. This barrier 

increases with the trap filling in the cluster, providing stronger repulsion of carriers and reducing 

the effective capture cross section. Theoretical analysis of the model leads to an equation that fits 

the experimental results in [55] 

Nt

s 3 10

sts p

3 2 10~ ≤≤ 10~ − st p2 ≥10

 ( ) opo o

ttA

C

C

+= /1ln Δ (2.23) 

where 

feN

kT A

t o ⋅⋅ = 2 ; ttho

o

o

vn

A

t

⋅〉〈⋅ σ = 2

; is the free carrier density beyond the region 

occupied by the cluster potential. The cluster potential is , where is the density of 

carriers captured at , and is the model-dependent coupling factor. Theory also fits the 

data previously obtained for 

no

Φ s ⋅= fN Ns

p = tt f

α -Si solar cells, Ge0.3Si0.7/Si heterojunctions and plastically 

deformed GaAs (see references in [54]). For CuInGaSe2 cells of various In/Ga ratios studied in 

[55], the values were in the range of to s. Trap densities determined from the 

region, where 

ot 4 10− 2 10−

)( p Δ tC saturated, were rather high, from to , close or 15 ⋅104 317 102~ − ⋅ cm

52
greater than the “C-V doping level”. Assuming uniformly charged spherically shaped clusters of 

radius R (coupling factor 3/ ε f), the authors estimated 2 = eRf R ≈ −559 nm and to 

. The authors recognize that the capture cross sections are surprisingly small but 

they have not yet proposed an unambiguous explanation. 

22 104 − σ t ⋅≈

223 103 cm − ⋅

Grain boundaries (GB)

It is mentioned in [55] that “the calculated radial dimensions of the hypothetical charging 

spheres are of the same order of magnitudes as the smallest polycrystalline ‘grains’... This 

dimensional coincidence may imply that the charging is taking place at the surface of the grains.” 

We have recalculated the radius values using the same data and found that they are of the order 

of hundreds of nanometers, which makes the grain boundaries model even more probable. 

However the analysis carried out in [55] for the charged spherical trap clusters, should be revised 

with respect to two dimensional grain boundaries. We have done that and made some estimates 

for CdTe based on the information we obtained previously when studying the GB electronic 

properties in CdTe thin films [56-58]. 

 The CdTe films were deposited on glass substrates at IEC (physical vapor deposition) 

and FS (vapor transport deposition). Some films were treated with CdCl2 and doped with Cu at 

CSM. We also studied films prepared at NREL with a close space sublimation method. Those 

films were deposited on CdS/TCO/glass substrates and underwent a standard postdeposition 

treatment. Then they were lifted-off from the device and mounted on a glass substrate. 

By using impedance spectroscopy [57, 58] we were able to separate and independently 

study individual contributions of grain boundaries (GB) and intragrain (IG) material in film 

resistance and capacitance: , ,CRR GBIGGB and . Major results for all films: CIG

• The RGB values are orders of magnitude greater than . RIG

• Positive charge captured by grain boundary states provided a potential barrier for holes. The 

estimated barrier height Φ varied in dark from to depending on post-deposition 

treatment and doping.

3.0 8.0 eV

• Illumination could decrease RGB significantly by reducing barrier height.

• Doping with Cu reduces dark by more than two orders of magnitude. Potential barrier 

thickness W (depleted thickness) estimated from capacitance was of the order of 200-500Å 

in the Cu-doped films.

RGB

• Space charge density in the depleted layer adjacent to GB calculated based on Φ and 

turned out to be very high: , sometimes up to . This indicates 

that the doping level 

W 317 10~/ − ρ cme 318 10 − cm

A −= NNN D in vicinity of GBs is much higher than NC−V . The 

same result was obtained in [38] from measurements and modeling the 

dependencies.

GB (TR )

• Integrated density of charged states per unit GB area estimated from the 

studies is in the range of ~

DGB

)(VC biasGB

21312 1010 − − cm .

 The following important conclusions from these results are relevant to the problem of 

slow traps and their contribution to the transients observed in CdTe cells: 

53
• The GB state density per unit volume is high. For cube-shaped grains with a grain size 

, 

NGB

d GB = ( GB /3 dDN ). For =1μmd and 213 10 − D = cm , . 317 103 − NGB ⋅= cm

• The GB repulsive potential barrier makes capture cross section (CCS) for holes very low. 

CCS for a single deep state in the neutral GB, , should be multiplied by a Boltzmann 

factor 

o

GB σ

[−Φ /exp kT] for electrically charged GB with a potential barrier height of Φ . For 

example, assuming (reasonable value for the neutral defect), we 

obtain ( and ( at room 

temperature.

216

GB 10)0( cm − Φσ ≈=

GB σ 221 eV 101)3.0 cm − Φ ⋅≈= GB σ 225 eV 104)5.0 cm − Φ ⋅≈=

• Since the potential barrier increases with the density of charged GB states, , the capture 

cross section, hence the capture rate, reduces rapidly with the number of trapped holes. For a 

charged plane (like GB), 

+ NGB

2 2

)( + ×=∝ GB Φ eNfQ . A small change in the amount of trapped 

holes provides a change in the barrier height of 

)/)((2 + + +≈ GB NNN GBGB δΦΦδ (2.24) 

 

Thus, if we start with Φ = 5.0 eV and then increase density of trapped holes by 10%, the 

potential barrier grows by 0.1eV and the capture rate decreases 50x at T=300 K. A 

increase will reduce the rate about two thousand times. That means that we should not expect 

any significant change in the GB states occupation in any reasonable time of observation of 

capture process, say in hours or even days. 

%20

• It is easy to show that the kinetics of trap filling for reasonable time (small changes in + NGB) 

is described by the equation similar to Eq. 2.23: [ +∝ 1/ln)( ] +

GB otttN . That increases the 

probability that the long term transients in capacitance observed in CIGS cells in [55] were 

due to filling GB states. 

• The potential of the positively charged GB is repulsive for holes but attractive for electrons. 

Therefore the GB capture cross section and the capture rate for electrons is orders of 

magnitude higher than for holes, as seen in our studies of photoconductivity of CdTe 

polycrystalline films. Conductivity increases rapidly and greatly with illumination, due to the 

reduction of the potential barrier caused by a decrease in the GB positive charge. The latter is 

provided by capturing of photogenerated electrons that recombine with holes trapped by GB 

states. After the light was switched off, restoration of the film resistance takes hours. In a 

wide time range (from ~10s and not close to saturation) the time dependence of the resistance 

change is Δ ∝ lntR which means that slow capturing traps are the major mechanisms of 

potential barrier restoration. The rate of capturing reduces with time of observation.

• These are all grounds to expect manifestation of the GB deep states in transients measured on 

CdTe solar cells. Analysis of the data will be complicated due to presence of both hole and 

electron capture/emission processes. GB provides acceptor-like and donor-like states in the 

gap, thus hole- and electron-traps. Detailed studying of transients can provide additional 

information about the GB electronic properties and their dependence on cell processing and 

degradation. It would be useful to compare results of studies on cells with those conducted on 

CdTe films, especially extracted from the cells with the lift-off technique.

54
2.3.4 Transients. 

In previous sections we considered transients mostly as an additional tool for detecting 

and studying deep states. But the problem could be viewed from another, more fundamental, 

side. What are the mechanisms of transients in electrical and photoelectric characteristics that 

were being observed in CdTe cells for a long time but not studied thoroughly so far? Can studies 

of transients provide better understanding of basic issues behind cell performance and stability, 

for example, identify the defects effecting electronic properties of a cell, migration and mutation 

of these defects, their influence on cell degradation, etc.? 

Strong support to the necessity of the systematic transient studies is provided by 

discussion of this subject (March 2002) initiated by Ken Zweibel. E-mail messages exchanged 

between members of CdTe team (26 messages in a week) helped to better understand the current 

status of transient studies in CdTe, to discuss definitions, in particular the terms “transients” and 

“degradation”, to formulate the problems, and discuss some possible approaches to their 

solution. 

 One obvious problem is the influence of transients on cell characterization results. 

Since it is impossible to avoid the effect of transients, we must develop the procedures of 

measurement and data treatment that minimize their impact. As a first step, the CdTe Team 

needs to standardize conditions of measurements, which will make comparable the results 

obtained at various facilities. Transients manifest themselves not only in J-V characteristics. As 

the examples we can refer to the problem of C-V profiling in the presence of slow traps 

considered in Sec.2.3.1 or transients in conductivity of thin films (Sec. 2.3.3). Before developing 

standardized procedures of measurements and data treatment we need to study in detail and 

better understand transients of various characteristics of devices and materials of interest. 

 Transients can be caused solely by changes in trap occupation as well as by generation, 

mutation and migration of defects, and also by combination of these mechanisms. Discriminating 

these two mechanisms is an important but not simple problem, and cannot be solved based only 

on significantly different characteristic times of the processes or their reversibility. 

 The discussion mentioned above has shown that the whole CdTe National Team 

considers transients in CdTe cells as an important problem that should be addressed in the basic 

studies. At the same time it was mentioned more than once that there has been a lack of studies 

of this kind and information we have is far less complete and detailed than say for amorphous 

silicon. The publications on CdTe by Sasala & Sites [59, 60] and by MacMahon [61] should be 

mentioned. All three are devoted to transients in J-V characteristics at varying temperature due to 

light or bias switch. Some non-systematic data on capacitance transients can be found, mostly on 

hysteresis in C-V profiles. 

 Publications on capacitance transients in amorphous silicon provided vast information on 

metastable effects/defects in this material. Capacitance transients are mostly due to slow changes 

in the space charge density after switching bias or light. Therefore interpretation is easier than for 

transients in J-V dependencies or photoconductivity. From the magnitude of change in 

capacitance one can estimate the charged defect density, no matter whether these changes are 

due to change in filling existing traps or generation of new atomic/structural defects. The sign of 

the transient allows discrimination of majority- and minority-carrier traps. The capacitance 

magnitude and time dependence studied at various temperatures provide the characteristic time 

55
of traps, their energy levels, capture cross section, concentration or DOS function in the case of 

continuous distribution of states. If transients are due to generation of new defects or their 

mutation, the activation energy of the processes can be derived from the TtC ),( dependencies. 

 Capacitance transients in differently processed cells, both as-prepared and stressed, are an 

important part of our studies. We believe that these studies combined with intentional variations 

in deposition and postdeposition procedures, in sort, amount and method of introducing of 

dopant, and accompanied with structural and compositional studies, can also provide significant 

progress in identification of defects responsible for specific electronic states. The current 

situation is far from being perfect. Indeed, in many cases different authors attribute the same (or 

almost the same) energy level to different defects, even if the level is determined with the same 

method. Conversely, different levels are ascribed to the same defect. Frequently, a significant 

discrepancy is seen in the energy of the same state/defect obtained by measurements and 

theoretically. A good example is a variety of energy levels ascribed to the Cu substitute for Cd. It 

should be mentioned that the problem of identification is even more severe for polycrystalline 

CdTe, where manifestations of the same defect can differ to some extent for locations in the 

grain bulk and in the GB region. 

2.4 New System for Measuring Admittance in a Wide Temperature Range 

 To realize the program of studying deep states with electrical methods a new measurement 

system has been built. It is designed to provide admittance spectroscopy (AS) measurements at 

different biases, capacitance-voltage profiling (CV), current-voltage characterization (JV), and 

measurements of transients (Tr) in all characteristics listed above. All measurements can be 

conducted at various temperatures, in dark and under illumination. 

The experimental ‘parameter testing space’ for the admittance measurements (AS, CV, Tr) 

covers five dimensions: 

1. AC current frequency from 25Hz to 30MHz 

2. DC Bias from –3V to +3V. A wider range can be used at the risk of damaging the sample. 

3. Temperature ranges from –190o

C to +80oC. We can maintain sample within one °C for 10 

minutes and for a longer time at temperatures near room temperature. 

4. Light of intensity up to 1 Sun is injected through a fiber cable. Spectral content is varied using 

filters. 

5. Sampling time ranges from the shortest interval of about 1.5 seconds to about 600 seconds 

(keeping temperature within one degree C). It is possible to sample up to several days at room 

temperature. 

Temperature

A custom designed sample stage was built with the principal goal of providing thermal 

stability. This is achieved through an insulated copper chamber with a large (15kg) copper mass. 

The high heat conductivity of copper reduces thermal gradients within the chamber and the large 

mass (heat capacity) provides thermal stability for the time sufficient to perform each type of 

measurement at practically constant temperature. The chamber is heated or cooled beyond the 

testing temperature(s), allowed to settle, and then as it slowly (about 0.1 °C per minute or 

slower) returns to room temperature, tests are programmatically triggered at preset temperatures. 

Temperature is monitored using a K-type thermocouple connected to an Omega I-series 

controller which is in turn connected to a PC through a RS232 port. 

56
Sample Stage

The sample rests on 5cm x 5cm glass rectangle that allows illumination from underneath. 

The back contact and front contact sample probes are Copper-Tin-Zinc alloy, shaped to have an 

approximately 1mm2

 flat surface for contact with a sample. They are mounted on a custombuilt spring cantilever that is controlled with a set screw. The sample itself is located within a 

5cm diameter by 5cm high copper cylinder inner chamber. The chamber has notches to allow 

the probes access to the sample. This inner chamber reduces convection around the sample and 

helps stabilize the temperature and reduce thermal gradients. 

The inner chamber is enclosed in an outer chamber consisting of copper sheets and 

2.54cm (1 inch) thick copper slabs. This outer chamber weighs 15kg and provides thermal mass 

to stabilize the temperature in addition to screening out electromagnetic noise. It is grounded to 

the LCR meters. The chamber is encased in three layers of insulation foam board with a total 

thickness of 11.43 cm (4.5 inches). 

Heating and Cooling

Heating is provided by four 6.4mm (1⁄4 inch) by 11.4cm (4.5 inch) 35W cartridge heaters 

(140W total). These are inserted into bored holes in the copper slabs. The sample can be heated 

up to 80 °C. This upper limit is because of the melting temperature of the foam board insulation 

which is less than 100 °C. 

Cooling is provided by pumping liquid nitrogen through 6.4mm (1⁄4 inch) diameter copper 

tubing that is wrapped around the outer chamber copper slabs. The sample can be taken down to 

near liquid nitrogen temperatures. 

Light 

Light is conveyed to the sample through the insulation and copper chambers with a fiber 

cable 1.25cm (1/2 inch) in diameter. The light source is provided by a Fiber Lite A240P power 

supply and the accompanying Fiber Lite A240L 150W light source. An intensity of up to about 

1 sun is possible. Band pass filters can be inserted to control the wavelength from 500nm to 

950nm. 

Time 

The fastest LCR meter trigger response time is 1.3 seconds. A sampling length of 600 

seconds is possible within a temperature range of 1C (heating/cooling temperature gradient of 

0.1C per minute). At room temperature, tests can be run for several days for studying long-term 

transients. 

Thermal IV characterization 

The copper sample stage can also be used to generate J-V curves in a wide temperature 

range, the same as for admittance measurements. The most desirable approach is to reroute the 

contact leads to a Keithley source meter and to run the appropriate HPVEE driver program. 

However, access to a Keithley source meter is limited. Therefore, an alternate system has been 

developed using the LCR meter as a DC power supply and an available Keithley Digital Multi 

Meter. Unfortunately, to date, the JV curves generated in this manner have been of inferior 

quality. This problem will be addressed as time permits. Currently we measure JV characteristics 

at room temperature with a separate system that allows observation of transients caused by the 

light pulse and bias voltage switch. The time of transient observation is up to hours. 

57
2.5. Preliminary Experimental Results Obtained with the New System 

In this section we present some results of initial studies aimed at exploring conditions and 

procedures of measurements to define those providing more visible manifestations of deep states, 

hence more opportunities for their detection and studying. 

2.5.1 Bias induced transients 

Measurements were conducted on the CdS/CdTe cells supplied by Dr. D. Albin (NREL). 

Table 2.3 Variations in processing specifications and front contact structure in the cells studied 

Cell ID SnO2 type CBD CdS 

time, min 

CSS O2

press., torr 

CdTe 

thickn., μm 

CdCl2 treat. 

time, min 

Precontact. 

etch 

D131A Bilayer 35 0.8 8.9 10.0 Std. NP 

D133D Bilayer 37.5 0.8 9.0 10.0 Std. NP 

D135B Bilayer 40 0.8 8.3 10.0 Std. NP 

D114B Single l-er 35 1.0 8.5 15.0 200:50BrM 

D117A Bilayer 35 1.0 8.7 15.0 200:50BrM 

D115B Bilayer 35 0.8 9.7 6.0 200:50BrM 

D116D Bilayer 35 0.8 9.1 13.5 200:50BrM 

D217C Bilayer 37 0.8 9.2 8.0 200:50BrM 

D220B Bilayer 37 1.4 8.8 7.0 200:50BrM 

CdS layers were prepared with the chemical bath deposition, and CdTe films were 

deposited with close space sublimation. For these first tests a wide range of processing 

conditions was examined. J-V dependencies measured at CSM demonstrated close efficiency 

values, except for cell D131A that has obvious shunting effect and was excluded from further 

measurements. The results of admittance measurements presented below were selected to 

illustrate the major statements and proposed approaches in Sec. 2.3. 

58
Capacitance (parallel) Vs Elapsed Time

0.00E+00

2.00E-09

4.00E-09

6.00E-09

8.00E-09

1.00E-08

1.20E-08

0 200 400 600 800

Elapsed Time (s)

Cp (Farad cm-2)

D133D

D135B

D114B

D117A

D115B

D116D

D217C

D220B

9/18/2003

 Freq=10kHz; 

Figure 2.7 Capacitance transients at T=23oC for the samples listed in Table 2.3, except D131A. 

The bias voltage sequence is: 0 V Æ -1 V Æ 0 V 

As seen in Fig. 2.3, the capacitance transients of significant magnitude are observed for 

all cells. As mentioned in Sec. 2.4, the LCR meter trigger response time is in the range of 

seconds, therefore we lose the capacitance transients with shorter characteristic times. However, 

the equipment is available that allows observation and measurement of transients at much shorter 

time if needed (e.g., that created by Dr. D. Young at NREL). More detailed studies of 

admittance, including capacitance transients were conducted on the cell D220B. In particular, 

transients were measured over a much longer time range, up to st (~55 hours). 5 ⋅= 102

The results are presented in Fig. 2.8 with a logarithmic time scale. As shown by these tests, 

long-term capacitance transients were observed that required several days for total recovery of 

the initial capacitance. 

The evolution of capacitance transients with cell cooling is illustrated by Fig. 2.9a. 

Fig. 2.9b shows only transients observed after voltage switching back from -1 V to 0 V. Curves 

show a decrease in magnitude and in the rate of capacitance change with cooling. 

59
Capacitance vs. Elapsed Time

3.00E-09

3.50E-09

4.00E-09

4.50E-09

5.00E-09

5.50E-09

6.00E-09

6.50E-09

7.00E-09

1 10 100 1000 10000 1E+05 1E+06

Elapsed Time (s)

Cp (Farad cm-2)

D220B 9/18/2003 

Freq=10kHz; T=23C

 (a) 

0.00E+00

5.00E-09

1.00E-08

1.50E-08

2.00E-08

2.50E-08

3.00E-08

3.50E-08

1 10 100 1000 10000 10000

0

1E+06

Elapsed Time (s)

Cp (Farad cm-2)

D220B 9/18/2003 

Freq=10kHz T=23C

 (b) 

Figure 2.8 Capacitance transients at T=23o

C for the cell D220B in a wide time range. 

(a) The bias voltage sequence is: 0 V Æ -1 V Æ 0 V; (b) after switching back to V= 0 

60
Capacitance vs. Elapsed Time

0.00E+00

1.00E-09

2.00E-09

3.00E-09

4.00E-09

5.00E-09

6.00E-09

7.00E-09

8.00E-09

9.00E-09

1.00E-08

0 200 400 600 800

Elapsed Time (s)

Cp (F/cm^2)

-85 C

-55 C

-25 C

5 C

20 C

9/19/03, 10kHz

 (a) 

Capacitance vs. Elapsed Time

3.00E-09

4.00E-09

5.00E-09

6.00E-09

7.00E-09

8.00E-09

9.00E-09

0 50 100 150 200 250 300

Elapsed Time (s)

Cp (F/cm^2

)

-25 C

-15 C

-5 C

10 C

9/19/03, 

10kHz

 (b) 

 Figure 2.9 Capacitance transients at reduced temperatures for the cell D220B. 

 (a) Bias voltage sequence: 0 VÆ -1V Æ 0V; (b) expanded view following return to 0 V. 

 

To find possible manifestations of the single trap levels or narrow bands, we analyzed the so 

called DLTS function at each temperature. According to the simple single level 

theory, the dependencies or should have a peak at 

= tddCS )(ln/

tS )( tS )(ln t =τ where τ is the characteristic 

time of the trap. As seen from Fig. 2.10, the peaks were observed at all temperatures in the range 

tested. Position of the peak, hence the τ value, shifted toward longer times with cooling. With a 

common procedure of τ T)( analysis, using the Arrhenius plot, we have estimated activation 

61
energy of the trap level eVE and capture cross section, a = 39.0 220 10 cm − σ ∞ = . When cooling 

to -70 o

C we have found another trap state: { eVE ; a = 42.0 218 10 ⋅= cm − σ ∞ }. It should be 

mentioned that so far we did not try to separate contributions in activation energy of the trap 

level energy and potential barrier for capturing, therefore the capture cross section derived from 

Arrhenius plot is denoted as σ ∞ . A similar analysis of the transients after voltage switch from 0 

V to -1 V revealed two more levels: { eVE ; a = 47.0 216 10 ⋅= cm − σ ∞ } and { ; 

}. 

a = 88.0 eVE

212 10 ⋅= cm − σ ∞

 Capacitance deivative upon log time

-1.00E-09

-9.00E-10

-8.00E-10

-7.00E-10

-6.00E-10

-5.00E-10

-4.00E-10

-3.00E-10

-2.00E-10

-1.00E-10

0.00E+00

10 100 1000

Elapsed Time (s)

t*dCp/dt (F/cm^2)

-25 C

-20 C

-15 C

-10 C

-5 C

0 C

5 C

10C

9/19/03, 

10kHz

Figure 2.10 DLTS function for the capacitance dependence on time after the voltage switch 

from -1V to 0 V, in the temperature range of 10o

C to -25oC. Cell D220B. 

 Fig. 2.11 shows capacitance transients for the cell D217C at elevated temperatures. As 

expected, both magnitudes of transients and the rate of change increased with higher 

temperature. The data for the short times after switching voltage to -1 V are not available as 

explained above. Therefore we were not able to search for relatively “fast” states with τ δ 1s. But 

evidence of the slower states can be seen at longer times. The tendency to saturation in the C(t) 

dependence is seen for the reverse bias applied as temperature increases. For the highest 

temperatures of measurements (70, 75, 80o

C), saturation is followed by the additional increase. 

62
Capacitance vs. Elapsed Time

3.50E-09

4.00E-09

4.50E-09

5.00E-09

5.50E-09

6.00E-09

6.50E-09

7.00E-09

7.50E-09

1 10 100 1000

Elapsed Time (s)

Cp (F/cm^2)

80 C

75 C

70 C

65 C

55C

45 C

D217C 10/21/03,

10kHz, 

 Figure 2.11 Capacitance transients at elevated temperatures for the cell D217C. 

 

 The DLTS function analysis reveals peaks in the S(t) dependence (see Fig. 2.12) whose 

position on the time scale rapidly shifts with temperature. This can be due to the presence of a 

new level with high activation energy. 

All the states detected so far from the transient analysis did not reveal themselves in our 

AS measurements because their characteristic frequencies, at reasonable temperatures 

are beyond the limits of LCR meters. At long times of observation, transient demonstrate very 

slow changes, maybe caused by specific capture-time dependence due to collective action of 

closely distanced traps (grain boundaries, or trap clustering). 

−1

t =τω

We have also measured C-f and C-V profiles on all cells. C(f) and G(f) dependencies 

demonstrated a peak in the range of ~ 105

 kHz or lower at lower temperatures and higher at the 

elevated temperatures. The Arrhenius plot in the temperature range of 220 K to 340 K provided 

an activation energy of 0.32 eV and capture cross section of ~10-16 cm2

. Both C-V and C-f 

dependencies manifested a significant variation with temperature. Figs. 2.13 and 2.14 illustrate 

changes at higher temperatures. 

63
Capacitance derivative upon log time

0.00E+00

2.00E-11

4.00E-11

6.00E-11

8.00E-11

1.00E-10

1.20E-10

1.40E-10

1.60E-10

1.80E-10

2.00E-10

10 100 1000

Elapsed Time (s)

t*dCp/dt (F/cm^2)

80 C

75 C

70 C

65 C

55 C

45 C

D217C,10/21/03, 10kHz 

Figure 2.12 DLTS function for the data presented in Fig. 2.11. The S(t) peaks are well seen 

at T >70o

C indicating presence of a single trap level/narrow band. 

Capacitance vs. Frequency

4.0E-09

5.0E-09

6.0E-09

7.0E-09

8.0E-09

9.0E-09

1.0E-08

10 100 1000 10000 100000 1000000

Frequency (Hz)

Cp (F/cm^2)

75 C

65 C

50C

40C

9/18/03

Figure 2.13 Variation of the C-f dependence with varying temperature in the elevated 

temperature range. Cell D220B. 

64
Capacitance (parallel) Vs Voltage

0.0E+00

2.0E-09

4.0E-09

6.0E-09

8.0E-09

1.0E-08

1.2E-08

1.4E-08

1.6E-08

1.8E-08

2.0E-08

-2.5 -1.5 -0.5 0.5

Voltage (V)

Cp (F/ cm-^2)

75 C

65 C

50 C

40 C

10/18/03, 

10Khz

Figure 2.14 Variation of the C-V dependence with varying temperature in the elevated 

temperature range. Cell D220B. 

The temperature range of measurements was extended down to -130o

C. Capacitance 

continually decreased over the entire temperature range. For example, its value measured at Vbias 

= 0 and f = 10 kHz, decreased 4.2 times from T= + 80oC to T= -130oC. This effect can be 

explained partially by reducing contribution into measured capacitance of the oscillating trapped 

charge due to the exponential increase in characteristic time with cooling. However, the free 

carrier freeze-out effect cannot be excluded. If considerable contribution of the second effect is 

found, that would indicate a significant role of deep states in controlling carrier concentration. In 

turn, it will bring us again to the problem of concentration of the defects and their 

compensation, as well as of the origin of the state with 

CuCd

a ≈ 32.0 eVE detected by AS 

measurements. 

 Variation of polarity of the “charging pulse” can significantly influence transients and 

provides additional opportunities to detect and study traps. Capacitance transients at opposite 

polarities of the applied bias are compared in Fig. 2.15. It is seen that magnitude of a transient is 

much greater at forward bias (FB) than at reverse bias (RB). Difference between the capacitance 

values at the beginning and end of the FB transient is 24nF/cm2

 (maybe higher, because there is a 

delay between the bias switch and start of recording, and also the FB steady state was not 

reached). It is significantly greater than the initial, 6nF/cm2

, and final, 9nF/cm2, measured 

capacitance values. If we attribute the transient to slow traps, we must conclude that their density 

is greater (maybe much greater) than the concentration of free carriers and faster traps. 

The DLTS function for FB (Fig. 2.15 b) demonstrates a well pronounced minimum at 

t that can be considered as a manifestation of a very slow single level (narrow band) trap. 

No extrema are seen in the RB DLTS function. Generally, this should not be surprising. Indeed, 

≈ 280s

65
transients at FB and RB, even if controlled by the same trap, are provided by opposite processes: 

emptying and filling traps and vice versa depending on the trap type (for holes or electrons), 

while the emission and capture rates can be significantly different. 

dC/dlnt vs. lnt

-1.00E-08

-8.00E-09

-6.00E-09

-4.00E-09

-2.00E-09

0.00E+00

2.00E-09

4.00E-09

10 100 1000

Elapsed time, s

dC/dlnt, F/cm^2

0, -1, 0 V

0, +0.5, 0 V

 (a)

Capacitance transients. Test freq.: 100KHz

0.00E+00

5.00E-09

1.00E-08

1.50E-08

2.00E-08

2.50E-08

3.00E-08

3.50E-08

4.00E-08

10 100 1000 10000

Elapsed Time (s)

Cp (Farad cm-2 )

0, -1, 0 V

0, +0.5, 0 V

 (b) 

Figure 2.15 Capacitance transients measured after switching bias from 0 V to V=+0.5V (red) 

and from 0 V to -1 V (blue). Cell D220B, T≈22o

C, f=100 kHz.

66
In this respect it is very useful to compare transients recorded under applied bias and after 

switching back to zero bias (relaxation stage). DLTS functions look different for the two stages, 

sometimes they are rather complex revealing several extrema, both minima and maxima. Thus, 

comparing transients at different sequence of bias applied and at different polarity one can obtain 

more detailed and reliable information on the trap type/s, emission and capture rates. 

It should be mentioned, that a near total recovery of the capacitance value occurs in 

several hours after switching back to zero bias in transients presented in the figures above. 

Reversibility may be considered as an additional argument that the observed transient are caused 

by a change in the slow trap filling with carriers, but not by generation, transformation or 

migration of the charged defects. 

2.5.2 Light induced transients 

Slow light induced capacitance transients have been observed at room temperature at 

wavelengths below and above the CdTe bandgap (wavelength cutoff λ≈825nm). Light intensities 

were very low, of the order of 0.1mW/cm2

. This excludes any significant changes in temperature, 

verified by the thermocouple readings that did not change with illumination within the accuracy 

of 0.1o

 K. Based on preliminary measurements of the temperature coefficient of capacitance, this 

may lead to thermally induced capacitance variations not exceeding 0.01nF/cm2

, whereas the 

observed light induced changes were in the range of 0.2 to 1 nF/cm2

. 

 Figure 2.16 shows capacitance versus time for a 30 second initial period in dark, a 45 

minute (2700s) light pulse, and a 45 minute recovery period on the CdTe/CdS cell 217C supplied 

by D. Albin (see Table 2.3). The changes in capacitance indicate changes in depletion width due 

to changes in net space charge density (SCD). The SCD variations are inferred to result from net 

charging and discharging of traps. 

 A pronounced difference can be seen between the above band gap (680nm) and below 

band gap (1064nm, 1300nm) wavelengths. With the above band gap wavelength, following an 

initial spike, the capacitance drops below the pre-illumination level before gradually increasing. 

Both during and following the illumination the capacitance does not reach saturation indicating 

very long characteristic times (τ > 1000s). Differences in starting capacitance during the initial 

30 s period prior to lighting are due to remnant trap charges from previous testing. To start with 

the same capacitance the sample needed to be kept in the dark overnight indicating τ ~ hours. 

The light source was a 150W halogen bulb filtered with narrow band (~20nm) Corian filters 

to select the wavelength incident upon the solar cell. Light intensities for the 680nm (1.82eV) 

and 1064nm (1.17eV) wavelengths were measured with a silicon based optical power meter. For 

the 1300nm (0.95eV) wavelength that is below the 1.1 eV silicon band gap, no direct light 

intensity measurements were made. In this instance the rheostat setting was left the same as for 

the 1064nm wavelength and the light intensity was assumed to be approximately the same. 

 Figure 2.17 shows capacitance versus time for light intensities of 0.025, 0.050, 0.100, 

0.200, and 0.400 mW/cm2

. For each test, the sample was kept in the dark overnight prior to the 

test so as to start with a comparable capacitance value. In spite of this, there remained minor 

starting capacitance differences of up to 0.1 nF. On a couple of tests the sample was kept in the 

dark for 60 hrs over a weekend (not shown in figure 2.17) and this caused the starting 

capacitance to drop by an additional 0.2nF. Even with these offsets, the structure of the light 

induced CTr curves was essentially the same. Thus, overnight in the dark was determined to be 

sufficient for these low light CTr tests. 

67
4.E-09

5.E-09

6.E-09

1 10 100 1000 10000

Elapsed Time (s)

Cp (Farad cm-2)

680nm (1.82eV)

1064nm (1.17eV)

1300nm (0.95eV)

Figure 2.16 Light induced capacitance transients for 0.1mW/cm2 lighting intensity and 

wavelengths of 680nm, 1064nm, and 1300nm. Testing voltage frequency is 100kHz, sequence is 

30s dark, 2700s light, 2700s dark. Sample D217C (Table 2.3) 

4.E-09

5.E-09

6.E-09

1 10 100 1000 10000

Elapsed Time (s)

Cp (Farad cm -2)

0.025 mW

0.050 mW

0.100 mW

0.200 mW

0.400 mW

Figure 2.17 Capacitance transients induced by the sub-bandgap (1064 nm) light of various 

intensities. 

68
 In all cases there was no measurable electrical potential generated by the cell from this 

sub - bandgap light source. There was an initial increase in capacitance indicating a net increase 

in the negative space charge density (SCD) for the p-type CdTe material. There is a subsequent 

decrease in capacitance followed by a gradual increase. The drop in capacitance never reached 

the pre-illumination level. During the 45 minute illumination period the capacitance increase 

never saturates. Since the light source is sub-bandgap, these phenomena are a result of direct 

interaction of photons with deep traps. This involves the promotion of electrons from the valence 

band to the trap or from the trap to the conduction band. This direct photon interaction is 

superimposed on the thermal emission and capture of holes and electrons by the trap. 

 When the light is switched off, as the cell returns to its pre-illumination equilibrium, there 

is a spike in capacitance followed by a gradual decrease in capacitance. This is a typical transient 

response. It takes at least several days in the dark for the cell to return to equilibrium. The 

magnitude of the effect increases with intensity but the basic structure remains the same. The 

spikes at the beginning of illumination and after the light is switched off are limited by the 3 

second capacitance reading interval of our instrumentation and the approximately 2 seconds for 

the bulb to fully illuminate and extinguish. 

 The preliminary experiments with light indicate that further studies of light induced 

transients along with those caused by bias changes will provide new opportunities for detecting, 

characterization and identification of deep electronic states in CdTe solar cells. 

2.6 Summary 

 

1. The basics of admittance spectroscopy (AS) as a method for detection of deep states and 

studying their properties were presented and discussed along with two modifications of AS. 

(Sections 2.2.1 and 2.2.2) 

2. Our studies of CdTe cells using AS and modified AS revealed various deep states with 

concentration, spatial distribution and characteristic time, that depend on deposition 

technique, postdeposition treatment, and contacting procedure (Sec. 2.2.3). A common 

feature of the cells fabricated at different facilities (FS, NREL, UT, FS/CSM) was a high trap 

concentration, , exceeding doping level, , determined by C-V profiling. In some 

cells density of detected trap states was of the order of 10

Nt N −VC

16 cm3

. Experimental data also 

indicated presence of significant amount of slower traps not detectable with the standard AS 

equipment . 

3. The common approach to the analysis of AS and DLTS measurements is based on the theory 

valid for low trap concentration much smaller than the doping level ( ). In the case 

of CdTe this approach should be revised based on more adequate theory and numerical 

modeling using more reliable physical models. The common equations used for trap 

concentration estimates, actually provide the ratio . Analysis presented in Sec. 2.3.1 

shows that at τ this ratio is underestimated, especially if the levels are located close to 

Fermi level. On the other hand, the doping level derived from C-V profiling, can be 

strongly overestimated if . Numerical estimates of apparent doping level, , 

compared to the real one, N , are presented for CdTe with two trap levels having 

characteristics close to those determined by AS. 

t << NN

t / NN

Nt N

N −VC

t > NN N −VC

69
4. In Sec. 2.3.2 the technical limitations on the characteristic time of traps available for 

detection by AS are considered. As possible solutions of the problem, variation of 

temperature and measurements of transients are discussed. 

5. The procedure for determining trap cross section is considered in Sec. 2.3.3, in particular 

with respect to the traps with the repulsing potential barrier. The latter is considered in more 

detail for the barrier provided by collective action of closely spaced traps, e.g., trap clusters 

and especially grain boundary states. The latter are considered as the most important origin 

of very slow traps. The trap filling dependence on time is specific for these “collective” trap 

states which can be used for their identification. 

6. The current state of the transient studies in CdTe is unsatisfactory (Sec. 2.3.4). The transients 

have been observed but not studied properly. Studying of capacitance transients is the most 

promising method for detection slow/deep states, estimating trap concentration, 

distinguishing between the majority- and minority-carrier traps. 

7. A new measurement system recently developed in our facilities is described in Sec. 2.4. The 

system is designed for studying the cell admittance, in particular, C-f and C-V dependencies, 

along with transients in a wide temperature range, from below -100o

C to +80oC. 

Measurements can be conducted in dark and under illumination of varying intensity and 

spectrum. 

8. The preliminary results of measurements presented in Sec 2.5 show that the system provides 

the expected/designed options. Measurements revealed transients, including very long-term 

ones. Analysis of transients based on DLTS function detected at least five levels or narrow 

bands with characteristic times in the range of −10010~ ss not available for common AS. 

Analysis of temperature dependencies using the Arrhenius plot allowed estimates of 

activation energy and capture cross section. We noted very slow transients a, non-exponential 

decays, and very low capture cross sections that could be attributed to grain boundaries. A 

significant capacitance dependence on temperature in the range of -130 to +80o

C was found. 

Possible mechanisms of dependence were briefly discussed. 

3. EFFECT OF CdCl2 TREATMENT, DOPING WITH Cu, AND STRESSING ON 

DEEP ELECTRONIC STATES (DES) 

To date all high performance CdTe solar cells have included CdCl2 treatment and Cu 

doping in a postdeposition heat treatment [62]. While the complex interaction between these two 

additives is known to favorably change the defect structures that control performance, the 

understanding of this process remains incomplete. To help address this, we used admittance 

spectroscopy (AS) and capacitance transients (CTr) to study differences in deep electronic states 

(DES) for CdTe cells produced with and without both Cu and CdCl2. This study is a 

continuation of our previous efforts to monitor deep states behavior in cells under varying 

processing procedures [14] 

3.1 Samples 

The cells for this study were produced at NREL by Dr. D. Albin. They were grown on 

Corning 7059 glass substrates with a bi-layer (500 nm F-doped/100 nm undoped) tin-oxide 

70
structure used as the front transparent contact. The CdS window layer was deposited by chemical 

bath deposition (CBD) to a thickness of ~80nm followed by close-spaced sublimation (CSS) of 

the CdTe absorber layer to a thickness of ~8μm. The CdCl2-treated devices were annealed at 

400o

 C in a CSS-arrangement using anhydrous CdCl2 as a source in an ambient of 100 torr 

Oxygen/400 torr Helium for 7 min. All devices were then etched with a nitric-phosphoric acid 

(NP) etch consisting of 1:88:35 parts HNO3:H2PO4:H2O. Back contact carbon-based pastes 

(Electrodag 114) with and without Cu additives (Cu1.4Te powder) were applied to the CdTe 

surface by brushing. The back contact areas are approximately 0.5cm2

 and a 260oC anneal in 

helium was applied to activate the contact. To avoid back to front contact electrical shorting, a 

~1 mm lateral margin of solar cell material was retained around the back contact. This enhances 

current collection at the back contact edges yielding higher short-circuit current values than 

would be obtained without the margin. The results presented in this section (Sec. 3) were 

obtained mostly on four cells with different combinations of CdCl2 treatment and Cu doping: 

[No-Cu, Yes-CdCl2], [Yes-Cu, Yes-CdCl2], [Yes-Cu, No-CdCl2], [No-Cu, No-CdCl2]. 

Light JV curves for each cell are shown in Fig. 3.1. The cells treated with CdCl2

performed significantly better than the non CdCl2 treated cells for all metrics including: 

efficiency (η); open-circuit voltage (Voc); short-circuit current (Jsc); and fill factor (FF). The cells 

treated with Cu had better η, Voc, and FF. The best cell with the combination CdCl2 and Cu 

treatment had a slightly lower Jsc than the CdCl2 only cell. Rollover can be seen in the cells 

processed without Cu due to the back contact Schottky barrier [63]. 

-25

-15

-5

5

15

-0.1 0.4 0.9 Volts mA/cm^2 

No Cu & Yes CdCl2, η=11.1%

Yes Cu & Yes CdCl2, η=12.4%

Yes Cu & No CdCl2, η= 9.0%

No Cu & No CdCl2, η= 7.3%

Figure 3.1 Light JV curves for the four CdTe solar cells studied. 

71
3.2. Admittance Spectroscopy Measurements and Analysis 

3.2.1 AS spectra and their analysis. Trap signatures 

As explained in Sec. 2.2.1, when the testing AC signal angular frequency, ω, significantly 

exceeds the trap characteristic frequency, ωt=1/τt, the trapped electrical charge cannot follow the 

AC signal resulting in a lower measured capacitance, , and saturation in the AC conductance, 

, dependence. Trap characteristic frequency can be determined experimentally from 

the plots of either 

Cp

Gp ( ) ω ωt

p dCd ωω or p dGd ω versus ω. Both have peaks at ω =ωtpeak . Since 

the characteristic frequency strongly (exponentially) depends on temperature, trap oscillating 

capacitance measured at fixed AC signal frequency, is also temperature dependent. Namely, its 

contribution is negligible at low temperatures where ωt <<ω , while reaches its maximum and 

saturates at high temperatures where ωt >>ω . To demonstrate trap contribution into 

capacitance, distinguish traps with different characteristic frequencies (times), and estimate their 

relative densities, we will use in this section images combining frequency and temperature 

dependences of capacitance and AC conductance as seen from the figures below. The magnitude 

of the trap contribution to the capacitance, , indicates the relative trap concentration. This is 

applicable even when the trap concentration is higher than the doping level as is often the case 

with polycrystalline thin film CdTe cells. 

Ct

The measured capacitance C(ω) and conductance G ω)( are modeled here by: 

β ωω

ω 2

)/(1

)(

t

t

d

C CC

+

+= ; β

β

ωω

ωω

ω ω 2

2

)/(1

)/( )()(

t

t CGG ttd

+

+= (3.1) 

 and represent the base depletion capacitance and the cell DC differential conductance, 

respectively. represents the maximum low frequency trap capacitance. We have added a 

phenomenological term, 

Cd Gd

Ct

β , in the exponent to help with fitting to the experimental data. This is 

similar to the stretched exponential parameter that is used to model many natural phenomena

[64]. The capacitance transition from to can be spread over a wide range of 

frequencies and often a good fit is obtained with 

+CC td Cd

β ≈ 5.0 . This could be caused by a finite width 

band of trap energy levels. The peak location in plots of either p dCd ωω or p dGd ω versus 

ω is again at ω =ωt and does not depend on the β value. 

We used Ct for relative comparisons of trap concentrations and have avoided using 

common quantitative formulas because the trap concentrations are high relative to the free carrier 

concentration, violating underlying formula assumptions. Besides, the trap concentrations vary 

spatially further complicating quantitative analysis. We verified that the p dCd ωω peaks were 

not manifestation of RC circuit back contact Schottky barrier. It was done by adding known 

resistors in series and checking that ω peak did not change. It did not change also with 

application and varying of the forward bias around +1 V, which should change resistance of a 

Schottky barrier and related 1 )(

− ω peak = RC . 

72
 AS differences were observed between each of the four differently processed cells and 

the most noticeable were between the Cu- doped and undoped cells. This can be readily seen by 

comparing the graphs in Fig. 3.2 which show the AC conductance versus temperature. 

1.E-08

1.E-06

1.E-04

1.E-02

-200 -100 0 100

Temperature C

Conductance (S/cm^2)

H1 H2 

(A) 

1.E-08

1.E-06

1.E-04

1.E-02

-200 -100 0 100

Temperature C

Conductance (S/cm^2)

H3 H1 

(B) 

Figure 3.2 AC conductance versus temperature lines shown for AC frequencies ranging from 

1kHz (bottom line) to 1MHz (top line). The upper graph (A) is for the cell with both the Cu and 

CdCl2 treatment, and the lower graph (B) is for the cell with only the CdCl2 treatment. The 

detected majority carrier traps labeled H1 through H3 are shown with lines connecting the 

conductance peaks. 

73
The upper Fig. 3.2A shows the cell with the combined Cu and CdCl2 treatment and Fig. 

3.3B shows the cell with the CdCl2 only treatment. Each line corresponds to a fixed AC signal 

frequency with frequencies ranging from 1kHz to 1MHz. The peaks on the lines indicate traps 

with the characteristic frequency corresponding to the line frequency at the peak’s temperature. 

The families of peaks clearly show the temperature to characteristic frequency relationships for 

multiple traps. Lines connecting the peaks have been added and are labeled to identify the 

different trap signatures that are subsequently discussed. 

A total of three distinct hole trap signatures were analyzed. Table 3.1 lists the traps along 

with the cells in which they were detected 

Table 3.1. Activation energy, Ea, apparent cross section, σ, and trap contribution to capacitance, 

Ct, for traps detected with AS in cells with different combinations of Cu and CdCl2. 

Trap Cu CdCl2 Ea σ Ct Temp. Range for the Arrh. plot 

 (eV) (cm2

) (nF/cm2) (K)

H1 No Yes .128±.003 8E-14 3.0 83 to 108 

 Yes Yes .133±.006 4E-14 2.1 83 to 118 

 Yes No .128±.005 2E-14 1.1 83 to 118 

 No No .145±.007 1E-12 2.5 83 to 108 

------------------------------------------------------------------------------------------------------------------------ 

H2 Yes Yes .305±.010 9E-14 0.9 183 to 238 

 Yes No .292±.009 6E-14 0.5 173 to 233 

------------------------------------------------------------------------------------------------------------------------ 

H3 No Yes .475±.006 7E-13 0.6 243 to 310 

 No No .464±.009 8E-13 0.5 238 to 300 

The trap signatures in the table were grouped based on a combination of their activation 

energy and their detection temperature range. Temperature and frequency dependent capacitance 

for all four combinations of Cu and CdCl2 are presented in Fig. 3.3. 

74
0,0E+00

2,0E-09

4,0E-09

6,0E-09

8,0E-09

1,0E-08

1,2E-08

-190 -140 -90 -40 10 60

Temperature, C

Cp (F/cm^2)

No Cu, Yes CdCl2

0,0E+00

2,0E-09

4,0E-09

6,0E-09

8,0E-09

1,0E-08

1,2E-08

-190 -140 -90 -40 10 60

Temperature, C

Cp (F/cm^2)

Yes Cu, Yes CdCl2

0,0E+00

2,0E-09

4,0E-09

6,0E-09

8,0E-09

1,0E-08

1,2E-08

-190 -140 -90 -40 10 60

Temperature C

Cp (F/cm^2)

Yes Cu, No CdCl2

0,0E+00

2,0E-09

4,0E-09

6,0E-09

8,0E-09

1,0E-08

1,2E-08

-190 -140 -90 -40 10 60

Temperature C

Cp (F/cm ^2)

No Cu, No CdCl2

Figure 3.3 Temperature dependence of capacitance at frequencies ranging from 1Khz (top line) 

to 1MHz (bottom line). 

 Frequency dependent capacitance at a fixed temperature indicates trap contribution into 

the measured capacitance. In agreement with the temperature and frequency dependent AC 

conductance in Fig. 3.2, the graphs in Fig. 3.3 demonstrate presence of at least three different 

types of traps. Trap labeled above as H1 manifests itself at low temperatures (Tδ-130 C). At the 

lowest temperature (-190 C) and highest frequency (1MHz), capacitance of all four cells is close 

to the value determined with the equation = εε o / dC (d is the CdTe thickness). Capacitance 

75
increases with reducing frequency because frequency of voltage oscillations,ω, comes closer to 

the trap characteristic frequency, ωt . As temperature increases to T≈-130 C, dependence on 

frequency becomes minimal indicating that above this temperature 16 102 − ⋅> s t πω and 

trapped charge follows applied voltage oscillations even at f=1MHz. At further increase in 

temperature the frequency dependence again becomes significant because now new slower (and 

deeper) traps start to contribute to capacitance, although the high frequency capacitance does not 

change for a while until the ωt value exceeds ω due to temperature increase. After that, 

capacitance increases with temperature at all frequencies. An appropriate analysis of the 

C ω T),( dependencies reveals the H3 trap in the [No Cu – Yes CdCl2] spectra and the H2 trap 

in the cells doped with Cu. 

 The C( f ) dependence not disappearing at higher temperatures (up to T=+85 C) could be 

considered as a manifestation of multiple slower traps or wide trap level band. 

 At fixed temperature, difference between high frequency and low frequency capacitance 

which provides an estimate of theCt value, is proportional to the trap density. Therefore, it 

follows from the figures above that the H1 density is the greatest for the CdCl2 treated, but not 

doped with Cu, cell. For the [Cu+CdCl2] cell, the H2 density is high, while Cu doping alone, 

without CdCl2, leads to a significantly lower H2 density. 

 

3.2.2. Attribution of detected traps 

 We have mentioned previously that there is a lot of ambiguity and discrepancies in 

attribution of the deep electronic states to particular defects. However, keeping reservations, we 

would like to discuss possible attribution for the traps we have labeled as H1, H2 and H3. 

 A strong H1 signature was observed in all four cells with Ea matching the 0.13 eV singly 

ionized cadmium vacancy acceptor defect [65, 20] as well as the chlorine A-center 

 acceptor defect [65, 22]. Perhaps we deal with both presented in different proportions 

in differently treated cells. The NP etch leaves behind a ~0.1μm thick free Te layer at the back 

surface and a Te enriched layer underneath [66] . This serves as a source of diffusing in the 

CdTe depth. The C

− VCd

+−

Cd − ClV Te

2

− VCd

t values indicate that the H1 density decreases with the addition of Cu which 

is consistent with Cu substitutions on cadmium vacancies meaning partial replacement of 

with . Just on this effect was based an explanation of temperature dependence of the EL 

pattern in the Cu-dot cells (see Sec.1). In turn, the H1 concentration increases with the addition 

of CdCl

− VCd CuCd

2 which is consistent with the formation of . +−

Cd − ClV Te

2

 The H2 signature was only detected in cells doped with Cu. The Ea ~0.30 eV is close to 

values observed by others [21, 67] and we thus inferred that this was a defect. The 

H2 concentration was higher with CdCl

CuCd CuCd

2 implying that the defects induced by CdCl2 increased 

the concentration of . CuCd

 The H3 signature with an Ea of 0.47 eV was only detected in cells with no intentionally 

added Cu. This energy level has been detected by others [67, 68] and in two instances has been 

attributed to [65, 69] in strong discrepancy with the theoretical estimates for this defect 

[20]. Besides, we note that H1 which we attributed to the singly ionized remains detectable 

2− VCd Ea − VCd

76
with Cu, while H3 does not, although both are ascribed to the cadmium vacancy. We also would 

not exclude that H3 is caused by the negative U-center . The apparent discrepancy between 

our measured E

2− Tei

a of 0.47 eV and the calculated transition (0/-2) energy of 0.57 eV [20] can be 

eliminated by suggesting the U value of –0.2 eV. We have observed the H3 trap only in the cells 

subjected to the NP etch, but did not see it in the cells etched with Br: Methanol. NP etch 

provides a strong excess of Te in the top layer of CdTe. One can suggest that due to that the 

interstitial Te concentration can be significant in CdTe, at least at the region not too remote from 

the back contact. Disappearance of the H3 manifestation in Cu doped cell could be attributed to 

masking by H2 contribution into admittance. Another, “more scientific”, explanation could be 

that high Cu concentration leads to high negative space charge density in the depletion region. 

This, in turn, leads to redistribution of the CdTe band bending in the way that increases distance 

between the crossing point of the H3 level and back contact. Since excess Te diffuses from the 

back contact, it is reasonable to suggest that the density sharply decreases with increase in 

this distance making H3 not detectable. However, we remain cautious about assigning H3 to a 

specific defect. The H3 β values were higher than average, possibly indicating a narrower 

energy band for this defect. 

Tei

3.3 Deep States Studied with Capacitance Transients

 

 As discussed in previous sections, the finite LCR meter frequency range puts limitations 

on the characteristic frequency of the traps that can be detected and studied with admittance 

spectroscopy. As an alternative method, we have used the capacitance transient measurements 

which allow detection of very slow states with the characteristic times in the range of 

seconds, minutes, etc. The transient effects presented and analyzed in this section were induced 

by changes of the bias applied to a cell in various sequences: Zero Bias (ZB) Æ Forward Bias 

(FB), ZBÆ Reverse Bias ( RB), FBÆZB, RBÆZB, FBÆRB. Since the procedure of 

measurements in our measurement system (Sec. 2.4) provides 

−1 =ωτ tt

TVtGC ),,(, t dependences in 

one temperature run, we have used the for the trap detection analysis of C t)( and at 

different temperatures. Activation energy and apparent cross section, as before for AS, were 

derived from the Arrhenius plot. The sign of the DLTS function peak (maximum or minimum) 

allowed to distinguish the majority (hole) and minority (electron) carrier traps. 

tG )(

3.3.1 Trap Signatures. Arrhenius plots 

Figure 3.4 shows the Arrhenius plots obtained with transient measurements along with those 

obtained with AS measurements. The principal difference between these two groups is in the 

range of characteristic times. Traps H1, H2, H3 had characteristic times short enough (“fast 

traps”) to be detected with AS. Traps E2, E3, H4, H5, H6 (“slow traps”) could be detected only 

via transient measurements since the temperature range was limited to T~85 C to avoid 

irreversible effects caused by heating. Note that temperature ranges where we were able to detect 

and study manifestations of specific traps were narrower for slow traps studied with C-Tr. This 

leads to a larger inaccuracy in determining activation energy from the slope of a plot and capture 

cross section from the intersection with the 1/T axis. In particular, it is not clear whether the plots 

labeled H4 and H5 represent different traps or the same one.

77
It is seen from Fig. 3.4 and Table 3.2 that the set of detectable traps varies with the cell 

treatment. Namely, E2, H6 were found only in a cell with (CdCl2+Cu) treatment. These states 

were not seen if one of the treatments was missed. E3, H4 were seen only without Cu doping, H5 

with [Yes Cu - No CdCl2] treatment. As seen from the table, all slow traps have low or moderate 

capture cross section, except H5 with 211 10 cm − σ ≈ . 

Table 3.2. Traps detected with the capacitance transient (C-Tr) studies

Cu / CdCl2 Hole/Electron trap a ,eVE 2 σ ,cm

Yes /Yes E2 0.21 1.0x10-17

No / Yes E3 0.40 4.3x10-15

No / Yes; No / No H4 0.51; 0.53 1.7x10-15; 6.2x10-15

Yes / No H5 0.70 8.7x10-12

Yes / Yes H6 0.83 8.3x10-17

Arrhenius plots derived from the AS and C-Tr 

measurements

1,E-08

1,E-06

1,E-04

1,E-02

1,E+00

1,E+02

2 4 6 8 10 12

1000/T (K^-1)

1/(

*T^2)

No Cu, Yes CdCl2

Yes Cu, Yes CdCl2

Yes Cu, No CdCl2

No Cu, No CdCl2

H3 H2 H1

H6

E3 E2

H5 H4

Figure 3.4 Arrhenius plots for traps detected by AS (H1, H2, H3) and those detected by C-Tr 

(H4, H5, H6, E2, E3). H1 to H6 are traps for holes; E2 and E3 are traps for minority carrier 

electrons. 

78
3.3.2 Relative densities of different traps 

 It would be instructive to compare the different trap densities in differently treated cells. 

For the H1, H2, H3 states we have done it using the C T ω),( dependencies presented in Fig. 

3.2. For long term transients we used the magnitude of capacitance transients and the magnitude 

of a peak in the plot of DLTS function vs. time. Although there should be inaccuracies in the 

numerical estimates, the sequence of the determined Ct values, probably, correctly represents 

ordering of the traps with respect to their density. 

 The H1 concentration (presumably related to − VCd and d − ClV TeC A-center) are high. 

H1 states were found in all cells. Diffusion of Cu reduces density of H1 states, supposedly due 

to replacement of VCd by CuCd . Concentration of H2 states detected only in Cu-doped cells 

was also in the order of one nF/cm2

, but lower than the H1 density. The same order of magnitude 

has the concentration of H3 states except Cu-doped cells, where it was undetectable. 

Concentration ofH4, H5, E2 and E3 states in the cells where they could be detected is below 

0.1nF. State H6 that was found only in a cell [Yes Cu / Yes CdCl2], demonstrates the highest Ct

value of 7.5 nF/cm2

, which corresponds to trap density exceeding 1015cm3. 

Table 3.3. Transient capacitance Ct for various types of traps 

Cell type

No Cu / Yes CdCl2 Yes Cu/Yes CdCl2 Yes Cu / No CdCl2 No Cu / No CdCl2

Trap type Ct, nF/cm2

H1 3.0 2.2 1.1 2.4 

H2 - 1.0 0.5 - 

H3 - 0.6 - 0.5 

H4 0.07 - - 0.07 

H5 - - 0.08 - 

H6 - 7.5 - - 

E2 - 0.025 - - 

E3 0.08 - - - 

3.4 Effect of Stress 

 The cells prepared as described in Sec. 3.1, were subjected to accelerated degradation in 

light of 1 Sun intensity at 1000

C, under open circuit conditions. The stressed cells were 

characterized in the same way as the as prepared ones. The J-V characteristics are presented in 

Fig. 3.5 for all four types cells before and after stress and before stress. 

 Table 3.4 presents common cell parameters: efficiency η , open circuit voltage , 

short circuit current 

Voc

sc J , fill factor FF , “shunt resistance” 

0

1 )/( =− = V sc dVdJR , “series 

resistance” 

0

1 )/( =− = J oc dVdJR

Table 3.4 Cell parameters before and after stress 

79
Cell Before/afte

r 

stress 

η

(%) 

Voc

(V) 

sc J

(A/cm2

) 

FF

(%) 

Rsc

(Ω-cm2) 

Roc

(Ω-cm2) 

No Cu Before 11.1 0.782 24.16 58.9 1350 11.2 

Yes 

CdCl2

after 9.9 0.757 23.82 54.7 912 13.7 

Yes Cu Before 12.4 0.818 23.88 63.3 578 5.2 

Yes 

CdCl2

after 11.7 0.788 23.91 62.1 992 5.7 

Yes Cu Before 9.0 0.720 22.03 56.6 276 6.3 

No CdCl2 after 7.7 0.692 20.25 54.6 376 7.7 

No Cu Before 7.3 0.688 21.16 50.1 287 10.5 

No CdCl2 after 6.7 0.683 20.24 48.4 286 13.75 

Degradation of J-V dependences was not too severe for all cells. The efficiency decreased 

by about 11 relative percents for all cells. Decrease in Voc did not exceed 30 mV; the highest 

decrease in Jsc by 1.8 mA/cm2

 was for the [Yes Cu –No CdCl2] cell, for other cells it did not 

exceed 0.3 mA/cm2

. Fill factor decreased by 4.2% for the [No Cu –Yes CdCl2] cell and only by 

2% or less for other cells. Roc increased slightly in all cells. Rsc reduced from 1350 to 912 Ω-cm2

for the [No Cu –Yes CdCl2]; for other cells it increased or stayed the same. 

When presenting effect of stress on the trap system, we confine ourselves to two cells, 

both doped with Cu, one of treated and the other not treated with CdCl2. It is seen that plots 

relating to the same trap states shift after stressing, indicating decrease in capture cross section, 

most significant (almost two orders of magnitude) for H1 and E2 states in the [Yes Cu – Yes 

CdCl2] cell (Fig. 3.6) Changes in activation energy are less significant. They are within the 

accuracy of measurements and data treatment, except for H2 state that we are prone to attribute 

to the CuCd defect. For this state increases after stressing from ~0,3 to ~0.35 eV. By the 

way, just in this range vary the estimates of for 

Ea

Ea CuCd acceptor in Ref. 21, obtained from 

Hall coefficient temperature dependencies in CdTe:Cu single crystals. 

80
J-V Characteristics before stressing 

-30

-20

-10

0

10

20

30

40

-0,5 0 0,5 1 1,5

Voltage (V)

Current density (mA/cm-2) 

No Cu, yes CdCl2

Yes Cu, Yes CdCl2

Yes Cu, No CdCl2

No Cu, No CdCl2

 (a)

J-V characteristics after 100 hr stress

-30

-20

-10

0

10

20

30

40

-0,5 0 0,5 1 1,5

Voltage, V

Current density (mA/cm-2) 

No Cu,Yes CdCl2

Yes Cu, Yes CdCl2

Yes Cu, No CdCl2

No Cu, No CdCl2, ---

-

 (b) 

Fig. 3.5. Current-Voltage characteristics of [No Cu – Yes CdCl2] and [Yes Cu – Yes CdCl2]. 

cells. (a) – before stress, (b) – after stress. 

 

81
Arrhenius plots for as-prepared and stressed cells 

1,0E-08

1,0E-06

1,0E-04

1,0E-02

1,0E+00

1,0E+02

1,0E+04

2 4 6 810 12

1000/Temperature (K)

1/(Time*Temperature^2)

Yes Cu, Yes CdCl2, unstr

Yes Cu, Yes CdCl2, str

Yes Cu, No CdCl2, unstr

Yes Cu, No CdCl2, str

Figure 3.6 Arrhenius plots for cells [Yes Cu – Yes CdCl2] and [Yes Cu – Yes CdCl2] before 

(empty marks) and after stress (filled marks). 

 

 Table 3.5 shows the tendencies of the trap density changes due to stress. As previously, 

we suppose that the transient capacitance magnitude could be used as an indicator. 

Table 3.5. Transient capacitance Ct for various types of traps before and after stress 

Cell type

Yes Cu – Yes CdCl2 Yes Cu – No CdCl2

Before stress After stress Before stress After stress 

Trap 

type Ct (nF/cm2

) 

H1 2.2 1.0 1.1 0.02 

H2 1.0 1.4 0.45 0.25 

H5 - - 0.08 0.025 

H6 7.5 9.5 - - 

E2 0.025 0.06 - - 

 The Table shows that the Ct value (hence trap density) for H1 states visibly decreases 

after stress. In the CdCl2 treated cell Ct reduces more than two times, in the non-treated cell 

about fifty times. The following possible explanation could be proposed for this difference. 

82
• Let us assume, as in Sec. 3.2.2, that the H1 group is actually composed by states of almost 

the same activation energy (0.13-0.14 eV) provided by two types of defects: cadmium vacancies 

and ( d − ClV TeC ) A-centers. Contribution of A-centers into H group is comparable to that of 

cadmium vacancies in the CdCl2 treated cells, whereas in the untreated cells concentration of the 

former is much less than of the latter. 

• Probably diffusion of Cu during stressing causes its spatial redistribution. This, in turn, leads 

to a significant decrease in the VCd density near location of the crossing point ( t= EE F ) due to 

replacement of VCd by CuCd . Therefore density of H1 states reduces dramatically in the 

untreated cell where majority of H1 states are produced by VCd . 

• Let us suppose that density of A - centers is much less sensitive to concentration of Cu 

atoms. Their contribution into the H1 density becomes dominant after replacement of VCd by 

CuCd and defines the limit to which NH1 reduces due to stressing. 

We have suggested in Sec. 3.2.2 that the H2 states ( a ≈ 3.0 eVE ) are provided by the 

CuCd acceptor defect. Table 3.5 shows that theCt value (also NH 2 ) is lower in the treated cell 

than in the untreated one. Moreover, in treated cell it increased by 40 % after stress, while 

reduced by 45% in the untreated cell. Probably these facts should be interpreted as manifestation 

of interaction of Cu-related and Cl-related defects. In particular, the donor states should 

compensate the 

ClTe

CuCd acceptors. On the other hand, we should consider possible influence of 

auto-compensation these two defects. Spatial redistribution and possible mutation of both types 

of defects (e.g., transitions Cd ↔ CuCu i controlled by Fermi energy) can be behind observed 

differences and stress effects. However, due to the complexity of phenomena and lack of direct 

information of the defect nature, its concentration and spatial distribution, explanations and 

conclusions are still too speculative and ambiguous. 

The third type of states, H6 ( a ≈ 8.0 eVE ), was observed only in the cell treated both 

with Cu and CdCl2. The estimated density, N H 6 , at crossing point exceeds . In the 

cell doped with Cu but untreated, H6 trap was undetectable. Possible explanation is that H6 is 

provided by some complex that involves both Cu and Cl. Note, the stress led to increase in 

315 10 − cm

Ct

by about 30%. This high density trap level located close to the CdTe midgap might be an 

effective recombination center and deserves to be studied thoroughly. 

While H1, H2, H6 states in Cu doped cells demonstrated rather high concentrations, thus 

affecting cell performance and stability, other states had much lesser density, or even were 

undetectable. In future studies, H1, H2, H6 states should be studied more thoroughly in larger 

number of cells, as prepared as well as subjected to stresses of longer duration and under various 

conditions. 

3.5 Summary 

• Combination of admittance spectroscopy and capacitance transient measurements allowed 

detection and characterization of multitude of deep electronic states (traps) in CdTe solar cells. 

83
• The set of detected traps, their signatures, and evolution caused by stress, vary with the post 

deposition treatment (Cu/no Cu and/or CdCl2/no CdCl2). 

• Traps of higher density include those labeled as H1, H2, H6 with activation energy of 

~0,13eV , ~0.3eV, and ~0.8eV respectively. 

• The H1 group was observed in all cells and may be attributed to the mixture of two acceptor 

type defects: VCd and )( Cd − ClV 2 A-center with indistinguishable activation energies. In 

CdCl2 treated cells both defects are presented in comparable concentrations, in untreated cells 

VCd strongly dominates. 

• The H2 states were detected only in Cu doped cells and were attributed toCuCd acceptor 

defects. In CdCl2 treated cells the H2 density is higher, maybe due to the auto-compensation 

effect with CuCd acceptors and donors involved. ClTe

• The H6 states were detected in cells CdCl2 treated and then doped with Cu. With only one or 

none of these treatments the states were undetectable. It was assumed that the H6 state is 

provided by some complex defect with Cu and Cl involved. Concentration of these states/defects 

was estimated to be in the range of 1015cm3

. 

• Concentration of detected states has changed after stress (1 Sun light, 1000

C, 100 hours). 

Variation in the changes from trap to trap and cell to cell could be explained in the way 

consistent with the states attribution to the specific defects. 

84
4. SCANNING TUNNELING MICROSCOPY AND LOCAL SPECTROSCOPY OF 

THE BACK CONTACT INTERFACE 

4.1. High Resolution Current Mapping at p+-ZnTe/p-CdTe Back Contact Interface 

 A number of microscopic probes have been used successfully on polycrystalline solar cell 

materials. Examples are cathodoluminescence spectroscopy [7] and near-field scanning optical 

microscopy [11, 72]. Under favorable circumstances these techniques can map carrier 

generation/recombination or impurity concentration with spatial resolution of about 100 nm. 

Scanning capacitance microscopy in an atomic force microscope has recently been used to 

determine the potential distribution at single grain boundaries, critical to charge collection and 

recombination [70]. A key device property, carrier transport across contacts and in the absorber 

itself, remains poorly understood, yet is crucial for optimizing the short-circuit current and 

efficiency of a solar cell. 

 We have demonstrated a novel technique, charge injection spectroscopy (CIS) in a 

scanning tunneling microscope (STM), which can provide detailed maps of carrier transport near 

the interface between the polycrystalline absorber layer of a thin film solar cell and an adjacent 

ohmic contact. We use this technique to image near-surface transport between a p-CdTe surface 

and a p+

-ZnTe back contact interface with lateral resolution below 20 nm, about 2 orders of 

magnitude below the absorber grain size. Charge transfer between the p+

-ZnTe contact and the pCdTe absorber is highly non-uniform, and involves large areas with roughly constant high 

resistance coexisting with nanoscale high-conductance pathways. A majority of the cell current 

flows along these pathways, which are often localized at grain boundaries, thus highlighting an 

important role of grain boundaries as efficient near-contact transport channels. Conducting paths 

found within a few of the grains suggest that specific intragrain defects may also cause a 

lowering of the contact resistance. Identification and promotion of such beneficial defects could 

result in substantially reduced overall contact resistance. 

 The samples used were standard CdTe/CdS heterojunction structures in ‘superstrate’ 

configuration from First Solar, LLC (Fig. 4.1) with our standard ZnTe:Cu contact. STM 

experiments were performed on the ZnTe surface layer under dark conditions and at room 

temperature, using a home made tunneling microscope operating in air or high vacuum. 

Electrochemically etched tungsten tips were used as emitters, while the STM collector contact 

was made to exposed sections of the SnO2 cell front contact. In this configuration, we 

simultaneously measured the surface topography via constant-current STM and current-voltage 

(I-V) spectra on a regular grid of points across the field of view (FOV). From the measured I-V 

spectra, current values at constant bias voltage were extracted to produce high-resolution twodimensional current maps, I(x,y). STM measurements were complemented by XTEM, performed 

in a Philips CM200 microscope at 200 keV beam energy, on samples prepared by tripod 

polishing and brief low-energy ion milling. 

 Fig. 4.1 illustrates the layout of our STM experiments. With emitter and collector 

contacts on opposite sides of the solar cell, carriers injected from the tip into the p+

-ZnTe contact 

layer have to undergo transport into and through the entire device structure to contribute to the 

overall current measured at the SnO2 collector contact. Conventional STM on single crystals is 

surface sensitive, and contrast is usually dominated by surface states of the sample. On a highly 

heterogeneous polycrystalline device structure, additional contrast may arise if the overall local 

conductance of contact and cell is low, i.e., comparable to the conductance of the tunneling gap. 

85
Suppose, for example, highly insulating grains with a substantial potential barrier at the 

ZnTe/CdTe interface, separated by grain boundaries with higher conductance and locally 

reduced contact barrier. Carriers injected into the ZnTe layer above a grain [Fig. 4.1, position (1)] 

will then spread in the ZnTe to the surrounding grain boundaries, where they are transferred into 

the p-CdTe absorber and to the STM collector contact. The equivalent electrical resistance in this 

case is a sum of the tunneling resistance [Rt], the position-dependent resistance to carrier 

spreading in the ZnTe [Rs(x,y)], and the equivalent resistance of current transport across the 

contact and through the cell [Rc]: Rtot(1) = Rt + Rs(x,y) + Rc. If, however, the tip is placed over 

an area with low contact barrier [e.g., a grain boundary, position (2)], no spreading in the ZnTe 

occurs and only two terms remain: Rtot(2) = Rt + Rc. 

Figure 4.1 XTEM cross-section of a p-CdTe/n-CdS solar cell, showing the cell structure and 

illustrating the contact configuration used in our STM measurements. 

 Measuring, at constant tip-collector bias and as a function of tip position, the current 

injected into the cell should thus provide detailed maps of non-uniform carrier transport near the 

ZnTe/CdTe back contact interface. In particular, this technique should be sensitive to highconductance pathways due to locally reduced contact resistance, e.g., at grain boundaries or in 

‘defective’ grains. A relatively resistive contact layer (such as the ZnTe contact used here) is key 

for obtaining current contrast, since for a highly conductive (metallic) overlayer the spreading 

resistance Rs(x,y) is small compared to the other resistance terms, thus providing negligible 

contrast. Other STM-derived techniques, such as ballistic electron emission microscopy have 

been developed to map the local height of potential barriers, e.g., at Schottky contacts on single 

crystal substrates. In comparison to this three-terminal technique, CIS is relatively simple and 

can be used to reliably map local contact resistances on complex polycrystalline photovoltaic 

devices. 

86
 Fig. 4.2 (a) shows a current map obtained at Ut = 2V on a p+

-ZnTe capped p-CdTe/n-CdS 

solar cell. Dark (bright) shading in the image marks areas in which the current injected into the 

cell is low (high). 

 Figure 4.2 Identification of the contrast mechanism in STM current maps on CdTe/CdS cells. 

(a) Grayscale STM current map at 2V bias (field of view: 2 × 2 μm2

). (b) Comparison of line 

profile across a single grain (between arrows in (a)) with model calculations for an effective 

injector radius of 10 nm. The inset illustrates the geometry assumed in the calculation. 

 The image shows substantial contrast in the form of large areas with uniformly low 

current, separated by bands of much higher conductance. We can use the data obtained on a 

single uniform grain (circle) and the surrounding grain boundary to verify the contrast 

mechanism proposed above, and to estimate the spatial resolution achievable in CIS. The 

expected current contrast due to a position dependent spreading resistance Rs(x,y) was calculated 

for a simplified geometry of circular slab of ZnTe (thickness h) surrounded by a collector contact 

at its periphery (inset of Fig. 4.2 (b)). Current I injected at a distance d from the center of the slab 

will spread toward this collector contact. We have numerically computed the total resistance to 

current flow in the slab, as a function of the position and effective radius (reff) of the injector. The 

resulting current at constant injector-collector bias Ut was compared with the experimental data. 

For reff = 10 nm, we obtain excellent agreement between the computed and observed current 

distribution across a single grain. This result not only confirms the assumed contrast mechanism 

based on spreading in the ZnTe overlayer, but suggests that it should be possible to map 

preferential conducting paths near the absorber/contact interface with lateral resolution of about 

20 nm. 

 Simultaneous measurements of sample topography and of current maps provide 

information on the spatial distribution of high conductance pathways at an ohmic contact to a 

solar cell, in our case at the p+-ZnTe/p-CdTe interface. A correlation of topography with current 

contrast is given in Fig. 4.3. Panels 4.3 (a) and (b) show the simultaneously measured 

topography and current image within a field of view of 6 × 6 μm2

. The topographic scan [fig. 4.3 

(a)] shows rounded elevated features, typically about 1.5 μm in diameter, separated by welldefined deep trenches. Based on comparison with XTEM, we associate the elevated areas with 

individual grains, and the intervening trenches with grain boundaries. The current map [Fig. 4.3 

(b)] shows large areas with roughly constant low current (dark), indicative of sample areas with 

low transmission probability across the ZnTe/CdTe interface, alternating with narrow, elongated 

conducting bands (bright) in which the contact is highly transparent. 

87
Figure 4.3 (a) Topographic image obtained on a ZnTe capped CdTe/CdS cell (field of view: 6 ×

6 μm2

, height scale: 0.7 μm). (b) Current map of the same region at 2V tip-collector bias (current 

range: 0 – 6 nA = Imax). (c) Overlay of topographic data (color) with contours of the 2V current 

map. Light and dark gray areas, indicating current levels of 0.3 Imax and 0.85 Imax, respectively, 

trace troughs associated with grain boundaries (B) and some ‘leaky’ grains (G). 

 The position of these conducting paths is strongly correlated with grain boundaries. Fig. 

4.3 (c) illustrates the correlation between topographic features and conductive pathways via a 

superposition of the topographic image with contours of constant current obtained from the 

current map. Regions of high current have a strong tendency to trace grain boundaries (B). The 

intragrain material generally shows rather uniform, low current, i.e., appears to be substantially 

more resisitive than grain boundaries. An exception is a few ‘leaky’ grains (G), in which a high 

density of microscopic conducting paths are observed. 

 Our data show substantial contrast on a nanometer length scale in the local transport 

properties of the p+

-ZnTe/p-CdTe interface of p-CdTe/n-CdS solar cells. The observed features 

appear unrelated to the grain structure of the small-grain ZnTe contact, but instead trace the 

topography of the underlying CdTe with its much larger grain size. This observation confirms 

that our data indeed map transport across the ZnTe/CdTe interface, and in the CdTe absorber 

near this interface. Current flow between absorber and back contact is highly non-uniform, with 

few low-resistance areas likely carrying a large fraction of the overall current of the cell. A 

majority of these areas trace grain boundaries, which indicates that grain boundaries may play a 

crucial role as conducting channels for carrier transfer into the cell back contact. Given that 

performance of polycrystalline CdTe/CdS solar cells is superior to single crystalline devices, a 

beneficial role of grain boundaries in carrier collection and transport, see e.g. [71, 11]. We 

provide the first observation of preferential current transfer into contacts via grain boundaries. 

Importantly, some intragrain material also shows efficient current transfer into the contact. 

Comparison with XTEM images suggests that structural defects or an accompanying segregation 

of impurities may be responsible for a local lowering of the contact barrier within some grains. 

This observation points toward a potential route for achieving substantial gains in cell efficiency 

88
via lowering of the back contact resistance by promoting these beneficial defects, and 

concomitant increases in short circuit current and fill factor.

4.2. New Experimental Capabilities 

 While the initial experiments discussed above involved local I-V measurements between 

ZnTe-capped CdTe/CdS cells in air, we have developed new experimental capabilities, direct 

conductance (dI/dV) mapping and imaging in high vacuum. These new capabilities will be 

discussed briefly below. 

4.2.1. High-resolution conductance mapping 

 We have developed the capability of obtaining, simultaneously with topographic images, 

conductance (dI/dV) curves on a regular grid within the image field of view. Similar to the I-V 

measurements, from which high-resolution current maps could be derived, the dI/dV data allow 

us to produce maps of local conductance, dI/dV(x,y), near the back contact of a thin film solar 

cell. 

 First results were obtained on a new batch of p+-ZnTe capped p-CdTe/n-CdS solar cells 

with layout as shown in Fig. 4.1. Our previous measurements showed symmetric I-V curves, i.e., 

negligible rectification. In our new data, in contrast, a strong dependence of conductance on bias 

polarity (i.e., rectification) is observed, the origin of which requires further clarification. Fig. 4.4 

shows representative conductance curves measured within a grain, and at a grain boundary of a 

device that has undergone CdCl2 treatment and Br:MeOH etching prior to evaporation of thin 

Cu and ZnTe layers. 

Figure 4.4 Conductance spectra obtained at a grain boundary (red) and in intragrain material 

(blue). V > 0 corresponds to reverse bias of the main CdTe/CdS junction. Negative (positive) 

bias in fig. 4 corresponds to forward (reverse) bias of the p-CdTe/n-CdS junction of the cell. 

 We have obtained simultaneously measured maps of topography and local conductance, 

as shown in Fig. 4.5. The topographic image (Fig.4.5 (b)) shows, as before, rounded grains with 

diameter of about 1 μm, separated trenches that we identify as grain boundaries. The 

89
conductance map at positive bias (Fig. 4.5 (a)) shows the same general contrast distribution that 

was observed in current maps: high conductance preferentially near grain boundaries, uniform 

low conductance when the tip is placed over a grain. The conductance image at negative bias, 

however, deviates from this behavior. Instead of conducting paths near grain boundaries, we 

observe contrast that correlates much less with topographic features. 

Figure 4.5 Simultaneously measured topography and conductance maps (FOV: 2 × 2 μm). 

(a) dI/dV at V = +1 V (bright: high conductance). (b) Topography. (c) dI/dV at V = - 2.3V. 

 If we assume that the conductance images of Fig. 4.5 reflect high conductance pathways 

across the ZnTe/CdTe interface, we thus conclude that in these new devices clear differences 

exist between electron and hole transport across that contact. Currents of hole (majority carrier) 

injection into the CdTe are generally low, and strongly localized at grain boundaries. A 

substantial barrier to hole injection appears to exist at the contact to intragrain material. Electron 

injection, on the other hand, is more efficient and often shows minima at the grain boundary 

itself, surrounded by areas with higher conductance. A conclusive interpretation of these initial 

results of dI/dV maps requires additional measurements and modeling, and will be presented in 

the future. 

4.2.2 Measurements in high- and ultrahigh vacuum 

 Most of the results shown above were obtained in a compact STM system, operating in 

air. Etched tungsten tips were generally used as probes for these measurements, since they can be 

formed into very sharp tips, required for scanning the rough surface of a solar cell. However, 

tungsten tips tend to oxidize over the extended times required to scan high-quality conductance 

maps. We have thus changed our experimental setup to be able to measure in high- or ultrahigh 

vacuum (Fig. 4.6). Incompatible materials, such as solder joints, were replaced with vacuum 

(UHV) compatible materials that can withstand baking temperatures up to 150°C required for 

rapid pump-down to UHV. In addition, we have implemented a heating system that will allow us 

to anneal samples in vacuum, for surface cleaning, or to induce grain regrowth or impurity 

diffusion. 

90
Figure 4.6 Compact UHV STM system for imaging and spectroscopy on solar cell 

materials. 

5. NEAR FIELD SCANNING OPTICAL MICROSCOPY OF PHOTO-CURRENT 

AND PHOTOLUMINESCENCE IN CdTe SOLAR CELLS 

 Our prior studies of thin film CdTe cells using near-field microscopy (NSOM) have 

shown that cross sectional photocurrent collection (PCC) measurements of cleaved (or broken) 

cells can provide high spatial resolution information on the width of the collection region, the 

sulfur content in the CdTe, and diffusion of S into the material [11]. During this contract we have 

worked upon extension of this technique capabilities. Particularly, the NSOM on sub-grain 

length scales, both cross-sectional and in-plane, has been developed providing imaging 

photocurrent collection and also photoluminescence. NSOM of electroluminescence is under 

development. 

 As shown in Sec.1, using the signatures of specific defects in the EL and PL spectra one 

can study spatial distribution of the defects on the ~10μm scale. With NSOM the same is 

possible to do on the sub-grain lengthy scale. PL can also provide information about the spatial 

distribution and nonuniformity of recombination processes. 

 One of the reasons for this development work was to allow a broader range of samples to 

be characterized. In our previous studies, the presence of the glass superstrate precludes high 

spatial resolution photocurrent in plan view. With photocurrent, we are restricted to samples 

fabricated into a cell. With PL we can study films in plan view before back contact fabrication. It 

would be advantageous, to be able to study films at a series of steps along the fabrication process 

to learn what each step does on a microscopic scale. 

 In this section we present the data that illustrate new possibilities we got for studying 

photo-electric properties of material on the sub-grain scale. Systematic NSOM studies of cells 

and constituent films, correlated with variation in processing and stress procedures, are the 

subject of continuing and planned for the near future experiments. 

91
5.1. Plan-View Imaging of Photocurrent 

5.1.1. CdTe/CdS cell 

 The plan-view NSOM image obtained once [11] on a CdTe cell, after an access for the 

optical probe was provided to the TCO surface (mechanical polishing and etching of glass), 

demonstrated in some locations higher PCC for the grain boundary regions than for the grain 

bulk. This result (if confirmed in other experiments) considered to be of significant importance 

providing better understanding of the grain boundary (GB) effect on the cell performance. The 

PCC plan-view studies were partially initiated by speculations on possible beneficial effects of 

GBs later presented in [13]. 

 It was very difficult to repeat these in plane studies because CdTe cells are usually of a 

superstrate configuaration. The early technique we used for the TCO exposure was complicated 

and usually damaged a cell. The problem was solved recently after the University of Toledo 

provided us with a cell of a substrate configuration. The CdTe/CdS device has been grown on a 

Molybdenum substrate using an RF sputter deposition, then went through a CdCl2 treament 

followed by depositing ITO via RF sputtering. A HeNe laser with wavelength of 632.8 nm was 

used as the excitation source in the photocurrent measurements. Results of this experiment (Fig. 

5.1) consistently show a reduced PCC from grains as compared to the intergrain regions. Several 

images taken at random points on the device show qualitative agreement with one another and 

demonstrate on average a 5% difference in the photocurrent between these regions in favor of 

grain baoundaries. The 2x2 μm2

 images shown in Fig. 5.1 were generated with two different 

programs to aid in the visualization of the topography vs. photocurrent where the topography is 

on the left and photocurrent is on the right. As one can see, the photocurrent in the two higher 

grains is lower with an increase in photocurrent around the grains. 

 Recently, the NSOM measurements of the PCC on VTD CdTe cells with the transparent 

back contact, ZnTe:Cu, also showed enhanced PCC from the intergrain regions [72].. The 

investigated cell was of a superstrate configuration. Thus, a similar effect of grain boundaries on 

PCC was observed on differently processed cells with the light absorbed close to the CdTe/CdS 

interface (our case) and close to the top surface of CdTe [72]. 

Figure 5.1. Correlation between a.) topography and b.) photocurrent plan-view images on CdTe 

cell of substrate configuration fabricated at the University of Toledo 

92
5.1.2. CIGS/CdS cell 

 As a substrate configuration is very suitable for in-plane imaging of PCC, a CIGS/CdS 

cell was also tested with NSOM. The primary reason for investigating this sample was to check 

whether the area where the ITO and CdS were lifted off contributed considerably into measured 

photocurrent and hence into estimated cell efficiency than the traditional scribing technique. The 

CIGS cells were prepared with standard procedure, then the new etching process was applied and 

efficiency was measured. Several scans were taken in the etched region at different locations 

that show very little or no PCC in this region. Fig. 5.2 shows an image obtained with the 26x26 

μm2

 scan and the excitation wavelength of 632.8 nm. The bottom left corner presents the region 

with the CdS and ITO still attached. Toward the top right corner, the ITO and CdS have been 

lifted off. The image on the left is the topography and the image on the right is photocurrent. 

Figure 5.2. Correlation between a.) topography and b.) photocurrent on CIGS at the etch step. 

 Fig. 5.3 shows significant difference between images obtained with the excitation 

wavelengths of 6.32.8 nm and 457 nm. 

Figure 5. 3 Difference in PCC at the etch stop for wavelengths of a) 632.8 nm and b) 457nm 

93
The PCC studies were conducted on the CIGS cell. Fig 5.4 shows the correlation between 

the topography and the PCC images of the same fragment of the cell area. The scan range is of 

5x5 μm2

 with the outline of the grains marked with a black line in both images. An increase in 

photocurrent at the grain boundaries is seen relative to the grain interior. This is in contrast to a 

report by McDaniel and Hsu [73], in which they found that cells with grains larger than 10 μm or 

smaller than 1μm showed decreased photocurrent at the grain boundary as compared to the grain 

interior. However, the cells they studied had lower efficiencies than those used in this 

experiment, and they also had a much thicker (450 nm) TCO layer. 

Figure 5.4 Topography (a) and photocurrent collection (b) images for the CIGS cell 

5.2 NSOM Studies of Photoluminescence on CdTe/CdS Cells 

 Recently we have become able to perform room temperature NSOM photoluminescence 

(PL) measurements on the substrate configuration UT CdTe cells. Fig. 5.5 shows the near- field 

PL spectrum excited by the 488 nm (the photon energy is above the CdS bandgap) laser 

irradiation through the front TCO contact. 

Two broad peaks are seen in the spectrum centered at the photon energy of 1.718 eV (λ= 

722 nm) and 1.468 eV (λ==845 nm), above and close to the CdTe band gap, respectively. PL 

spectrum was also measured excited by irradiation with the wavelength ranging from 457 nm to 

632 nm. As the excitation source went to longer wavelengths, the magnitude of the high energy 

PL peak decreased gradually and disappeared altogether at 632 nm wavelength. One can 

conclude that this peak is generated in the CdS layer and apparently is generated by the band-todefect level transitions, whereas the lower energy peak is generated in the CdTe layer and may 

be attributed to the band-to-band transitions. A significant difference in magnitude of the 

observed peaks is determined by the fact that the cell is irradiated from the CdS side of 

CdTe/CdS junction, hence the exciting light with λ=488 nm is absorbed predominantly by the 

CdS layer. 

94
Near-Field Photoluminesence of CdTe

3000

5000

7000

9000

11000

13000

15000

1.38 1.48 1.58 1.68 1.78 1.88 1.98

Energy (eV)

Intensity (a.u.)

Near-Field Spectrum of CdTe

CdTe

CdS

Figure 5.5 Near field PL spectrum obtained on a cell of substrate configuration produced at UT. 

λexcit. = 488nm . Measurements were done at nominal temperature of 250

C . 

 Fig. 5.6 shows the plan-view topography and PL images taken from the same 

fragment of a cell. The excitation source used to produce the PL was 488 nm and the images 

were not taken simultaneously therefore the two PL images show a shift due to the drift of the 

piezoelectric tubes. The PL from the CdS layer was taken with the spectrometer centered at 750

nm and spectral PL data was acquired between 712 nm and 774 nm, therefore allowing 

integrating PL intensity underneath the broad peak. As for the PL from the CdTe layer, the 

spectrometer was centered on 850 nm and spectral data was acquired between 829 nm and 866 

nm. To demonstrate the similarity between two PL patterns, the CdS data was normalized to 

the CdTe thereby showing that the CdS follows somewhat the same pattern as the CdTe although 

there is a subtle difference. 

 The correlation between the PL and topography images is seen in two different length 

scales. The first characteristic length is of the order of at least several grain sizes, and possibly 

comes from the roughness of the Mo substrate. Shorter length scale, quite visible in the 

topography image, but also observable in the PL pattern, is of the order of a single grain size. 

The latter is much smaller for the films deposited with the RF sputtering than with CSS, VTD, 

Gas Jet, etc. The contrast in the PL intensity, better seen in the CdTe range, although small in 

both ranges, is mostly in favor of the grain boundary regions. 

95
Figure 5.6 Correlation between topography (a) , CdS PL (b), and CdTe PL (c) in plan-view on 

the CdTe UT cell with the black line indicating the same PL structure in both spectrum. 

 

 

 

The next experiment indicated that the NSOM PL image contrast could be controlled to 

some extent by applying bias voltage to a cell. Fig. 5.7 shows the NSOM PL spectrum excited 

with a source of λ=488 nm under zero, forward and reverse bias. Fig. 5.8 presents the NSOM 

PL images for λ=750 nm and λ=850 nm taken from the same area at the reverse, zero and 

forward biases. The PL excitation source was of λ=488 nm. 

96
Near-Field Photoluminesence Plots on a biased CdTe cell

3000

5000

7000

9000

11000

13000

15000

1.38 1.48 1.58 1.68 1.78 1.88 1.98

Energy (eV)

Intensity (a.u)

0 Volt forward

1 Volt Forward

2 Volt Forward

1 Volt Reversed

2 Volt Reversed CdTe

CdS

 

Figure 5.7 Effect of bias on the NSOM PL images. 

Figure 5.8 Spatial distribution of the NSOM PL intensity at the bias voltage indicated in the 

Figure. The PL was excited by the light of λ=750 (top images) and of λ=850 nm light (bottom 

images). 

97
 The effect of bias is obvious. Both reverse and forward biases enhanced the mean PL 

intensity and redistributed it spatially. In particular, in some areas the contrast has changed 

between what we believe to be the grain boundary region and the grain bulk. For example, the 

mean PL intensity in the II quadrant of images from the biased cell (for both positive and 

negative bias) is higher than that from the unbiased cell. Then, in the same quadrant one can see 

enhanced PL from the chains of circular spots which represent grains according to the 

topography image. 

 It might be that by changing conditions of the experiment, e.g., by using the excitation 

source of lower photon energy that will be absorbed only in CdTe, we can have stronger control 

of the PL images so that they better manifest spatial non-uniformity of PL. These studies are now 

conducted in our laboratory. 

 Finally, cross-sectional measurements have been made on the CSM CdTe devices were 

grown with GJD. The results obtained from this experiment are shown in Fig. 5.8. The top left 

image is the topography with the glass on the left side and the device on the right side. This is 

more easily seen in the top right PL image in which the glass is the blue part of the image and the 

device can be seen starting in the middle. This image is a scan of 3x3 microns. The spectrometer 

was measuring PL only in the CdTe range. The bottom image shows the combined PL and 

topography images. PL is represented by the color and topography with the texture. 

Figure 5.9 Cross-sectional image of CSM CdTe devices with a.) topography, b.) PL of the 

CdTe spectral peak and c.) combination of PL and Topography with PL represented by the color 

and topography with the texture. 

98
 As expected, the maximum PL intensity is observed from the CdTe portion closest to the 

window layer from which the exciting light comes. Reducing of the incident light intensity due 

to absorption in CdTe causes the PL intensity fading with distance. Probably, more interesting 

and instructive could be nonuniformity observed in PL in the direction parallel to the 

device/glass boundary, that is at supposedly the same distance from the window layer. However 

it is too early to make some conclusions based on still preliminary measurements. 

99
6. CELL PROCESSING 

 In this Project the modification of both of our deposition techniques (electrodeposition, 

ED, and gas jet deposition, GJD) were undertaken. This was driven by efforts to improve our 

ability to produce cells rapidly and with sufficient control over all conditions to yield high 

reproducibility. First, the work has concentrated on increasing both the flexibility of the CdTe 

deposition systems (small areas and easily modified source materials) and the uniformity of the 

deposited film, however maximizing of device efficiency was also a goal of these activities. 

6.1. Electrodeposition 

 Previously the CdTe electrodeposition at CSM was conducted using a 4-electrode system 

with a 20 liter bath [74]. This system was donated by Ametek where it was used as a pilot plant. 

While good quality CdTe/CdS solar cells were produced at CSM in the past (NREL verified best 

of 12.9%), it has been difficult to maintain the system, and its large solution volume created a 

large amount of hazardous waste each time the bath composition had to be changed. In addition, 

since the bath is not in continuous use, startup time was lengthy. To address these issues, we 

have replaced the Ametek system with a 3-electrode configuration with a 2.5-liter volume bath. 

This setup increases lab safety and reduces both hazardous waste production and bulk chemical 

requirements. It also allows us to easily modify bath composition and temperature or utilize 

multiple baths. 

 Now, the solution is contained in a pyrex beaker 10 cm tall and 19 cm in diameter. The 

beaker sits on a stir/hot plate, which is used to stir the solution and maintain a desirable 

temperature. In operation, the beaker contains approximately 2 L of 1M cadmium sulfate with 

600mg tellurium oxide and sufficient hydrochloric acid to make the pH ~1.8 at 700

C. The pH is 

monitored with a temperature compensated pH electrode, and the working electrode potential is 

controlled by a potentiostat with a Ag/AgCl reference electrode. A serious of Cd bars and a 

platinum mesh has been used as counter electrodes. 

 Deposition from the solution described above at -0.6 V Ag/AgCl resulted in current 

densities of ~1.8 mA/cm2

, and produced CdTe films approximately 3μm thick in 50 minutes. It is 

speculated that the deposition rate could be further increased by optimizing the solution flow 

pattern and rate, and possibly further increasing the tellurium concentration. However the 

achieved deposition rate is much higher than it was in the previous AMETEK system: 3-8 hours 

to deposit 3 μm film. The system used by British Petroleum before they abandoned CdTe cells 

grew films even slower. 

 The achieved higher deposition rate stimulated to continue optimization of ED system 

and the CdTe film and cell processing. However, we had to stop this work and concentrate our 

further efforts on development of the GJD system and processing. One of the reasons was that 

after BP abandoned CdTe cell area, the only US company in the field has become First Solar, 

LLC, which successfully uses the vapor transport deposition, VTD, technique. Our GJD 

technique, in principle is similar, to the FS VTD. In both cases the deposited substance is 

delivered by gas flow to the substrate. 

100
6.2. Gas Jet Deposition System and the Cell Fabrication

 In our GJD system, cadmium and tellurium vapors are produced by heating a quartz tube 

containing CdTe chunks to approximately 800 °C. Helium is passed through this tube, carrying 

the Cd and Te to a heated substrate where it reacts to form a CdTe film. The deposition chamber 

is pumped with a mechanical pump, and maintains a pressure of approximately 460 mtorr with a 

140 sccm helium flow through the CdTe source tube. The substrate is heated with a resistive 

element, and is kept at a desired temperature which varied in our optimization experiments in the 

range of 380-450 °C.

 Cells were grown on both single and bi-layer TCO/glass substrates. Our first experiments 

were done with commercial 3 mm thick soda lime glass coated with SnO2:F as single layer 

substrates. Additional coating at CSM with the undoped SnO2 provided bi-layer substrates. In 

our recent experiments we have used substrates made on 1 mm thick 7059 glass, with the TCO 

consisting of 500 nm SnO2:F covered with 100 nm undoped SnO2. Those were supplied by Dr. 

Dave Albin's group at NREL. 

 CdS was deposited on the substrates using a chemical bath deposition process. In this 

process, the substrates are first cleaned by boiling them in a solution of 20 mL Micro-90 

detergent in 1200 mL deionized water and rinsing them in 5 changes of boiling deionized water. 

The cleaned samples were suspended in 1500 mL beaker in a heated ultrasonic bath. The beaker 

also contains a T-shaped rod connected to an electric motor to provide stirring. A solution of 900 

mL deionized water containing 0.265 g cadmium acetate and 1.540 g ammonium acetate was 

stirred, ultrasonically agitated, and heated to 80 °C. When the solution reached 80 °C, 0.380 g 

thiourea dissolved in 100 mL deionized water and then 30 mL 29% ammonium hydroxide were 

added. This solution was stirred and ultrasonically agitated at 80 °C for 1 hour, after which the 

samples were removed and ultrasonically agitated in 80 °C water for 10 minutes. The process 

was then repeated, yielding CdS films usually 325 nm thick. 

 The CdS films were exposed to a 1 M CdCl2 mist for 45 s, and then placed in a 450 °C 

tube furnace. The tube furnace has a volume of approximately 9.3 L and was purged with 

nitrogen flowing at 71 standard liters per hour. The furnace temperature was maintained at 450 

°C for 50 minutes, and then allowed to cool to room temperature overnight with the nitrogen still 

flowing. Typically, the furnace takes 3 hours to cool from 450 °C to 100 °C. The samples were 

removed from the furnace the next morning, rinsed in boiling deionized water, and stored in a 

desiccator. 

 CdTe films were deposited on the CdS substrates using the GJD system. A plot of the 

process temperatures as a function of time for a typical growth is shown in Fig. 6.1. It should be 

noted that the temperature values in the figure are the thermocouple readings that can 

considerably differ from the real temperature. After loading the substrate, the chamber was 

evacuated, and the helium carrier gas was started flowing through the source tube. After the 

chamber pressure reached a steady state, the substrate heater was turned on (A) and allowed to 

reach approximately 400 °C at its surface. This takes approximately 25 minutes. Then the frit 

heater is turned on (B) and allowed to warm up for 10 minutes. Radiation and conduction from 

the frit heater cause both the substrate and source to warm slightly. The source heater is then 

turned on (C) and allowed to warm to ~700 C and dwell there for 10 minutes. Radiation from the 

source heater warms the sample holder to approximately 435 °C. The shutter is then opened (D), 

which allows CdTe deposition and warms the substrate holder to approximately 455 °C. For the 

T-t profile shown in Fig. 6.1, the deposition proceeded for 20 minutes and then the shutter was 

101
closed and the source and substrate heaters were turned off (E). The frit heater was allowed to 

run another 10 minutes before it was turned off (F), in an effort to prevent the frit from being 

clogged with CdTe. The source tube and sample holder were allowed to cool to 50 °C before the 

chamber was opened for sample removal. 

Figure 6.1. Nominal temperatures of a source and substrate versus time for a typical growth 

process 

 Deposited CdTe/CdS films were annealed placed in a graphite tray and covered with 1 

mm glass source plates that had been exposed to a 1 M CdCl2 mist for 2 minutes. The separation 

between the samples and the source plates was approximately 2 mm. The sample tray was placed 

in a 390 °C tube furnace with a 9.3 L internal volume that was purged with dry air at a rate of 71 

standard liters per hour. The furnace temperature was maintained at 390 °C for 20 minutes, and 

then allowed to cool to room temperature overnight with the dry air purge running. 

 The annealed samples were rinsed in boiling deionized water for 1 minute and blown dry 

in preparation for making the back contacts. The samples were etched with 0.04 M bromine in 

methanol for 10 s, rinsed in methanol for 1 minute, and blown dry. The etched samples were 

immediately placed in a vacuum evaporator (approximately 5 minutes in atmosphere after the 

etch), in which were deposited 0.7 nm of copper followed by 50 nm of ZnTe. The ZnTe:Cu 

coated samples were annealed in a 9.3 L tube furnace. The samples were loaded into the furnace 

at room temperature, and it was purged with nitrogen at 142 standard liters per hour for 20 

minutes. The purge was then reduced to 71 standard liters per minute, and the temperature was 

ramped to 200 °C in 15 minutes. The furnace was then turned off, and the temperature allowed to 

drop to 100 °C over 2 hours. The annealed samples were placed in a vacuum chamber and an 

array of gold dots 0.32 cm in diameter and 250 nm thick were deposited on them through a 

shadow mask. 

102
 The CdTe and CdS in a 5 mm strip along the perimeter of the samples was removed with 

a razor blade, and indium was applied to the exposed TCO with an ultrasonic soldering iron to 

form the front contact. 

 With the specifications described above or with some modifications numerous cell were 

grown, mostly on soda-lime-glass/SnO2 substrates with highest efficiency of ~10%. However, 

combinations of processing parameters are still far from optimum. The ongoing experiments 

indicate that there is a room for a significant improvement of a cell performance. 

6.3 Characteristics of Recently Fabricated GJD Cells 

Here we present some results obtained on the cells grown recently with some 

modifications of processing procedure on different glass/TCO substrates. 

 Cells were grown on 1.5”x1.5” substrates consisting of either a commercial 3mm thick 

soda lime glass with a single layer of tin-oxide TCO or 1mm thick borosilicate glass (Corning 

7059) with either a NREL produced single layer or bi-layer (500 nm F-doped/100 nm undoped) 

tin oxide TCO. The cell preparation process consisted of the following six steps: 

• CDB CdS deposition with three 35 minute cycles of heated and agitated solution consisting 

of ammonium acetate, cadmium acetate, ammonium hydroxide, and thiourea. 

• Gas Jet deposition with the CdTe chunk source temperature at 700o

C and the nominal 

substrate temperature at 380o

C. 

• One or more CdCl2 ultrasonic nebulizer mist treatments onto a warmed substrate (125o

C) 

followed by a 35 minute anneal at 410o

C. 

• The CdTe top surface etch of 10 seconds in a 0.02 M bromine methanol solution 

• Immediately following the etch, a thermal evaporation of 10 Å of Cu followed by 500 Å of 

ZnTe with a final anneal of 20 minutes above 180o

C (maximum temperature ~190oC). 

Thermal evaporation of 0.079cm2

 gold dot back contacts, approximately 50 per substrate. 

The first three batches of cells whose characteristics are shown were subjected to a single 

two minute exposure to CdCl2 mist. The fourth batch, as indicated in Fig.6.2, was exposed eight 

times, 16 minutes total. Histograms in Fig.6.2a demonstrate distribution of about 50 cells on each 

substrate over cell efficiencies. Efficiency is the highest (up to 12.4%) for the cells of the 4th

batch (1mm borosilicate glass, 16 min CdCl2). Most of cells demonstrate η11%. The average 

efficiency of the batch 3cells grown on the same kind of bi-layer substrate, but with a single 2 

min CdCl2, is about 10%. Efficiency decreases as we go to single-layer layer TCO substrates. 

Decrease is stronger for the commercial 3mm soda lime glass substrate, and some weaker for the 

NREL 1mm corning glass substrate. Both single-layer batches demonstrate much greater nonuniformity of efficiency over the substrate area than the bi-layer ones. 

• When analyzing the Voc, Jsc, FF, Rsc and Rs data, note that the colors of marks for 

specific batches are different form those for histograms. The highest values of Voc, Jsc and FF 

are observed for the cells of the 4th batch. Most of these cells have reasonably high Rsc and low 

Rs values. Single-layer-substrate batches have lower average Rsc and higher average Rs values 

than the bi-layer-substrate batches, as well as lower average fill factor. The most dramatic effect 

on cell efficiency is provided by Voc. The lowest efficiency cells of all batches have a very low 

Voc value. The short circuit current has the narrowest spread of values over the batch (Fig. c). 

103
Figure 6.2 Characteristics of cells grown using GJ-deposition of CdTe on different substrates 

with different CdCl2 treatments indicated in the figures. (a) Histograms of cell efficiencies. (b), 

(c), (d), (e), and (f) represent respectively open circuit voltage, short circuit current density, fill 

factor, “shunt resistance”, and “series resistance”, versus cell efficiency. 

123 4 5 6 7 8 9 10 11 12 13

0

5

10

15

20

25

Cell Count

Percent Efficency

3mm single TCO Efficiencies Histogram 1mm single NREL TCO 

1mm bilayer NREL TCO 

1mm bilayer NREL TCO 8x2min CdCl2

Voc vs Eff GasJet 2005-04-14

0.00

0.10

0.20

0.30

0.40

0.50

0.60

0.70

0.80

0.90

0.00% 5.00% 10.00%

Eff %

Voc (V)

1mm single NREL TCO 

3mm single TCO

1mm bilayer NREL TCO 

1mm bilayer NREL TCO 8x2min CdCl2

Jsc vs Eff GasJet 2005-04-14

0.00

5.00

10.00

15.00

20.00

25.00

0.00% 2.00% 4.00% 6.00% 8.00% 10.00% 12.00%

Eff %

Jsc (mA/cm2)

1mm single NREL TCO 

3mm single TCO

1mm bilayer NREL TCO 

1mm bilayer NREL TCO 8x2min CdCl2

FF vs Eff GasJet 2005-04-14

0

10

20

30

40

50

60

70

0.00% 2.00% 4.00% 6.00% 8.00% 10.00% 12.00%

Eff %

FF (%)

1mm single NREL TCO 

3mm single TCO

1mm bilayer NREL TCO 

1mm bilayer NREL TCO 8x2min CdCl2

(a) 

(b)

(c) (d) 

RscLight vs Eff GasJet 2005-04-14

0

200

400

600

800

1000

1200

1400

0.00% 2.00% 4.00% 6.00% 8.00% 10.00% 12.00%

Eff %

RscLight (O hm cm 2)

1mm single NREL TCO 

3mm single TCO

1mm bilayer NREL TCO 

1mm bilayer NREL TCO 8x2min CdCl2

RocLight vs Eff GasJet 2005-04-14

0

5

10

15

20

25

30

35

0.00% 2.00% 4.00% 6.00% 8.00% 10.00% 12.00%

Eff %

RocLight (O hm cm 2)

1mm single NREL TCO 

3mm single TCO

1mm bilayer NREL TCO 

1mm bilayer NREL TCO 8x2min CdCl2

(e) (f) 

104
 

 This nonuniformity of the cell parameters over the substrate area correlates with the 

results of comparative studies of electroluminescence on the single-layer and bi-layer cells (see 

Sec.1). However those are the non-uniformities in different length scales: tens-hundreds of 

microns for the EL-detected and millimeters-centimeters for those indicated by Fig. 6.2. 

 It is clear from the results presented and discussed above that 

• Bi-layer substrates are much better than the single-layer ones and with the CdTe GJ 

deposition technique can provide opportunity cells with efficiency above 12%. By the way, the 

best CSM ED cell of 12.9% efficiency was grown on bi-layer substrate supplied by the 

University of South Florida. 

• Varying and optimizing CdCl2 treatment may led to higher uniformity of a cell/module 

parameters hence to higher efficiency [6]. 

• It is reasonable to expect that further improvement could be reached by optimizing (reoptimizing) not one processing parameter but their combinations. For example, it was shown not 

once and for different processing technologies that with the bi-layer substrates one can reduce 

the CdS thickness, hence the Jsc value, without reducing Rsc (shunting) and FF. Since anneal 

with CdCl2 promotes inter-diffusion and consumption of CdS, we have to adjust to each other 

and optimize simultaneously thickness of CdS, its postdeposition anneal (with or without CdCl2), 

and CdCl2 treatment of CdTe/CdS structure. 

 Some results of our studies, not presented here, have given hints on what processing 

parameters should be changed/optimized in the first turn. We hope we will have an opportunity 

to continue and accomplish optimization of processing technology based on GJD. 

105
7. SUMMARY 

In accordance with our general research approach and specific goals of studies 

formulated in the Preface, our work upon this project was mostly devoted to making up a 

deficiency in fundamental knowledge and understanding of some basic issues behind the 

CdTe/CdS cells performance and stability. Namely, we were focusing on electronic properties of 

defects that may control the majority carrier concentration, mechanisms of the dopant 

compensation, recombination processes, distribution of electrostatic potential in a cell, specifics 

of the electron transport. In particular, we tried to get new information about the grain boundary 

effect on electrical and optical properties of the cell fine-grain material. To fulfill these tasks we 

have developed new methods and techniques (or adjusted existing ones) for material 

characterization as well as modified the approaches to the data analysis and interpretation. The 

most significant progress in characterization methods was made by developing equipment, 

measuring procedures, and data analysis for electroluminescence (EL) studies, admittance 

spectroscopy (AS), capacitance transients (C-Tr) measurements, and also in development of high 

spatial resolution methods, such as tunneling microscopy (STM) and near field scanning optical 

microscopy (NSOM). In addition to the development of characterization methods and their 

application to the defect studying, the Report also presents our activities aimed at development 

and improvement of the cell processing methods. Main results and accomplishments of our work 

under this subcontract are summarized below 

Luminescent Studies

• It was found that the spatially resolved measurements of electroluminescence (EL) are easy 

to perform on CdTe cells at various temperatures ranging from the room temperature to the 

rather low one (~40 K in our measurements). With a quite available instrumentation, the EL 

pattern could be resolved on the length scales from several microns to centimeters. Usually EL 

measurements demanded small current densities of the order of 101

 mA/cm2, or even below this, 

which excluded the cell damaging in the process of measurements.

• EL mean intensity and pattern (bright spots on dark background) vary significantly 

depending on processing technology variations and cell stress conditions and duration. High 

sensitivity of EL to the cell degradation makes EL measurements an attractive method for early 

predictions of the cell/module lifetime. 

• Simultaneous measurements of spatially resolved EL and PL from the same cell area allows 

to separate nonuniformities in carrier transport and in recombination rate as sources of 

nonuniformity in EL. 

• EL measurements have shown that nonuniformity in electron transport are effected by both 

back and front contacts. Buffer layer between the TCO and CDS significantly reduces the 

nonuniformity. 

• Patterned doping with Cu and patterned CdCl2 treatment proposed and applied in this project 

demonstrated possibility to reveal nonuniformity in defect distribution. The Cu-related defect 

signatures in the EL spectra were used to detect these defects; spatial distribution of the defects 

was studied with the EL scans. A significant influence of CdCl2 was demonstrated on 

accommodation and manifestation of Cu atoms. 

• EL studies have provided and independent evidence of photoconductivity in the CdS layer 

and revealed long-lasting (hours-days) effect of exposure to laser irradiation with the photon 

106
energy above the CdS bandgap. This provides new options for investigations of the CdS 

photoconductivity mechanisms, in particular, for identification of defects responsible for the 

photoeffect, their generation, transformation and migration under illumination. 

Admittance spectroscopy (AS) and capacitance transient (C-Tr) measurements 

• A traditional AS and modified AS were used for detection and studying characteristics of 

deep electronic states in the absorber CdTe layer. It was found that characteristic time, 

concentration of this states, distribution over CdTe thickness and over energy depend on 

deposition technique, postdeposition treatment, and contacting procedure. 

• An important result of these studies was a high trap concentration found in cells fabricated at 

various facilities (FS, NREL, UT, FS/CSM), which exceeded doping level, , determined 

by C-V profiling. In some cells density of detected trap states was of the order of 10

N −VC

16 cm3

. 

Experimental data also indicated presence of significant amount of slower traps not detectable 

with the standard AS equipment. 

• The common AS measurement procedures and approaches to the data analysis were revised 

based on published theory and our own experience of the AS studies on CdTe cells. The 

common approach used for determining trap characteristics from the AS results is valid only for 

low trap concentration, much smaller than the doping level ( ). For CdTe (high trap 

density) common approach underestimates trap concentration. Our numerical modeling 

evaluated a correction factor that should be used to make estimates more accurate. The 

magnitude of this factor is especially high if the traps levels are located close to Fermi level (like 

for Cu

t << NN

Cd defects in CdTe). On the other hand, the doping level derived from C-V profiling, 

, can be strongly overestimated if , which was proved with numerical modeling of 

apparent doping level, , compared to the real one, N , for two trap levels with 

characteristics close to those determined by AS. 

N −VC t > NN

N −VC

• The technical limitations on the AS method were discussed. Specifically, the characteristic 

time of traps available for detection with usual LCR meters was considered. As possible 

solutions to the problem, variation of temperature (though also limited) and measurements of 

transients (for very slow traps) were discussed. 

• The problem of determining trap cross section was analyzed for the case of repulsing 

potential barrier provided by the collective action of closely spaced traps at the grain boundaries. 

The latter are considered the most important origin of very slow traps. The trap filling 

dependence on time is specific for this “collective” trap action, which can be used for the GB 

states identification. 

• The current state of the transient studies in CdTe is unsatisfactory. Studying of capacitance 

transients (C-Tr) is the most promising method for detection slow/deep states, estimating trap 

concentration, distinguishing between the majority- and minority-carrier traps. This report 

contains numerous results of the C-Tr studies in CdTe cells. 

• A new measurement system was developed and used for studying the cell admittance, in 

particular, C-f and C-V dependencies, along with transients in a temperature range from -190o

C 

to +85o

C. Measurements can be conducted in dark and under illumination of varying intensity 

and spectrum. Transients were induced by bias pulses of different polarity and duration, as well 

as by light pulses of different photon energies, including those lower than the CdTe bandgap. 

• Combined AS and C-Tr measurements with this new system were used to study effects of Cu 

doping and CdCl2 treatment on trap states in CdTe cells. Activation energies and apparent 

107
capture cross sections (trap signatures) were determined for at least five hole traps and two traps 

for minority carrier electrons. Concentrations of the traps and their changes after stress varied 

significantly for different combinations of Cu (yes, no) and CdCl2 (yes, no) treatments. 

• Some of the traps were attributed to specific defects, such as VCd, (VCd-ClTe) A-centers, CuCd, 

Tei. The latter trap with Ea ≈ 0.47 eV was observed only in cells subjected to NP etch that 

provided free Te layer on the top of CdTe and excess Te underneath. A deep trap with Ea ≈ 0.8 

eV and concentration exceeding 1015 cm-3 was found only in cells with combined Cu and CdCl2

treatment. It could be suggested that this trap is related to some complex involving Cu an Cl. 

High spatially resolved measurements

• A novel technique, charge injection spectroscopy (CIS) with a scanning tunneling 

microscope (STM) was used to map carrier transport across the interface between CdTe and 

ZnTe:Cu back contact. The lateral resolution below 20 nm was demonstrated. 

• Charge transfer between CdTe and ZnTe was highly non-uniform. A majority of the current 

flows along the pathways usually localized at grain boundaries. The grain bulk regions show 

rather uniform, low current, indicating substantially more resistive back contact. However, 

conducting paths were also found within some grains suggesting that specific intragrain defects 

also may reduce back contact resistance. 

• Near field scanning optical microscopy (NSOM) was used for mapping lateral distribution 

of photocurrent collection. These measurement were done on CdTe cells of substrate 

configuration supplied by the University of Toledo. This configuration provided an access of the 

optical tip to the TCO surface making possible near field measurements. Spatial resolution was 

better than 100 nm. 

• The photocurrent images combined with the topography ones demonstrated unambiguously 

enhanced current collection from the grain boundaries as compared to the intragrain regions. A 

similar effect was observed at the CIGS/CdS cells (NREL) of high efficiency. 

• In addition to photocurrent mode of NSOM, the photoluminescence mode was developed in 

this project. First NSOM PL images, both plan-view (UT cells) and cross sectional (CSM cells) 

confirmed possibility to study nonuniform distribution of the PL intensity on the subgrain size 

scale. The contrast between the intergrain and intragrain regions varied with the bias application 

providing the opportunity to control the sensitivity of measurements to spatial variations in 

recombination rate. 

Cell processing

• A new gas jet deposition system (GJD) for the CdTe deposition was developed where the 

deposited substance or its components are delivered by gas flow to the substrate. The mthod is 

similar to the VTD used by First Solar, LLC. 

• The deposition rate controlled by the source and substrate temperatures reaches 20 μm/min, 

although in our studies we used more moderate rates of ~4-5 μm/min. 

• The studies aimed at optimization of processing procedure are far for completion. The best 

cells grown on bi-layer substrates with the CBD CdS, GBD CdTe and ZnTe:Cu/Au back contact 

demonstrated efficiencies of ~12.5%. However recent experiments showed that there is a room 

for significant further improvement. As more promising directions of studies we see combined 

optimization of CdCl2 treatment, postdeposition treatment and thinning of CdS, and optimization 

of the back contact structure, in particular, thickness of ZnTe and content of Cu in it. 

108
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52. J.H. Scofield, “Effects of series resistance and inductance on solar cell admittance 

measurements”, Solar Energy Materials and Solar Cells 37, 217 (1955). 

53. S. W. Johnston, R.S. Crandall, “Evidence of the Meyer-Neldel Rule in InGaAsN alloys: 

Consequences for Photovoltaic Materials”, MRS Spring 2003, S. Francisco, CA, April 2003. 

54. R.S. Crandall, J. Electron. Mat., 9, 713 (1980) 

55. D.L. Young, K. R. Ramanathan, M. Contreras, J. Abushama, R. S. Crandall, “A Barrier To 

Trap Filling in CuIn1-xGax Se2”, MRS Spring 2003, S. Francisco, CA, April 2003. 

56. L.M. Woods, D.H. Levi, V. Kaydanov, G.Y. Robinson, and R.K. Ahrenkiel, “Electrical 

Characterization of CdTe Grain-Boundary Properties from As Processed CdTe/CdS Solar 

Cells”, in Proc. 2nd World Conf. on PV Solar Energy Conversion, July 1998, Vienna, 

Austria, pp. 1043-1046; “Electrical Characterization of Etched Grain-Boundary Properties 

from As-Processed p-CdTe Based Solar Cells”, Proc. NCPV PV Progr. Rev. Meeting, 

Denver, CO 1998, AIP CP462, pp.499-504. 

57. A.S. Gilmore, V. Kaydanov, U. Laor, T.R. Ohno, B. McCandless, “ AC Characterization of 

Grain Boundary Electronic Properties in CdTe Thin Films”, Proc. NCPV Progr. Rev. 

Meeting 2000, Denver, CO, April 2000, pp.259-260. 

58. V.I Kaydanov and T.R. Ohno, “Process Development and Basic Studies of Electrochemically 

Deposited CdTe-Based Solar Cells”, Final Technical Report, 15 May 1988-17August 2001, 

NREL/SR-520-31777, Colorado School of Mines, March 2002. 

59. R.A. Sasala and J.R. Sites, “Transient Voltage of Thin-film Polycrystalline Solar Cells”, AP. 

Conf. Proc. 268 , PV Advanced R&D Project, Denver, CO, 1992, pp.218-227. 

60. R.A. Sasala and J.R. Sites, “Time –Dependent Voltage in CuInSe2 and CdTe Solar Cells”, 

Proc. 23d IEEE PVSC, p.543 (1993). 

 

61. T.J. MacMahon, “Dark Curernt Transients in Thin-Film CdTe Solar Cells”, Proceedings 

 of the 29th IEEE PV SC, May, 2002, New Orleans, LA., pp. 768-771. 

62. Brian E. McCandless and James R. Sites, in Handbook of Photovoltaic Science and 

Engineering, edited by Antonio Luque and Steven Hegedus, John Wiley & Sons Ltd, West 

Sussex, England, p. 633, (2003). 

113
63. A. Niemegeers, M. Burgelman, “Effects of the Au/CdTe back contact on IV and CV 

characteristics of Au/CdTe/CdS/TCO solar cells”, Journal of Applied Physics, 81 No. 6 

p2881-2886. (1997). 

64. Reuven Chen, “Apparent stretched-exponential luminescence decay in crystalline solids”, 

Journal of Luminescence, 102-103, p. 510-518 (2003). 

65. A. Castaldini, A. Calvallini, B. Fraboni, P. Fernandez, J. Piqueras, “Deep energy levels in 

CdTe and CdZnTe”, Journ. of Appl. Physics, 83 No 4 p212-2126. (1998). 

66. D. Albin, R. Dhere, Z. Wu, T. Gessert, M.J. Romero, Y. Yan, S. Asher, “Perturbation of 

Copper Substitutional Defect Concentration in CdS/ CdTe Heterojunction Solar Cell 

Devices”, Proceedings of the Materials Research Society Conference, Symposium F, (San 

Francisco, California, USA, April 2002). 

67. A. Balcioglu, R. K. Ahrenkiel, F. Hasoon, “Deep-level impurities in CdTe/CdS thin-film 

solar cells”, J. Appl. Phys., 88 No 12 p7175-7178. (2000). 

68. M. A. Lorenço, W.L. Ng, K. P. Homewood, K. Durose, “A deep semiconductor defect with 

continuously variable activation energy and capture cross section”, Appl. Phys. Lett., 75 

 No 2 , pp. 277-279. (1999). 

69. P. Emanuelsson, P. Omling, B.K. Meyer, M. Weinecke, M Schenk, “Identification of 

cadmium vacancy in CdTe by electron paramagnetic resonance”, Physical Review B 47 

15578 (1993). 

70. I. Visoly-Fisher, S.D. Cohen, D. Cahen, “High Resolution Characterization of a Single Grain 

Boundary in CdTe/CdS Solar Cells”, MRS Spring 2003, S. Francisco, CA, April 2003. 

 

71. V. Kaydanov, P. Meyers, “Some Possible Effects of Grain Boundaries on Thin Film Cell 

Performance”, in “National CdTe R&D Team Meeting Minutes”, Golden, CO, January 

2000, Appendix 33. 

72. S. Smith, P. Zhang, T. Gessert, and A. Mascarenhas“Near-field optical beam-induced 

 currents in CdTe/CdS soar cells: Direct measurement of enhanced photoresponse at grain 

 boundaries”, Appl. Phys. Lett., 85, No. 17, pp. 3854-3856 (2004). 

73. A.A. McDaniel, J.W.P. Hsu, A.M. Gabor, “Near-field scanning opticl microscopy studies of 

 Cu(In,Ga)Se2 solar cells”, Appl.Phys. Lett. 70, No. 30, pp. 3555-3557 (1997). 

74. W. Song, “Development of High Efficiency Polycrystalline CdTe/CdS Solar Cells 

Using Electrodeposition”, Ph.D. Dissertation, Colorado School of Mines, Golden, Colorado, 

1999. 

114
9. ACKNOWLEDGEMENTS 

 Many people, in addition to those at the Colorado School of Mines, have contributed to 

this work over the past year. 

 We are very thankful to Peter Meyers and Anke Abken of First Solar, LLC., for 

supplying the materials for CdTe cell preparation at our facilities and also the completed cells 

(stressed and unstressed) for studies. 

 Dr. D. Albin (NREL) has provided invaluable contributions in our studies of defects in 

CdTe cells. He supplied us with the cells produced with varied processing procedures, and 

participated in planning of experiments and discussion of results. 

 We greatly appreciate the opportunity to collaborate with our partners in the National 

CdTe R&D team: Prof. J. Sites, T. Nagel, Prof. W. Sampath, A. Enzeroth, K. Barth (Colorado 

State University); Prof. A. Compaan, Prof. V. Karpov, Dr. D. Shwydka et al. (University of 

Toledo); Prof. C. Ferekides (university of S, Florida) 

It is hard to underestimate the value of discussions of our research approaches and results 

that we have had with the individuals mentioned above and Alan Fahrenbruch, Bolko von 

Roedern, Tim Gessert, Brian McCandless, R. Crandall, D. Young, S. Johnston and other 

scientists involved in fabrication and studies of thin film solar cells. 

 Finally we greatly value the encouraging interest in our activities, discussions and 

suggestions we receive from the management of Thin Film Partnership Program: Kenneth 

Zweibel (our subcontract technical monitor), Bolko von Roedern and Harin Ullal. 

115
10. APPENDICES 

10.1 Personnel 

The names, titles, and representative responsibilities of the individuals contributed to this work are 

summarized below. 

• Timothy R. Ohno, Associate Professor of Physics: Photovoltaic Development, Surface 

Physics, Electroluminescence, Principal investigator

• Victor I. Kaydanov, Research Professor of Physics: Photovoltaic Development, Electron 

Transport , Studying of the Defect Electronic States, Principal Investigator 

• Reuben T. Collins, Professor of Physics: Electronic and Optical Properties of 

Semiconductors, NSOM, Photo- and Electroluminescence 

• Colin Wolden, Associate Professor of Chemical Engineering: Gas Jet Deposition System 

Design 

• Peter Sutter, Associate Professor: Scanning Tunneling Microscopy and Spectroscopy 

• Joe Beach, Post. Doc. Fellow, then Research Professor: Cell Processing and 

Characterization 

• Angelo S. Gilmore, Graduate Research Assistant, then Post. Doc. Fellow: Admittance 

Spectroscopy, Hall effect 

• Scott Feldman, Graduate Research Assistant: Cell Electroluminescence and 

Photoluminescence, Micro-nonuniformity (Defended Ph.D thesis in May 2005) 

• James Kestner, Graduate Research Assistant: Developing of GJD System, Cell 

Processing (Defended PhD thesis 2003) 

• Fred Seymour, Graduate Research Assistant: Studying of Deep Electronic States by means 

of Admittance Spectroscopy and Transients; Developing of Database 

• Lorelle Mansfield, Graduate Research Assistant: Cell Processing and Characterization, 

AFM Measurements 

• John Yarbrough, Graduate Research Assistant: Near Field Scanning Microscopy 

116
10.2 Laboratory Improvements 

 Principal changes at CSM revolved around both processing and characterization during this 

project. As detailed in section 6, the electrodeposition method was modified to increase growth 

rate, decrease bath size and simplify the tellurium supply. It was decided midway through to 

focus on the higher growth rate GJD, which more closely resemble processes in use by our 

industrial partner First Solar. The GJD deposition system is described in detail in Reference 34, 

and was developed initially under subcontract ZAK-8-17619-03, with ITN Energy Systems. 

 The most significant change in equipment for characterization was the development of the 

temperature controlled admittance measurement system detailed in section 2.4. The high 

temperature stability, as well as high sensitivity afforded by the Agilent LCR meters and 

Keithley source meter, resulted in the excellent quality data reported in section 2. A very 

important part of this system is the flexible software written in Agilent VEE that permits 

automated data acquisition. 

 Both the EL and PL measurements were enhanced during the project by acquisition of an 

Acton 300i spectrometer with a Princeton Instruments Spec-10:100BR Si CCD array detector 

which allows for simultaneous spatial and spectral resolution of EL or PL performed on the exact 

same spot of a solar cell. As described in Sections 1 and 5, high-speed parallel detection of 

spectra and in some cases spatial mapping allowed studies of non-uniformities. Our EL and PL 

measurements allow for many degrees of freedom. Spatial and spectral changes in luminescence 

can be seen due to changes in temperature, electrical bias (for both EL and PL), laser power (for 

PL), and light bias (for EL). By altering these parameters, we can emphasize the signal from 

different defects. The combination of this detection with the Ti-sapphire laser from the previous 

project provided a powerful tool of EL/PL and NSOM. 

High spatial resolution techniques showed two developments during this period. A 

variety of spectroscopic and spatial mapping techniques were shown in section 4 using STM. 

These were used during the first half of the project, until PI Sutter relocated to Brookhaven 

National Laboratory. The second more promising technique, NSOM, resulted in two new 

instruments during this project, funded by NSF projects DMI-0340259 and DMR-0103945 (PI 

Collins). The system used for cross sections and PL used an inchworm approach and tuning fork 

force feedback. 

117
10.3 Publications 

1. F. Seymour, V. Kaydanov, and T.R. Ohno, “Study of Meyer-Neldel rule behavior 

detected with slow capacitance transients in CdTe solar cells”, Proc. of MRS Spring 

Meeting, March 28-April 1, 2005, S. Francisco, CA. 

2. F.H. Seymour, V. Kaydanov, T.R. Ohno, and D. Albin, “Cu and CdCl2 influence on 

defects detected in CdTe solar cells with admittance spectroscopy”, submitted to Appl. 

Phys. Lett., May 2005. 

3. S.D. Feldman, L. Mansfield, T.R. Ohno, V. Kaydanov, J.D. Beach, and T. Nagle, “Nonuniformity mitigation in CdTe solar cells: the effects of high-resistance transparent 

conducting oxide buffer layers”, Proc. 31st IEEE PVSC Conf., January, 2005, Orlando, 

FL. 

4. F. Seymour, V. Kaydanov, T.R. Ohno, “Study of deep electronic states in CdTe solar 

cells through the detection and DLTS treatment of slow transients”, Ibid 

5. S.D. Feldman, T.R. Ohno, R.T. Collins, and V. Kaydanov, Defect studies in CdTe solar 

cells using spatially and spectrally resolved electro-optical methods“, Solar Energy 

Technologies Progr. Review. Meeting, October 2004, Denver, CO. 

6. F. Seymour, V. Kaydanov, T.R. Ohno, “Study of deep electronic states in CdTe solar 

cells with capacitance transient measurements”, Ibid 

7. T. R. Ohno, S. D. Feldman, F. Seymour, J.D. Beach, L. Mansfield, R. T. Collins, and V. 

Kaydanov “Defect physics and chemistry in thin film CdTe Solar Cells”, Ibid

8. J. Yarbrough, R.T. Collins, J. D. Beach, V. Kaydanov, and T.R. Ohno , “Multiple 

Wavelength Near Field Scanning Optical Microscopy Study of Thin Film Polycrystalline 

Solar Cells”, Ibid

9. S.D. Feldman, R.T. Collins, V. Kaydanov, and T.R. Ohno, “Effects of Cu in CdS/CdTe 

solar cells studied with patterned doping and spatially resolved luminescence”, Appl. 

Phys. Lett. 85, 2004, pp.1529-1531. 

10. J. M. Kestner, S. McElvain, S. Kelly, L. M. Woods, T. R. Ohno, and C. A. Wolden, “An 

Experimental and Modeling Analysis of Vapor Transport Deposition of Cadmium 

Telluride”, Sol. Energy Mater. Sol. Cells 83, 2004, pp. 55-65. 

11. P. Sutter, E. Sutter, and T.R. Ohno, “High-resolution mapping of nonuniform carier 

transport at contacts to polycrystalline CdTe/CdS solar cells”, Appl. Phys. Lett. 84, No. 

12, 2004, pp.2100-2102. 

12. A.S. Gilmore, V. Kaydanov, T.R. Ohno, “The Study of Deep Levels in CdS/CdTe Solar 

Cells Using Admittance Spectroscopy and its Modifications”, MRS Spring 2003, S. 

Francisco, CA, April 2003. 

13. P. Sutter, P. Zahl, E. Sutter, V. Kaydanov, T. Ohno, “High-Resolution Electron Structure 

Mapping on Polycrystalline CdTe Solar Cells by Scanning Tunneling Microscopy”, Ibid

14. S.D. Feldman, T.R. Ohno, V. Kaydanov, and R.T. Collins, “Development of 

Electroluminescence Imaging Characterization for CdS/CdTe Solar Cells”, Ibid.

15. J.M. Kestner, Sarah McElvain, and Colin Wolden, “Vapor Transport Deposition and 

Characterization of Polycrystalline CdTe Solar Absorbers”, Ibid.

16. S.D. Feldman, F.H. Seymour, T.R. Ohno, V. Kaydanov, and R.T. Collins, “Development 

and Application of Electroluminescence Imaging for CdS/CdTe Solar Characterization”, 

NCPV and Solar Progr. Rev. 2003, Denver, CO, March 24-26, 2003. 

118
17. J. Kestner, S. McElvain. C.A. Wolden, T. Ohno, and Lawrence Woods, “ Temperature 

Dependence of Growth Rate and Morphology of Vapor Transport Deposited CdTe Thin 

Films”, Ibid. 

18. S. Gilmore, V. Kaydanov, T. R. Ohno, D. Rose, S. D. Feldman, P. Erslev, “Treatment 

Effects on Deep Levels in CdTe Based Solar cells”, Proceedings of the 29th IEEE PV SC,

May, 2002, New Orleans, LA., p. 604. 

19. A.S Gilmore, A. Al-Kaoud, V. Kaydanov, T.R. Ohno, “Mobility in SnO2:F Thin 

Polycrystalline Films”, Material. Res. Soc. Symp. Proc. Vol. 668, San Francisco, CA, 

April 16-20, 2001, pp. F3.10.1-F3.10.6. 

20. A.S. Gilmore, V. Kaydanov, T.R. Ohno, D. Grecu and D. Rose, “ Impedance 

Spectroscopy and Hall Measurements on CdTe Thin Polycrystalline Films”, Material. 

Res. Soc. Symp. Proc. Vol. 668, San Francisco, CA, April 16-20, 2001, pp. H5.10.1-

H5.10.6 

21. S.W. Townsend, T.R. Ohno, V. Kaydanov, A.S. Gilmore, J.D. Beach, “The Influence of 

Stressing at Different Biases on the Electrical and optical Properties of CdS/CdTe Solar 

Cells”, Material. Res. Soc. Symp. Proc. Vol. 668, San Francisco, CA, April 16-20, 2001, 

pp. H5.11.1-H5.11.6 

22. T.R. Ohno, E. Sutter, J. Kestner, A.S. Gilmore, V. Kaydanov, C.A. Wolden, P.V. 

Meyers, L.Woods, M.J. Romero, M.M. Al-Jassim, S.Johnston, “Microscopic 

Characterization of Polycrystalline APCVD CdTe Thin Film PV Devices”, Material. Res. 

Soc. Symp. Proc. Vol. 668, San Francisco, CA, April 16-20, 2001, pp. H6.5.1-H6.5.6 

23. A.S. Gilmore, V. Kaydanov, T.R. Ohno, D. Rose, “Admittance Spectroscopy of Deep 

Levels in CdTe Solar Cells”, NCPV Progr. Rev. Meeting, 2001, Oct. 2001, Lakewood 

CO, pp. 187-188. 

24. S. Feldman, S. Townsend, R.T. Collins, V. Kaydanov, and T.R. Ohno, 

“Electroluminescent and Photoluminescent Studies of Stressed and Unstressed CdTe/CdS 

cells”, NCPV Progr. Rev. Meeting, 2001, Oct. 2001, Lakewood CO, pp. 199-200. 

119
10.4 Presentations 

1. F.H. Seymour, V.Kaydanov, T.R.Ohno, D. Albin, “Cu and CdCl2 Influence on Deep Defects 

Detected in CdTe Solar Cells”, National CdTe R&D Meeting, Golden, May 5-6, 2005, 

Golden, CO. 

2. S.D. Feldman, “Electroluminescence Studies”, National CdTe R&D Team Meeting. 

Perrysburg, OH, Feb. 26-27, 2004. Appendix 6 in “Minutes 

3. S.D. Feldman, “Effects of Cu Studied with Non-Uniform Doping and Spatially Resolved 

Characterization”, Ibid. Appendix 13 in “Minutes”. 

4. T.R. Ohno, “Defect Chemistry - What do we know?”, Ibid. Appendix 10 in “Minutes” 

5. F. Seymour, V. Kaydanov, T.R. Ohno, “Transient Studies in CdTe Solar Cells”, Ibid. 

Appendix 15 in “Minutes”. 

6. V. Kaydanov, “Capture Cross-Section”, Ibid. Appendix 16 in “Minutes”. 

7. F. Seymour, V. Kaydanov, and T.R. Ohno,”Study of Deep Electronic States in CdTe Solar 

Cells with Capacitance Transient Measurements”, Rocky Mountain Chapter AVS 

symposium August 12, 2004. 

8. S.D. Feldman, V. Kaydanov, and T.R. Ohno,”Study of Deep Electronic States in CdTe Solar 

Cells with Capacitance Transient Measurements”, Rocky Mountain Chapter AVS 

symposium August 12, 2004. 

9. S.D. Feldman, “, T.R. Ohno, V. Kaydanov, and R.T. Collins, “Back Contact Nonuniformity 

Studied with Luminescence Technique”, in “National CdTe R&D Team Meeting Minutes”, 

Golden, CO, July 10-11, 2003. Appendix 6 in “Minutes”. 

10. S.D. Feldman, T.R. Ohno, V. Kaydanov, R.T. Collins, “Electroluminescence Studies”, in 

“National CdTe R&D Team Meeting Minutes”, Golden, CO, Oct. 31- Nov. 01, 2002. 

Appendix 8 in “Minutes”. 

11. V. Kaydanov, “Capacitance/Conductance Spectroscopy of Deep States and Hole 

Concentration in CdTe Solar Cells”, Ibid. Appendix 32 in “Minutes”. 

12. Scott Feldman, “Electroluminescence Studies”, National CdTe R&D Team Meeting, FSEC, 

Cocoa, Florida, March 14-15, 2002. Appendix 8 in “Minutes”. 

13. A. S. Gilmore, “Drive Level Capacitance Profiling”, Ibid. Appendix 34 in “Minutes”. 

14. Scott Feldman, “Solar Cells and Grad. School. What is not to Love?” (Invited talk)

Colloquium at Colorado College, Colorado Springs, January 30, 2002. 

15. T.R. Ohno “ Photovoltaics “, (Invited talk), Colloquium at the University of Colorado Denver / 

Metro State University, Denver, CO, April 5, 2002. 

120
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1. REPORT DATE (DD-MM-YYYY)

February 2007 

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Subcontract Report 

3. DATES COVERED (From - To)

16 October 2001 – 31 August 2005 

5a. CONTRACT NUMBER

DE-AC36-99-GO10337 

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4. TITLE AND SUBTITLE

Studies of Basic Electronic Properties of CdTe-Based Solar Cells 

and Their Evolution During Processing and Stress: Final Technical 

Report, 16 October 2001 – 31 August 2005 

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NREL/SR-520-41129 

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PVB75201 

6. AUTHOR(S)

V.I. Kaydanov and T.R. Ohno 

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Physics Department 

Colorado School of Mines 

Golden, Colorado 80401 

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14. ABSTRACT (Maximum 200 Words)

This report describes basic issues behind CdTe/CdS cell performance and stability, such as the nature and electronic 

properties of impurities and defects that control the majority carrier concentration, mechanisms of dopant compensation, 

recombination processes, their nature and properties, migration and transformation of defects under various processing, 

stress, and operating conditions. We believe that a better basic understanding of the specific influence of grain 

boundaries, especially for fine-grain materials such as those making up CdTe-based cells, is now one of the most 

important issues we must address. We need to clarify the role of grain boundaries in forming the film electronic 

properties, as well as those of the p-n junction. This report presents studies relevant to the problems mentioned above 

that were carried out at Colorado School of Mines during this Thin Film PV Partnership subcontract. 

15. SUBJECT TERMS

Grain boundaries; electroluminescence; photoluminescence; charge injection spectroscopy; scanning tunneling 

microscopy; electrodeposition; admittance spectroscopy; capacitance transients; deep electronic states; chemical-bath 

deposition; post-deposition annealing; gas-transport deposition; Gas-Jet system; optimization; 

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