Subcontract Report

NREL/SR-520-45398

April 2009

Fabrication and Physics of CdTe 

Devices by Sputtering 

Final Report

1 March 2005 – 30 November 2008

A. Compaan, R. Collins, V. Karpov, and 

D. Giolando

University of Toledo

Toledo, Ohio
National Renewable Energy Laboratory

1617 Cole Boulevard, Golden, Colorado 80401-3393

303-275-3000 • www.nrel.gov

NREL is a national laboratory of the U.S. Department of Energy

Office of Energy Efficiency and Renewable Energy

Operated by the Alliance for Sustainable Energy, LLC

Contract No. DE-AC36-08-GO28308

Subcontract Report

NREL/SR-520-45398

April 2009

Fabrication and Physics of CdTe 

Devices by Sputtering 

Final Report

1 March 2005 – 30 November 2008

A. Compaan, R. Collins, V. Karpov, and 

D. Giolando

University of Toledo

Toledo, Ohio

NREL Technical Monitor: Bolko von Roedern

Prepared under Subcontract No(s). ZXL-5-44205-01
This publication was reproduced from the best available copy

Submitted by the subcontractor and received no editorial review at NREL

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iii

Table of Contents

Table of Contents ......................................................................................................................... iii

List of Figures .................................................................................................................................v

List of Tables ............................................................................................................................... vii

1. Introduction and Executive Summary ..................................................................................1

2. Ex Situ Spectroscopic Ellipsometry ......................................................................................4

2.1 Etch back studies of CdTe materials and solar cells .........................................................5

2.1.1 Verification of the etching procedure .........................................................................5

2.1.2 Etch back studies of CdTe materials ...........................................................................6

2.1.3 Etch back studies of CdTe solar cells .........................................................................7

2.2 Ex situ SE measurements of CdTe solar cells ..................................................................8

2.2.1 TEC-15 optical properties ...........................................................................................8

2.2.2. Optical modeling of etch-smoothened CdTe solar cells ............................................9

2.2.3. Through-the-glass measurements of the CdTe solar cell ............................................10

2.3 Future directions .............................................................................................................12

3. Real Time Spectroscopic Ellipsometry: Dielectric Functions and Structural 

Evolution ..............................................................................................................................14

3.1 Overview of CdTe and CdS dielectric functions ............................................................15

3.2. CdTe and CdS structural evolution on Si substrates as measured by RTSE .................16

3.2.1. Role of substrate temperature for CdTe and CdS ....................................................16

3.2.2. Role of Ar pressure for CdTe...................................................................................19

3.3 Future directions .............................................................................................................20

4. In-depth Analysis of the Dielectric Functions of CdTe and CdS .....................................22

4.1 Analysis methodology .......................................................................................................23

4.2 Critical point broadening parameters and grain structure ...............................................24

4.3 Critical Point Energies and Strain ...................................................................................25

4.4 Summary: Effects of temperature, grain size, and stress on the critical points of CdTe 

and CdS ...........................................................................................................................27

4.5 Ternary alloys between CdS and CdTe ..........................................................................28

4.5.1 Dielectric functions of ternary alloys ........................................................................28

4.5.2 Analysis of alloy formation at CdS/CdTe and CdTe/CdS interfaces .......................29

4.6 Future directions .............................................................................................................29

5. HREM, EXAFS and XES Studies of CdTe cells ................................................................31

5.1 Introduction .....................................................................................................................31

5.2 Sample Preparation .........................................................................................................31

5.3 Voids in CdTe and CdTe/CdS Interface .........................................................................32

5.4 Cu Distribution and Local Structure in CdTe cells .........................................................33

5.4.1 EDS mapping ............................................................................................................33

5.4.2 X-ray Fluorescence ...................................................................................................33

5.4.3 High Resolution TEM and Fine-probe EDS .............................................................34

5.4.4 EXAFS ......................................................................................................................36

5.5 S at CdTe Back Contact ..................................................................................................37

5.6 Conclusions .....................................................................................................................38
iv

6. Potential Distribution During Magnetron Sputtering Deposition....................................39

6.1 Introduction ........................................................................................................................39

6.2 Cathode DC Bias.............................................................................................................40

6.3 Substrate Self-Bias ..........................................................................................................40

6.4 Plasma Potential ..............................................................................................................41

6.5 Conclusions .....................................................................................................................41

7. Ultra Thin Devices ................................................................................................................43

7.1 Presence of an HRT layer ...............................................................................................43

7.2 CdS thickness ..................................................................................................................43

7.3 CdTe thickness ................................................................................................................43

7.4 CdCl2 treatment. ..............................................................................................................43

7.5 Thickness of the as-deposited Cu layer. .........................................................................47

7.6 Cu diffusion time. ...........................................................................................................48

7.7 Cell performance statistics for optimized cells ...............................................................48

7.8 Conclusions .....................................................................................................................55

8. Shunt Passivation - Electrochemical Polymerization Treatment of CdTe Solar Cell 

Devices. .................................................................................................................................56

8.1 Introduction .....................................................................................................................56

8.2 Characterizing the new insulating thin film (conducting polymer) ................................56

8.3 Effect of electrochemical treatment on copper diffusion ................................................59

9. A theory of Admittance Spectroscopy for Thin Films Photovoltaics ...............................62

9.1 Introduction .....................................................................................................................62

9.2 Theory .............................................................................................................................63

9.3 Practical Implications ......................................................................................................64

9.4 Conclusions .....................................................................................................................65

10. References (from section 1 through 9) ..............................................................................66

11. Publications .........................................................................................................................67

11.1 Refereed papers published or in press ..........................................................................67

11.2 Poster or Oral Presentations ..........................................................................................69

11.3 Publications and presentations at DOE Program Reviews ...........................................70

12. Project Personnel ................................................................................................................71

12.1 Postdoctoral Associates / Research Assistant Professors .............................................71

12.2 Graduate Students (with Principal Advisor) .................................................................71

12.3 Undergraduate Students ................................................................................................71

12.4 Technical Assistants ......................................................................................................71
v

List of Figures

Figure 2.1 The void volume fraction of CdTe during a low temperature deposition (130°C). ... 5

Figure 2.2 E1 (upper panel) and width ΓE1 (lower panel) of CdTe bulk layer. ............................ 6

Figure 2.3 E1 transition of CdTe bulk layer for a sputtered CdTe solar cell. .............................. 7

Figure 2.4 Broadening parameter ΓE1 of CdTe bulk layer for a sputtered CdTe solar cell ......... 7

Figure 2.5 Dielectric function of TEC-15 glass ........................................................................... 8

Figure 2.6 Dielectric functions of SnO2:F. .................................................................................. 9

Figure 2.7 Structural model for the CdTe solar cell after the 15th Br2+methanol etch. ............. 9

Figure 2.8 Ex situ SE spectra in (ψ,∆) from the CdTe surface and from the prism/glass side. . 10

Figure 2.9 The structural parameters deduced from fig 2.8. ...................................................... 11

Figure 2.10 CdS and CdTe/CdS interface layer thicknesses deduced from fig 2.8. .................. 11

Figure 2.11 Ellipsometric spectra for a CdTe solar cell obtained after CdCl2 treatment. ......... 12

Figure 3.1 Complex dielectric functions ε of typical polycrystalline CdTe and CdS films. ..... 15

Figure 3.2 Surface roughness thickness (ds) for CdTe films on c-Si wafer at different Td. ...... 16

Figure 3.3 ds vs. db evolution for CdS films sputtered on c-Si wafer at different Td. ................ 16

Figure 3.4 Void volume fraction (fv) for CdTe films sputtered on c-Si wafer held at 

different Td. .............................................................................................................. 17

Figure 3.5 fv evolutions for CdS films sputtered on c-Si wafer held at different Td. ................ 18

Figure 3.6 Effective rate of material growth (excluding voids) as a function of Td .................. 18

Figure 3.7 Surface roughness evolution for CdTe sputtering with different Ar pressures ........ 19

Figure 3.8 Stepwise void evolution for the CdTe films of Fig. 3.7. .......................................... 19

Figure 3.9 Effective CdTe growth rate for the films of Fig. 3.7. ............................................... 20

Figure 4.1 The 2nd derivative spectra of dielectric functions ε of polycrystalline CdTe and 

CdS.. ......................................................................................................................... 23

Figure 4.2 The broadening parameters for the E0, E1-A, and E1-B CPs for CdS films 

deposited at different temperatures. ......................................................................... 24

Figure 4.3 Grain size for the CdS films of Fig. 3.3. .................................................................. 24

Figure 4.4 The CdS E1-A and E1-B broadening parameters plotted vs. the reciprocal of 

grain radius in Fig. 20. ............................................................................................. 25

Figure 4.5 The 2nd derivative spectra of room temperature ε for c-CdTe and the CdTe films 26

Figure 4.6 15°C E0 CP energies for CdTe films sputtered at different Td. ................................ 27

Figure 4.7 Dielectric functions of co-sputtered CdSxTe1-x alloy films at 190°C. ...................... 28

Figure 4.8 Composition profiles x(db) in CdSxTe1−x ................................................................. 29

Figure 5.1 Cross-sectional HAADF and STEM-BF images of standard CdTe device. ............ 32

Figure 5.2 HAADF image and EDS mapping of Cd, Te, S, Cu and Au on the same area. ....... 33

Figure 5.3 The schematic structures of the samples and X-ray fluorescence spectra ................ 34

Figure 5.4 High resolution TEM image at the CdTe/Au interface. ........................................... 35

Figure 5.5 Phase-uncorrected χ(R) functions of semi-cell structure sample and theoretical 

fitting compared with Cu2O reference; .................................................................... 36

Figure 5.6 Sulfur L2,3 XES spectra of as-grown CdS reference, as-grown and CdCl2 treated 

CdTe/CdS, and a CdSO4 (sulfate) reference. ........................................................... 38

Figure 6.1 Experimental setup for potential distribution and the Langmuir probe 

measurements. .......................................................................................................... 39

Figure 6.2 Cathode DC bias as a function of pressure and RF power. ...................................... 40

Figure 6.3 Substrate self-bias bias as a function of pressure and RF power. ............................ 40
vi

Figure 6.4 Plasma potential as a function of pressure and RF power.. ...................................... 41

Figure 7.1 CdCl2 treatment temperature profiles for nominal 3 and 30 minute treatments. ...... 44

Figure 7.2 A schematic representation of “nominal” vs. “actual” treatment time. .................... 44

Figure 7.3 Arrhenius plots of diffusion coefficients for Cl and Cd in bulk CdTe and Cd 

diffusion along grain boundaries in CdTe. .............................................................. 45

Figure 7.4 Relative contribution of the ramp-up and ramp-down time for a 30-minute treatment 

at 387 oC ................................................................................................................... 45

Figure 7.5 Relative contribution of the ramp-up plus ramp-down time for t a three-minute 

treatment at 387 oC ................................................................................................... 45

Figure 7.6 CdCl2 treatment time vs. CdTe thickness and the linear fit for the “corrected” time.

.................................................................................................................................. 46

Figure 7.7 Treatment time chosen for samples of different thickness. ...................................... 47

Figure 7.8 A typical sample used in this study. ......................................................................... 48

Figure 7.9 A sketch of a sample with individual dot cells. ........................................................ 50

Figure 7.10 Box-and-whiskers plot of the efficiency of the 10 best cells from each sample .... 50

Figure 7.11 Short-circuit current of the 10 best cells from each sample ................................... 51

Figure 7.12 Quantum efficiency at zero bias conditions ........................................................... 51

Figure 7.13 J-V characteristics of the cells from the Figure 7.11. ............................................. 52

Figure 7.14 Open-circuit voltage of the 10 best cells from each sample. .................................. 52

Figure 7.15 Fill factor of the 10 best cells from each sample. ................................................... 53

Figure 7.16 Quantum efficiency at variable bias. Thicker cells. ............................................... 54

Figure 7.17 Quantum efficiency at variable bias. Thinner cells. ............................................... 55

Figure 8.1 The base (i.e., neutral) polyaniline ........................................................................... 56

Figure 8.2 XPS spectrum of a TEC-15 glass substrate used as a control .................................. 57

Figure 8.3 XPS spectrum of a TEC-15 glass substrate coated with the conducting polymer. .. 58

Figure 8.4 ESI-MS spectra of the newly formed thin film after rinsing. ................................... 59

Figure 8.5 Top surface view of the CdTe solar cell sample. ...................................................... 60

Figure 8.6 Voc and Jsc of 2 hour copper diffusion of an electrochemically treated and 

untreated cells. ......................................................................................................... 60

Figure 8.7 FF and efficiencies of a 2 hr copper diffusion of an electrochemically 

treated and untreated cells ........................................................................................ 61

Figure 9.1 Equivalent circuit of a device ................................................................................... 62

Figure 9.2 Examples of four possible 1D device scenarios. ...................................................... 63

Figure 9.3 Prediction of capacitance as a function of frequency for a small 

2D substrate cell. ...................................................................................................... 64
vii

List of Tables

Table 4.1 Single crystal CP Energies and Widths, Temperature Coefficients of CP Energies 

and Widths, Electron Group Speeds, and Stress Coefficients of CP Energies for 

Major CPs for CdTe and CdS .................................................................................... 27

Table 5.1 Comparison of Cu Kα and Mo Kα intensities. ............................................................ 35

Table 5.2 Theoretical fitting parameters of EXAFS spectra of partial cell structure and 

Cu2O reference. ........................................................................................................... 36

Table 7.1 Benchmarks for CdCl2 treatment time of different CdTe thickness and 

corresponding best cell efficiency .............................................................................. 46

Table 7.2 Sample fabrication details .......................................................................................... 49
1

1. Introduction and Executive Summary 

There were three overall emphases in this subcontract: 1) improving our understanding of 

key aspects of the CdS/CdTe solar cell device physics, 2) improving our understanding of 

magnetron sputtering and increasing the sputter deposition rate while maintaining high device 

quality, and 3) reducing the thickness of CdTe layers in the CdS/CdTe cell below 0.5 μm while 

maintaining voltage and fill factor.

In Phase III of this subcontract, we have completed our characterization of the optical and 

morphological properties of CdS, CdTe, and the alloy CdSTe films using spectroscopic 

ellipsometry (Sections 2, 3 and 4). We have performed high resolution energy dispersive x-ray 

spectroscopy (EDS), x-ray absorption fine structure spectroscopy (XAFS) and x-ray emission 

spectroscopy (XES) to study the location and chemical bonding of Cu and oxygen in completed 

CdTe cells (Section 5). We have studied the properties of the sputtering plasma with the goal,

inter alia, of understanding the origin of and controlling the formation of dust on and in the 

deposited film (Section 6). A particular emphasis was placed on studies of ultra-thin CdS/CdTe 

devices with CdTe layers less than 1 μm thick (Section 7). We have developed an 

electrochemical approach to shunt passivation (Section 8). And finally, we have developed a 

theory of admittance spectroscopy that is applicable to thin-film PV with resistive electrodes

(Section 9). 

Highlights of accomplishments during Phase III include: 

• A new through-the-glass optical approach has been developed for on-line measurements 

of CdTe solar modules by spectroscopic ellipsometry (SE). This approach consists of 

blocking the top surface reflection from the glass and only measuring the beam that is 

reflected from the TEC-15/solar cell interface. The analysis provides information on the 

TEC-15 layers, the interface roughness between the CdS and the TEC-15 SnO2:F layers, 

as well as the thickness of the CdS, the combined roughness-interaction thickness 

between the CdS and CdTe, and the thickness of the CdTe.

• Real time SE performed on CdTe prepared by magnetron sputtering as a function of Ar 

pressure demonstrates that the lowest attainable Ar pressure leads to the most dense and 

uniform films throughout deposition to a 1 μm thickness; increasing Ar pressure from 

this minimum leads to more rapid void development with film thickness. The surface 

roughness evolution also depends strongly on the Ar pressure. The lowest Ar pressure 

leads to a smooth stable surface throughout film growth; increasing Ar pressure leads to 

much faster roughness development, possibly due to reduced surface diffusion, a 

mechanism that can lead to smoothening of the surface. 

• The critical point (CP) widths and energies in the dielectric functions of CdS and CdTe 

have been parameterized in terms of key film properties, including temperature, stress, 

and grain size: 

 Γ = Γb(T0) + CTΓ * (Tm – T0) + (hvg/R) 

 E = Eb(T0) + CTE * (Tm – T0) + CPP 

where Γb and Eb are the width and CP energy of the single crystal; T0 is a reference 

measurement temperature, normally room temperature; CTΓ and CTE are the temperature 

coefficients of the CP width and CP energy, respectively; h is Planck's constant, vg is the 
2

excitation group velocity, and R is the grain size. This parameterization has applications 

in monitoring and spatially mapping systems that deduce film properties on a solar cell 

production line. 

• Consistency in the analysis of the CdS and CdTe dielectric function CPs in terms of 

measurement temperature Tm, grain size R, and stress P, has been achieved. The 

following consistencies are possible: (i) for a given solar cell, Tm for CdTe and CdS are 

the same; (ii) for a series of cells of the same component materials (CdTe and CdS) but 

with different characteristics, the single crystal CP values, Γb and Eb, the CP coefficients, 

CTΓ and CTE, and the electron group speeds νg, are all constants. As a result, this 

parameterization reflects not only important fundamental information on the electronic 

structure, but also provides a useful “interlocking” mechanism that can significantly 

reduce correlation and instrumentation-related drift when determining properties on a 

production line. 

• High-angle annular dark field and STEM bright field images reveal the presence of a 

modest number of macroscopic voids having typical dimensions of ~0.15 μm, with a void 

fraction estimated at ~1%, in the CdTe. This macroscopic void fraction observed by 

microscopy is less than the overall void fraction of ~4% revealed in the ellipsometry 

studies of CdTe films grown on c-Si under similar conditions. 

• EDS mapping of TEM cross sections, done in collaboration with the University of 

Michigan, show that, after back-contact diffusion, the 3.5 nm of Cu used in the back 

contact mostly remains at the CdTe/Au interface or alloys with the Au layer. Much 

smaller amounts of Cu appear in the CdTe layer with some higher accumulation in the 

CdS layer. The higher amount of Cu in the CdS layer compared with CdTe is consistent 

with SIMS studies of CdS/CdTe cells reported by others. Little evidence was found for 

Cu accumulation at grain boundaries. 

• Synchrotron-based studies (at the Argonne APS) of Cu kα x-ray fluorescence and XAFS 

at the k-absorption edge of Cu indicate that most of the Cu in completed CdS/CdTe cell 

structures is oxidized in the form of Cu2O. However, if the structure has not received a 

prior CdCl2 treatment, the Cu is not oxidized. Although the XAFS signal is not spatially 

selective, the data are consistent with the EDS signal that arises from the CdTe/Au 

interfacial layer.

• X-ray and ultraviolet photoemission spectroscopy done on completed cells in 

collaboration with the University of Nevada Las Vegas, show the presence of S at the 

back contact. Much of this S has been oxidized to the form of SO4 in completed cells. 

This S diffusion and oxidation apparently occurs during the CdCl2 treatment.

• Using Langmuir probe studies of the sputter plasma we find that the substrate self-bias 

potential is depends on the condition of the sputter cathode ground shroud. A bare 

metal shroud leads to negative substrate self-bias of a few volts, whereas a CdTe-coated 

shroud yields a positive substrate self-bias of a few volts. This shift in substrate 

self-bias and corresponding shifts in the plasma potential are correlated with the density 

of pinhole formation in the deposited film. 
3

• We studied the dependence of dot cell efficiency on CdTe thickness ranging from a 

maximum of 2.5 μm to a minimum of 0.3 μm. Cell performance was optimized to first 

order in CdCl2 treatment as well as Cu back contact thickness and diffusion time.

• A systematic comparison was made between very thin CdTe cells on standard Pilkington 

TEC15 glass as well as TEC15 glass with a high resistivity transparent (HRT) coating. 

• We have achieved, on HRT-coated TEC15 glass, best cells with efficiencies of

o 12% @ 0.97 μm of CdTe,

o 10% @ 0.5 μm of CdTe, and

o 6% @ 0.3 μm of CdTe. 

• We have developed an electrochemical polymerization of a polyaniline-based solution 

that yields promising results for shunt passivation of thin-film CdTe cells. The 

characteristics of the polymerized film were determined by XPS and by Electron Spray 

Ionization Mass Spectroscopy. 

• We have developed a theory of admittance spectroscopy that applies to devices with 

resistive electrodes as is often the case for thin-film PV. (Electrodes need not be 

equipotentials as is usually assumed.) The model uses the previously introduced concept 

of a decay length, L0, for electric perturbations in the lateral direction and also includes 

two new decay lengths that describe the spatial dependence for ac perturbations of the 

distributed shunt resistance, LR, and capacitance, LC. 
4

2. Ex Situ Spectroscopic Ellipsometry

This section describes the techniques of ex situ spectrocopic ellipsometry (SE) that have 

been developed and applied to materials and solar cell devices over the subcontract period. 

Two measurements methodologies have been developed to study solar cells: reflection from the 

back, i.e., from the free CdTe surface before back contact formation, and reflection from the 

front, i.e., through the glass, which is possible at any stage of the processing. 

The challenge of back-side measurement is the significant roughness that exists on the free 

CdTe surface, which can be up to 10% or more of the total thickness. As a result, thick CdTe 

films (> 1 μm) may show only a weak specularly reflected beam with dominant non-specular 

scattering component. To overcome this problem and to exploit the full power of SE as a 

sensitive probe of specularly reflecting interfaces, a step-by-step Br2-methanol etching method 

has been developed for both smoothening and optical depth profiling of the optical structure of 

CdTe materials and solar cells. In this procedure, a single step consists of immersing the CdTe 

thin film or device briefly (5 s) in a Br2-methanol solution, cleaning the etched surface in pure 

methanol, drying it by N2 gas, mounting the sample on the ellipsometer stage, and finally 

measuring it by SE at a single angle of incidence. The use of a multichannel ellipsometer with 

a measurement time of a few seconds enables multiple steps to be performed in a matter of 

minutes. 

In this section, the validity of the step-by-step etching method has been demonstrated 

through its application to a CdTe film on a crystalline Si wafer whose void profile was 

previously determined in a real time SE measurement. The void profiles observed during 

growth and during the step-by-step etch back process were found to be in good agreement. 

This result indicates that step-by-step etching can be considered as time-reversed real time SE, 

whose capability is limited only by the patience of the user in performing the number of etch 

steps required for suitable depth resolution. The layer thickness removed in each step can be 

controlled by the concentration of Br2 in methanol The step-by-step etching process has been 

applied to CdTe thin films on Si wafers before and after different treatments in order to compare 

the structure of the films. CdTe films subjected to annealing in Ar were compared with those 

subjected to CdCl2 treatment using comparable temperature profiles in both cases. The process 

has also been applied to CdCl2 treated solar cells before back contact formation. The extracted 

information on the CdTe includes void profiles from the dielectric function amplitudes, strain 

profiles from the critical point energies, and grain size profiles from the critical point widths. 

Information on the underlying structure of solar cells has also been obtained with confidence, for 

example, the thickness and void fraction of the CdS and the thickness and nature of the 

CdS/CdTe interface -- which are expected to be unchanging as the step-by-step etching proceeds.

The front-side approach is desirable as it can be applied to CdTe modules in any stage of the 

production process and can be implemented on-line, whereas the disadvantage of the back-side 

approach is that it is destructive, much more time consuming, and thus not applicable for on-line 

analysis. The challenge of the front side approach is to minimize controllably or to eliminate 

the top surface reflection from the glass. In these studies, two methods have been developed for 

this purpose. In the first method, a 60° prism is contacted to the top glass surface with index 

matching fluid, and in the second method, the beam from the top surface is spatially isolated and 

blocked. The first method was exploited most extensively in this research effort whereas the 

second method was developed in the last phase of the subcontract and is only now being more 

fully developed. Measurements of the solar cell from the glass-side using a prism coupler, and 
5

from the CdTe side using the step-by-step etch process have been yielded information in good 

agreement.

 Details of these studies are provided in the highlights of the following sub-sections. A 

significant component of the research on ex situ SE involves developing a database of dielectric 

functions. Three different sources can be used, depending on the situation. (i) Dielectric 

functions can be drawn from the literature; (ii) dielectric functions can be deduced from data 

collected in our laboratory based on in situ real time measurements during film growth; and (iii) 

dielectric functions can be extracted from data collected in our laboratory based on ex situ 

measurements. In this section, the dielectric functions of the components of TEC-15 glass are 

reported as an example of source (iii) above; the section following this one provides examples of 

the determination of CdS and CdTe dielectric functions from source (ii). 

2.1 Etch Back Studies of CdTe Materials and Solar Cells

2.1.1 Verification of the etching procedure

Figure 2.1 The void volume fraction as a function of CdTe bulk thickness obtained from 

RTSE data collected during a low temperature deposition process (130°C) on native oxide 

covered crystalline silicon substrates and during a post-deposition etching process of the 

same sample. The rf power to the CdTe target was 50 Watt and the Ar pressure was 

18 mTorr. 

• The discrete points in Figure 2.1 were collected after each Br2+methanol etching step, 

and match deposition results (solid line) very well (within the confidence limits in the 

analysis). The RTSE results were obtained for a film deposited at low temperature that 

showed a clear structural transition to high void fraction at a bulk thickness of 1500 Å, 

believed to be induced by stress. This served as an excellent test sample since the void 

fraction and structure spanned a wide range. 

• This result demonstrates that the etching process is the reverse of the deposition process, 
6

and that etching does not modify the underlying film structure. Thus, a capability of 

etch-back in conjunction with SE measurements has been demonstrated for 

post-deposition depth-profiling. 

• In the above analysis, the restriction to the high photon energy spectral range (5.0-6.5 

eV) implies a higher sensitivity to the surface region of the film. When the surface 

roughness thickness becomes a significant fraction of the penetration depth, the optical 

model may be interpreting the minima in the surface profile as bulk material and this 

may account in part for the high void volume fractions.

2.1.2 Etch back studies of CdTe materials

Figure 2.2 Critical point energy E1 (upper panel) and width ΓE1 (lower panel) as functions of 

CdTe bulk layer thickness during etching by Br2+methanol for CdTe films deposited on crystal Si 

substrates all deposited under the same conditions (temperature: 188°C, rf power: 60 Watt, and 

Ar pressure: 18 mTorr) and then processed in three different ways: (i) as-deposited, (ii) annealed 

at 387°C in Ar for 30 min, and (iii) CdCl2 treated at 387°C in dry air for 5 min (not including ramp 

time). The deviations at low thickness are due to the onset of semi-transparency for the E1

critical point energy. 

• For the as-deposited film, the red-shift of E1 relative to the single crystal value suggests 

significant strain in the film over the studied depth range of 1500-2000 Å; The maximum 

E1 energy shift of −0.12 eV at a depth of 1500 Å converts to a stress level of 0.6 GPa 

which is also consistent with the energy shifts in the E0 band gap. The lower panel shows 

that the as-deposited film has a very large broadening parameter ΓE1 ~ 0.6 ± 0.15 eV, 

indicative of a very small grain size (~ 10 nm).

• Upon Ar annealing of the film, the strain nearest the substrate is significantly reduced as 

the grain size increases (reduced ΓE1). Even after 30 min of annealing in Ar, however, 

there is no significant reduction in the grain size within 500 Å of the surface, and the 

strain in this region increases somewhat relative to the as-deposited film (as indicated by 
7

the lower E1 energy).

• A 5 min CdCl2 treatment leads to an E1 energy within 10 meV of the single crystal value 

throughout the thickness, suggesting a fully strain-relaxed film. In addition, ΓE1 has been 

reduced significantly to a constant value of ΓE1 ~ 0.30 ± 0.02 eV throughout the bulk of 

the film, indicating a significant increase in grain size.

2.1.3 Etch back studies of CdTe solar cells (CdCl2 treated TEC-15/CdS/CdTe, 

Sample # asc34)

Figure 2.3 Energy of the E1 transition as a function of CdTe bulk layer thickness in successive 

Br2-methanol etching steps for a sputtered CdTe solar cell. Also shown is the constant energy 

of the E1 transition of thin film CdTe (3300 Å thick) fabricated on a c-Si wafer substrate after 5 

minutes CdCl2 treatment (sample (iii) of fig 2.2). 

The E1 energy of the CdTe layer remains close to the single crystal value [1] from 1000 Å to 

1.8 μm, indicating stress relaxation throughout the thickness of the solar cell. 

Figure 2.4 Broadening parameter ΓE1 as a function of CdTe bulk layer thickness in successive 

Br2-methanol etching steps for a sputtered CdTe solar cell. Also shown is the constant width of 

the E1 transition of thin film CdTe (3300 Å thick) fabricated on a c-Si wafer substrate. 

• Because the value ΓE1 ~ 0.26 ± 0.01 eV at the film surface side is less than that of single 

crystal CdTe, it suggests more careful measurements of the single crystal are needed. It is 

likely that surface scattering in the epitaxially grown single crystal [1] is the origin of the 

larger ΓE1 value.
8

• The gradual increase in ΓE1 as the interface to the CdS is approached indicates that the 

electron mean free path is decreasing toward this interface There may be two causes of 

this behavior: 

(i) a reduction in grain size toward the CdS interface; or (ii) S diffusion into the CdTe.

2.2 Ex situ SE Measurements of CdTe Solar Cells 

2.2.1 TEC-15 optical properties

Figure 2.5 Optical properties expressed in terms of the dielectric function for three of the four 

components of TEC-15 glass including (a) the soda lime glass substrate, (b) the near-substrate 

undoped SnO2, and (c) the SiO2 interlayer.

• Dielectric functions of the components of TEC-15 glass have been determined including 

(a) the soda lime glass substrate, (b) the near-substrate undoped SnO2, and (c) the SiO2

interlayer. These can be used in modeling the structure of the CdTe solar cell on the 

TEC-15 glass substrate.
9

Figure 2.6 Dielectric functions of SnO2:F, the top-most thickest layer of the TEC-15 glass. For 

photon energies above 5 eV, the SnO2:F is opaque, and the optical properties are extracted from 

(Ψ, Δ) via inversion, correcting for surface roughness.

The dielectric function of SnO2:F, the top-most thickest layer of the TEC-15 substrate, has 

been determined for use in modeling the structure of CdTe solar cells on TEC-15 glass 

substrates.

2.2.2. Optical modeling of etch-smoothened CdTe solar cells [2] (CdCl2 treated 

TEC-15/CdS/CdTe, Sample # asc34).

CdTe/void = 0.5/0.5 35 ± 0.4 Å CdTe surface 

roughness

CdTe bulk

CdTe/CdS

interface

CdS bulk

CdS/SnO2:F

interface

SnO2:F

SiO2

SnO2

Soda lime glass

CdTe/void=0.96±0.002/0.04±0.002 14529± 32 Å

CdTe/CdS = 0.5±0.02/0.49±0.02 935± 10 Å

CdS/void = 0.93±0.001/0.07±0.001 1247± 25 Å

CdS/SnO2:F = 0.5/0.5 296 Å

SnO2:F = 1.00 3178 Å

SiO2 = 1.00 215 Å

SnO2 = 1.00 267 Å

glass = 1.00 semi-inf.

TEC15

structure

Figure 2.7 Structural model for the CdTe solar cell after the 15th Br2+methanol etch step used 

for surface smoothening. This model provides the best fit to ex situ SE data collected from the 

free CdTe surface.

• A multilayer stack has been established that successfully simulates SE data for 

determination of the structure and composition of the individual layers of the etched CdTe 

solar cell on TEC-15 glass substrates. 

• It incorporates (i) an optical structure for TEC-15 consisting of SnO2 (267 Å); SiO2 (215

Å); and SnO2:F (3178 Å); (ii) an interfacial roughness layer between the TEC-15 and the 

CdS whose thickness is chosen to match the surface roughness thickness measured from 
10

the uncoated TEC-15 and whose composition is a fixed 0.5/0.5 effective medium mixture 

of the overlying and underlying materials. 

• A single effective medium layer of CdS+CdTe of variable composition satisfactorily

represents the complex layer at the CdS/CdTe interface; (see discussion of Fig. 2.10). The 

CdS/SnO2:F interface layer in this model is assumed to be the result of the surface 

roughness on TEC-15. 

• A wide photon energy spectral range (1.0 ~ 6.0 eV) is used in this analysis in order to 

determine the complete film structure including the TCO coatings on TEC glass. Thus, the 

0.04 void fraction in the CdTe layer is an optical average throughout the layer thickness.

2.2.3. Through-the-glass measurements of the CdTe solar cell

Figure 2.8 Ex situ SE spectra in (Ψ, Δ) (symbols) (a) from the free CdTe surface after 8 

Br2+methanol etching steps and (b) from the prism/glass side without etching. The best fit 

results are given as the solid lines. In (b), a prism was affixed to the TEC-15 substrate with 

index-matching fluid to eliminate the top surface reflection from the top side (i.e., sun side) of 

the glass.

• In both studies, the best fit results (solid lines) yield the solar cell structural parameters, 

including the thicknesses of the CdTe roughness, CdTe bulk, CdTe/CdS interface, and CdS 

bulk layers, as well as the volume fractions of CdS/CdTe in the interface layer and void in 

the CdS layer. 
11

Figure 2.9 A comparison of the structural parameters deduced from spectra collected from the 

CdTe surface after eight etching steps (left) and from spectra collected through the prism/glass 

(right). 

• Excellent agreement in the critical CdS and CdTe/CdS interface layer thicknesses and 

compositional parameters are obtained. Agreement in the CdTe bulk and roughness layer 

thicknesses is not expected, however, since the CdTe film has been etched for the back 

side measurements (left).

Figure 2.10 CdS and CdTe/CdS 

interface layer thicknesses 

deduced from spectra collected 

through the prism/glass (solid 

line) and from spectra collected 

from the CdTe surface in 

successive etches (points, 

dotted line extreme).

• The CdS layer and CdTe/CdS interface thicknesses have been deduced from spectra 

collected at the CdTe free surface in 24 successive etches. 

• These independent values for CdS/CdTe interface layer thickness and CdS bulk layer 

thickness should be constant since the etching does not affect the sub-surface material, and 

any variations provide a measure of the uncertainty in these values.

• Maximum deviations of ±2-3% from the average values are obtained and the average 

values lie within the confidence limits of the analyses performed on spectra collected 

through the prism/glass

• The (CdS+CdTe) interface layer is modeled as a vol. fraction mixture of CdS/CdTe; 

however, this mixture merely provides a dielectric function that approximates that of the 

interface region and should not be interpreted physically. In fact, this layer requires a 

more complex model that includes interdiffusion using CdTe1-xSx and CdS1-xTex alloy 
12

dielectric functions. Further study of the dielectric function of an alloy series is reported 

in Sec. 4.

Figure 2.11 Ellipsometric spectra (symbols) as measured through the glass without a prism for a 

CdTe solar cell obtained after CdCl2 treatment and before back contact formation. Also shown is 

a best fit (solid lines).

• An new through-the-glass optical approach has been developed for on-line measurement of 

CdTe solar modules. This approach consists of blocking the top surface reflection from the 

glass and only measuring the beam reflected from the TEC-15/solar cell interface. 

• These more expanded results provide information on the starting TEC-15, namely, the 

thickness of the undoped SnO2 at the glass interface, the thickness of the SiO2 layer, and the 

thickness of the doped conducting SnO2:F layer. In terms of the solar cell properties, the 

analysis provides information on the interface roughness between the CdS and the SnO2:F, as 

well as the thickness of the CdS, the combined roughness-interaction thickness between the 

CdS and CdTe, and the thickness of the CdTe.

2.3 Future Directions

We are currently seeking improvements in the above approaches while simultaneously 

applying them to assist research and development efforts at University of Toledo and production 

efforts in collaboration with Toledo area glass and CdTe module manufacturers. This 

represents a continuing research program and the following directions are currently being 

pursued. 
13

• Improvements in the etch-back process are being sought using improved etching 

chemistries that minimize the thickness of the residual amorphous Te layer on the 

surface. Single-crystal CdTe provides a convenient test surface for evaluating the a-Te 

layer thickness in etch-back procedures. For the current etch-back analysis and any 

future ones to be developed, it is important to incorporate the a-Te layer in the data 

analysis. Although the layer is very thin, accurate modeling of the layer may improve 

the ability to extract other details of the overall sample structure.

• Improvements in the dielectric function database used in ex situ measurements are 

continually being sought. This includes an improved set of dielectric functions of 

TEC-15 components achieved through recent advances in modeling, as well as 

additional parameterization of CdS and CdTe dielectric functions versus temperature, 

strain, grain size, alloying, and defects. New and ongoing studies of ion-implanted 

CdTe as well as CdTe and CdS with low levels of S and Te alloying, respectively, are 

motivated by the need to develop new databases. 

• Module scanners having the capability of off-line and on-line mapping are being 

developed for use by local manufacturers with the assistance of a U.S. optical 

instrumentation company. The ex situ measurement technology developed in this 

subcontract is to be exploited in this application.
14

3. Real Time Spectroscopic Ellipsometry: Dielectric Functions and Structural 

Evolution

In this section, the application of real time SE to understand better the growth mechanisms 

of sputtered CdTe and CdS films is described. The focus is on the use of smooth, native-oxide 

covered crystal Si wafer substrates. The thin SiO2 layer simulates a disordered glass surface 

whereas the very smooth, abrupt interface between the native oxide and underlying silicon makes 

it possible to explore the monolayer level interactions during the nucleation and coalescence of 

the overlying film. Once coalescence occurs, then it is expected that the film growth on Si 

wafers more closely reflects the deposition of actual solar cell layers on TCO-coated glass, albeit 

with a much smoother surface. The studies during the subcontract period focused on technique 

development, and an in-depth analysis of three series of depositions, a temperature series for 

CdTe, a temperature series for CdS, and a pressure series of CdTe.

 In terms of technique development, a number of capabilities have been established for 

in-depth analysis of growth: 

• separation of the film into bulk and surface roughness layer components whose 

thicknesses can be tracked versus time;

• determination of deposition rates including the instantaneous bulk layer rate and the total 

effective rate, the latter including the contribution from both roughness and bulk layer 

components; and 

• determination of step-wise void profiles to simulate the continuous variation in film 

density during growth. 

 Using these capabilities various interesting features have been observed as a function of 

deposition temperature and pressure. For example, the initial stages of growth of both CdTe 

and CdS show features consistent with a transition from a Vollmer-Weber type growth 

mechanism at low temperatures, with dominant initial clustering, to Stranski-Krastinov growth at 

high temperatures, with dominant initial layer-by-layer growth -- followed by clustering. 

 After nucleation and coalescence for CdTe, the resulting void fraction depends sensitively 

on substrate temperature and Ar pressure with the higher temperature and pressure films showing 

a higher void volume fraction. Thus, the densest CdTe films are obtained at the lowest 

temperatures and pressures. Films prepared at 180°C are extremely dense but undergo a 

microstructural transition to high void density after a thickness of ~1500 Å, apparently induced 

by significant compressive strain in the dense films. This effect in conjunction with the general 

increase in void volume fraction with increasing substrate temperature leads to a minimum in the 

void volume fraction for 3000 Å thick CdTe films deposited at the intermediate temperature of ~ 

230°C. The results as a function of pressure are surprising in that at the lowest attainable 

deposition pressure, the highest densities and smoothest films are obtained throughout the 

deposition process. In fact 1 μm thick films can be prepared at 2.5 mTorr Ar pressure with void 

fractions no greater than 0.03 and with surface roughness thicknesses no greater than 20 Å 

throughout the entire deposition process. For CdS, the observed variations in the void fraction 

with temperature are much weaker than those for CdTe. 

 In addition to providing structural information, the real time SE measurements can be applied 

to extract the dielectric functions of the depositing layers. The dielectric functions are most 

accurately obtained in the thin layer regime (500 - 1000 Å) before the development of extensive 

surface roughness and void structures that generate non-uniformity with depth. Typically the 

depositions are suspended at a thickness in this regime for stepwise cool-down, enabling the 
15

dielectric function to be obtained versus temperature, starting from the deposition temperature 

and ending down at room temperature. Then the room temperature dielectric functions of films 

prepared under different deposition conditions can be compared based on SE measurements 

performed under fixed environmental conditions (at room temperature and under vacuum). 

This also enables greater averaging times for higher precision spectra. 

 Dielectric functions obtained under these conditions tend to be more accurate than those by 

any other method because: (i) they are obtained in situ which avoids native oxides; (ii) they 

employ the substrate optical properties and characteristics (e.g., native oxide thickness) in the 

optical model that were obtained from measurements made prior to film growth on the substrate, 

and (iii) they employ surface roughness thicknesses in the optical model that were obtained in 

situ during film growth. As a result, these dielectric functions are superior for use as reference 

data in ex situ analysis of solar cell structures as described in the previous section. Furthermore, 

such dielectric functions can then be analyzed carefully to develop databases that enable one to 

extract information on grain size, defect density, strain, and temperature. The results of this 

latter effort are presented in Section 4.

3.1 Overview of CdTe and CdS Dielectric Functions

Figure 3.1 15°C complex dielectric functions ε of typical polycrystalline CdTe (upper) and CdS 

(lower) films magnetron sputtered using the indicated deposition temperatures (Td). Additional 

parameters are as follows -- CdTe: rf power of 60 W and Ar pressure of 18 mTorr; CdS: rf power of 

50 W and Ar pressure of 10 mTorr. Also plotted in the upper panel is the room temperature ε of 

single crystal CdTe [1].

• The film dielectric functions are determined from in situ spectroscopic ellipsometry (SE) data

taken after cooling the film down to 15°C from Td and from film structural parameters 
16

(thickness, roughness, ...etc.) deduced from real time SE (RTSE) data taken during 

deposition. The critical points (CPs) are marked.

• The differences in CP features (resonance energy, broadening,...etc.) are associated with 

important material properties (temperature, stress, grain size, ...etc.) that depend on 

preparation conditions as explained in Sec. 4.

3.2. CdTe and CdS Structural Evolution on Si Substrates as Measured by RTSE

3.2.1. Role of substrate temperature for CdTe and CdS

Figure 3.2 Surface roughness 

thickness (ds) evolution with bulk 

film thickness (db) for CdTe films 

magnetron sputtered on c-Si wafer 

substrates held at different 

temperatures (Td) but with otherwise 

identical deposition parameters (rf 

power of 60 W and Ar pressure of 18 

mTorr). 

• The nucleation stage (left) shows a monotonic change from the Volmer-Weber (V-W) 

growth mode, or island growth mode, at lower Td to the Stranski-Krastanov (S-K) growth 

mode, or layer-island growth mode, at higher Td. 

• The longer term ds evolution (right, when db > 2000 Å) shows monotonic trend of surface 

smoothening at higher Td. 

• Both trends are consistent with enhanced surface diffusion at higher Td. 

Figure 3.3 ds vs. db evolution 

for CdS films sputtered on c-Si 

wafer substrates held at different 

Td but with identical deposition 

parameters of fig 3.2. 

• A monotonic change is observed from V-W growth mode with small nuclei height at lower 

Td to S-K growth mode with large nuclei height at higher Td. The longer term ds evolution 

(right) resembles that observed for CdTe over a similar range of db.
17

Figure 3.4 Stepwise evolution of void volume fraction (fv) for the CdTe films of Fig. 3.2 

magnetron sputtered on c-Si wafer substrates held at different Td. 

• For all Td, fv increases monotonically with increasing bulk layer thickness. 

• In the thin film regime (db < 500 Å), fv increases monotonically with increasing Td. 

• The lowest Td film having the lowest fv value in the initial stages, however, exhibits an 

abrupt structural transition at db ~ 1500 Å where fv increases by more than eight times 

within a narrow db range. This transition is attributed to strain relaxation and the effect is 

suppressed with increasing Td, as the initial void fraction increases. 

• fv in the top-most layer of the final film is shown as a function of Td in the inset. The 

highest density in the near-surface region of the final film (~ 3300 Å) is obtained near Td ~ 

230°C. 

• It is reasonable that both surface roughness and voids provide means for strain relaxation 

in the films. 
18

Figure 3.5 fv evolutions 

for the CdS films of Fig. 

3.3 sputtered on c-Si 

wafer substrates held at 

different Td. 

• Void development for the deposited CdS films on crystalline Si is relatively weak 

compared to CdTe in the corresponding thickness range of interest for CdS (db < 1400 Å).

• Stepwise void analysis of the CdS film grown at Td=320°C gives a final fv of 0.01 (using 

RTSE data in full spectral range 0.75~6.5 eV, similar to that of Fig. 3.4). Other samples 

have similarly small results. No clear trend vs. Td is observed.

• Continuous void analyses using RTSE data in the spectral range 4.5~6.5 eV, which is more 

sensitive to the surface of the sample, lead to a final near surface value fv = 0.02±0.01 for 

all the CdS films, again comparatively small.

Figure 3.6 Effective rate of material 

growth (excluding voids) as a function of 

Td for the CdTe and CdS films of Fig. 3.2 

and Fig. 3.3.

• The counterintuitive increase of rate with increasing Td for CdTe films may be attributable to 

a reduction in Ar atom density with the increase in gas temperature at constant pressure and 

volume. 

• The general trend of decrease of rate with increasing Td for CdS films is consistent with a 

reduction in the sticking probability for the species at higher Td.
19

3.2.2. Role of Ar pressure for CdTe 

Figure 3.7 Surface 

roughness evolution for 

magnetron sputtered CdTe 

with different Ar pressures 

and otherwise identical 

parameters, including a 

substrate temperature of 

230°C and an rf power of 60 

W.

• The surface roughness evolution strongly depends on the Ar pressure. Extremely low Ar 

pressure leads to a smooth surface throughout film growth; increasing Ar pressure leads to 

much faster roughness development, probably due to reduced surface diffusion which can 

smoothen the surface; but this effect reaches a maximum at ~18 mTorr; further increasing Ar 

pressure causes a reduction in surface roughness.

Figure 3.8 Stepwise void 

evolution for the CdTe films 

of Fig. 3.7.

• A trend similar to that of surface roughness evolution is observed. Extremely low Ar 

pressure leads to a dense and uniform film through the deposition; increasing Ar pressure 
20

leads to more rapid void development; but this effect reaches a maximum at ~25 mTorr; 

further increasing Ar pressure causes less voids in the film.

Figure 3.9 Effective CdTe growth rate for the films of Fig. 3.7.

• A clear decrease of rate with increasing Ar pressure is observed, due to stronger scattering 

of species with higher Ar atom density in the gas phase.

3.3 Future Directions

 The future directions for continuing real time SE studies of CdS and CdTe layers are quite 

clear. In future research, the analysis of the structural evolution in terms of bulk layer thickness, 

surface roughness layer thickness, and void volume fraction must be streamlined for depositions 

on crystalline Si wafer substrates. Then a wider range of deposition conditions can be explored 

expeditiously, including the roles of pressure for CdS growth, and sputtering power and substrate 

bias for CdS and CdTe growth. In addition, once streamlining has been achieved for more rapid 

turn-around in data analysis, then the ability to perform multistep processing becomes possible in 

order to control separately the interface, bulk, and surface structural properties, based on the 

understanding provided by real time SE. 

 In additional future studies, the focus will be on substrates that are relevant for solar cell 

fabrication. As a first step, 1300 Å thick CdS films will be deposited on Si wafers, as this leads 

to roughness layers no thicker than about 15 Å on the surfaces of the final films. Then CdTe 

films can be deposited in situ on the ultra-smooth CdS surfaces under different conditions. In 

this case -- in addition to the nucleation, growth, and structural characteristics of the CdTe film --

the alloying interaction of the depositing CdTe with the underlying CdS can be quantified. 

Initial studies in this area have been undertaken in the final phase of the subcontract, as will be 

highlighted in the next section; however, this work must be expanded to include variations in 

substrate temperature, Ar pressure, etc., for the overlying CdTe deposition. 
21

 The final step in such studies is to develop an understanding of how CdS grows on the 

transparent conducting oxide (TCO) surfaces used in the actual solar cells. The challenge in 

this case is to understand the chemical interactions that occur between the CdS and the TCO 

even in the presence of a rather thick roughness layer on the starting TCO surface. Because of 

such thick roughness layers, studies of the TCO/CdS interface are now proceeding in an ex situ 

mode since it is not clear how much additional useful information can be extracted in real time 

from such a difficult optical problem. As a result, initial attempts in studying TCO/CdS 

interface formation by real time SE may focus on TCO layers specially deposited for their 

smoothness, in order to better understand the chemical interactions, before proceeding to the 

more challenging case where roughness and chemical interactions must be handled 

simultaneously. 
22

4. In-Depth Analysis of the Dielectric Functions of CdTe and CdS

The previous section described how accurate dielectric functions have been extracted from 

in situ measurements performed after suspending the CdS or CdTe deposition at a thickness 

within the range of 500 - 1000 Å and cooling the resulting thin film to room temperature. 

These deduced dielectric functions have been analyzed assuming the standard parabolic band 

model in the neighborhood of the band structure critical points. An entire series of spectra 

collected at different measurement temperatures during cooling provide the critical point 

energies as a function of temperature. These results can then serve as a database for accurate 

substrate temperature calibration based on the critical point energy shifts that occur upon cooling 

the CdS or CdTe thin film sample from an unknown temperature to a known temperature. 

The absolute critical point energies cannot be used for temperature calibration since these 

can shift significantly from sample to sample due to strain in the thin films. As a result, the 

critical point energies at a fixed temperature are found to provide information on strain. For 

CdTe deposited at different substrate temperatures, the strain shifts in the critical points energies 

obtained from room temperature dielectric functions are the largest for films deposited at low 

temperatures. The magnitude of this shift appears to correlate with the appearance of an abrupt 

structural transition in the subsequent evolution of the void volume fraction, which indicates that 

void development serves to relax strain in the network. 

The critical point widths in the dielectric functions of CdS and CdTe are consistent with a 

model in which the lifetimes of the excitations are limited by scattering at defects, attributed to 

grain boundaries. We have associated a group velocity with each critical point of CdS and 

CdTe that relates the carrier mean free path and mean free time between grain boundary 

scattering events. The latter is inversely proportional to the broadening parameter of the critical 

point. With such a model, an optically-deduced grain size can be obtained, and particularly for 

CdS, the grain size is observed to increase systematically with substrate temperature.

These approaches that relate the dielectric function critical point energies and widths to 

strain and grain size, respectively, enable one to establish a database for the analysis of ex situ 

SE data. This database has been applied to CdCl2 treated CdTe thin films in order to extract the 

strain and grain size as a function of depth in step-by-step etching procedures. In this way, it 

has been demonstrated that the CdCl2 treatment significantly relaxes the strain in the network in 

addition to increasing the grain size since the critical point energies and widths approach those of 

the single crystal upon treatment. 

 The dielectric functions of alloys between CdTe and CdS have been determined using real 

time SE of thin films deposited by co-sputtering at low temperatures to avoid phase separation. 

This provides an additional database which has been demonstrated for depth profiling the 

composition when CdTe is deposited on CdS and vice versa. 
23

4.1 Analysis Methodology

Figure 4.1 The 2nd derivative spectra of typical 15°C dielectric functions ε of polycrystalline 

thin films: (upper) the Td =237°C CdTe film of Fig. 3.2; and (lower) the Td =160°C CdS film of Fig. 

3.3.

• The model fits are based on the critical point parabolic band approximation: 

 ε = Σ An (En − E − iΓn)

μn

 exp(iφn) (1)

where An, En, Γn, μn, and φn are the amplitude, band gap (resonance energy), broadening 

parameter (width), exponent, and phase of the nth CP, respectively. These parameters 

are used to deduce physical information on the sample.
24

4.2 Critical Point Broadening Parameters and Grain Structure 

Figure 4.2 The 

broadening 

parameters at 15°C for 

the E0, E1-A, and E1-B 

CPs for the CdS films 

of Fig. 3.3; the lowest 

values are circled.

• The CP features of the Td =310°C film with the smallest Γn (circled) are even sharper than 

those of single crystal wurtzite CdS, as measured in this study and by S. Ninomiya et al. 

This is likely due to near-surface polishing damage on the single crystal. Thus, Γn for this 

sample can be taken as intrinsic to single crystal wurtzite CdS (denoted Γbn, b: "bulk"), and 

can be related to its band structure.

Figure 4.3 Grain size for the CdS films of Fig. 3.3, estimated from the near E0 band structure of 

wurtzite CdS, using the ΓE0 values in Fig. 4.2.
25

• According to the parabolic band approximation, the electron group velocity near E0 is vg,E0

= { Γb,E0mh*/[me*(me* + mh*)] }1/2, where me* and mh* are electron and hole effective 

masses respectively. From the literature and from this work, vg,E0 of CdS was estimated 

to be ~ 2.2x105 m/s.

• If the dominant broadening effect in the films is taken to be the limited excitation lifetime 

due to grain boundary scattering, then the equation below can be applied: 

 Γn = Γbn + (hvgn /R) (1) 

where vgn is the group speed associated with the nth CP, h is the Plank’s constant, and R is 

the deduced grain size (radius). 

• Using the values of vg,E0 and Γb,E0 for single crystal wurtzite CdS, R of polycrystalline CdS 

films in Fig. 19 can be calculated versus Td. The effect of Td on R is strong: doubling T 

from ~150°C to ~300°C increases the grain size in as-deposited CdS films by ~30 times. 

Figure 4.4 The CdS E1-A and E1-B broadening parameters plotted vs. the reciprocal of the 

deduced grain radius Fig. 4.3. The group speeds of excited electrons associated with the E1-A 

and E1-B transitions are calculated from the linear fits and Equation (2).

• The linear relation between Γn and R−1 in Equation (2) should be applicable to all CPs, not 

only E0, but with different slopes determined by vgn and different intercepts determined by 

Γbn. As a check of the correctness of the concept, the broadening parameters ΓE1-A and 

ΓE1-B of the CdS samples are plotted versus R−1 (deduced from ΓE0) in Fig. 5.4. Equation 

(2) is closely followed. The group speeds associated with the E1-A and E1-B excitations are 

estimated from the slopes of the linear fits to be ~3.3x105 m/s and ~8.8x105 m/s, 

respectively.
26

Figure 4.5 The 2nd derivative spectra (symbols) of room temperature ε for c-CdTe, and 15°C ε

for the CdTe films with Td =188°C and Td =267°C. The model fits (solid lines) are based on a 

parabolic band critical point model. 

• The blue shift of E0 resonance energy for CdTe films relative to that of c-CdTe is obvious, 

being as large as ~55 meV. In general, the lower the deposition temperature, the larger 

the blue shift. This shift is considered to be caused by stress in the deposited films (see Fig. 

4.6).

4.3 Critical Point Energies and Strain
27

Figure 4.6 15°C E0 CP 

energies for CdTe films in 

Fig. 3.2. The stress (right 

scale) is calculated from 

the indicated equation. The 

horizontal dash represents 

the room temperature E0

for c-CdTe which is 

assumed to be stress free.

• Previous measurements that relate hydrostatic pressure to the fundamental band gap in 

single crystal CdTe by Dunstan et al. [3] have established a pressure coefficient CP(E0) of 

0.065 eV/GPa. Although the stress in the CdTe films is uniaxial rather than hydrostatic, the 

estimate may be a reasonable one because the optical electric field is strongly refracted into 

the material and thus probes the high stress direction. 

• For Td above 220°C, these db ~1000 Å CdTe films pass through a relaxation whereby the 

stress decreases by a factor of two over a ~50°C range. The very high stress levels in the 

two lowest Td films are consistent with the observed low fv in this thickness range. For 

db >1000 Å, these low temperature films undergo abrupt structural transitions in which fv

increases significantly (see Fig. 3.4). For the two higher Td films, fv for db ~1000 Å is 

higher and no structural transition is observed. Thus, a high initial stress is correlated with 

the appearance of a subsequent microstructural transition in the evolving film.

4.4 Summary: Effects of Temperature, Grain Size, and Stress on the Critical 

Points of CdTe and CdS

Table 4.1 Single crystal CP Energies and Widths, Temperature Coefficients of CP Energies and 

Widths, Electron Group Speeds, and Stress Coefficients of CP Energies for Major CPs for CdTe 

and CdS

CdS Eb(T0) 

(eV)

Γb(T0) 

(eV)

CTΓ

(eV/°C)

CTE

(eV/°C)

vg

(m/s)

CP

(eV/GPa)

E0 2.38 0.071 2.0x10-4 -4.1x10-4 2.2x105 0.045

E1-A 4.82 0.21 4.8x10-4 -5.3x10-4 3.3x105 0.089

E1-B 5.50 0.64 16x10-4 -8.6x10-4 8.8x105 0.028

CdTe Eb(T0) 

(eV)

Γb(T0) 

(eV)

CTΓ

(eV/°C)

CTE

(eV/°C)

vg

(m/s)

CP

(eV/GPa)

E0 1.49 0.041 1.2x10-4 -3.7x10-4 2.3x105 0.065

E1 3.31 0.30 2.2x10-4 -7.3x10-4 8.1x105 -0.20

E1+∆1 3.89 0.29 5.1x10-4 -2.5x10-4 6.9x105 0.23

E2 5.16 0.92 18x10-4 -2.7x10-4 4.2x105 0.16
28

• In the experimental range, parameterization can be applied to relate CP features to key film 

properties, including temperature, stress, and grain size:

 Γ = Γb(T0) + CTΓ * (Tm – T0) + (hvg/R) (3a) 

 E = Eb(T0) + CTE * (Tm – T0) + CPP (3b) 

where Γb and Eb are the width and CP energy of the single crystal; T0 is a reference 

measurement temperature, normally room temperature; CTΓ and CTE are the temperature 

coefficients of the CP width and CP energy, respectively; h, vg and R have the same 

interpretations as previously. This parameterization is potentially useful for spatially 

scanning and monitoring system for film properties on a solar cell production line. 

• For a given layer, all the CPs share the same measurement temperature Tm, grain size R and 

stress P; furthermore, for a given solar cell, Tm for CdTe and CdS are the same. In addition, 

for a series of cells of the same component materials (CdTe and CdS) but with different 

characteristics, the single crystal CP values, Γb and Eb, the CP coefficients, CTΓ and CTE, and 

the electron group speeds vg, are all constants. As a result, this parameterization reflects not 

only interesting characteristics, but also provides a useful “interlocking” mechanism that can 

significantly reduce correlation and instrument-related drift.

4.5 Ternary Alloys Between CdS and CdTe

4.5.1 Dielectric functions of ternary alloys

Figure 4.7 Dielectric functions of co-sputtered CdSxTe1-x alloy films at 190°C.

• A rate vs. rf power calibration was performed for both CdTe and CdS. Different expected 

x values (as indicated) were achieved by applying different rf powers to the CdTe and CdS 

target (as indicated) during co-sputtering. 

• Dielectric functions of co-sputtered CdSxTe1-x films vary consistently with expected x. 

This set of dielectric functions can serve as a database in the investigation of diffusion 

profile at the CdTe/CdS interface during junction formation.
29

4.5.2 Analysis of alloy formation at CdS/CdTe and CdTe/CdS interfaces 

Figure 4.8 Composition profiles 

x(db) in CdSxTe1−x for depositions at 

190°C for CdTe on CdS (upper) and 

CdS on CdTe (lower).

• A virtual interface approximation is applied to the RTSE data to find x for the topmost ~8 Å 

of deposited material during interface formation. 

• In both deposition sequences, there is a ~40 Å layer with a strong gradient in x, possibly 

generated by ion impact in the sputtering process. Beyond this thickness, S diffusion into 

CdTe exhibits a tail with x ~ 0.02; whereas for the inverted structure, Te diffusion into CdS 

exhibits a negligible tail.

4.6 Future Directions

 Although progress has been made in understanding the effects of temperature, strain, grain 

size, and Te and S alloying on the dielectric functions and critical points of CdS and CdTe, as 

exemplified in this section, many of the concepts and modeling approaches are oversimplified 

and need to be developed in greater depth. 

• The temperature dependence of the critical points of the dielectric functions of CdTe and 

CdS have been well-established, and the observed temperature coefficients are 

independent of the film characteristics of strain and grain size within experimental 

confidence limits; however, the effect of alloying on these coefficients has yet to be 

established. 

• In the case of strain, a simple model of hydrostatic pressure has been used to explain the 

shifts in the E0 fundamental gap of CdTe. Using these E0 shifts as a measure of the 

pressure, linear pressure coefficients can be derived that account for the observed shifts in 
30

the other critical points. However, such a model may not be expected to work in all 

circumstances. Because the strain is greatest in the plane of the film, the simple model 

will not be applicable when the film is preferentially oriented. Thus, future efforts will 

focus on the determination of the full spectroscopic strain-optic tensor using the 

recently-developed Mueller matrix ellipsometry method designed for anisotropic optical 

responses. Under ideal circumstances, information on crystallite orientation may be 

accessible by determination and consideration of the energy shifts of the full set of 

critical points. 

The model for grain size is similarly over-simplified, based on a model of spherical, 

randomly-oriented grains. To assess the robustness of the model, defects other than grain 

boundaries are being introduced into single crystal CdTe as scattering centers to determine if the 

same set of group velocities can explain the broadening that is observed. This effort will 

continue in future research. 
31

5. HREM, EXAFS and XES Studies of CdTe cells

5.1 Introduction

We have performed a series of studies to identify the location and bonding of copper in our 

sputtered CdTe cells. These studies used EXAFS measurements at the Argonne Advanced Light 

Source, high resolution electron microscopy at the University of Michigan, and photoelectron 

spectroscopy at the University of Nevada Las Vegas.

Through the collaboration with the Electron Microbeam Analysis Laboratory (EMAL) at the 

University of Michigan, a subnanometer Cu-rich layer has been observed at the interface 

between the CdTe and the Au contact in High Resolution Transmission Electron Microscopy 

images (cross sectional) from our complete cell structure. The diffusion of Cu into the Au layer 

is also confirmed by cross-sectional, fine-probe EDS spectra and fine-probe EDS mapping. We 

used a JEOL 3011 ultra-high resolution microscope (~0.17nm) and a JEOL 2010F analytical 

electron microscope that can be operated either in TEM or STEM mode (~0.17 nm). 

In order to study the local structural changes of copper in completed CdTe cells using 

EXAFS at the Advanced Photon Source at Argonne, we prepared sputter-deposited CdS and 

CdTe layers on fused silica substrates. With this structure similar to a complete solar cell, a 

built-in field at the p-n junction is established which is expected to affect the diffusion of copper 

atoms through CdTe into CdS, yet without x-ray fluorescence contamination from the soda-lime

glass normally used. The probable existence of Cu2O is confirmed by EXAFS collected at 

Materials Research Collaborative Access Team (MR-CAT) beamline at the Advanced Photon 

Source (Argonne, IL). A Cu/Au alloy structure is also found through the theoretical fitting and 

confirmed by our x-ray fluorescence study of peeled-off Au contact layer (Phase I Quarter 3 

report).

In this final report we focus on the analysis of the High Resolution Electron Microscopy 

studies and of an interfacial copper-oxide layer between the CdTe and Au layers in our standard 

sputtered CdS/CdTe devices. 

5.2 Sample Preparation

Our standard CdS/CdTe devices are fabricated by magnetron sputtering in a conventional 

structure on 3 mm TEC-7 glass. The 0.13 μm CdS and 2.2 μm CdTe films were deposited 

without vacuum break followed by a 30 minute CdCl2 vapor treatment with the temperature of 

sample at 397 °C and CdCl2 source at 392 °C. As the next step, 40Å Cu and 200Å Au layers 

were evaporated onto the chloride-treated films. The devices were then finished with a low 

temperature “diffusion” process at 152 °C in ambient air for 52 minutes. The average 

performance of these cells is: 

Eff = 11.9%, Voc = 800mV, Jsc = 20.7 mA/cm2 and FF = 71.6%. 

In order to study the bonding and migration of copper in a structure as close as possible to 

our standard cells, we sputter-deposited at 250 °C 0.13 μm CdS and 2.3μm CdTe layers on fused 

silica substrates without TCO coating, but otherwise the same as the deposition on regular TEC 

glass. A standard vapor CdCl2 treatment was carried out, followed with 35Å Cu and 200 Å Au 

depositions by evaporation. The process was finished with the same 45 minute thermal activation 

at 150 °C in ambient air. Cu K-edge EXAFS spectra and x-ray fluorescence (XRF) spectra were 

collected from these samples. Data were obtained at the MR-CAT beamline at the Advanced 

Photon Source with a 13-element high purity Ge detector as discussed in our publications.[4] 
32

The above partial cell samples were prepared for EXAFS and XRF at the APS synchrotron 

source. Conventional cells were chosen for cross-sectional high resolution electron microscopy 

(HREM) studies in EMAL, including HRTEM, Fine Probe X-ray Energy Dispersive 

Spectroscopy (EDS), EDS mapping and High-Angle Annular Dark-Field (HAADF). 

Since the typical scattering volume of 300 keV electrons in bulk CdTe is on the order of 

microns, in order to obtain high resolution EDS mapping, a thinning process to reduce the 

sample thickness down to electron transparency (typically 60-100nm) must be carried out. 

Sample preparation was done mostly at the University of Michigan. Details of the thinning 

process were discussed in our previous quarterly report (Phase III Quarter 2).

5.3 Voids in the CdTe and at the CdTe/CdS Interface

We collected both High-Angle Annular Dark-Field (HAADF) and scanning transmission 

electron microscopy bright field (STEM-BF) images on the same area of our sample as shown 

below in Fig. 5.1. Apparent voids show up in the 2.2 μm thick CdTe layer as dark spots (lower

average Z number for electron diffraction) in Fig. 5.1a and bright spots (lower mass density 

giving higher electron transmission) in Fig. 5.1b. These voids have a lateral dimension of ~0.15 

μm and appear to increase slightly in density toward the back contact. This gradient is 

qualitatively similar to that indicated by ellipsometry but the average volume void fraction 

accounted for by these microscopic voids is estimated at ~3% which is close to the volume void 

fraction, fv, obtained from ellipsometry analysis of CdTe cells on TEC glass. (See Fig. 2.7.) 

Under most conditions, as-grown CdTe films on Si wafers (without CdCl2 treatment) show 

considerably higher void fractions by ellipsometry. (See Figs. 3.4 and 3.8.) Fig. 5.1 also shows 

the some voids are visible in STEM micrographs at the CdS/CdTe interface.

Figure 5.1 Cross-sectional (a) HAADF and (b) STEM-BF images of standard CdTe device. 

Voids are circled for emphasis.

EDS elemental mappings were obtained by collecting x-ray signals from the same 

cross-sectional area as Fig. 5.1 to identify the distribution of the major elements in the sample 

from glass through TCO, CdS, CdTe to the back metal layer. The cell structure is labeled in the 

upper left image of Fig. 5.2 which is a tilted HAADF in low resolution. Other images are the 

x-ray distribution maps of Cd, Te, S, Cu and Au. EDS mapping was performed in the STEM 

mode with an electron beam probe size 5 Å. The dimension of the x-ray emission area is usually 

less than 10 times the probe size (normally 2~3 times) with this sample preparation.
33

The Cd and Te maps show relatively lower concentrations (dark spots as circled in Fig. 5.2) 

of both elements at the same locations as the dark spots in the HAADF image (Fig. 5.1a). This 

confirms the existence of voids in the CdTe layer. Low Cd density is also found at the CdS/CdTe 

interface which matches the locations of void segments in HAADF image.

Figure 5.2 HAADF image and EDS mapping of Cd, Te, S, Cu and Au on the same area. [The 

apparent Te distribution that appears in the SnO2 layer is due to signal leakage of the 

Sn Lβ1 (3662.8 eV) peak into the Te Lα1 (3769.3 eV)]. 

5.4 Cu Distribution and Cu Bonding in CdTe Cells 

5.4.1 EDS mapping

A high concentration of Cu in the back metal contact layer (where 3.5 nm was deposited 

initially) is confirmed by this mapping (the two lower-right images in Fig. 5.2). Although we 

expected the Cu to diffuse primarily into the CdTe, Cu is found distributed throughout the Au 

back contact. EDS mapping in other areas from the same sample confirms similar diffusion of 

Cu into the Au. This indicates the possible formation of a Cu-Au alloy and is further suggested 

by our x-ray fluorescence and EXAFS theoretical fitting, as discussed in following sections. 

Another feature clearly seen is the relatively higher concentration of Cu in the CdS layer 

than in CdTe. This result has been observed in SIMS studies of CdS/CdTe cells prepared by 

other methods.[5] We carefully compared the Cu mapping and HAADF image and found lower 

Cu concentration in the void segments at interface than in the CdS segments. This suggests some 

of the copper atoms diffuse from the back surface through the CdTe layer during back contact 

activation and accumulate in the CdS. 

The Cu fluorescence signal from the SnO2 (TCO) layer is also relatively stronger than from 

the CdTe layer but weaker than from CdS. 

5.4.2 X-ray Fluorescence

 The high concentration of Cu at the back contact was also observed in peeled-off metal back 

contact segments in x-ray fluorescence studies. In Phase I (Phase I Quarter 3 report), concurrent 

with our EXAFS studies at MR-CAT, we used x-ray fluorescence together with the peel-off 
34

technique as a convenient method to determine the total amount of copper in different layers 

(metal or CdTe/CdS/SnO2) of the partial cell samples. 

 By taking advantage of the intense (peak brilliance at 6.5keV: 9.6x1018 ph/sec/mrad2/mm2 

/0.1% bandwidth) and stable x-ray beam of the APS and the high purity Ge multi-element 

detector of MR-CAT beamline, we were able to investigate low level Cu concentrations in our 

CdTe cells (typically ~0.3% averaged through the structure). Furthermore, by using an 

epoxy-based lift-off or peel-off process we could separate the Au layer from the CdTe (Fig. 5.3a) 

and independently measure the amount of Cu in the contact and in the residual structure of the 

partial cell. 

We have measured the x-ray fluorescence excited at 10 keV, just above the Cu K-edge, to 

obtain maximum sensitivity to the Cu. We found the amount of Cu in the remaining structure of 

CdTe/CdS/glass to be much lower than in the peel-off metal/epoxy layers. As shown in Fig. 5.3a, 

the Cu Kα intensity of the remaining structure (sample B) is ~10 % of the one of the complete 

partial cell (sample A), while the intensity of the peeled off metal layer (sample C) on epoxy is 

90% of that of sample A. Thus, only a small portion of Cu actually diffuses into sputtered CdTe 

after the 45 minutes of heating at 150 °C. The ~90% of Cu peeled off with the Au/epoxy layer 

and our EDS mapping are evidence indicating that even after the thermal activation or diffusion

process, most of the Cu actually stays at either the interface of the Au layer and CdTe film or 

diffuses into the Au layer. Furthermore, much of the Cu that does migrate toward the junction 

ends up in the CdS. 

1mm fused silica

2.3μm CdTe

35Å Cu 200Å Au

1mm fused silica

2.3μm CdTe

200Å Au

0.13μm CdS

A

B

C Epoxy

a)

0.13μm CdS

b)

0

20000

40000

60000

80000

100000

120000

140000

0 2000 4000 6000 8000 10000

hv (eV)

intensity (count/180s)

CdS/CdTe film w/ Au

CdS/CdTe film w/o Au

peeled off Au layer

0

4000

8000

12000

16000

A 

B

C

Figure 5.3 a) the schematic structures of the samples; b) X-ray fluorescence of CdS/CdTe film 

with Cu/Au metal layer (A), film without metal layer (B) and the peeled off Au layer (C). 

Note: the peak at 8048 eV is Cu Kα fluorescence generated under exposure to the10 

keV x-ray beam.

5.4.3 High Resolution TEM and Fine-probe EDS

A high resolution investigation into the Cu distribution in the region near the Au back contact 

was needed to provide better evidence of the Cu location. Such study requires nanometer scale 

resolution, since the evaporated Cu layer is only 3.5 nm. We have used High Resolution 

Transmission Microscopy (HRTEM) with point-to-point resolution of 1.7 Å together with x-ray 

energy dispersive spectroscopy (EDS).
35

Figure 5.4 a) High resolution TEM image at the CdTe/Au interface. b) Cross-section TEM image 

of CdTe and Au layer. Circles are the locations where the fine-probe EDS spectra were 

taken. Broken line indicates the grain boundary of CdTe.

Table 5.1 Comparison of Cu Kα and Mo Kα intensities.

Position Cu/Mo peak intensity 

ratio

eds-b 1.61

eds-c 1.73

eds-a 2.24

eds-e 0.32

eds-d 0.27

eds-f 0

As shown in Fig. 5.4a, cross-sectional HRTEM image demonstrates a continuous interfacial 

layer with thickness of approximately 4 Å (which is brighter than the CdTe and Au layers). Such 

interfacial layer between CdTe and Au can be observed everywhere along the interface between 

CdTe and Au. 

Fine-probe EDS spectra from CdTe, Au and the interfacial layer confirm higher copper 

concentration around the CdTe/Au interface. EDS spectra (refer to Phase II Annual report for 

more details) across this interfacial layer were collected from positions as labelled in Fig. 5.4b. 

Intensity ratios of Cu and Mo Kα peaks were compared to determine the relative Cu 

concentration as summarized in Table 5.1. (Mo Kα arises from the mounting ring and is a 

convenient reference.) 

 As shown in Fig. 5.4, the spectra were collected on spots aligned along the direction of a 

grain boundary of CdTe. The highest Cu concentration occurs at the CdTe/Au interface (position 

eds-a), which confirms the interfacial layer at CdTe/Au interface (eds-a) in Fig. 5.4 is Cu-rich, 

with the Cu/Mo intensity ratio of 2.24. For the two positions in the Au layer (spots eds-b and 

eds-c), the Cu concentration is relatively lower than interface but comparable. Entering the CdTe 

layer, EDS indicates a sudden drop of Cu fluorescence signal to around one-tenth of that at 

interface for eds-e and eds-d. Increasing distance from the back contact yields weaker Cu signals. 

The Cu/Mo ratios of positions e and d are 0.32 and 0.27, respectively (Table 5.1). The Cu signal 

at position f, located at the grain boundary only 70 nm away from Au contact, is already 

undetectable. Thus, very little Cu appears to be located at grain boundaries. 

a) b)
36

These fine-probe EDS spectra together with the EDS mapping confirm the interdiffusion 

between Cu and Au layers justifying the hypothesis for our theoretical fitting of the EXAFS data, 

as discussed in the following section. The HRTEM and fine-probe EDS directly show the high 

concentration of copper at the CdTe/Au interface and in the Au layer. But any Cu above the 

detection limit of the fine-probe EDS mapping in the CdTe is within a region approximately 20 

nm or less away from the contact/CdTe interface. There is no evidence indicating higher Cu 

concentrations at the CdTe grain boundary than in the grain interiors. 

5.4.4 EXAFS 

Besides the high concentration of Cu as observed at the interface of CdTe and Au and inside 

the Au layer, investigation of local structure and chemical bonding of copper atoms in complete 

cell structures through EXAFS analysis is helpful to understand the role of copper at the back 

contact. 

0

1

2

3

4

5

0 1 2 3 4 5 R(Å)

data

fitting

Cu2O

|χ(R)|(Å-4)

a)

-5

-4

-3

-2

-1

0

1

2

3

4

5

0 1 2 3 4 5

R(Å)

data

fitting

Re[χ(R)](Å-4)

b)

Figure 5.5 a) Phase-uncorrected χ(R) functions of semi-cell structure sample and theoretical 

fitting compared with Cu2O reference; b) the real part of χ(R) functions of experiment data 

and theoretical fitting. (Imaginary parts of the χ(R) function are not)

Table 5.2 Theoretical fitting parameters of EXAFS spectra of partial cell structure and Cu2O 

reference.

bond 

Scattering N ∆N (+/-) R(Å) ∆R(±) σ2

(10-3Å) ∆σ2

(±10-3Å) ∆Eo (eV)

Semi cell structure without TCO (SiO2:Al/0.13 μm CdS/ 2.3 μm CdTe/35 Å Cu/ 200 Å Au)

Cu-O(Cu20) 1.4 0.3 1.84 0.02 1.6 fixed

Cu-Au(CuAu) 11.9 3.0 2.59 0.06 21.1 8.2 -3.07 ± 4.09

Cu-Cu(CuAu) 0.9 0.22 2.63 0.11 11.7 12.6

Cu2O

Cu-O(Cu20) 1.5 0.39 1.84 0.004 1.6 0.9 8.31± 0.32

Cu-Cu(Cu20) 20.1 5.12 3.04 0.01 27.6 1.5

 Our EXAFS data on the partial cell structure samples indicates a strong peak arising from 

the first nearest neighbor - oxygen atom shell located at the same position -1.43Å in |χ(R)| 

spectrum as the peak of the first nearest neighbor bond (Cu-O) in Cu2O (Fig. 5a). The peak width 

is different from that of Cu2O, but the real and imaginary parts of the χ(R) function in the region 
37

from 1.2 Å through 1.7 Å indicate this narrowing is due to scattering intensity leakage from the 

other two peaks at 2.0 Å and 2.73 Å. This suggests that most of the copper in the complete cell 

forms a copper oxide structure, which is consistent with our earlier work on Cu-diffused, 

CdCl2-treated CdTe film. [6] 

 These two peaks at 2.0 Å and 2.73 Å are also not generated from scattering by sulphur, 

tellurium (as in Cu2Te), chlorine (as in CuCl or CuCl2), or copper (as in metal Cu crystal) 

neighbors. But given the small amount of Cu in the sample and the finite sensitivity of the x-ray 

detector, we cannot conclude that Cu2S and Cu2Te do not exist in the cell at low levels. What 

appear in the EXAFS spectra in this case should be the dominant chemical phases of copper in 

the samples. These are Cu2O and Cu-Au as discussed below. 

 Model fitting to the two EXAFS peaks was informed by our EDS mapping, x-ray 

fluorescence study of peeled-off Au contact layers, HREM images and fine-probe EDS as 

discussed in previous sections. Scattering paths of Cu-Au and Cu-Cu from a model structure of 

the ordered 50:50 Cu-Au alloy are fitted to the spectrum together with the Cu-O path from a 

Cu2O reference; the result of which is shown in Fig. 5.5. Our fitting gives the Cu-Au and Cu-Cu 

path lengths of 2.59 Å and 2.63 Å, respectively (Table 5.2). The coordination number of the Au 

and Cu neighbor shell provided in the theoretical fitting is 11.9 and 0.9, respectively. The copper 

atoms may locate at the grain boundary of the Au grains or defects of Au crystal.

Given the theoretical fitting results of EXAFS data from the partial cell structures and the Z 

contrast of the HREM images at the CdTe/Au interface, we consider it most likely that the 

interfacial Cu-rich layer should be a copper-oxide layer with approximate thickness of 4Å. Since 

the Cu-O and Cu-Cu bond lengths in Cu2O are 1.84 Å and 3.04 Å (Table 5.2), respectively, such 

a presumed 4 Å Cu2O layer is so thin that not more than two Cu-O layers could be formed in this 

layer. This explains the absence of Cu-Cu bonds in our EXAFS spectra. 

5.5 S at CdTe Back Contact (EDS and XES Studies)

Another significant feature indicated by the EDS mapping is the presence of some sulfur at 

the back surface of the CdTe (See Fig. 5.2). The EDS map shows the S at the CdS layer, but 

there is also a significant increase in sulfur at the back of the cell at the interface between the 

CdTe and Cu/Au. The thickness of the high sulfur concentration layer at the back surface is 

approximately 100nm. The sulfur concentration in this layer is much lower than in CdS but 

higher than in CdTe and SnO2 as indicated by the qualitative differences in brightness. 

The chemical form of this interfacial sulfur at CdTe/back contact, was studied in 

collaboration with Lothar Weinhardt and Clemens Heske at the University of Nevada, Las Vegas 

using X-ray Emission Spectroscopy (XES) at the Advanced Light Source, Lawrence Berkeley 

National Lab. They used our standard sputtered CdS/CdTe films (0.13μm and 2.3μm, 

respectively) on TEC-7 together with an as-grown CdS film (1.8μm) on fused silica. CdTe/CdS

films both with and without standard vapor CdCl2 treatment were studied. Sulfur 2p core holes 

are generated by the x-ray beam with energy of 200 eV, which has an attenuation length in CdTe 

of 257nm (Model available at http://www-cxro.lbl.gov/

Fig. 5.6 shows the S L2,3 spectra for as-grown and chloride-treated CdTe/CdS films, an 

as-grown CdS film, and a CdSO4 reference. While no S can be detected on the CdTe surface of 

the untreated CdTe/CdS/TEC-7 sample (probe depth ~250 nm), we find a clear S L2,3 signal in 

the range from 145 eV through 153 eV for the CdCl2 treated sample, namely a dominant peak 

from S 3s states at approximately 148 eV and two sharp peaks just above 150 eV corresponding 

).
38

to the Cd 4d-derived bands. This signal is primarily comprised of CdS-like sulfur, as well as 

some sulfur in an oxidized (sulfate) environment. This can be seen by comparing the difference 

spectrum with a CdSO4 reference in the energy range from 153 eV through 167 eV with 140 

times intensification on the spectrum of the chloride-treated sample. 

Figure 5.6 Sulfur L2,3 XES spectra of 

as-grown CdS reference, as-grown and 

CdCl2 treated CdTe/CdS, and a CdSO4

(sulfate) reference. The spectrum of CdCl2

treated CdTe/CdS is primarily comprised of 

CdS-like sulfur below 153 eV and 

sulfate-like sulfur above 153 eV.

As a conclusion of our EDS mapping of sulfur and XES surface state study, the CdCl2

treatment apparently leads to S diffusion and accumulation at the back surface of CdTe and some 

of the diffused sulfur is oxidized to form a sulfate. We suggest this CdSO4 can stabilize the 

higher concentration of S at the back contact compared with the bulk of the CdTe film. 

5.6 Conclusions

This work using EDS mapping, XRF, HRTEM together with fine-probe EDS provides direct 

evidence of Cu redistribution in conventional sputtered CdS/CdTe cell structures (that have 

received CdCl2 activation treatments). Approximately 90% of the 35Å of deposited Cu remains 

at the back contact after the standard fabrication process, including CdCl2 treatment. Some of the 

Cu at the back contact forms a subnanometer copper-rich layer at the CdTe/Au interface. The 

rest of the Cu at the back contact diffuses into the Au layer. The remaining ~10% of the 

deposited Cu ends up mostly in the CdS. 

Our EXAFS modeling suggests that the subnanometer copper-rich layer at the CdTe/Au 

interface has the bonding arrangement of Cu2O, at least for sputtered cells prepared with vapor 

CdCl2 activation (with no post-chloride etching) before evaporation of Cu and Au. Such a 

cuprous-oxide layer may serve to limit some diffusion of copper into the CdTe layer, although it 

is clear that some Cu does migrate into the CdS.
39

6. Potential Distribution During Magnetron Sputtering Deposition

6.1 Introduction

In the progress report for the Second Quarter of Phase 3 we presented the results of the 

potential distribution during magnetron sputtering. The Langmuir probe, made of a 10 mm long, 

70 micron diameter Pt wire, was placed in front of the center of the substrate 10 mm towards the 

CdTe target. 10 mm of wire protruded with the rest of the wire isolated from the plasma by the 

glass capillary tube and fed through the chamber wall for easy readout. We used a Keithley 2400 

SourceMeter to collect the I-V data. Two other feedthroughs were used for the substrate bias 

wire and target self-bias wire. The substrate holder was isolated from the chamber walls with a 

dielectric piece and normally during the deposition the substrate potential was floating. The setup 

shown on the Fig. 6.1 allowed us to measure the self-bias potential of the substrate as well as to 

apply a desired bias over the range of -50 to 50 V in order to check on the corresponding changes 

in plasma potential.

In order to cover the most useful region in the phase space of deposition parameters we 

performed all the measurements for the Ar pressure range of 5 mTorr to 50 mTorr and RF power 

of 5 W to 60 W. In addition, data were taken in two different configurations: with a freshly 

cleaned ground shroud (bare metal) and a ground shroud with a thick CdTe coating, equivalent to 

the coating that is normally built up after about 200 hours of deposition. 

Tec-7 substrate

Sputtering chamber

CdTe Target

Langmuir probe

Substrate holder

Ground Shield

Plasma

Substrate bias wire

Target bias wire

Figure 6.1 Experimental setup for potential distribution and the Langmuir probe measurements.

Below is a brief summary of the results with some important conclusions. 
40

6.2 Cathode DC Bias

0 10 20 30 40 50 60

-220

-200

-180

-160

-140

-120

-100

-80

-60

 5 mTorr, Clean GS

 10 mTorr, Clean GS

 18 mTorr, Clean GS

 30 mTorr, Clean GS

 50 mTorr, Clean GS

 5 mTorr, Covered GS

 10 mTorr, Covered GS

 18 mTorr, Covered GS

 30 mTorr, Covered GS

 50 mTorr, Covered GS

Cathode DC Bias, V

RF Power, W

Figure 6.2 Cathode DC bias as a function of pressure and RF power. Clean ground shroud

(dotted lines) and coated ground shroud (solid lines).

The dependence of target dc self-bias on pressure is rather weak while it steadily becomes 

more and more negative when the RF power increases (Fig. 6.2). Also there is a noticeable 

difference between clean and covered ground shroud conditions – the cathode DC bias is always 

less negative for the same pressure and power conditions when the ground shroud is covered

with an insulating coating of CdTe. 

6.3 Substrate Self-Bias

0 10 20 30 40 50 60

-8

-6

-4

-2

0

2

4

6

8

10

 5 mTorr, Clean GS

 10 mTorr, Clean GS

 18 mTorr, Clean GS

 30 mTorr, Clean GS

 50 mTorr, Clean GS

 5 mTorr, Covered GS

 10 mTorr, Covered GS

 18 mTorr, Covered GS

 30 mTorr, Covered GS

 50 mTorr, Covered GS

RF Power, W

Substrate Self-Bias, V

Coated

Bare

Figure 6.3 Substrate self-bias bias as a function of pressure and RF power. Clean ground 

shroud (dotted lines) and coated ground shroud (solid lines)

 Our standard deposition configuration maintains the substrate at floating bias that changes 

with both pressure and RF power as shown on the Fig. 6.3. It becomes more positive (or less 
41

negative in some cases) when both pressure and power increase. We found the largest 

differences between the cases of clean and coated ground shroud. For the case of a clean ground 

shroud the substrate is always found under negative self-bias while it eventually becomes 

positive when enough CdTe coating is developed on the shroud. We believe this behavior can

explain the observed change in pinhole density – extremely high right after the cleaning of the 

ground shroud and diminishing as the total deposition hours increases. Higher pinhole 

production can be explained by the presence of positively charged dust particles inside the 

chamber that get attracted to the substrate when its substrate self bias potential is negative but 

otherwise do not cause a problem. 

6.4 Plasma Potential

We found a very weak dependence of plasma potential on the RF power with a tendency to 

increase with increasing power. The plasma potential is independent of pressure within the 

experimental error (Fig. 6.4). The largest change in plasma potential observed was between the 

cases of coated and clean ground shrouds. The plasma potential is about 5 V higher in the case of 

the of the coated ground shroud. 

0 10 20 30 40 50 60

0

5

10

15

20

 5 mTorr, Clean GS

 10 mTorr, Clean GS

 18 mTorr, Clean GS

 30 mTorr, Clean GS

 50 mTorr, Clean GS

 5 mTorr, Covered GS

 10 mTorr, Covered GS

 18 mTorr, Covered GS

 30 mTorr, Covered GS

 50 mTorr, Covered GS

Plasma potential, V

RF power, V

Figure 6.4 Plasma potential as a function of pressure and RF power. Clean ground shroud (dotted 

lines) and coated ground shroud (solids).

6.5 Conclusions

We measured the dc cathode bias, substrate self bias and plasma potential as functions of 

sputter gas pressure and applied RF power. We found a substantial difference between conditions 

resulting from using a freshly cleaned ground shroud vs. one that had been in the chamber for 

about 100 hours of deposition. 

This explains the reduction of the deposition rate that is observed as the CdTe coating on the 

ground shroud builds up. A coated ground shroud results in a smaller potential difference 

between the plasma and the target DC bias that results in less integrated sputtering time over 

each RF cycle. This also relates to the higher pinhole density right after the cleaning of the 
42

ground shroud, since the ground shroud cleaning procedure leads to the decrease of the substrate 

self bias by roughly 10 V which is enough to make it slightly negatively biased as opposed to the 

positive self bias which we believe is typical for the major part of the target maintenance cycle. 

As a result of this study we extended our typical 100 deposition hour ground shroud cleaning 

cycle by 5 times and no negative effects were observed other than a decrease of the deposition 

rate that can be compensated by increasing the RF power or using lower pressure sputtering 

conditions. High quality films with very low pinhole density yielding above 13% efficiency cells 

have been deposited under these conditions on commercial soda-lime glass.
43

7. Ultra-Thin Devices 

Earlier in this phase we reported our studies on ultra-thin (about 0.5 micron CdTe) cells. 

Even though an efficiency of 10% was achieved for these devices, the cells still suffered 

significantly from losses in all three important figures of merit (i.e. JSC, VOC and FF). While a 

loss in JSC with thin CdTe is fairly well understood as arising mainly from deep penetration loss 

in the IR part of the spectrum), losses in VOC and FF are more complex. These two parameters 

can be affected in general by six variables. Three are briefly summarized: 1) the presence of an 

HRT layer, 2) the CdS thickness, and 3) the CdTe thickness. Four variables are independently 

discussed in subsequent sections: 4) the CdCl2 treatment parameters, 5) the amount of copper 

used in the back contact, and 6) the duration/temperature of the final back contact diffusion. 

Finally, in Section 7.7 performance statistics are presented for optimized cells. 

7.1 Presence of an HRT Llayer 

It has been shown previously that adding an HRT layer between the CdS and the 

conventional high conductivity SnO2:F coated glass is beneficial, especially for cells with thin 

(below 80 nm) CdS layers. Our study of the effect of an HRT layer was discussed in our Phase 3, 

Quarter 4 report. In general the HRT layer allows thinner CdS with higher current and less loss 

in VOC which typically happens with CdS layers below 80 nm. 

7.2 CdS Thickness 

We reported on the optimum thicknesses of the CdS layer for cells deposited on both bare 

Tec-15 (about 80 nm) and HRT-coated Tec-15 (about 45 nm). In this study we kept the CdS 

layer thickness close to 60 nm for all of the samples independent of the CdTe thickness. This 

approach has likely decreased the maximum efficiency due to a slight loss in Jsc caused by the 

excessive absorption in CdS, but on the other hand gave us an extra assurance that the n-side of 

the photovoltaic junction is robust and not likely to influence the observed VOC and FF changes. 

7.3 CdTe Thickness 

We prepared a set of samples with the following CdTe thicknesses: 0.38 μm, 0.5 μm, 0.65 

μm, 0.8 μm, 1.1 μm, 1.45 μm, 1.85 μm, 2.2 μm and 2.6 μm. Thickness was measured by a 

DEKTAK profilometer and represents a mean thickness of the sample averaged over 35 small 

area (0.062 cm2

) cells on each sample (a quarter piece of our typical 3”x3” substrate). We will 

specifically identify this gradient from center to edge where appropriate. All samples were 

deposited at 18 mTorr of Ar pressure with 20 W of applied RF power. 

7.4 CdCl2 Treatment 

We have observed previously some dependence of cell parameters on all the variables 

associated with CdCl2 treatment such as the treatment type (vapor or wet), treatment temperature 

and treatment duration. In this study we used a wet treatment meaning that a room temperature 

saturated CdCl2-methanol solution was applied directly to the CdTe surface and allowed to dry 

forming a relatively uniform coating on the sample. After the samples were dried, they were 

annealed in dry air for variable times with a temperature profile that reached 387 oC. A typical 

temperature profile for the treatment is shown in Fig. 7.1. The flat regions of the two curves 
44

represent the nominal treatment time as it would be entered into the computer program that 

controls the system. In reality the cells undergo some treatment during the ramp-up and 

ramp-down time as well (Fig. 7.2). The total amount of treatment that the sample receives is an 

integral value of both and can be represented as the area below the curve if the Arrhenius-like 

behavior of the different process that form a core of CdCl2 treatment is also taken into account. 

Fig. 7.3 represents the temperature dependence of bulk diffusion coefficients for Cd and Cl –and 

the grain boundary diffusion coefficient for Cd. Cd (bulk and GB) self-diffusion coefficient are 

taken from,[7] and DCL(BULK) is taken from [8]. From data of Fig. 7.3 and Fig. 7.4 one can 

estimate a relative contribution of the treatment during the ramp-up and ramp-down time to the 

entire treatment and adjust to a nominal treatment time with a step function temperature-time 

profile. Fig. 7.5 shows the values for three different processes for a 30 minute treatment and Fig. 

7.6 – for a three-minute one. This estimation should not be taken as a full quantitative analysis 

but we believe that it gives useful guidelines for choosing CdCl2 treatment times for the cells 

with different CdTe thicknesses. 

0 1200 2400 3600 4800 6000

0

50

100

150

200

250

300

350

400

 30 min, 

 

 3 min, 

 

Temperature, o

C

Time, seconds

Figure 7.1 CdCl2 treatment temperature profiles for nominal 3 minute and 30 minute treatment 

times.

0 1200 2400 3600 4800 6000

0

50

100

150

200

250

300

350

400

0.5 μm CdTe

 3 min

Temperature, o

C

Time, seconds

0 1200 2400 3600 4800 6000

0

50

100

150

200

250

300

350

400

2.5 μm CdTe 

 30 min

 

Temperature, o

C

Time, seconds

Figure 7.2 A schematic representation of “nominal” (square areas on the graphs) vs. “actual” 

(the whole area under the curve) treatment time.
45

Figure 7.3 Arrhenius plots of diffusion coefficients for Cl in bulk CdTe (DCl(Bulk) - red), Cd 

self-diffusion in bulk CdTe (DCd(Bulk) - green), and Cd diffusion along grain boundaries

in CdTe (DCd(GB) – blue). The three different curves are for 1 mTorr (solid), 10 mTorr 

(dashed) and 100 mTorr (dotted) partial pressures of CdCl2) 

8.72

91.3%

ClBulk VCdBulk

5.56%

94.4%

VCdGB @ 10 mTorr

7.17%

92.8%

Figure 7.4 Relative contribution of the ramp-up and ramp-down time (light blue segment) 

calculated for the three different processes from the Fig. 7.3 for the case of a 

30-minute treatment at 387 o

C 

ClBulk

48.2%

51.8%

VCdBulk

37%

63%

VCdGB @ 10 mTorr

43.6%

56.4%

Figure 7.5 Relative contribution of the ramp-up plus ramp-down time (green segment) calculated 

for the three different processes from the Fig. 7.3 for the case of a three-minute 

treatment at 387 o

C 
46

As an example of this correction for ramp-up and ramp-down, we can reanalyze our 

previously established benchmarks that are proven to give reasonably good results in terms of 

cell performance (Table 7.1 – “nominal” treatment time column). If treatment time is plotted 

against CdTe thickness (Fig. 7.6, open squares) it does not intercept the origin, while if you 

consider the ramp-up plus ramp-down contribution to be equivalent to approximately 3 minutes 

of treatment at constant temperature (387 o

C) (see Fig. 7.6) and correct the time correspondingly 

(“actual” treatment time in the Table 7.1) then the results can be easily approximated by a 

straight line passing through the origin (Fig. 7.6). 

Table 7.1 Benchmarks for CdCl2 treatment time of the samples with different CdTe thickness and 

corresponding best cell efficiency

CdTe 

Thickness

CdCl2 treatment time 

(nominal)

CdCl2 treatment time

(actual)

Best cell 

efficiency

2.5 30 33 14%

2.25 25 28 13.7%

1.32 15 18 11%

0.97 10 13 12%

0.75 5 8 8%

0.5 3 6 10%

0.3 1 4 6 %

0.0 0.5 1.0 1.5 2.0 2.5

0

5

10

15

20

25

30

 CdCl2 treatment time, min

CdTe thickness, μm

Figure 7.6 CdCl2 treatment time (“nominal” – open squares and “actual” - blue dots) vs. CdTe 

thickness and the linear fit for the “actual” time.

In order to test this hypothesis we prepared a set of samples with different CdTe thicknesses 

as described above and treated them with times that would approximately follow the straight line 

from the Fig. 7.6 but with a small variation such that each sample of a certain thickness was 

treated for two different times closely spaced along the time axis. Fig. 7.7 illustrates this 
47

approach. Two separate sets of values at the time axis correspond to “nominal” and “actual” 

treatment time.

0.0 0.5 1.0 1.5 2.0 2.5 3.0

0

5

10

15

20

25

30

35

40

4

2

7

10

15

20

25

31

ssc32

ssc34

ssc31

ssc33

ssc37

ssc35

ssc30

ssc38

CdCl2 treatment time, min

CdTe thickness, μm

ssc28 (2.84, 2.6, 2.15)

(2.24, 2.0, 1.85)

(2.0, 1.83, 1.53)

(1.6, 1.35, 1.25)

(1.1, 1.1, 0.95)

(0.83, 0.75, 0.65)

(0.68, 0.65, 06)

(0.52, 0.46, 0.4)

(0.37, 0.35, 0.3)

35

Figure 7.7 Treatment time chosen for samples of different thickness. Arrows indicate the two 

different treatment time chosen. Values on the time axis are “actual (black) and 

“nominal” (red) treatment time.

7.5 Thickness of the As-Deposited Cu Layer 

UT’s typical Cu/Au back contact deposited by thermal evaporation was used in this study. 

Since we have previously found some dependence of cell efficiency on the as-deposited Cu layer 

thickness as the CdTe thickness is changed, this parameter was included into the study in 

addition to the CdCl2 treatment time variation. As described before, two sister quarter pieces 

from each sample of a certain thickness were treated for a longer time than the two others. After 

that, one sample with a longer treatment time was completed with a back contact having a Cu 

layer thinner than the other one. Samples with shorter CdCl2 treatment time were processed 

similarly (Fig. 7.9). For the samples with CdTe layer thicker than 1.5 micron, 10 Å Cu / 200 Å 

Au and 30 Å Cu / 200 Å Au back contacts were used and for the thinner ones the 10 Å Cu / 200 

Å Au and 20 Å Cu / 200 Å Au combination was chosen. See Table 7.2 for detailed information 

about each sample. 
48

Shorter CdCl2

Thicker Cu

Shorter CdCl2

Thinner Cu 

Longer CdCl2

Thicker Cu

Longer CdCl2

Thinner Cu

Figure 7.8 A typical sample used in this study. Each quarter-piece receives a certain combination 

of CdCl2 treatment and back contact. 

7.6 Cu Diffusion Time 

Again, our previous experience suggests that Cu diffusion time should progressively get 

shorter as the thickness of the CdTe layer decreases. We are yet to determine the optimum 

conditions for all the different thicknesses of CdTe, especially on the thinnest side of the series 

presented here, but we followed our previous benchmarks that were proven to be useful (see 

Table 7.2 for more information). 

7.7 Cell Performance Statistics for Optimized Cells

Finally in this section, we present performance statistics (VOC, JSC, and FF) for cells 

optimized over the various parameters discussed above.
49

Table 7.2 Sample Fabrication Details

Sample ID Substrate CdS, 

nm CdTe, μm

CdCl2

treatment, min Cu 

,Å 

Au, 

Å 

Cu 

diff, 

minal actual min

SSC28

A

HRT

Tec 15 60 

2.84, 2.5, 2.15 

35 38 10

200 

45 

B 30

C

31 34 

10

D 30

SSC38

A

2.24, 2.0, 1.85 

10

B 30

C

25 28 

10

D 30

SSC30

A

2.0, 1.83, 1.53 

10

B 30

C

20 23 

10

D 30

SSC35

A

1.6, 1.35, 1.25 

10

30 B 30

C

15 18 

10

D 20

SSC37

A

1.1, 1.1, 0,95 

10

20 B 20

C

10 13 

10

D 20

SSC33

A

0.83. 0.75, 

0.65 

10

15 B 20

C

7 10 

10

D 20

SSC31

A

0.68, 0.65, 0.6 

10

10 B 20

C

4 7 

10

D 20

SSC34

A

0.52, 0.46, 0.4 

10

7 B 20

D

2 5 

10

C 20

SSC32 B 0.37, 0.35, 0.3 10 5 A 20

The back contact area of 0.062 cm2 was defined by a mask with 35 dots per quarter piece 

sample as pictured in Fig. 7.10. Individual cells are indexed in such a way that cells with smaller 

numbers are closer to the middle of the sample and the larger number is closer to the outer 

regions. Since our cells normally have a thickness gradient from the center to the edge as 

reflected in the CdTe thickness column in the Table 7.2 (three values correspond to the thickness 
50

measured close to the cells # 1, # 9 and # 25 correspondingly) this sometimes provides valuable 

information about fine structure in the variance of the cell performance vs. CdTe thickness.

Figure 7.9 A sketch of a sample with 

individual dot cells. Numeration starts from 

the center of the piece and dot cells with 

lower numbers typically have slightly 

thicker CdS and CdTe layers.

Typically we report statistical results for the whole ensemble of 35 cells for each sample, but

in this particular case we had to take a different approach. This is a consequence of an unusually 

large pinhole density in all the samples regardless of CdTe thickness. (We recently found the 

high pinhole density was a result of a particular step in our sample handling procedure that has 

now been changed. Although normally we keep our samples in plastic boxes without anything 

being in contact with the film, in this case they were stored in polyethylene bags that proved to 

be detrimental for the cells. We have observed pinholes appearance after the sample has been 

stored in the bag even for a short period of time. Electrostatic interactions between the plastic 

and the sample surface is the plausible cause, but further investigation is needed. 

As a result of film damage presumably caused by the plastic bags, the statistics for 35 cells 

ended up being greatly affected and not representative of a proper fabrication process. Therefore 

we had to restrict our analysis to the best 10 out of 35 cells on the each sample. We believe that 

this approach is more appropriate in this particular case and the results presented below nicely 

represent the CdTe thickness dependence that was actually sought.

Figure 7.10 

Box-and-whiskers plot of 

the efficiency of the 10 

best cells from each 

sample

1 2 9 10 25

4 3 8 11 24

5 6 7 12 23

16 15 14 13 22

17 18 19 20 21

26 27 28 29 30

35 34 33 32 31
51

Fig. 7.10 represents the efficiency dependence on CdTe thickness. Following the recipes 

presented in the Table 7.2 we have been able to fabricate cells with an average efficiency greater 

than 12 % for all the thicknesses above 1 μm. It looks from our data that this thickness represents 

the first tipping point below which a significant efficiency drop is observed. The second critical 

value appears to be around 0.5 μm below which a more rapid efficiency drop begins. It also can 

be seen from the figure that variations in as-deposited Cu (red vs. blue symbols) have a stronger 

effect on efficiency than variations in CdCl2 treatment time (solid vs. dashed). 

0

5

10

15

20

25

0.35 0.5 0.65 0.85 1.1 1.45 1.85 2.0

20/30 o

A Cu

10 o

A Cu

JSC, mA/cm2

CdTe thickness, μm

2.6

Figure 7.11 Short-circuit current of the 10 best cells from each sample

450 500 550 600 650 700 750 800 850 900

0

10

20

30

40

50

60

70

80

90

100

 1.5 μm

 1.8 μm

 2.0 μm

 2.6 μm

QE, %

Wavelength, nm

450 500 550 600 650 700 750 800 850 900

0

10

20

30

40

50

60

70

80

90

100

 1.5 μm

 1.8 μm

 2.0 μm

 2.6 μm

QE, %

Wavelength, nm

Figure 7.12 Quantum efficiency at zero bias conditions
52

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8

-25

-20

-15

-10

-5

0

5

 1.5 μm

 1.8 μm

 2.0 μm

 2.6 μm

Current density, mA/cm2

Voltage, V

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8

-25

-20

-15

-10

-5

0

5

 0.35 μm

 0.5 μm

 0.65 μm

 0.85 μm

 1.1 μm

Current density, mA/cm2

Voltage, V

Figure 7.13 J-V characteristics of the cells from the Figure 7.11.

Fig. 7.11, Fig. 7.13 and Fig. 7.14 represent the second order metric for the same sets of cells. 

Short-circuit current (Fig. 7.11) starts to decrease slightly when the thickness of CdTe layer is 

less than one micron due to deep penetration loss first and the more dramatically due to weaker 

collection which appears to be wavelength independent as can be seen from the QE graphs in Fig. 

7.12. On top of that there are slight variations in transmission of the CdS layer that can be 

attributed either to the position of the cell on the sample (CdS thickness gradient, similar to that 

of CdTe), or nominal as-deposited CdS thickness fluctuations from sample to sample, or amount 

of CdTe consumed by interdiffusion during the CdCl2 treatment process.

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.35 0.5 0.65 0.85 1.1 1.45 1.85 2.0

20/30 oA Cu

10 o

A Cu

VOC, V

CdTe thickness, μm

2.6

Figure 7.14 Open-circuit voltage of the 10 best cells from each sample.

Open-circuit voltage (Fig. 7.14) follows approximately the same dependence on CdTe 

thickness as does JSC – remaining virtually the same for all the samples thicker than one micron 

and gradually decreasing for the thinner ones. This is consistent with the collection problems of 
53

the thin samples observed in JSC graphs before. One interesting observation is the prominent 

effect of Cu on the cells with thicker CdTe layers (one micron and above) - an additional 20 Å 

of as-deposited Cu can increase the average VOC by as much as 50 mV in some cases (1.85 

micron sample on the Fig. 7.14). A similar dependence was observed in the fill factor as well 

(Fig. 7.15). 

0

10

20

30

40

50

60

70

0.35 0.5 0.65 0.85 1.1 1.45 1.85 2.0

20/30 oA Cu

 10 oA Cu

FF, %

CdTe thickness, μm

2.6

Figure 7.15 Fill factor of the 10 best cells from each sample.

We obtained bias-dependent QE measurements on one of the best samples from each 

thickness point to investigate further the issue of carrier collection. It is remarkable that there is 

virtually no difference between the QE curve taken at zero bias and at reverse bias (-0.5 V and -1 

V) for all the samples with CdTe layer thicker than 1 micron. There is also a very little change in 

the overall QE shape except for a slight hint of deep penetration loss (Fig. 7.16 and the top left 

graph on the Fig. 7.17). 
54

450 500 550 600 650 700 750 800 850 900

0

10

20

30

40

50

60

70

80

90

100 2.6 μm CdTe

Wavelength, nm

QE, %

 0 V

 -1 V

 -0.5 V

 0.3 V

 0.5 V

450 500 550 600 650 700 750 800 850 900

0

10

20

30

40

50

60

70

80

90

100 2.0 μm CdTe

Wavelength, nm

QE, %

 0 V

 -1 V

 -0.5 V

 0.3 V

 0.5 V

450 500 550 600 650 700 750 800 850 900

0

10

20

30

40

50

60

70

80

90

100 1.8 μm CdTe

Wavelength, nm

QE, %

 0 V

 -1 V

 -0.5 V

 0.3 V

 0.5 V

450 500 550 600 650 700 750 800 850 900

0

10

20

30

40

50

60

70

80

90

100 1.5 μm CdTe

Wavelength, nm

QE, %

 0 V

 -1 V

 -0.5 V

 0.3 V

 0.5 V

Figure 7.16 Quantum efficiency at variable bias. Thicker cells.

The results change significantly when the CdTe layer thickness decreases below one micron. 

The difference between zero-bias QE and reverse-bias QE becomes more apparent and shows no 

wavelength dependence below the CdS absorption edge. QE also shows an overall decrease that 

is not compensated even by application of a strong negative bias and therefore cannot be 

attributed solely to a weaker electric field at the junction. We believe that it indicates some 

unidentified recombination mechanisms within the space charge region that are not present in the 

thicker cells. Plausible explanations can range from imperfections at the CdS/CdTe interface that 

are not fully cured because of the shorter CdCl2 treatment of the thinner cells, higher grain 

boundary surface to the volume ratio in the thinner cells due to smaller average grain size, or 

proximity of the CdTe/back contact junction with a high surface recombination velocity. 
55

450 500 550 600 650 700 750 800 850 900

0

10

20

30

40

50

60

70

80

90

100 1.1 μm CdTe

Wavelength, nm

QE, %

 0 V

 -1 V

 -0.5 V

 0.3 V

 0.5 V

450 500 550 600 650 700 750 800 850 900

0

10

20

30

40

50

60

70

80

90

100 0.85 μm CdTe

Wavelength, nm

QE, %

 0 V

 -1 V

 -0.5 V

 0.3 V

 0.5 V

450 500 550 600 650 700 750 800 850 900

0

10

20

30

40

50

60

70

80

90

100 0.65 μm CdTe

Wavelength, nm

QE, %

 0 V

 -1 V

 -0.5 V

 0.3 V

 0.5 V

450 500 550 600 650 700 750 800 850 900

0

10

20

30

40

50

60

70

80

90

100 0.65 μm CdTe

Wavelength, nm

QE, %

 0 V

 -1 V

 -0.5 V

 0.3 V

 0.5 V

Figure 7.17 Quantum efficiency at variable bias. Thinner cells.

7.8 Conclusions

We investigated the dependence of the cell performance over a wide range of CdTe 

thicknesses (0.35μm...2.6μm). We find that that an average efficiency of greater than 12% can 

be achieved for cells as thin as 1 micron with the appropriate post-deposition treatment and 

optimized back contact scheme. Over CdTe thicknesses of one micron and above, we find the 

effect of the back contact scheme (as-deposited Cu thickness) to be more dramatic than variation 

in CdCl2 treatment time. Insufficient Cu caused average VOC reduction of as much as 50 mV in 

some cases, while the use of adequate amounts of Cu yielded VOC greater than 800 mV 

consistently for all samples within this thickness range. 

We also found that further reduction of CdTe thickness leads to a gradual decrease in

performance although 10% efficiency can be obtained with 0.5 micron CdTe. We established 

some benchmarks for CdCl2 treatment and back contact processing for these samples with CdTe 

thickness less than one micron that yield reasonably good results although more comprehensive 

studies need to be done to resolve the issue of VOC and FF losses and collection probability less 

than unity from the space charge region in the thin cells.
56

8. Shunt Passivation - Electrochemical Polymerization Treatment of CdTe Solar

 Cells

8.1 Introduction

Conducting polymers are of significant interest given their possible application in numerous 

research areas including electronics, energy storage, and medicine [9]. Among such polymers, 

polyaniline has been studied extensively and is finding increasing uses in various fields of 

technology such as in gas sensors, electronics and anticorrosion coatings [10]. This polymer is 

readily prepared electrochemically from aqueous solution (anodic polymerization onto the 

electrode surface), is insoluble in water and has high chemical stability [11,12]. As shown in Fig. 

8.1 below, the value of Y in each individual unit X may be Y = 1, the fully reduced form 

(leucoemeraldine base), Y = 0.5, the form that is half oxidized (emeraldine base, most stable 

form), and Y = 0, the fully oxidized form (pernigraniline base) [9]. 

N N NH NH

1-Y Y X

Figure 8.1 The base (i.e., neutral) polyaniline, which is half reduced/oxidized, is thought to be of 

this general structure.

The electrochemical polymerization technique is an interfacial reaction, performed by applying a 

voltage across a conductive substrate and an inert electrode, both placed inside an electrolyte rich 

solution containing a selected monomer, which for this study was aniline. The aniline monomer 

reacted with the exposed surface of the conductive substrate (the transparent conducting oxide 

(TCO) coated glass) to form a poorly conducting polymer (polyaniline) on the surface. In this

study we anticpated growing polyaniline in the pinhole areas that have the TCO layer or CdS 

layer of the CdTe solar cell exposed. The CdTe solar cell is built on a glass substrate which is 

coated with a TCO, then sputter deposited with CdS (n-type) and CdTe (p-type) semiconductors 

before finally depositing the metal back contact (Cu-Au). Occasionally, pinhole defects are 

created during the fabrication process. In such cases, our goal was to selectively coat any 

portions of the TCO and/or CdS layer that are exposed through the semiconductor layers, 

without affecting the surface of the CdTe layer. After the electrochemical polymerization 

treatment, an insulating material (polyaniline) is expected to fill the pinholes and prevent the 

formation of an electrical contact (shunt) between TCO /CdS and the Cu-Au back contact. We 

have succeeded in this goal and we were able to obtain promising results that indicates both the 

deposition of the polymer on the solar cell device and reduction of shunting of the CdTe solar 

cell.

8.2 Characterizing the New Insulating Thin Film (Conducting Polymer) – using 

X-ray Photoelectron Spectroscopy (XPS) and Electron Spray Ionization – Mass 

Spectroscopy (ESI-MS) Techniques

The monomer solution (standard): A freshly prepared solution containing 0.2 M Aniline, 0.1 

M p-toluenesulfonic acid (p-TSA), 2.0 M sodium chloride in water was stirred for half an hour, 
57

providing a solution with a pH ~6.2. Since aniline is photo-sensitive, the entire process was done 

inside a hood with the lights off. Aniline, and its solution, oxidizes on exposure to air (oxygen) 

and light, as seen by a change in color (from colorless to dark brown). 

Electrochemical treatment technique (standard): A potentiostat (D.C. power supply), 

containing two wires terminating with an alligator clip, were used to apply a potential between a 

TEC-15 glass substrate and a platinum electrode used as an auxiliary electrode. TEC-15 is a 3 

mm thick glass substrate coated with a transparent conductive oxide (SnO2:F). The SnO2:F layer 

acts as the anode/positive electrode and the platinum electrode as the cathode/negative. The 

electrodes where placed in a beaker filled with one inch height of the monomer (aniline) solution. 

The TEC-15 glass substrate is submerged half way into the solution. A voltage of 2.0 Volt was 

applied for 20 minute to polymerize the aniline. After the polymer treatment the device was 

rinsed with de-ionized water and dried with a stream of compressed air. The new film coating 

has a thickness of 2500 + 50 Å, measured using a DEKTAK depth profiler and 1.305 kΩ/□ sheet 

resistance (poor conductor). 

Figure 8.2 XPS spectrum of a TEC-15 glass substrate used as a control - not coated with 

the conducting polymer. 

The sample that is now half coated with the new insulating polymer thin film is cut into half. 

Both the coated and the uncoated side of the TEC-15 were examined by X-ray photoelectron 

spectroscopy (XPS) analysis with Cu Kα photons – hυ = 8048eV. The kinetic energy (KE)

distribution of the emitted photoelectrons is measured with an electron energy analyzer and the 

binding energy (BE) is obtained from 

 BE = hυ – KE. 
58

The samples were measured using a Kraos Axis Ultra XPS, at the University of Michigan – 

EMAL, with sample chamber base pressure of ~5.6x10-9 mTorr. The survey scans of the 

uncoated/control and the coated thin film are shown in Figures 8.2 and 8.3 below: 

Figure 8.3 XPS spectrum of a TEC-15 glass substrate coated with the conducting polymer. 

In these XPS spectra, we anticipated seeing the presence of oxygen and carbon in both cases, 

due to contamination/dust particles on the sample surface. However, the key elements to look for 

are ‘Sn’ and ‘N’. For the uncoated sample used as a control (with no polymer film coating), we 

anticipated seeing the presence of SnO2 (Sn and O elements). As shown in Fig. 8.1, we can 

clearly see ‘O’ and ‘Sn’ peaks, with binding energies of 532 eV and 487 eV respectively. On the 

other hand, for the sample with the new thin-film coating, Fig. 8.2, we do not see any ‘Sn’ peaks, 

but rather we see a new ‘N’ peak with a binding energy of 398 eV. This information provides 

two insights:

1. an indication that the sample surface is covered with a new material, and

2. that the new thin film might be a polymer – polyaniline. 

Among all the chemicals used in this experiment, ‘N’ is present only in the aniline 

monomer (solution). At this point, we know that the aniline monomer is part of the new thin-film 

coating, but we cannot confirm that it is polyaniline. We used Electron Spray Ionization – Mass 

Spectroscopy (ESI-MS) to determine the molecular weight of the new thin film coating to 

answer this question. The ESI-MS instrument used for this experiment is located at The 

University of Toledo – Instrumentation Center. 

A standard aniline solution was prepared and the standard electro-chemical polymerization 

treatment was applied to a TEC-15 glass substrate with a voltage of 2.0 V for 20 minutes. After 

the polymerization treatment, the device was rinsed with deionized water and dried with a stream 

of compressed air. The new film coating that is formed on the TEC-15 glass substrate is then 

rinsed from the substrate using ethanol and collected in a beaker. The rinsed solution was then 

diluted further with ethanol in a test tube before running the ESI-MS. 
59

As shown in Fig. 8.4 (a and b) below: we can see the major peaks of polyaniline oligomers 

[9]; 183 m/z, 278 m/z, 365 m/z, 454 m/z, 545 m/z, 634 m/z, 725 m/z, 816 m/z, 907 m/z and 

996 m/z. These results match the data reported by Haiteng Deng and Gary J. Van Berkel at Oak 

Ridge National Laboratory, on a similar experiment [9]. As indicated in both spectra our newly

grown film is a mixture of aniline oligomers, ranging from a dimer to hendecamer (11-mer).

Figure 8.4 ESI-MS spectra of the newly formed thin film after rinsing, showing a mixture of 

oligomers (n-mers). (a) ESI-MS spectra reading of the rinsed solution with the 

instrument - target mass sensitivity set to 250 m/z. (b) ESI-MS spectra reading of the 

rinsed solution with the instrument – target mass sensitivity set to 750 m/z.

In the spectra of Fig. 8.4 a and b, the ESI-MS mass target sensitivity was set to 250 m/z and 

750 m/z respectively. For this reason, in Fig. 8.4a, the tetramer (4-mer) appears to have a higher 

concentration, than the other oligomers. Similarly, in Fig. 8.4b, the heptamer (7-mer) appears to 

have a higher concentration, than the other oligomers. 

In conclusion, we have obtained convincing information from both XPS and ESI-MS 

characterization techniques, indicating the growth of a new film on the substrate and the 

formation/presence of polyaniline. 

8.3 Effect of Electrochemical Treatment on Copper Diffusion 

A CdTe solar cell sample was prepared on an HRT-coated TEC-15 glass substrate. Both the 

CdS and CdTe layers were sputtered in the AJA-system. After the deposition on the CdTe layer, a 

~ 28 minute, 385 °C CdCl2 treatment was done. The samples had noticeable pinholes and were 

therefore ideal tests for the electrochemical polymerization treatment. Using the standard aniline 

solution, the standard electrochemical treatment was performed. Similar to the previous 
60

experiment, as shown in Fig. 8.5 below, half of the sample was electrochemically treated and the 

other half was left untreated as a control. 

Figure 8.5 Top surface view of the CdTe solar cell sample. (a) Sample placement inside the aniline 

solution - half of the sample immersed in the aniline solution. (b) Sample after Cu-Au 

deposition – 15 dot cells and 25 dot cells in the untreated and treated areas 

respectively.

After electro-polymerization the sample was metalized through a mask to form dot cells with 

Cu/Au, 35Å / 200 Å, respectively, by evaporation and then unintentionally diffused for 2 hours 

instead of the standard 45 minutes at 150 °C. I-V characteristics were measured using the Oriel

solar simulator.

(a) (b)

Figure 8.6 A 2 hour copper diffusion of an electrochemically treated and untreated CdTe solar cell 

device. (a) Open-circuit voltage (b) Short-circuit current.
61

(a) (b)

Figure 8.7 A 2 hr copper diffusion of an electrochemically treated and untreated CdTe solar cell device. 

(a) Fill factor and (b) Efficiency measurements.

 After 2 hours of copper diffusion treatment, our expectation was to see very poor solar cell

performance. However, as shown in Fig. 8.7b, the cells from the treated area have an average 

cell parameters of: VOC=0.79 V, JSC=20.2 mA/cm2, FF=64 % and efficiency =10.2 %. The cells 

from the untreated area have averages of: VOC=0.50 V, JSC=12.9 mA/cm2

, FF=33.3 % and 

Eff=2.85 %. Additionally, the data scatter in all categories was very narrow for the treated as 

compared to the untreated area cells. This indicates that pinholes are filled with an insulating 

material (polyaniline) and good shunt passivation appears to have been obtained. 

 However, conclusive results will await further tests on devices with our standard 45 minutes 

of copper diffusion rather than 2 hours of copper diffusion and using devices with a variety of 

pinhole densities. However, there is some indication that the electro-polymerization is providing 

is creating an ultra thin layer of polyaniline (conductive) on the CdTe surface that acts not only 

as a shunt Passivation but also as a diffusion barrier for the copper metal. Based on this result, 

this experiment opens a new door to the electrochemical treatment and further experiments are in 

progress. 
62

9. A Theory of Admittance Spectroscopy for Thin Film Photovoltaics

9.1 Introduction

Admittance spectroscopy has long been a routine characterization technique in 

semiconductor science and technology.[13] It provides information about the space charge 

distribution (capacitance-voltage, C-V) and the density of states distribution 

(capacitance-frequency, C-F) in crystalline, amorphous, and polycrystalline semiconductor 

materials and devices.[14] The standard understanding of admittance characterization is based on 

the model of a leaky flat plate capacitor. The admittance measurements conducted at different 

frequencies (ω), biases (V), and temperatures (T) are interpreted assuming a model where all the 

physical quantities (such as defect states and doping profiles) vary along one spatial coordinate 

perpendicular to the system electrodes. This assumption implies that the electrodes are 

equipotentials, so that there is no lateral current in the system. However, many practically 

important device structures, such as photovoltaics, have sufficiently resistive electrodes that are 

not equipotentials and exhibit significant currents in the lateral directions.

The purpose of this work is to introduce a theory of admittance characterization for a broad 

range of systems with resistive electrodes. The theory provides a practical method to effectively 

determine specific device parameters, such as shunt resistance, lateral resistance, 

non-uniformities, capacitance, and others. More specifically, we consider a distributed system 

exhibiting leaky photo-diode and capacitive properties, whose discrete equivalent circuit is 

shown in Fig. 9.1. The fundamental element of the system includes: 1) a photodiode 

characteristic of the semiconductor material; 2) a capacitance related to the material response to 

an ac voltage; and 3) a shunt resistance, all in parallel with each other. This model and the 

theoretical development make no reference to specific material combinations. Also, we consider 

both one-dimensional (1D) and two-dimensional (2D) systems.

Figure 9.1 Equivalent circuit of a device with a 

resistive electrode in the lateral direction and 

distributed diode, capacitance, and shunt 

resistance in the transverse direction.

A similar but simpler model, without distributed capacitance and shunt resistance, was 

analyzed earlier to describe dc operations of photovoltaics.[15] The previous work introduced a 

characteristic decay length, L0, of a small electric perturbation in the lateral direction. It was 

shown that the decay length delineates the region in which current is collected (the active area) 

and, therefore, influences the I/V characteristics of the device. In the present work, the concept of 

decay length is extended to include systems with distributed shunt resistance and capacitance, 

resulting in two new characteristic lengths, LR and LC, that describe the decay of an ac 

perturbation in the system and determine the frequency-dependent admittance. Simultaneously, 

various regimes of applied dc voltage are considered. We show that an understanding of these 

decay lengths plays a vital role in effective device characterization.
63

9.2 Theory

Our methodology consists of a qualitative analysis to provide a general overview of the 

phenomena and to calculate estimates of the frequency-dependent admittance. Then, starting 

from first principles, we present a rigorous derivation of the exact results for various testing 

biases and device structures. Using these results, we provide a detailed description of integrity 

and uniformity diagnostic techniques for thin-film photovoltaic devices with examples of 

practical applications.

A complete description of the decay length approach requires that we clearly define all 

conceivable scenarios. First of all we note the division into the regimes of geometrically large 

(l >> L) and small (l << L) cells, each of which can be either 1D or 2D systems. In addition, one 

has to discriminate between two types of electrode configurations, namely, substrate (e.g. 

CuIn(Ga)Se2 or a-Si:H) and superstrate (e.g. CdTe) devices. Therefore, there are a total of eight 

conceivable nominally different scenarios, with the four 1D scenarios presented in Fig. 9.2. A 

small electric perturbation at the input of the circuit in Fig. 9.1 will decay in the lateral direction 

over the characteristic length, L, determined by the electrode resistance and current leakage 

through the transverse resistors, diodes, and capacitors. The decay length L, together with the 

linear dimension of the electrode l, determines the active region of the system as shown for the 

various scenarios in Fig. 9.2. 

Figure 9.2 Examples of four possible 1D device scenarios: (a) small, substrate; (b) large, 

substrate; (c) large, superstrate; and (d) small, superstrate. The device components are 

labeled as M, T, S, and G for metal, TCO, semiconductor, and glass, respectively.

In the framework of the qualitative analysis, the characteristic decay length is estimated as 

the minimum of the three partial lengths, LR, LC, and Ld (decay length of an ac voltage due to 

diodes). Straightforward calculations show that R = RL / ρ , L 1/( cρω ) C = and Ld depends on the 

applied dc bias and is related to the previously defined dc decay length, L0. In these equations, ω

is the testing frequency, R is the specific shunt resistance, is the sheet resistance of the resistive 

electrode, and c is the specific capacitance. By applying qualitative arguments it is shown that 

the measured capacitance splits into high and low frequency regimes. In the low frequency 

regime, the capacitance is constant and depends on device parameters, while in the high 

frequency regime the measured capacitance is frequency dependent with ~ cC /(ωρ) for a large 

superstrate device.

In the rigorous derivation we provide a general formalism for calculating the

frequency-dependent admittance for a broad range of devices and testing scenarios. We show 

that the rigorous results are similar to the qualitative results with the two frequency regimes of 

the measured capacitance and conductance defined by a characteristic frequency, ωo, given as 

1/( )/1/1)( 2 2

o = ρω o + LLc R . Using our general formalism, we are able to provide exact results for 
64

the eight different scenarios discussed above. We show that for the cases of small superstrate 

cells our results reduce to the typical analysis of admittance results applied in the current 

standard practice, while for other types of cells there are important differences that must be 

considered for proper device characterization.

9.3 Practical Implications

We believe that our work introduces a diagnostic technique applicable to practically any

semiconductor junction. Here we discuss in more detail the case of thin-film photovoltaics to 

provide examples of device parameter evaluation. For definiteness, we focus on the substrate 

type of photovoltaics, particularly CuIn(Ga)Se2 with the typical structure (glass / metal / 

CuIn(Ga)Se2 / CdS / buffer) and the following parameters: buffer (TCO) layer sheet resistance of 

ρ ~ 10 Ω/□; current density jo ~ 0.03 A/cm2 (for 1 sun light intensity); shunt resistance R ~ 104

Ω⋅cm

2

; and capacitance c ~ 1 nF/cm2. Corresponding to these parameters are the decay lengths 

Lo ~ 0.3 cm and LR ~ 30 cm, and the characteristic frequency ωo ~ 109 rad/s. Note, however, that 

the current jo can be decreased and the length Lo increased by, respectively, four and two orders 

of magnitude by decreasing the light intensity from 1 sun to the typical ambient room light. In 

addition, we assume a testing frequency range of ω~ 1 to 10 rad/s, corresponding to the 

capacitive decay length LC ~ 10 - 104

cm.

Consider as an example a small cell of radius cm deposited as a current collection terminal 

on the buffer layer; the other terminal is connected to the metal electrode buried under the 

semiconductor. The inequalities and correspond to the low frequency regime of a 2D small cell 

with the conductance and capacitance given by, 

)/ln(

12

aL

G

ρ

π = and 

[ ] 



 





+ = )(

arctan

)/ln(

1

222

22

2

RoC

Ro

LLL

LL

aL

C

ρω

π

Measuring G and C under these conditions will give information about ρ and jo. The capacitance 

as a function of frequency is presented in Fig. 9.3.

Figure 9.3 Prediction of capacitance as 

a function of frequency for a small 2D 

substrate cell under the conditions 

discussed in the running text. The 

measured capacitance is constant in the 

region ω << ωo and goes as 1/ω in the 

region ω >> ωo, where ωo ~105

 rad/s.

Also, in the low frequency regime the measured frequency-dependent capacitance will have 

to be attributed to the intrinsic c(ω), if any, which was considered constant our formulation, thus 

giving the standard admittance spectroscopy information about the material related to space 

charge density and defect energy spectra. We note that the typical frequency dependence for c(ω)

is logarithmically weak and can hardly be mistaken for the above predicted C ∝ 1/ω in the high 

frequency region. 

Under the same conditions, decreasing the light intensity will eventually put the system into 

the regime where LR << Lo and LR becomes the predominant (smaller) decay length. This 

crossover, after which C and G become independent of light intensity, will determine the shunt 
65

resistance R. One conceivable outcome of such experimenting is that the crossover will take 

place at a light intensity which is above the expected value due to the presence of a shunt at 

distance LO from the cell. Therefore, our proposed capacitance diagnostics have the potential of 

detecting shunts and their distribution in the system. Another effect of low light intensity is that 

the characteristic frequency will change from ωo ~ 109 to 105 rad/s, well into the testing 

frequency range (as shown in Fig. 9.3), which makes it possible to observe the crossover 

between the high and low frequency regimes, thus determining ωo and related parameters.

Our work also provides additional examples related to the effects of different test biases as 

well as methods to measure other system parameters, such as Voc and ideality factor. We also 

detail how these diagnostic techniques can be applied to thin-film photovoltaics before the 

device is finished so that information related to system leakage can be determined prior to 

metallization.

9.4 Conclusions

In conclusion, we have developed a theory of ac response by systems of distributed diodes in 

parallel with resistors and capacitors connected through resistive electrodes that can represent 

many practical devices including photovoltaics and Schottky junctions. In particular, we have 

shown that: 

1. There exist three lateral decay lengths related to the system diodes, shunt resistors, and 

capacitances respectively that determine the effective frequency-dependent lateral decay 

length L and describe the physics of ac response in such distributed systems; 

2. The response is frequency-independent below a certain characteristic frequency, above 

which it strongly depends on testing frequency; 

3. The 1D and 2D systems behave similarly in the large device regime, l >> L, where l is the 

device lateral dimension, while in the small device regime, l << L, 2D systems exhibit 

certain unique behavior; 

4. Both the capacitance and conductance are described by closed form analytical expressions as 

functions of frequency and dc bias and are parametrically dependent on system material 

characteristics;

5. Our theoretical results establish a basis for a type of admittance characterization applicable 

to a wide variety of semiconductor structures including photovoltaics and Schottky junctions. 
66

10. References (from Sections 1 through 9) 

 

[1] B. Johs, C. Herzinger, J. Dinan, A. Cornfeld, J. Benson, Thin Solid Films 313, 137 (1998). 

[2] J. Chen, J. Li, D. Sainju, K. D. Wells, N. J. Podraza, and R. W. Collins, Proc. 4th World 

Conference on Photovoltaic Energy Conversion, (IEEE, Piscataway NJ, 2006), p. 475. 

[3] D. Dunstan, B. Gil, C. Priester, K. Homewood, Semicond. Sci. Technol. 4, 241 (1989). 

[4] X. Liu, A. Compaan and J. Terry, “X-ray absorption fine structure study of aging behavior of 

oxidized copper in CdTe films”, 31th IEEE Photovoltaic Spec. Conf., pp 267-270 

(0-7803-8707-4/05/.2005 IEEE.). 

[5] Visoly-Fisher, I., K. D. Dobson, J. Nair, E. Bezalel, G. Hodes and D. Cahen, Adv. Funct. 

Mater., 13(4), 289 - 299 (2003).

[6] X. Liu, A. Compaan and J. Terry, “Cu K-edge X-ray fine structure changes in CdTe with 

CdCl2 processing” , Thin Solid Films, 480-481 (2005), pp. 95-98. 

[7] B. E. McCandless, “Diffusion in CdS/CdTe thin film couples,” Proc. 16th European PVSEC, 

2000, 349. 

[8] D. Shaw and E. Watson. The diffusion of chlorine in CdTe. J. Phys. C: Solid State Phys.: 17 

(1984) 4945-4950. 

[9] Deng, Haiteng, Van Berkel, Gary J.; Anal. Chem. 1999, 71, 4284-4293. 

[10] Liao, Chuanping, Gu, Mingyuan, Thin Solid Films, 2002, 408, 37–42. 

[11] E.T. Kang, K.G. Neoh, K.L. Tan, Prog. Polym. Sci. 1998, 23, 277. 

[12] G. Min, Synth. Met. 1999, 102, 1163. 

[13] K. Schroder, Semiconductor Material and Device Characterization, John Wiley, 1998. 

[14] P. Blood and J. W. Orton, The electrical characterization of semiconductors: majority 

carriers and electron states, Academic Press, NY 1992. 

[15] V. G. Karpov, G. Rich, A. V. Subashiev, G. Dorer, J. Appl. Phys. 89, 4975 (2001). 
67

11. Publications

11.1 Refereed Papers Published or in Press

 

1. “Piezo-effect and physics of CdS based thin-film photovoltaics”, Diana Shvydka, J. 

Drayton, A. D. Compaan, and V. G. Karpov, Appl. Phys. Lett. 87, pp. 123505-1-3 

(2005). 

2. “Physical model of CdS based thin-film photovoltaic junctions”, M. L. C. Cooray and V. 

G. Karpov, Appl. Phys. Lett., 88 , p. 093508 (2005). 

3. “Cu K-edge EXAFS Studies Of CdCl2 Effect On CdTe Solar Cell”, Xiangxin Liu, A. D. 

Compaan, and Jeff Terry, MRS Symp. Proc. 865, edited by W. Shafarman, T. Gessert, S. 

Niki, S. Siebentritt, F4.2 (2005). 

4. “Photoluminescence from Ion Implanted CdTe Crystals”, Xiangxin Liu and Alvin D. 

Compaan, MRS Symp. Proc. 865, edited by W. Shafarman, T. Gessert, S. Niki, S. 

Siebentritt, F5.25 (2005).

5. “Photoluminescence and Extended X-ray Absorption Fine Structure Studies on CdTe 

Material”, Xiangxin Liu, Ph.D. Thesis: The University of Toledo, 2006 (unpublished, 

available at OhioLINK- Electronic Theses and Dissertations 

Center, http://www.ohiolink.edu/etd/).

6. “Spatial and Temporal Variations in Electronic Transport Through a CdTe-Based 

Schottky Barrier”, Diana Shvydka, V. Parikh, V.G. Karpov and A.D. Compaan, MRS 

Symp. Proc. 865, edited by W. Shafarman, T. Gessert, S. Niki, S. Siebentritt, p. F12.2, 

(2005). 

7. “Physics of CdTe Photovoltaics: From Front to Back”, V.G. Karpov, Diana Shvydka, and 

Yann Roussillon, MRS Symposium Proceedings 865, edited by W. Shafarman, T. 

Gessert, S. Niki, S. Siebentritt, p. F10.1, San Francisco, March 28-April 1 2005.

8. “Lateral Nonuniformity and Mesoscale Effects in Giant Area Electronics, V. G. Karpov, 

Diana Shvydka, and Yann Roussillon”, MRS Symposium Proceedings 870E, edited by M. 

S. Shur, P. Wilson, M. Stutzmann, p. H2.2, San Francisco, March 28-April 1 2005. 

9. “Power generation in random diode arrays”, Diana Shvydka and V. G. Karpov, Phys. Rev. 

B 71, pp. 115314-1-5 (2005). 

10. “Real Time Analysis of Magnetron-Sputtered Thin-Film CdTe by Multichannel 

Spectroscopic Ellipsometry”, Jian Li, Jie Chen, J. A. Zapien, N. J. Podraza, Chi Chen, J. 

Drayton, A. Vasko, A. Gupta, S. L. Wang, R. W. Collins, and A. D. Compaan, Mater. 

Res. Soc. Symp. Proc. 865, 9-14 (2005). 

11. “Propagating Electric Impulses in Thin Film PV”, T. K. Wilson, Diana Shvydka and V. 

G. Karpov, Proceedings of IEEE 4th World Conference on Photovoltaic Energy 

Conversion 2006 (IEEE, Piscataway NJ ), p. 471-474. 

12. “Phenomenological model of CdS based thin film photovoltaics”, M. L. C. Cooray and V. 

G. Karpov, Proceedings of IEEE 4th World Conference on Photovoltaic Energy 

Conversion 2006 (IEEE, Piscataway NJ), p. 542 - 545. 

13. “Piezo-photovoltaic coupling in thin-film CdS/CdTe solar cells”, Jennifer Drayton, M. 

Mitra, and Diana Shvydka, Proceedings of IEEE 4th World Conference on Photovoltaic 

Energy Conversion 2006 (IEEE, Piscataway NJ), p. 491 – 494. 

14. “Pressure dependence of photovoltaic parameters in thin film Cu(In,Ga)Se2 solar cells”, 

Diana Shvydka, J. Drayton1, M. Mitra, S.X. Marsillac, F. Jacob, , Proceedings of IEEE 
68

4th World Conference on Photovoltaic Energy Conversion 2006 (IEEE, Piscataway NJ),

p. 465 – 467. 

15. R.W. Collins, C. Chen, M.W. Horn, S. Pursel, and C. Ross,"The ultimate in real-time 

ellipsometry: multichannel Mueller matrix spectroscopy", Applied Surface Science 253, 

38-46 (2006). 

16. J. Li, J. Chen, N.J. Podraza, and R.W. Collins, "Real time spectroscopic ellipsometry of 

sputtered CdTe: effect of growth temperature on structural and optical properties",

Conference Record of the 2006 IEEE 4th World Conference on Photovoltaic Energy 

Conversion 2006, (IEEE, Piscataway NJ) p. 392-395. 

17. J. Chen, J. Li, D. Sainju, K.D. Wells, N.J. Podraza, and R.W. Collins, "Multilayer 

analysis of the CdTe solar cell structure by spectroscopic ellipsometry", Conference 

Record of the 2006 IEEE 4th World Conference on Photovoltaic Energy Conversion 

2006, (IEEE, Piscataway NJ, 2006) p. 475-478. 

18. R.W. Collins, A.D. Compaan, S. Marsillac, A. Parikh, J. Chen, and J. Li, "Optical 

analysis of II-VI alloys and structures for tandem PV", FY07 DOE Review (presented 

April 17-19, 2007). 

19. R. W. Collins, A.D. Compaan, J. Li, and J. Chen, "RTSE studies of the fabrication of 

high efficiency CdTe PV", FY07 DOE Review (presented April 17-19, 2007). 

20. J. Li, J.A. Stoke, N.J. Podraza, D. Sainju, A. Parikh, X. Cao, H. Khatri, N. Barreau, S. 

Marsillac, X. Deng, and R.W. Collins, "Analysis and optimization of thin film 

photovoltaic materials and device fabrication by real time spectroscopic ellipsometry", 

Photovoltaic Cell and Module Technologies, edited by B. von Roedern and A.E. 

Delahoy, (SPIE, Bellingham, 2007) vol. 6651, Art. No. 6651-07, p. 1-14. 

21. Victor Plotnikov and Alvin Compaan, “CdS/CdTe Solar Cells Made by High-rate 

Magnetron Sputtering,: Mater. Res. Soc. Symp. Proc. 1012, Y12.28 (2007). 

22. L. Weinhardt, S. Liu, J. Zhou, M. Baer, T. Hofmann, O. Fuchs, A. Compaan, X. Wu, 

and C. Heske, “X-ray and Electron Spectroscopy Investigation of Interfaces and Surfaces 

in CdTe Thin Film Solar Cells,” Mater. Res. Soc. Symp. Proc. 1012, Y7.6 (2007). 

23. V. Parikh, A. Vasko, A.D. Compaan, and S.X. Marsillac, “Transparent Back Contacts in 

CdTe/CdS: Evaluation for Tandem Cells,” Mater. Res. Soc. Symp. Proc. 1012, Y2.8 

(2007). 

24. V.G. Karpov, M.L.C. Cooray, and Diana Shvydka, “Physics of ultrathin photovoltaics,” 

Appl. Phys. Lett. 89, 163518 (2006). 

25. T. K. Wilson, Diana Shvydka and V. G. Karpov, “Propagating Electric Impulses in Thin 

Film PV”, Proc. 32nd IEEE Photovoltaic Specialists Conference-2006, Waikoloa, Hawaii 

May 7-12, 2006 (IEEE Piscataway, N.J.) p. 471. 

26. M. L. C. Cooray and V. G. Karpov, “Phenomenological model of CdS based thin film 

photovoltaics”, Proc. 32nd IEEE Photovoltaic Specialists Conference-2006, Waikoloa, 

Hawaii May 7-12, 2006 (IEEE Piscataway, N.J. p. 542. 

27. Jennifer Drayton, M. Mitra, and Diana Shvydka “Piezo-photovoltaic coupling in 

thin-film CdS/CdTe solar cells”, Proc. 32nd IEEE Photovoltaic Specialists 

Conference-2006, Waikoloa, Hawaii May 7-12, 2006 (IEEE Piscataway, N.J.), p. 491. 

28. Diana Shvydka, J. Drayton, M. Mitra, S.X. Marsillac, F. Jacob, “Pressure dependence of 

photovoltaic parameters in thin film Cu(In,Ga)Se2 solar cells”, Proc. 32nd IEEE 

Photovoltaic Specialists Conference-2006, Waikoloa, Hawaii May 7-12, 2006 (IEEE 

Piscataway, N.J.), p.465. 
69

29. M.L.C. Cooray and B.G. Karpov, “Long-range random potential in thin-film structures,” 

Phys. Rev. B. 75, 1 (2007). 

30. Xiangxin Liu, A. D. Compaan, Kai Sun and Jeff Terry, “EXAFS and High Resolution 

TEM Studies of Cu at the Back Contact in Sputtered CdS/CdTe Solar Cells”, 33rd IEEE 

Photovoltaic Specialists Conference, 2008. 

31. Victor Plotnikov, Xiangxin Liu and Alvin D. Compaan, “Studies of Cu location near the 

back contact of CdS/CdTe solar cells”, 33rd IEEE Photovoltaic Specialists Conference, 

2008. 

32. J. Li, N. J. Podraza, R. W. Collins, “Real time spectroscopic ellipsometry of sputtered 

CdTe, CdS, and CdTe1-xSx thin films for photovoltaic applications”, Physica Status Solidi 

(a), 205(4), 901-904 (2008).

33. P. Petrik, N. Q. Khánh, J. Li, J. Chen, R. W. Collins, M. Fried, G. Z. Radnóczi, T. Lohner, 

J. Gyulai, “Ion implantation induced disorder in single-crystal and sputter-deposited 

polycrystalline CdTe characterized by ellipsometry and backscattering spectrometry”, 

Physica Status Solidi (c), 5(5), 1358-1361 (2008). 

34. J. Li, J. Chen, R. W. Collins, “Analysis of the dielectric functions of CdS and CdTe for 

grain size, stress, and temperature: potentialities for on-line monitoring”, Conference

Record of the 33rd IEEE Photovoltaic Specialists Conference, (IEEE, Piscataway NJ, 

2008). 

35. Amorphous and Nanocrystalline Semiconductors: Science and Technology, edited by P. 

C. Taylor, H.M. Branz, R.W. Collins, and E.A. Schiff (Elsevier, Amsterdam, 2008) 850 

page book. 

 

11.2 Poster or Oral Presentations

1. “Understanding the Physics of CdS-based PV: Band Diagram, Interfaces, and 

Nonuniformities”, V. G. Karpov, D. Shvydka, J. Drayton, M. L. C. Cooray, and A. D. 

Compaan, DOE Solar Energy Technologies Program Review Meeting, November 7-10, 

Denver, Colorado, DOE/GO-1020006-2245, March (2006). 

2. “Piezo-effect in CdS-based solar cells”, Diana Shvydka, Jennifer Drayton, Mukut Mitra, 

Proceedings of the 19th National CdTe R&D Meeting, Editors: H. S. Ullal. P. V. Meyers, 

and C. Ferekides, Golden, Colorado, March 9-10 (2006). 

3. “Indicative facts and device model”, V. G. Karpov, D. Shvydka, J. Drayton, L. Cooray, 

and A. D. Compaan, Proceedings of the 19th National CdTe R&D Meeting, Editors: H. S. 

Ullal. P. V. Meyers, and C. Ferekides, Golden, Colorado, March 9-10 (2006). 

4. “Back contact and nonuniformity”, V. G. Karpov, D. Shvydka, J. Drayton, Y. Roussillon, 

and A. D. Compaan, Proceedings of the 19th National CdTe R&D Meeting, Editors: H. S. 

Ullal. P. V. Meyers, and C. Ferekides, Golden, Colorado, March 9-10 (2006). 

5. “Piezo-effect in CdS/CdTe solar cells”, Diana Shvydka, J.Drayton, V. G. Karpov and 

A.D. Compaan, National CdTe R&D Team Meeting; Golden, CO; May 5-6, 2005.

6. J. Li, A. Parikh, J. Chen, D. Sainju, L.R. Dahal, J.A. Stoke, N.J. Podraza, and R.W. 

Collins, “Accurate determination of dielectric functions of thin films and interfaces in 

photovoltaic devices: critical issues in optical modeling”, 2008 Materials Research 

Society spring meeting. 

7. J. Li, J. Chen, R. W. Collins, “Analysis of the dielectric functions of CdS and CdTe for 

grain size, stress, and temperature: potentialities for on-line monitoring”, the 33rd IEEE 

Photovoltaic Specialists Conference (2008). 
70

11.3 Publications and Presentations at DOE Program Reviews

1. Alvin D. Compaan, Robert W. Collins, Victor Karpov, Dean Giolando “Fabrication and 

Physics of CdTe Devices by Sputtering,” DOE Solar Energy Technologies Program 

Review/Peer Review Meeting, Denver, Apr 17-19, 2007. 

2. R. W. Collins, A.D. Compaan, Jian Li, and Jie Chen, “RTSE Studies of the Fabrication of 

High Efficiency CdTe PV,” DOE Solar Energy Technologies Program Review/Peer 

Review Meeting, Denver, Apr 17-19, 2007. (Manuscript available on-line at: 

http://www1.eere.energy.gov/solar/review_meeting/pdfs/p_15_collins_univ_toledo.pdf) 

3. A.D. Compaan, oral invited review presentation at the Solar Program Review, 

“Fabrication and Physics of CdTe Devices by Sputtering” Denver, CO, April 18, 2007. 

(Presentation available on-line at: 

http://www1.eere.energy.gov/solar/review_meeting/pdfs/tf_4_compaan_univ_toledo.pdf) 
71

12. Project Personnel

12.1 Postdoctoral Associates / Research Assistant Professors

1. Xiangxin Liu (Ph.D., University of Toledo, 5/2006)(5/06–) 

2. Diana Shvydka (Ph.D. University of Toledo, 5/2002)(3/1/05 - 10/31/2006) 

12.2 Graduate Students (with Principal Advisor) 

1. Victor Plotnikov (Compaan) 

2. Naba Raj Paudel (Compaan) 

3. Jian Li (Collins)

4. Anuja Parikh (Collins)

5. Jie Chen (Collins)

6. Lilani Cooray (Karpov) 

7. Misle Tessema (Giolando)

12.3 Undergraduate Students

NSF Research Experience for Undergraduates (REU), Summer 2006, 2007, 2008; 

(Supported by NSF with some matching from this contract.)

1. Nathan Reaver (advisor, Compaan) 

2. Ryan Zeller (advisor, Compaan) 

3. Craig McClellan (advisor, Compaan) 

4. Casey Bennett (advisor, Compaan)

5. Mary Lin (advisor, Compaan) 

6. Jason Owens (advisor, Compaan)

7. Sheriff Ceesay (advisor, Compaan)

12.4 Technical Assistants

Terry Kahle (3/03–) 
F1146-E(10/2008) 

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1. REPORT DATE (DD-MM-YYYY)

April 2009

2. REPORT TYPE

Subcontract Report

3. DATES COVERED (From - To)

3/1/2005 - 11/30/2008

4. TITLE AND SUBTITLE

Fabrication and Physics of CdTe Devices by Sputtering: 

Final Report, 1 March 2005 - 30 November 2008

5a. CONTRACT NUMBER

DE-AC36-08-GO28308

5b. GRANT NUMBER

5c. PROGRAM ELEMENT NUMBER

6. AUTHOR(S)

A. Compaan, R. Collins, V. Karpov, and D. Giolando

5d. PROJECT NUMBER

NREL/SR-520-45398

5e. TASK NUMBER

PVB91210

5f. WORK UNIT NUMBER

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University of Toledo

Dept. of Physics and Astronomy

Toledo, Ohio 43808

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ZXL-5-44205-01

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National Renewable Energy Laboratory

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NREL

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14. ABSTRACT (Maximum 200 Words)

The three overall emphases of this subcontract were the following: 1) improving our understanding of key aspects of 

CdS/CdTe solar cell device physics, 2) improving our understanding of magnetron sputtering and increasing the 

sputter deposition rate while maintaining high device quality, and 3) reducing the thickness of CdTe layers in the 

CdS/CdTe cell below 0.5 μm while maintaining voltage and fill factor.

15. SUBJECT TERMS

PV; device physics; solar cells; magnetron sputtering; spectroscopic ellipsometry; CdTe; CdS; deposition rate; device 

quality; admittance spectroscopy;

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