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

Office of Energy Efficiency & Renewable Energy

National Renewable Energy Laboratory 

Innovation for Our Energy Future 

Studies of Basic Electronic 

Properties of CdTe-Based Solar 

Cells and Their Evolution During 

Processing and Stress 

Annual Technical Report 

1 November 2005 — 31 October 2006 

J. Beach, F.H. Seymour, V.I. Kaydanov, 

and T.R. Ohno 

Colorado School of Mines 

Golden, Colorado 

Subcontract Report 

NREL/SR-520-41097 

January 2007 

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

Properties of CdTe-Based Solar 

Cells and Their Evolution During 

Processing and Stress 

Annual Technical Report 

1 November 2005 — 31 October 2006 

J. Beach, F.H. Seymour, V.I. Kaydanov, 

and T.R. Ohno 

Colorado School of Mines 

Golden, Colorado 

NREL Technical Monitor: Harin S. Ullal 

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

Subcontract Report 

NREL/SR-520-41097 

January 2007 

National Renewable Energy Laboratory

1617 Cole Boulevard, Golden, Colorado 80401-3393 

303-275-3000 • www.nrel.gov 

Operated for the U.S. Department of Energy 

Office of Energy Efficiency and Renewable Energy 

by Midwest Research Institute • Battelle 

Contract No. DE-AC36-99-GO10337
 

This publication was reproduced from the best available copy 

Submitted by the subcontractor and received no editorial review at NREL

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PREFACE

This report presents the CSM Team activities for the period (1 November 2005-31 

October 2006) under the extension of the subcontract ADJ-2-30630-05, “Studies of Basic 

Electronic Properties of CdTe-based Solar Cells”. The tasks for the continuing studies in the 

project extension were defined as follows: 

Task 1. Study of deep electronic states controlling Voc in CdTe/CdS thin film solar cells 

Task 2. Test different cell growth approaches that can lead to higher Voc in these cells. 

In our preceding studies in this project (see Final 2005 Report to NREL), combined 

admittance spectroscopy (AS) and capacitance transients (CTr) measurements were used to detect 

and characterize numerous deep electronic states (DES) in thin film CdTe/CdS cells. The AS 

approach was applicable to the DES with the characteristic trap times, , in the range of 

. The CTr measurements extended available the characteristic time range to 

minutes, hours and even days. A new measurement system was developed that permitted 

frequency- and bias-dependent admittance measurements in a wide range of temperatures. In 

addition, we could measure very long transients (up to tens of hours) at a fixed and very stable 

temperature. 

t τ

stos 2 7 10 10 − −

AS/CTr measurements and the data analysis led to important and sometimes unexpected 

results. Among the latter were: 

(a) Concentrations of detected traps, , in CdTe cells fabricated at different facilities are 

rather high, exceeding “doping level” commonly determined with the C-V profiling, 

. That makes application of common approaches to AS, DLTS and C-V data 

analysis based on the assumption of low relative trap density questionable. In particular, 

the trap concentration (from AS and DLTS) is underestimated while the doping level 

derived from C-V measurements can be strongly overestimated. 

Nt

N −VC

(b) Changes in processing that diminish cell efficiency often are accompanied with reduced 

estimated trap concentration. 

The AS and CTr measurements in a temperature range of –1900

C to ~ +800C were 

performed on CdTe cells subjected to various combinations of CdCl2 treatment and doping with 

Cu (the cells were provided by Dr. D. Albin (NREL)). These studies revealed traps for majority 

i
carrier holes, as well as for minority electrons with activation energy varying from 0.13 eV to 

~0.8 eV and apparent capture cross sections varying by four orders of magnitude. Manifestations 

of particular traps in as-prepared cells, specifically their concentrations, as well as their behavior 

under stress were influenced by variations in CdCl2 treatment and Cu doping. 

This report presents the results of the continued studies of DES (Task 1), including: 

(a) analysis of the factors that affect the trap signatures derived from AS and CTr measurements, 

such as activation energy capture cross sections, as well as the trap density estimates 

(b) comparative studies of the cells received from four different sources and prepared with 

significant variations in the cell structure and processing procedures 

 In our previous report we presented and discussed our activities in the cell processing 

aimed at improvement of the cell performance. Basic deposition techniques in use were: 

• Chemical bath deposition for CdS layer 

• Electrodeposition and gas transport deposition for CdTe 

• Thermal evaporation for the back contact structure: Cu/ZnTe/gold 

Deposited layers were subjected to various post-deposition treatments including: 

• Anneal of CdS with and without CdCl2 prior to the CdTe deposition 

• CdTe postdeposition treatment with CdCl2 

• Cu/ZnTe postdeposition annealing 

This report presents continuing studies aimed at the processing improvement and 

optimization (task 2). This time the only CdTe deposition method was gas transport deposition 

with the “Gas-Jet” system designed and developed at CSM. For CdS we added thermal 

evaporation. Since the performance of the cell was sensitive to the contact annealing stage, the 

post -Cu/ZnTe anneal was optimized. These and other modifications of the processing procedure 

led to improvement of the cell performance, in particular to increase of open circuit voltage, 

. The best cell grown on the tin oxide coated soda lime glass without front contact buffer, 

demonstrated an efficiency of 12.6% (not NREL verified). 

VOC

ii
TABLE OF CONTENTS 

PREFACE .......................................................................................................................................i 

TABLE OF CONTENTS ............................................................................................................. iii 

LIST OF FIGURES........................................................................................................................v 

LIST OF TABLES .......................................................................................................................vii 

1. FUNDAMENTALS OF ADMITTANCE CHARACTERIZATION TECHNIQUES ............1 

 1.1 Basic Equations ..................................................................................................................1 

 1.1.1 Admittance spectroscopy .........................................................................................1 

 1.1.2 Capacitance transients ..............................................................................................3 

 1.2 Stretched Exponential Parameter β ....................................................................................3 

1.3 Temperature Dependence of Characteristic Frequency/Characteristic Time.....................5 

 1.4 Estimating and with Arrhenius Plot.......................................................................8 Ea σ a

 1.5 Estimating Trap concentrations from Capacitance Measurements ....................................9 

2. SCAPS SIMULATION..........................................................................................................11

2.1 DES Concentration Gradient............................................................................................11 

2.2 Fermi Level Pinning, Distorted and Non-Exponential Behavior ...............................13 Ea

3. CELL PRODUCTION AND STANDARD TESTING PROCEDURES ..............................20 

 3.1 Production Processes for CdTe Solar Cells Used in Experiments ...................................20 

 3.1.1 Cells produced by D. Albin at NREL.....................................................................20 

 3.1.2 Cells produced at IEC.............................................................................................21 

 3.1.3 Cells produced by X. Wu at NREL........................................................................21 

3.2 Standard Testing Procedures ............................................................................................21 

 3.2.1 J-V and J-V-T tests.................................................................................................22

 3.2.2 Quantum efficiency ................................................................................................22 

 3.2.3 Capacitance-voltage profiling at varying temperature (C-V-T).............................22 

 3.2.4 Admittance spectroscopy (AS)...............................................................................23 

 3.2.5 Capacitance transients (CTr) ..................................................................................23 

 3.2.6 Temperature............................................................................................................23 

iii
4. RESULTS AND DISCUSSION ............................................................................................24 

 4.1 NREL (D.Albin) Cells......................................................................................................24 

 4.1.1 J-V, QE, C-V results...............................................................................................25 

 4.1.2 J-V-T Results..........................................................................................................27 

 4.1.3 AS Results ..............................................................................................................31 

 4.1.4 CTr Results.............................................................................................................33 

4.2 IEC Cells ..........................................................................................................................35 

 4.2.1 J-V, QE, C-V results...............................................................................................36 

 4.2.2 AS and CTr results .................................................................................................39 

 4.3 Cells Fabricated by X. Wu ...............................................................................................41

5. IMPROVEMENT OF THE CELL PERFORMANCE ..........................................................45 

6. REFERENCES.......................................................................................................................49 

7. APPENDICES........................................................................................................................51 

7.1 Personnel ..........................................................................................................................51 

7.2 Laboratory Improvements ................................................................................................51 

7.3 Publications/Presentations................................................................................................51 

iv
LIST OF FIGURES 

Figure 1.1 Capacitance and conductance as a function of Cp Gp

 AC signal frequency...........................................................................................................2 

Figure 1.2 Admittance spectroscopy example showing goodness of fit with 

 phenomenological parameter β=0.5 ...................................................................................4 

Figure 1.3 and Cp p ddC ln( ) ω vs. AC frequency at different temperatures 

 for a CdTe solar cell ...........................................................................................................5 

Figure 1.4. Conductance versus temperature for a range of frequencies. Gp

 The top red line corresponds to 1 MHz and the bottom line corresponds to 1 kHz. 

 The blue lines highlight peaks from two DES with different T )( ..................................6 ω t

Figure 1.5 Capacitance Cp versus temperature for a range of frequencies. The bottom 

 red line corresponds to 1 MHz and the top line corresponds to 1 KHz .............................7 

Figure 1.6. CTr measurements on CdTe cell. (tC ) p for the complete cycle (left) and 

( )

( )td

p tdC

ln

versus ln( )t for the forward bias +0.5V pulse (right)..........................................8 

Figure 2.1 Band diagrams for SCAPS model at -190o

C and -100oC. The conduction and 

 valence bands are in black, the Fermi level in blue, and the DES in red..................12 VCd

Figure 2.2 SCAPS simulated Cp versus T graph for a range of frequencies. The bottom red line 

corresponds to 1 MHz and the top line corresponds to 1 kHz .........................................13 

Figure 2.3 SCAPS simulation band diagrams for CdTe solar cells at T=-170o

C showing 

 Fermi level (blue line) and pinning by high concentration trap level (red line) for cell A 

 with 8 μm CdTe layer (left) and green line for cell B with 3 μm CdTe layer (right).......14 

Figure 2.4 SCAPS simulated 8 μm and 3 μm cells for T=-170o

C. DES energy levels 

 relative to Fermi level (left). Free carrier density (right) ................................................15 

 

Figure 2.5 SCAPS simulated 8 μm and 3 μm cells. Hole capture and emission rates (left) 

 and ionized DES density (right) .......................................................................................16 

Figure 2.6 SCAPS simulated 8 μm and 3 μm cells. DES hole capture/emission density at 

equilibrium (left) and DES characteristic frequency ft (right).............................................17 

Figure 2.7 SCAPS simulated 8 μm and 3 μm cells at -170o

C. Cp versus simulated AC 

signal frequency (left) and dCp/dln(ω) versus simulated AC signal frequency (right)....18 

 

Figure 2.8 Arrhenius plot with regression lines for SCAPS simulated 8μm and 3μm cells ....19 

v
Figure 4.1 J-V curves for NREL cells with yes/no Cu and yes/no CdCl2 treatments ..............25 

Figure 4.2 QE curves for NREL cells with yes/no Cu and yes/no CdCl2 treatments...............26 

Figure 4.3 Room temperature C-V profiles for NREL cells with yes/no Cu 

 and yes/no CdCl2 treatments ...........................................................................................27 

Figure 4.4 J-V-T curves for the yes-Cu/yes-CdCl2 cell at 10o C increments ranging from 

 20o

C to -40oC (blue), -50oC to -110oC (green), and -120oC to -180oC (red) ...................28 

Figure 4.5 and versus T graphs for yes/no Cu and yes/no CdCl VOC SC J 2 cells......................29 

Figure 4.6 versus T graphs for cells with yes/no Cu and yes/no CdCl ROC 2 treatments ..........30 

Figure 4.7 AS graphs of Cp versus T for NREL cells with yes/no Cu and yes/no CdCl2 

 treatments. Three detected DES signatures are labeled H1, H2, and H3...............................31 

Figure 4.8 The amplitude of capacitance transient caused by the FB charging pulse for 

NREL cells with yes/no Cu and yes/no CdCl

ΔC

2 treatments. Temperature dependence.....34 

Figure 4.9 Arrhenius plot for NREL cells with yes/no Cu and yes/no CdCl2 treatments. 

 H1, H2, H3 were detected with AS, and E2, E3, H4, H5 were detected with CTr..........35 

Figure 4.10 J-V curves for IEC cells with yes/no Cu and yes/no CdCl2 ....................................37 

Figure 4.11 QE curves for IEC cells with yes/no Cu and yes/no CdCl2.....................................37 

Figure 4.12 Room temperature C-V profiles for IEC cells with yes/no Cu 

 and yes/no CdCl2 .............................................................................................................38 

Figure 4.13 and versus T for IEC cells with yes/no Cu and yes/no CdCl VOC SC J 2 ..................39 

Figure 4.14 Cp versus T for IEC cells. Three detected DES signatures are labeled H1, 

 H2, and H3 .......................................................................................................................40 

Figure 4.15 Arrhenius plot for IEC cells detected with AS (H1, H2, H3) and CTr (E3, H5) ....40 

Figure 4.16 Arrhenius plot for DES in the cells produced by Wu and detected 

 with AS (top) and CTr (bottom).......................................................................................43 

Figure 5.1 J-V curves for RTA temperatures bracketing the optimum efficiency ...................46 

Figure 5.2 Voc vs. cell efficiency for cells processed with CBD or evaporated CdS, 

 and CdTe deposited in gas-jet system with N2 or He carrier gas .....................................47 

vi
LIST OF TABLES

Table 4.1 Performance parameters for Albin’s cells with yes/no Cu and yes/no CdCl2 ...........25 

Table 4.2 Estimate of the DES energy level based on freeze out temperature SC J

 for Albin’s cells with yes/no Cu and yes-CdCl2 treatments.............................................30 

Table 4.3 Summary of detected AS signatures H1 through H3 for NREL cells 

 with yes/no Cu and yes/no CdCl2 treatments ...................................................................32 

Table 4.4 Summary of detected CTr minority carrier trap signatures E2, E3, 

 and majority carrier trap signatures H4, H5 for NREL cells with yes/no Cu 

 and yes/no CdCl2 treatments ............................................................................................34 

 

Table 4.5 J-V performance results for IEC cells with yes/no Cu and yes/no CdCl2 ..................36 

Table 4.6 The DES signatures detected by AS ( H1 through H3) and CTr (E3, H5) ................39 

Table 4.7 J-V characterization results for cells from X. Wu......................................................42 

Table 4.8 DES signatures determined with AS and CTr in cells produced by X. Wu...............44 

Table 5.1 The effect of the RTA temperature on cell performance ...........................................46 

vii
1. FUNDAMENTALS OF ADMITTANCE CHARACTERZATION TECHNIQUES 

1.1. Basic Equations 

1.1.1 Admittance spectroscopy. 

A small amplitude (10mV) AC signal is applied to the device over a range of frequencies 

and the admittance Y and phase angle θ are recorded. From these the p-mode (parallel 

equivalent circuit of capacitor and resistor) capacitance C

f

p and conductance Gp are calculated as: 

( ) ( )

f

YSin

f

Y Cp π

θ

π 22

Im == , p Re( ) == (θ ) YCosYG (1.1) 

At sufficiently low frequencies and high temperatures the electrical charges for the traps 

near the “crossing point” (Fermi, EF , and DES energy, , levels are crossing) follow the AC 

voltage oscillations and directly contribute to the measured capacitance and AC conductance. At 

high frequencies or low temperatures these trap charges cannot follow the AC signal resulting in 

a lower measured capacitance and saturation in the 

Et

(ω) Gp dependence. The parallel mode 

capacitance ( ) ω and conductance Cp (ω) Gp , as a function of the AC signal angular frequency 

ω = 2π f are given by [2]: 

 ( ) ( )2

0

/1 t

t

p d

C CC

ωω

ω

+

+= , ( ) ( )

( )2

2

/1

/

t

t

o

t

p d

G GG

ωω

ωω

ω

+

+= , , (1.2) CG ωttt

00 =

where and represent the diode capacitance and conductance in the absence of traps, and 

 and are the maximum additional capacitance and conductance 

resulting from the traps; is the angular characteristic frequency, inverse of the trap 

characteristic time, . Figure 1.1 illustrates the capacitance and conductance as a function of 

frequency. 

Cd Gd

)0( 0 CC tt ω →= )( 0 GG tt ω ∞→=

ω t

t τ

1
0 G t

G d

ω t

ln (ω )

( ) G p ln

ln (ω ) ω t

0 C t

C d

C p

0 G t

G d

ω t

ln (ω )

( ) G p ln

ln (ω ) ω t

0 C t

C d

C p

Figure 1.1. Capacitance and conductance as a function of AC frequency. and are 

the depletion region contributions; and are the trap contributions to the 

capacitance and conductance. 

Cp Gp Cd Gd

( ) 0 Ct ω ( ) 0 Gt ω

The characteristic frequency corresponds to the inflection points of maximum slope and 

can be estimated based on Equations (1.3) or (1.4): 

ωt

( )

( )

( )

ω

ω

ω

ω

ωω p p

peakt

G

and

d

dC

for ln = (1.3) 

( ) ( )

ω

ω

ω

ω

ωω

d

dG

and

d

dC

for p p

t peak = 3 (1.4) 

This approach works well when there is a single DES level that stands out. 

Characteristic frequency is determined by the electron and hole capture ( c ) and emission 

( e) rates: 

nnpptt +++== ecec −1 τω (1.5) 

For holes 

TkEENvc (1.6) vppp BVF =σ −− /)(exp[

TkEENve (1.7) vppp BVt =σ −− /)(exp[

where is the hole capture cross section, is the hole thermal velocity, is the valence 

band effective density of states, and and are the Fermi, DES, and valence band edge 

energy levels, respectively. The equations are similar for the electron capture and emission rates, 

with respect to the conduction band. 

σ p p v Nv

EE tF , EV

2
1.1.2 Capacitance transients 

If a perturbation is applied to the diode electron system, such as light or electrical bias, 

shifts with respect to , and the trap occupancy changes to a new steady state level. When 

the perturbation is removed at the time moment 

EF

Et t n

t = 0 , relaxes from this steady state level 

 back to the initial equilibrium level 

nt

t(tn = 0) (tn ∞= ) t . For a single level DES, assuming that 

the process is reversible and that there are no meta-stable states, this occurs according to the 

exponential equation: 

 () ( ) ( ) ( ) ( ) −∞−∞= (tnnntn /exp0 τ ) tt t t t , (1.8) 

where is a characteristic time dominated by the largest value for , , or .(1.5). −1 =ωτ tt n c n e p c p e

Correspondingly the CTr is described by the equation 

( ) ( ) /exp τ 0

p td −+= tCCtC , (1.9) 

The characteristic time τ can be determined by looking for peaks in ( )

( )td

p tdC

ln

where . peak τ = t

1.2 Stretched Exponential Parameter β

The AS and CTr experimental data frequently did not adequately fit the basic equations 

above. A much better fit was found with the addition of a phenomenological stretched 

exponential parameter β in the basic capacitance equation. The modified equation becomes: 

 ( ) ( ) ( ) β

ωω

ω

2

0

/1 t

t

p d

C CC

+

+= (1.10) 

Figure 1.2 shows the AS curves for experimental data from a CdTe solar cell, a model fit with 

β = 5.0 , and a model fit with β =1. 

3
Freq (kHz)

Cp (nF cm-2)

Experimental

Model Beta=0.5

Model Beta=1.0

Freq (kHz)

dCp/dln(w) (nF cm-2)

1 10 100 1000 1 10 100 1000

0

1

2

3

4

-1.2

-0.9

-0.6

-0.3

0.0

Freq (kHz)

Cp (nF cm-2)

Experimental

Model Beta=0.5

Model Beta=1.0

Freq (kHz)

dCp/dln(w) (nF cm-2)

Freq (kHz)

Cp (nF cm-2)

Experimental

Model Beta=0.5

Model Beta=1.0

Freq (kHz)

dCp/dln(w) (nF cm-2)

1 10 100 1000 1 10 100 1000 1 10 100 1000

0

1

2

3

4

-1.2

-0.9

-0.6

-0.3

0.0

Figure 1.2 Admittance Spectroscopy example showing goodness of fit with phenomenological 

parameter β=0.5. 

 

Mathematical analysis shows that the relationship is independent of the β parameter, 

and there is no need to change the procedure of the determination from . 

= ωω peakt

ω t p ln/ tddC

The value of can be determined from the peak magnitude: 0 Ct

( ) 0 2 C Peak Magnitude t −×−=

β (1.11) 

Distortion of the AS spectrum that results in a better fit with stretched exponential dependence 

can be explained by the DES band, rather than a discrete level. Another possibility is a high 

density of a discrete level DES that causes Fermi level pinning and band bending in such a way 

that the AS signal is distorted. Simulations with the SCAPS software package indicate that this is 

a plausible explanation (see Section 2). 

The capacitance transient experimental curves are also better described with the stretched 

exponential equation: 

( ) ( ( ) ) β /exp τ 0 p d −+= tCCtC (1.12) 

Similarly to AS, the relationship in is independent of the β parameter. 

The value can be determined from the peak magnitude as: 

= ωω peakt p ddC ln/ ω

0 Ct

( ) 0 Peak Magnitude e Ct −×−=

β , (1.13) 

4
where e ( ) == 718.21exp . 

Similarly to AS, a good fit to the experimental data is often found with β 5.0~ . 

1.3 Temperature Dependence of Characteristic Frequency/Characteristic Time 

 The trap characteristic frequency is related to the capture and emission rates and therefore 

dependent on both Fermi and DES energy levels (see Eqs. (1.5)-(1.7)). As mentioned above the 

recharging of traps located near the crossing point dominates the frequency dependent component 

of measured admittance. Since at this point , capture and emission rates for the majority 

carriers are equal to each other and have the same temperature dependence, hence for holes 

= EE Ft

TkEENv (1.15) vpptt BVt == 2στω −− /)(exp[

Below we show three different representations of the temperature dependent AS spectra. 

Exponential dependence (T) is illustrated by Fig. 1.3. It plots versus frequency (left graph) 

and vs. frequency (right graph) for a range of fixed temperatures. The increase of 

with temperature can be seen with minima that correspond to on the vs. 

frequency graph. 

ω t Cp

p ddC ln/ ω

ωt ωt p ddC ln/ ω

Freq (kHz)

dCp/dln(w) (nF cm-2)

Freq (kHz)

Cp (nF cm-2)

 T=-150C

 T=-160C

 T=-170C

 T=-180C

 T=-190C

0.1 1 10 100 1000 0.1 1 10010 1000

0

1

2

4

3

-0.8

-0.6

-0.4

-0.2

0.0

0.2

Freq (kHz)

dCp/dln(w) (nF cm-2)

Freq (kHz)

Cp (nF cm-2)

 T=-150C

 T=-160C

 T=-170C

 T=-180C

 T=-190C

0.1 1 10 100 1000 0.1 1 10010 1000

0

1

2

4

3

-0.8

-0.6

-0.4

-0.2

0.0

0.2

Figure 1.3 and Cp p ddC ln( ) ω vs. AC frequency at different temperatures for a CdTe solar cell. 

5
This representation can become crowded and difficult to interpret in a case of wide range of 

temperatures and multiple DES detected. One alternative is to graph AC conductance Gp versus 

temperature at fixed frequenciesω . An example is shown in figure 1.4 for a CdTe solar cell with 

frequencies ranging from 1MHz (red line at top) down to 1 kHz (bottom line). The local peaks 

on the lines indicate a DES with the characteristic frequency corresponding to the fixed line 

frequency 

ωt

ω at the peak’s temperature T . In this example two DES outlined in blue are 

clearly visible as is the increasing with temperature. 

peak

ωt

Temperature oC

Gp (S cm-2)

-190 -140 -90 -40 10 60

10-8

10-7

10-6

10-5

10-4

10-3

10-2

DES #1

DES #2 1 MHz

1 kHz

Temperature oC

Gp (S cm-2)

-190 -140 -90 -40 10 60

10-8

10-7

10-6

10-5

10-4

10-3

10-2

DES #1

DES #2

Temperature oC

Gp (S cm-2)

-190 -140 -90 -40 10 60

10-8

10-7

10-6

10-5

10-4

10-3

10-2

DES #1

DES #2 1 MHz

1 kHz

Figure 1.4. Conductance G versus temperature for a range of frequencies. The top red line 

corresponds to 1 MHz and the bottom line corresponds to 1 kHz. The blue lines highlight peaks 

from two DES with different . 

p

T )( ω t

Another alternative is to graph versus at fixed . Figure 1.5 shows the same CdTe 

solar cell with frequencies ranging from 1 MHz (bottom line) to 1 kHz (top line). Here the DES 

can be seen as a spread in values over . The low temperature DES #1 (-190

Cp T f

Cp f o

C to ~ -100oC) 

is easily seen. The mid temperature DES #2 (-40o

C to 30oC) is less obvious but can be seen as a 

kink in the lines. At warmer temperatures (30o

C and above), the nearly constant spread 

in values at different indicates a continuous band of DES energy levels. Cp f

6
Temperature o

C

Cp (nF cm-2)

-190 -140 -90 -40 10 60

0

1

2

3

4

5

6

7

DES #1

DES #2

Continuous energy band DES

1 MHz

1 kHz

Temperature oC

Cp (nF cm-2)

-190 -140 -90 -40 10 60

0

1

2

3

4

5

6

7

DES #1

DES #2

Continuous energy band DES

Temperature oC

Cp (nF cm-2)

-190 -140 -90 -40 10 60

0

1

2

3

4

5

6

7

DES #1

DES #2

Continuous energy band DES

1 MHz

1 kHz

Figure 1.5 Capacitance Cp versus temperature for a range of frequencies. The bottom red line 

corresponds to 1 MHz and the top line corresponds to 1 kHz. 

In the temperature range -170o

C to -100oC the low frequency capacitance (top lines) 

decreases in magnitude with increasing temperature. This behavior contradicts with Eq. (1.1). In 

Sec.2, using simulations with SCAPS we will show that this effect can be explained by a steep 

defect concentration gradient near the back contact combined with the back contact Schottky 

barrier. As EF rises with increasing T , it crosses at different locations of decreasing density 

of the defect. 

Et

Figure 1.6 illustrates typical evolution of capacitance transients with temperature for the 

bias sequence: 30 s at zero bias, 300 s at forward bias +0.5 V, and a 300 s recovery period at zero 

bias. The minima in the 

Cp

d t

p tdC

ln

)(

 graph to the right shows reducing τ with increasing T.

7
0 200 400 600

Elapsed Time (s)

Cp(t) (nF cm-2)

T=85C

T=80C

T=75C

T=70C

T=65C

T=60C

1 10 100 1000

FB Pulse Elapsed Time (s)

dCp(t)/dln(t) (nF cm-2)

20

80 0

0

40

60 -2

-4

-6

-8

0 200 400 600

Elapsed Time (s)

Cp(t) (nF cm-2)

T=85C

T=80C

T=75C

T=70C

T=65C

T=60C

1 10 100 1000

FB Pulse Elapsed Time (s)

dCp(t)/dln(t) (nF cm-2)

20

80 0

0

40

60 -2

-4

-6

-8

Figure 1.6. CTr measurements on CdTe cell. (tC ) p versus t for the complete transient cycle 

(left) and ( )

( )td

p tdC

ln

versus ln( )t for the forward bias +0.5V pulse (right). 

1.4 Estimating E and a σ a with Arrhenius Plot 

Since in Eq. (1.15) v and , the characteristic time 2/1

T ∝ T 2/3V ∝ TN τ dependence on T

is linear on an Arrhenius plot of the form ( ) ][ln

1 2 − τT versus 1000 . A linear regression is used 

to estimate the activation energy and apparent capture cross section as: 

/T

Ea σ a

a b E 1000 ××−= kSlope (1.16) 

( )

( ) o

i a mmx

Y

/*1000.3

exp

21 σ = (1.17) 

where Slope is the regression line slope, is the regression line y-intercept at Yi T = ∞ , and 

 is the charge carrier’s relative effective mass. The activation energy approximation 

 is often assumed while more precisely , where 

mm o * /

a ≈ ΔGE a Δ += EHE σ ΔG is the Gibbs free 

energy, ΔH is the enthalpy, and is the potential barrier for carrier capture by a trap. Also, the 

apparent capture cross-section approximation is often made whereas more precisely, 

Eσ

≈ σσ ap

8
= na σχσ ∞ , where is the capture cross section at T of measurements, σ p σ ∞ = 

T ∞→

σ T)(lim , 

and is the entropy factor for the thermal emission. χ n

Meyer-Neldel Rule behavior 

A group of DES signatures determined by Arrhenius regression are said to obey 

the Meyer Neldel Rule (MNR) [3] when they form a straight line on a vs. plot, 

referred to as a Meyer-Neldel plot. This relationship indicates the same underlying DES in spite 

of a wide range of and values. This allows for the grouping of observed data 

points into DES “families” and simplifies classification of DES and identification of underlying 

defects. A number of possible explanations for MNR behavior have been proposed [4]. Early in 

this study, significant MNR behavior was observed with DES detected with CTr. It was mostly 

caused by instrumentation noise and aggravated by the narrow range where a specific trap is 

detectable. Subsequent improvements to the measurement system expanded the range of 

,{ } E σ aa

Ea (σ a ln )

Ea σ a ,{ } E σ aa

T

τ and 

consequently T , reducing this effect. 

1.5 Estimating Trap Concentrations from Capacitance Measurements 

Knowledge of the DES concentration is helpful in evaluating their impact on solar cell 

performance. The simple theory for a single trap level with uniform distribution of trap 

concentration and “doping level” , provides an equation [2]: Nt N

 ( ) /(/1

/1

tt

t dt

d

o

t

xNN

xx

N

N

C

C

⋅+

− = (1.18) 

) dx

where is the depletion width and is the “crossing point” - the distance from the interface 

to the point where the trap level crosses the Fermi level: . Eq. (1.18) is derived under 

assumption of low trap concentration. In the case of , Eq. (1.18) takes a simpler form: 

dx t x

= ExE Ftt( )

t << NN

⎟

⎟

⎠

⎞

⎜

⎜

⎝

⎛ −=

d

tt

d

o

t

x

x

N

N

C

C 1

2/1

⎟

⎠

⎞ ⎜

⎝

⎛ = U

U

N

N tt (1.19) 

9
where is the total band bending and . If to assume also 

<<1 (e.g., at forward bias applied), the equation transforms further to . If 

one observes , that means that the trap density is higher than the “doping level”. 

Indeed, sometimes we have observed a two- to three-fold decrease in capacitance value with 

increasing frequency. In this case the simple theory does not work. 

− += VVU biasib −≡ Ftt /)( eEEU

dt / xx td NNCCot ≈ //

≥ 1/ d o

t CC

If trap concentration is high and , the Fermi level may be pinned by the trap level 

. The influence of this effect was simulated with SCAPS and is discussed in Section 2. 

Additional problem comes from the uncertainty in spatial distribution of , or both and , 

and corresponding uncertainty in space charge density and electric potential distribution. Rather 

than make definite estimates of trap concentrations from capacitance measurements, we make the 

relative estimates of for different DES and differently processed cells based on the values 

derived using Eqs. (1.11) and (1.14). The same approach is used to see trends in the particular 

trap/defect concentration changes due to the cell stressing. 

<< NN t

Et

Nt N Nt

Nt

0 Ct

10
2. SCAPS SIMULATION 

The SCAPS package [5] allows a solar cell to be modeled with up to three DES with known 

, , and . The simulated AS results can be compared to experimental data and help 

validate or discredit different working hypotheses about the DES system. 

Ea σ a Nt

Two SCAPS CdTe solar cell models are discussed in this section to illustrate how 

simulations can help explain experimental observations. The first model shows that with a 

combination of a steep DES concentration gradient near the back contact, a back contact Schottky 

barrier, and high DES concentration relative to the doping level, AS results can show a 

decreasing capacitance with increasing temperature. The second model shows how the magnitude 

of an AS signal can be impacted by the cell width and how Fermi level pinning can distort the 

calculated activation energy and cause non-exponential behavior. 

2.1 DES Concentration Gradient 

A number of experimental AS results in this study have shown decreasing capacitance 

magnitudes with increasing temperature as was seen in Figure 1.5 for the low frequency 

capacitance (upper line) at T<-100o

C. These results are counter-intuitive since the increasing 

temperature leads to ionization of deeper states, which increases the space charge density and 

reduces the depletion width resulting in a higher measured capacitance. However, the SCAPS 

simulation shows that a combination of three conditions can cause this unexpected result 

observed experimentally. 

(a) The DES concentration has a gradient near the back contact. When modeling, we used 

, simulating cadmium vacancies ; the concentration was set to 

=3x10

a = 15.0 eVE − VCd

Nt

19cm-3 at the back contact and decreased exponentially to 3x1014cm-3 over a length 

of about 35nm. This gradient is reasonable since the cell post deposition NP etch is 

known to create a tellurium rich layer at the back contact. 

Nt

(b) There is a back contact Schottky barrier. The barrier height was set to = 0.5eV with 

respect to Fermi level. With this, when the Fermi level rises with increasing temperature, 

the crossing point will shift in the near-back contact region with a steep gradient. 

Φb

VCd Nt

11
(c) The shallow acceptor concentration is much lower than the DES concentration. This 

causes Fermi level pinning at the DES level and amplifies the AS signal. The shallow 

acceptor concentration was set to Na =1012cm-3. This is plausible if is taken as the 

primary dopant for the CdTe layer. The rest of the parameters are typical values for CdTe 

solar cells and were selected from two sources [6, 7]. 

VCd

Band diagrams for this model at -190o

C and -100oC are shown in figure 2.1. 

 

-1.5

-1.0

-0.5

0.0

0.5

1.0

1.5

01234

eV

5

Ec

Ev

Ef

VCd=Ev+0.15eV

-100o

C

-1.5

-1.0

-0.5

0.0

0.5

1.0

1.5

012345

eV

-190oC

Distance from back contact μm

-1.5

-1.0

-0.5

0.0

0.5

1.0

1.5

01234

eV

5

Ec

Ev

Ef

VCd=Ev+0.15eV

-100o

C

-1.5

-1.0

-0.5

0.0

0.5

1.0

1.5

012345

eV

-190oC

Distance from back contact μm

Figure 2.1 Band diagrams for SCAPS model at -190o

C and -100oC. The conduction and valence 

bands are in black, the Fermi level in blue, and the DES in red. VCd

 At -190o

C the Fermi level is pinned to the level almost all of the way to the back 

contact. The pinning near the back contact where the concentration is high greatly enhances 

the AS signal. However, at -100

− VCd

− VCd

o

C, due to the Schottky barrier band bending, the higher Fermi 

level is no longer pinned to the level in the vicinity of the back contact. The crossing point 

shifts to the region of much lower concentration and the AS signal is significantly reduced. 

− VCd

− VCd

The SCAPS simulated AS versus T diagram is shown in figure 2.2. The decreasing low 

frequency capacitance with increasing temperature is evident in the temperature range -170

Cp

o

C to -

120o

C. The effect observed experimentally (see Fig. 1.5) is smaller than in this simulated 

example but qualitatively is the same. 

12
The presence of a Schottky barrier manifested itself as an AS signal in the temperature 

range -80o

C to 40oC. This illustrates how the back contact barrier can be mistaken for a DES. In 

simulations the back barrier has consistently had β = 1. There is an unexplained kink in the 

capacitance values measured between 5o

C and 10oC. It is likely that this is an artifact of the 

software and is caused by numerical calculation problems. 

-190 -140 -90 -40 10 60

Temperature o

C

Cp (nF cm-2) 0

1

2

4

5

6

3

AS signal from Schottky barrier

AS signal from defect 0.15eV 

above valence band with steep 

concentration gradient near back 

contact.

-190 -140 -90 -40 10 60

Temperature o

C

Cp (nF cm-2) 0

1

2

4

5

6

3

AS signal from Schottky barrier

AS signal from defect 0.15eV 

above valence band with steep 

concentration gradient near back 

contact.

Figure 2.2 SCAPS simulated Cp versus T graph for a range of frequencies. The bottom red line 

corresponds to 1 MHz and the top line corresponds to 1 kHz. 

 2.2 Fermi Level Pinning, Distorted and Non-Exponential Behavior Ea

The simulation examples in this section examine the two cells that are identical except for 

the CdTe layer thickness, 8μm for cell A and 3μm for cell B. We start with the band diagrams 

and show how the characteristic frequency as a function of distance from the back contact is 

different for the two cells. This difference alters the AS signal from each cell and leads to 

differences in the calculated values for , , and Ea σ a β . 

13
The band diagrams for the two cells are shown in Figure 2.3 for . There is a 

DES in the model with , , and . There is a low 

shallow acceptor with concentration of . 

CT

o −= 170

a = 15.0 eVE 214 102 cmx a − σ = 314 103 − t = cmxN

312 10 − Na = cm

-2.5

-1.5

-0.5

0.5

1.5

02468

Distance from back contact μm

eV eV

Lateral extent of Fermi level (blue line) 

pinning by trap level (red line) is greater 

with thick CdTe layer 

Cell A, 8μm CdTe layer Cell B

3μm CdTe layer 

-2.5

-1.5

-0.5

0.5

1.5

0123

-2.5

-1.5

-0.5

0.5

1.5

02468

Distance from back contact μm

eV eV

Lateral extent of Fermi level (blue line) 

pinning by trap level (red line) is greater 

with thick CdTe layer 

Cell A, 8μm CdTe layer Cell B

3μm CdTe layer 

-2.5

-1.5

-0.5

0.5

1.5

0123

Figure 2.3 SCAPS simulation band diagrams for CdTe solar cells at T=-170o

C showing Fermi 

level (blue line) and pinning by high concentration trap level (red line) for cell A with 8 μm CdTe 

layer (left) and green line for cell B with 3 μm CdTe layer (right). 

The acceptor-type DES level is fixed at while the Fermi level 

varies as a function of the free carrier concentration at different distances from the back contact. 

Knowing the Fermi level, we can calculate the free hole concentration as a function of distance 

from the back contact with equation 

EE Vt =− 15.0 eV − EE VF

( )

⎟

⎟

⎠

⎞

⎜

⎜

⎝

⎛ −− = Tk

EE

Np

b

VF

V exp . Figure 2.4 (left) shows an 

expanded scale band diagram view of the acceptor DES energy level for the 8 μm and 3 μm cells 

with respect to the Fermi level. Figure 2.4 (right) shows the free majority carrier (hole) 

concentrations for the 8 μm and 3 μm cells as a function of distance from the back contact. The 

back contact and main junction depletion regions are evident from the low free carrier 

concentrations at both ends of the cells. The 3μm cell is nearly fully depleted across the CdTe 

layer. The 8μm cell has a free carrier concentration that exceeds the shallow defect doping 

14
concentration of in the central quasi-neutral or “non-depleted” region. The ionized 

DES is the source of these free carriers. 

312 10 − Nd = cm

0246 8

-0.04

-0.02

0.00

0.02

0.04

02468

eV 0.15eV DES Relative to Fermi

Distance from back contact μm

Fermi

8 μm cell

3 μm cell

109

1010

p 8μm cell

p 3μm cell

cm-3 Free Carrier Density (holes)

1011

1012

1013

0246 8

-0.04

-0.02

0.00

0.02

0.04

02468

eV 0.15eV DES Relative to Fermi

Distance from back contact μm

Fermi

8 μm cell

3 μm cell

109

1010

p 8μm cell

p 3μm cell

cm-3 Free Carrier Density (holes)

1011

1012

1013

Figure 2.4 SCAPS simulated 8 μm and 3 μm cells for T=-170o

C. DES energy levels relative to 

Fermi level (left). Free carrier density (right). 

 With the free carrier density known, it is possible to determine the DES capture rate for 

each DES as a function of distance from the back contact, using equations pvc , and = σ ppp

( )⎟

⎟

⎠

⎞

⎜

⎜

⎝

⎛ −− = Tk

EE

Nve

b

Vt

vppp σ exp . Since constEEt V − = , the emission rate will be constant as 

a function of distance from the back contact. With the emission and capture rates known, the 

ionization of the DES can be determined from equation 

pp

p

t

t

ce

e

N

n

+ = . Figure 2.5 (left) shows 

the hole capture and emission rates for the 8 μm and 3 μm cells as a function of distance from the 

back contact. Figure 2.5 (right) shows the ionized DES concentration as a function of distance 

from the back contact. 

nt

15
02468 0246 8

Distance from back contact μm

1

102

104

106

cp 8μm cell

ep 8μm cell

cp 3μm cell

ep 3μm cell

s-1 Hole Capture/Emission Rates

1012

1013

1014

1015

nt 8μm cell

nt 3μm cell

cm-3 Ionized DES Density nt

02468 0246 8

Distance from back contact μm

1

102

104

106

cp 8μm cell

ep 8μm cell

cp 3μm cell

ep 3μm cell

s-1 Hole Capture/Emission Rates

1012

1013

1014

1015

nt 8μm cell

nt 3μm cell

cm-3 Ionized DES Density nt

Figure 2.5 SCAPS simulated 8 μm and 3 μm cells. Hole capture and emission rates (left) and 

ionized DES density (right). 

As expected, when the DES level is significantly below the Fermi level, the DES are fully 

ionized (occupied with electrons) with . For the 8μm cell, in the quasi 

neutral central region, the ionized DES density approaches that of the free carrier density seen in 

figure 2.4 as a further indication that they are the dominant source of free carriers. 

314 103 − tt =≈ cmxNn

The capture density of holes by the DES is the product of the DES hole capture rate and 

concentration of DES available for capturing holes, that is . The emission density of holes by 

the DES is the product of the DES emission rate times the DES concentration available for 

emitting holes . At equilibrium, the capture and emission densities are equal. When there 

is a small shift in the Fermi level such as that caused by a LCR meter AC signal, the change in 

charge will be proportional to this capture/emission density. Thus, the parts of the cell with the 

highest capture/emission densities will make the greatest contributions to the measured 

capacitance. 

p c

nt

p e

− nN tt

Knowing the hole emission and capture rates it is possible to calculate the characteristic 

time from equation [2]. The electron capture and emission rates, 

are negligible because the DES energy level and p-type Fermi energy level are located far from 

the conduction band. Experimentally characteristic frequency is determined as the LCR 

ppnn +++= ecec −1

τ nn , , ec

−1

t = τω

16
meter angular frequency ω = 2πf at which ddC ln)( (ωω ) p has a peak. Figure 2.6 (left) shows 

the capture/emission densities for the DES as a function of distance from the back contact. 

Figure 2.6 (right) shows the DES characteristic frequencies as a function of distance from the 

back contact. The latter shows that characteristic frequencies determined in this way are 

significantly different for cells of different thicknesses: for the 8 μm cell while 

 for the 3 μm cell. 

t f

Hzft

5 ≈ 10

Hzxft

3 ≈ 105

02468 02468

102

103

104

105

106

s-1 DES Characteristic Frequency ft

ft 8μm cell

ft 3μm cell

1017

1018

1019

8μm cell

3μm cell

s-1 cm-3 Capture/Emission Density

Distance from back contact μm

02468 02468

102

103

104

105

106

s-1 DES Characteristic Frequency ft

ft 8μm cell

ft 3μm cell

1017

1018

1019

8μm cell

3μm cell

s-1 cm-3 Capture/Emission Density

Distance from back contact μm

Figure 2.6 SCAPS simulated 8 μm and 3 μm cells. DES hole capture/emission density at 

equilibrium (left) and DES characteristic frequency ft (right). 

Figure 2.7 shows the SCAPS simulated frequency dependencies of and Cp ddC ln(ω) p . 

The vs. graph (left) shows that the 8μm cell has a larger magnitude than 

that of the 3μm cell . This could be expected since in the former the region 

where is very close to

Cp f 0 2 39.1 − Ct = cmnF

0 2 18.0 − Ct = cmnF

Et EF is much thicker hence the number of DES contributing to the 

capacitance signal is much greater than in the thinner cell. This illustrates the complexity of 

estimates of from the value. In our example of two cells with the same DES concentration 

 but different thicknesses, the values differ by a factor of ~8. 

Nt

0 Ct

Nt

0 Ct

17
If the cell is sufficiently thin, it will be fully depleted and the DES level will not even 

approach the Fermi level making it undetectable with AS. This explains why cells produced by 

Colorado State University [7] that have a thin <2 μm CdTe layer had little or no detectable DES 

AS density (we do not present the data for these cells because the studies are not completed). 

 

102 103 104 105 106

0

1

2

3

nF cm-2 Capacitance

Cp 8μm cell

Cp 3μm cell

102 103 104 105 106

nF cm-2 dCp/dln(ω)

0.0

-0.5

-0.4

-0.3

-0.2

-0.1

0.1

8μm cell

3μm cell

Simulated LCR meter frequency (Hz)

ft ft

102 103 104 105 106 102 103 104 105 106

0

1

2

3

nF cm-2 Capacitance

Cp 8μm cell

Cp 3μm cell

102 103 104 105 106 102 103 104 105 106

nF cm-2 dCp/dln(ω)

0.0

-0.5

-0.4

-0.3

-0.2

-0.1

0.1

8μm cell

3μm cell

Simulated LCR meter frequency (Hz)

ft ft

Figure 2.7 SCAPS simulated 8 μm and 3 μm cells at -170o

C. Cp versus simulated AC signal 

frequency (left) and dCp/dln(ω) versus simulated AC signal frequency (right). 

 The graph dCp/dln(ω) vs. simulated AC signal frequency in figure 2.7 (right), makes 

evident the difference in the characteristic frequency minima. The 8μm cell has while 

the 3μm cell has corresponding to the dominant characteristic frequencies seen in 

figure 2.6 (right). This difference in leads to differences in the

Hzft

5 = 10

Hzxft

3 = 105

t f τ T)( dependencies that cause 

distortions in the calculated values for Ea and σ a . The dCp/dln(ω) signal spread spans three 

decades from f~103

 to f~106 Hz with the 8μm cell and only goes from 103 to 2x104 Hz with the 

3μm cell. The larger spread with the 8μm cell is caused by the larger range of characteristic 

frequencies that contribute to the AS signal. This causes the stretched exponential behavior that 

leads to values of β < 1. The asymmetry that seen in the 8μm dCp/dln(ω) curve is commonly 

observed in the experimental data. 

18
The Arrhenius plot for the simulated 8μm (red) and the 3μm (green) cells with the single 

DES of are shown in figure 2.8. For the 3μm cell, the Arrhenius 

regression indicates which is consistent with the input 

and where the capacitance signal is detected. For the 8μm cell, the Arrhenius regression 

indicates which shows the distortions to the regression 

slope that can occur. 

214 E eV 102,15.0 cmx a = σ a =

94.0,102,15.0 214

a = σ a = cmxeVE β =

≈ EE ft

45.0,102,10.0 215

a = σ a = cmxeVE β =

 

6 7 8 9 10 11 12

1000/Temperature(K)

0.1

1.0

10.0

100.0

1000.0

1/τT2 (s-1 K-2)

-170 oC

8μm cell

3μm cell

Regression line

6 7 8 9 10 11 12

1000/Temperature(K)

0.1

1.0

10.0

100.0

1000.0

1/τT2 (s-1 K-2)

-170 oC

8μm cell

3μm cell

Regression line

Figure 2.8 Arrhenius plot with regression lines for SCAPS simulated 8μm and 3μm cells. 

In summary, this simulation model offers a plausible explanation for a number of observed 

phenomena with experimental data collected from thin film CdTe solar cells with high 

concentrations of DES. These include: 

• Loss of AS signal for shallow DES when the CdTe layer is thin 

• Distortions to the measured values for and due to Fermi level pinning and the fact that 

is a function of both and . 

Ea σ a

ωt Et Ef

• Stretched exponential behavior when there is only a single discrete DES energy level. 

19
3. CELL PRODUCTION AND STANDARD TESTING PROCEDURES 

To date all high performance CdTe solar cells have included Cu and CdCl2 in postdeposition heat treatments [9]. While the complex interaction between these two additives is 

known to favorably change the defect structures and electronic properties that control 

performance, the understanding of this process remains incomplete. Therefore, this study has 

applied admittance spectroscopy (AS), capacitance transient (CTr), and standard solar cell 

analysis techniques to examine differences in deep electronic states (DES) for CdTe cells 

produced with and without both Cu and CdCl2. In addition, this study examines changes in DES 

with stressing. This section is divided into two subsections. The first one describes the production 

process for the cells used in the study. The second describes the standard testing procedures. 

3.1 Production Processes for CdTe Solar Cells Used in Experiments 

The 2x2 matrix of Cu/no Cu and CdCl2/no CdCl2 cells with replicates were obtained from 

four sources: Dave Albin at the National Renewable Energy Laboratory (NREL) Brian 

McCandless at the Institute for Energy Conversion (IEC), Xuanzhi Wu (NREL), and Alan 

Enzenroth at Colorado State University (CSU). 

3.1.1 Cells produced by D. Albin at NREL. 

The cells were grown on a single Corning 7059 glass substrate with a SnO2 bi-layer (500 

nm F-doped/ 100 nm undoped) as the front transparent contact. The CdS window layer was 

deposited by chemical bath deposition (CBD) to a thickness of ~80nm followed by close-spaced 

sublimation (CSS) of the CdTe absorber layer to a thickness of ~8μm. The substrate was then 

quartered and the two CdCl2-treated devices were annealed at 400o

C in a CSS-arrangement using 

anhydrous CdCl2 as a source in an ambient of 100 torr Oxygen/400 torr Helium for 7 min. All 

devices were then etched with a nitric-phosphoric acid (NP) etch. Back contact carbon-based 

paste (Electrodag 114) was applied to the CdTe surface by brushing. The two devices with Cu 

had Cu1.4Te powder added to the paste. 

The back contact areas are approximately 0.5cm2

 and a 260oC anneal in helium was applied 

to activate the contact. To avoid back to front contact electrical shorting, a ~1 mm lateral margin 

of solar cell material was retained around the back contact. This enhances current collection at the 

20
back contact edges yielding higher short-circuit current values than would be obtained without 

the margin. Two cells were produced for each Cu and CdCl2 category for a total of eight cells. 

3.1.2 Cells produced at IEC. 

The cells supplied by B. McCandless were grown on TEC15 glass TCO with a Ga coating 

oxidized at 500o

C to serve as the bi-layer. The CdS window layer was grown by chemical surface 

deposition where a solution at ambient temperature is applied to a pre-heated glass/TCO substrate 

producing a film with low particulate density. Then a CdCl2 vapor treatment is applied. The CdTe 

absorber layer was deposited by vapor transport at 550o

C on a moving substrate at a 2.5 cm/min 

translation speed, giving a 10 μm/min equivalent growth rate. The cells with CdCl2 were exposed 

to CdCl2 vapor at 415o

C for 25 minutes in an argon and oxygen atmosphere. Then all cells were 

sequentially etched in bromine-methanol, dichromate, and hydrazine. The cells with Cu had 

15nm of Cu deposited by evaporation and all cells had 50 nm of Ni deposited by evaporation. 

Each back contact area is 0.36cm2

 and a 25 minute anneal at 180oC in Argon was applied. 

The cell edge was scribed down to the front contact and an Indium solder strip was applied 

followed by a dip in a weak bromine methanol solution. Four replicates were produced for each 

category for a total of 16 cells. 

3.1.3 Cells produced by X. Wu at NREL 

The description of processing performance and some properties of the high-efficiency CdTe 

cells produced by Dr Wu can be found in Ref. [17]. We are not well aware of all the details of 

processing six cells we received from Dr. Wu for studies except those mentioned in Sec. 4.3. 

3.2 Standard Testing Procedures 

Upon receipt, all cells were subjected to the standard room temperature J-V analysis 

which is a fairly rapid procedure. Then, based on J-V results, cells were selected for QE testing 

and the full suite of temperature controlled testing including J-V-T, C-V-T, AS, and CTr. 

21
3.2.1 J-V and J-V-T tests 

After a dark J-V curve was taken, the light source was turned on and allowed to settle for 5 

seconds, and then a light J-V curve was taken. For the standard room temperature J-V 

measurements, the light source consists of five halogen light bulbs housed in a home built 

70x70x90cm box. The color spectrum of our light source is cooler than the standard sun with 

more intensity in the red and less in the blue. The measured efficiency values have typically been 

about one percent absolute lower than the official NREL determinations. 

For the temperature controlled current voltage (J-V-T) measurements, the insulated copper 

chamber and optical fiber cable light source were used. The light intensity of one sun is 

approximate and it is suspected to be non-uniform over the area of the cell. However, for each 

test sequence where the cell is left in place, the light intensity is reproducible. This is adequate 

since the goal is to compare J-V curves at different temperatures from a single run. The J-V-T 

measurements are taken while the chamber and cell are being cooled for a subsequent C-V-T set 

of measurements. During this time the Keithley source meter is connected to the chamber in place 

of the LCR meter. The first measurement is taken when the chamber reaches 20o

C and then at 

10o

C intervals down to near liquid nitrogen at -190oC. For comparisons between runs, a 

calibration factor is used that is the ratio of the value on the standard J-V system to the 

value at a nominal 20

SC J SC J

o

C. 

3.2.2 Quantum efficiency 

The CSM QE system consists of a sample stage with a pinhole to admit the single 

wavelength light, a monochromater to supply the single wavelength light, a chopper and lock-in 

amplifier to augment the resulting current signal, and a Keithley source meter to apply bias. No 

white light bias is applied as it overloads the current amplifier. 

3.2.3 Capacitance-voltage profiling at varying temperature (C-V-T) 

The thermally controlled C-V-T measurements apply a 10mV AC signal at 100kHz to 

obtain the admittance measurements. The LCR meter applies a bias sweep starting at -2V and 

incrementing in 50mV steps to +0.5V. An admittance measurement is recorded at each step. To 

check for hysteresis a reverse sweep immediately follows starting at +0.5V and decrementing in 

22
50mV steps down to -2V. The C-V measurements are taken in the dark at 10o

C intervals starting 

at -190o

C and going to 20oC. The apparatus setup is described in our Final 2005 Report to NREL 

[1] and in [8] 

3.2.4 Admittance spectroscopy (AS) 

The AS measurements applied a 10 mV AC signal with a frequency sweep starting at 25 

Hz and incrementing by 12 steps per decade up to 1 MHz. The signal was often noisy below 1 

kHz and below 250Hz it was so noisy that it was of no practical use, even with the doubly 

insulated BNC cables and the grounded copper chamber. An isolating power supply did not 

reduce the noise. The maximum frequency for the HP4284A LCR meter is 1MHz. No bias was 

applied during these measurements. 

3.2.5 Capacitance transients (CTr) 

 The CTr measurements consist of a timed interval sequence of admittance readings taken 

with a 10mV, 100kHz AC signal. The bias sequence started with 30s at 0V to establish a 

baseline, followed by a 300s filling pulse at +0.5V, and ended with a 300s 0V recovery period. 

A logarithmically increasing time interval between measurements was used. This did not cause 

any loss of information as the transients have an exponential (or close to that) time dependency. 

An advantage of the logarithmic time interval is reduced noise in the ( )

( )td

tdC

ln

versus ln(t)graph. 

With a fixed time measurement interval, the noise occurs near the end of the 300 s period where 

the Δln( )t in the denominator becomes very small and amplifies any Δ ( )tC signal noise in the 

numerator. 

3.2.6 Temperature 

The AS and CTr measurements are taken in sequence at preset temperatures ranging from 

85o

C to -190oC. The AS measurements are taken before the CTr measurements to minimize any 

residual transient contamination. This works under the assumption that the time between the 

measurements at subsequent temperatures is sufficient so that any residual transient from the CTr 

reading is negligible. 

23
The standard procedure is to heat the system first and allow it to gradually cool to room 

temperature. Soon afterwards the sample is cooled by liquid nitrogen and then allowed to 

gradually warm to room temperature. This has resulted in the best data stitching between the 

heating and cooling cycles. Room temperature capacitances changed by as much as 10% 

following a heating cycle. We suspect that this is caused by metastable states set at the modestly 

elevated temperature of 85o

C. If the cell remains at room temperature for a few days the 

measured capacitances return to their original value. However, if the cell is immediately cooled, 

the metastable state is frozen-in and remains for the duration of the cooling cycle resulting in 

better data stitching across room temperature. 

The preset test trigger temperature intervals range between 4o

C and 10oC. The tighter 

intervals are used in temperature ranges where signatures of interest are often found. 

4. RESULTS AND DISCUSSION 

Results of experiments conducted on completed CdTe solar research cells obtained from 

four different sources, are discussed in this section. Four post-deposition treatment categories 

from each source were studied: 

(1) Cells with yes-Cu/yes-CdCl2 treatment. 

(2) Cells with no-Cu/yes-CdCl2 treatment. 

(3) Cells with yes-Cu/no-CdCl2 treatment. 

(4) Cells with no-Cu/no-CdCl2 treatment. 

These cells were tested with J-V, QE, C-V, J-V-T, AS, and CTr. 

The results and discussion for the cells from different sources are presented in the following 

sections: 4.3.1- D. Albin’s cells; 4.3.3 – IEC cells; 4.3.3 - Wu’s cells; 4.3.4 – Enzenroth’s cells. 

4.1 NREL (D. Albin) Cells. 

The CdTe solar cells obtained from Dave Albin at NREL yielded the cleanest results and 

were the easiest to interpret. Experimental data was collected from both replicate cells from each 

of the four categories. However, differences between cells within each category were negligible 

and only the first cell results from each category are reported. 

24
4.1.1 J-V, QE, C-V results. 

Results of J-V measurements under illumination for each cell category are tabulated in table 

4.1. As expected, the yes-Cu/yes-CdCl2 cell had the highest efficiency η and the no-Cu/no-CdCl2

cell had the lowest η . Adding either the Cu or the CdCl2 treatment improved all of the 

performance metrics with one significant exception. The Cu treatment decreased the short circuit 

dynamic resistance (“shunt resistance”) and for the yes-CdCl RSC 2 cell it lowered the as 

well. The J-V curves are shown in figure 4.1 and the rollover effect is evident for the no-Cu cells 

as a result of the back contact Schottky barrier. 

SC J

Table 4.1 Performance parameters for Albin’s cells with yes/no Cu and yes/no CdCl2. 

η VOC

Cu/ CdCl2

% V

SC J

−2 cmmA

FF

%

RSC

2 Ωcm

ROC

2 Ωcm

yes/yes 12.4 0.82 23.9 63 578 5.2 

no/yes 11.1 0.78 24.2 59 1350 11.2 

yes/no 9.0 0.72 22.0 57 276 6.3 

no/no 7.3 0.69 21.2 50 287 10.5 

 

-30

-15

0

15

30

-0.50 0.511

Voltage

Current density (mA cm-2) 

.5

yes-Cu/yes-CdCl2

no-Cu/yes-CdCl2

yes-Cu/no-CdCl2

no-Cu/no-CdCl2

Figure 4.1 J-V curves for NREL cells with yes/no Cu and yes/no CdCl2 treatments. 

25
The quantum QE results are shown in figure 4.2. 

0

20

40

60

80

100

350 450 550 650 750 850

Wavelength (nm)

QE (normalized)

yes-Cu/yes-CdCl2

no-Cu/yes-CdCl2

yes-Cu/no-CdCl2

no-Cu/no-CdCl2

Figure 4.2 QE curves for NREL cells with yes/no Cu and yes/no CdCl2 treatments. 

The yes-CdCl2/no-Cu treatment enhanced overall current collection indicating improvement of 

carrier transport both in CdTe and CdS. The yes-Cu/no-CdCl2 treatment enhanced QE mostly in 

he short λ range. Adding the Cu treatment to the yes-CdCl2 cell decreased the long λ QE 

indicating that Cu attenuated the CdCl2 improvement to the carrier transport further from the 

junction. However, QE at the photon energy above the CdS bandgap is higher than for CdCl2

only. Based only on these data it is difficult to propose unambiguous explanations for the 

observed effects of Cu. 

C-V profiles taken at room temperature are shown in figure 4.3. The reverse bias voltage 

range is not sufficient at room temperature to reach the full depletion of the CdTe layer ~8 μm

thick. 

26
012345

Depletion width 

Na (cm-3

)

6

yes-Cu/yes-CdCl2

no-Cu/yes-CdCl2

yes-Cu/no-CdCl2

no-Cu/no-CdCl2

Zero Bias Rising 1013

1014

1015

μm

Figure 4.3 Room temperature C-V profiles for NREL (Albin’s )cells. 

 The yes-Cu cells had a lower apparent doping level than both CdCl2-treated and untreated 

cells. This looks surprising since introducing Cu is supposed to act as an acceptor doping. There 

is a possibility to explain this effect by pinning the Fermi level by partially compensated deep 

acceptor states provided by defects. In Sec.2, SCAPS SIMULATION, a similar effect was 

demonstrated for shallower DES at lower temperatures. 

CuCd

Hysteresis can be seen for the -2V to +0.5V to -2V bias voltage loop in the C-V profile for 

all of the cells (differences in on the left side of graph) and this is due to slow CTr’s as was 

discussed in section 1. This hysteresis is less pronounced for the yes-CdCl

Na

2 cells indicating that 

the CdCl2 treatment alters these DES. Possibly these changes reduce SRH recombination 

resulting in the improved current collection as seen in the QE measurements and JSC values. 

4.1.2 J-V-T Results 

 Figure 4.4 shows J-V curves for the yes-Cu/yes-CdCl2 cell for temperatures ranging from 

20o

C to -180oC in 10oC intervals. As the cell initially cools (blue curves), the increasing Schottky 

barrier rollover effect can be seen due to the exponential decrease with temperature of the reverse 

27
saturation current. As the cell continues to cool (green) the Schottky barrier increases further. As 

the cell cools further (red) decreases rapidly indicating that the free carriers are “freezing out” 

and that the cell is losing its ability to collect the photocurrent. The other three cells have the 

same general J-V-T structure. 

SC J

-30

-20

-10

0

10

20

30

-1 -0.5 0 0.5 1 1.5 2

J (mA cm-2)

Voltage

-30

-20

-10

0

10

20

30

-1 -0.5 0 0.5 1 1.5 2

J (mA cm-2)

Voltage

Figure 4.4 J-V-T curves for cell with the yes-Cu/yes-CdCl2 treatment at 10o C increments 

ranging from 20o

C to -40oC (blue), -50oC to -110oC (green), and -120oC to -180oC (red). 

The subtle differences are better highlighted by graphing and versus T as is shown for 

the four cells in figure 4.5. 

VOC J SC

28
0.0

0.2

0.4

0.6

0.8

1.0

1.2

-190 -140 -90 -40 10

Temperature (o

C)

Voltag

e

yes-Cu/yes-CdCl2

no-Cu/yes-CdCl2

yes-Cu/no-CdCl2

no-Cu/no-CdCl2

0

5

10

15

20

25

-190 -140 -90 -40 10

Temperature (o

C)

mA cm-2

Figure 4.5 and versus T graphs for cells with yes/no Cu and yes/no CdCl VOC SC J 2 treatments. 

 At room temperature the highest VOC value is demonstrated by the yes-CdCl2 cells, almost 

the same for yes- and no-Cu. The no-CdCl2 cells have the values equal to each other and 

lower by ~ 100mV than the former two cells. At T= -190

VOC

0

C both yes-Cu cells have higher 

than no-Cu cells: ~1180 mV and ~ 1150 mV respectively for no-Cl and yes-Cl cells. The lowest 

 has the yes-Cu/no-CdCl

VOC

VOC ~ 930mV 2 cell. 

SC J decreases modestly as T starts to decrease from room temperature and this may be 

attributed to narrowing absorbed solar spectrum caused by an increasing band gap. For CdTe, 

dTdE KmeV o

g −= /7.1 [9]. 

For the yes-CdCl2 cells, the carrier freeze out begins abruptly at lower T . It could be 

explained by freezing out the acceptor state that dominates the free carrier concentration in this 

temperature range. It happens when T becomes sufficiently low so that >>1, 

which eliminates the DES ionization and its contribution to the carrier transport. An approximate 

estimate of this acceptor energy level can be made by equating it to the Fermi level at the 

temperature where begins to drop and by neglecting the influence of light on the majority 

carrier holes concentration. The results of such estimates based on figure 4.5 data are presented in 

Table 4.2. 

− Ft /)( kTEE

SC J

29
Table 4.2 Estimate of the DES energy level based on freeze out temperature for Albin’s 

cells with yes/no Cu and yes-CdCl

SC J

2 treatments. 

Cu/CdCl2

NC-V at room T 

(cm-3) 

Freeze out

temperature (o

C) 

DES level estimated 

from Fermi level (eV) 

yes/yes 5x1013 -120 0.16 

no/yes 3x1014 -90 0.16 

 

The energy level of 0.16eV is close to that reported for the chlorine A-center 

[10,11], suggesting that it dominates the enhanced current collection in the yes-CdCl

+−

Cd − ClV Te

2

2 cells. This 

contrasts with the no-CdCl2 cells where there is no pronounced T -demarcation for the beginning 

of carrier freeze-out suggesting that current collection is enabled by DES with a range of defect 

energy levels. 

The graph of versus T in figure 4.6 shows the “series resistance” thermal dependencies 

for the differently processed cells. 

ROC

-190 -140 -90 -40 10

Temperature (o

C)

yes-Cu/yes-CdCl2

no-Cu/yes-CdCl2

yes-Cu/no-CdCl2

no-Cu/no-CdCl2

100

101

102

103

104

ROC (Ω cm2)

ROC crossover between

yes-Cu/yes-CdCl2 and no-Cu/yes-CdCl2 cells

-190 -140 -90 -40 10

Temperature (o

C)

yes-Cu/yes-CdCl2

no-Cu/yes-CdCl2

yes-Cu/no-CdCl2

no-Cu/no-CdCl2

100

101

102

103

104

ROC (Ω cm2)

ROC crossover between

yes-Cu/yes-CdCl2 and no-Cu/yes-CdCl2 cells

Figure 4.6 versus T graphs for NREL cells with yes/no Cu and yes/no CdCl ROC 2 treatments. 

30
The yes-Cu cells start off with the lowest resistances of about 5 Ω cm2

 at 20oC and steadily 

increase to about 1000 Ω cm2

 at -190oC. The no-Cu/no-CdCl2 cell starts off with 10 Ω cm2 at 

20o

C and climbs to about 1000 Ω cm2 at -80oC where it stays fairly constant to -190oC. The noCu/yes-CdCl2 cell starts off with 10 Ω cm2

 at 20oC and climbs to about 100 Ω cm2 at -30oC where 

it stays fairly constant to -190o

C. This sets up a series resistance crossover point between the yesCu and no-Cu/yes-CdCl2 cells at about -100o

C. The explanation relies on a combination of better 

transport properties from , depletion of by , and freeze out of around 100 K 

as discussed later. 

CuCd VCd CuCd CuCd

4.1.3 AS Results 

Figure 4.7 shows TC )( dependence in separate graphs for each of four (yes/no- CdCl p 2, 

yes/no-Cu) cells . 

nF cm-2 nF cm-2

0

2

4

6

8

10

12

-190 -140 -90 -40 10 60

0

2

4

6

8

10

12

-190 -140 -90 -40 10 60

0

2

4

6

8

10

12

-190 -140 -90 -40 10 60

0

2

4

6

8

10

12

-190 -140 -90 -40 10 60

Temperature (o

C)

yes Cu, yes CdCl2 no Cu, yes CdCl2

yes Cu, no CdCl2 no Cu, no CdCl2

H1

H1

H1

H1

H2

H2

H3

H3

nF cm-2 nF cm-2

0

2

4

6

8

10

12

-190 -140 -90 -40 10 60

0

2

4

6

8

10

12

-190 -140 -90 -40 10 60

0

2

4

6

8

10

12

-190 -140 -90 -40 10 60

0

2

4

6

8

10

12

-190 -140 -90 -40 10 60

0

2

4

6

8

10

12

-190 -140 -90 -40 10 60

0

2

4

6

8

10

12

-190 -140 -90 -40 10 60

0

2

4

6

8

10

12

-190 -140 -90 -40 10 60

0

2

4

6

8

10

12

-190 -140 -90 -40 10 60

Temperature (o

C)

yes Cu, yes CdCl2 no Cu, yes CdCl2

yes Cu, no CdCl2 no Cu, no CdCl2

H1

H1

H1

H1

H2

H2

H3

H3

 

Figure 4.7 AS graphs of Cp versus T for NREL cells with yes/no Cu and yes/no CdCl2 treatments. 

Three detected DES signatures are labeled H1, H2, and H3. 

31
Three distinct majority carrier or hole trap signatures were identified and analyzed. They were 

grouped by a combination of their activation energy, , and their detection temperature range. 

Differences were observed between each of the four differently processed cells and the most 

noticeable were between the yes-Cu and no-Cu cells. As explained in Section 1 each line 

corresponds to a fixed AC signal frequency ranging from 1 kHz (top line) to 1 MHz (bottom 

line). The analysis results are summarized in table 4.3. 

Ea

Table 4.3 Summary of detected AS signatures H1 through H3 for NREL cells with yes/no Cu 

and yes/no CdCl2 treatments. The estimates include a 75% linear regression confidence 

interval (experimental uncertainty not included). 

Ea

Ea Trap Cu/CdCl2 eV

σ a

2

cm

0 Ct

−2 cmnF β Temp 

K 

H1 yes/yes .133±.003 4x10-14 2.1 0.50 83 to 118 

no/yes .128±.002 8x10-14 3.0 0.48 83 to 108 

yes/no .128±.003 2x10-14 1.1 0.50 83 to 118 

no/no .145±.003 1x10-12 2.5 0.43 83 to 108 

H2 yes/yes .305±.005 9x10-14 0.9 0.49 183 to 238 

yes/no .292±.005 6x10-14 0.5 0.54 173 to 233 

H3 no/yes .475±.003 7x10-13 0.6 0.78 243 to 310 

no/no .464±.005 8x10-13 0.5 0.68 238 to 300 

A strong H1 signature was observed in all four cells with . This is consistent 

with reported values for the singly ionized cadmium vacancy [10] as well as the acceptor 

A center acceptor [10, 11]. The NP etch leaves behind a ~0.1μm thick free Te layer at 

the back surface and a Te enriched CdTe underneath [12] which serves as a source of as well 

as tellurium interstitials that can diffuse into the CdTe. The value at fixed frequencies 

decreases with increasing temperature indicating a steep concentration gradient near the back 

contact as shown by the SCAPS simulation in Section 2. Also, the values indicate that the 

concentration of H1 decreased with the addition of Cu which is consistent with Cu substitutions 

on cadmium vacancies that is replacement of shallower acceptor by the deeper one 

. This is also consistent with the previous observations of the temperature influence on the 

electroluminescence pattern in the cells with the intentional lateral nonuniformity of Cu-doping 

a ~ 13.0 eVE

Ea

− VCd

+−

Cd − ClV Te

2

− VCd

− Tei Cp

0 Ct

− CuCd− VCd

− CuCd

32
[16]. The concentration of H1 increased with the addition of CdCl2 which is consistent with the 

formation of . Thus, the H1 signature has been attributed to a combination of and 

A center . 

+−

Cd − ClV Te

2 − VCd

+−

Cd − ClV Te

2

The H2 signature was only detected in cells that had the Cu treatment. The is 

close to the values previously attributed to [13, 14]. We also assign this signature to the 

same defect. The H2 concentration was higher with CdCl

a ≈ 3.0 eVE

− CuCd

2 implying that the defects induced by 

CdCl2 (probably Cl-induced donor) increased the solubility of (well known effect of the 

Fermi level influence on the solubility of charged defects, see, e.g., [15]) . 

− CuCd

 The H3 signature with an of 0.47 eV was only detected in cells with no intentionally 

added Cu. It is possible that H3 is caused by the negative U-center . As mentioned above 

the NP etch can increase the tellurium interstitials. The apparent discrepancy between the 

measured and the calculated transition (0/-2) energy of 0.57 eV [15] can be 

eliminated by suggesting the negative 

Ea

2− Tei

Ea ≈ 47.0 eV

U-nergy value of –0.1eV. However, we remain cautious 

about assigning H3 to a specific defect. The H3 β values were higher (0.68 and 0.78), possibly 

indicating less Fermi level pinning for this defect (see results of simulation in Section 2). 

At warmer temperatures (30o

C and above), the yes-CdCl2 cells show a fairly constant spread in 

Cp values with frequency indicating a continuous band of energy levels possibly due to interfacial 

or GB surface DES. The no-CdCl2 cells show a growing magnitude with temperature possibly 

indicating a large magnitude defect beyond the measurement temperature range. 

4.1.4 CTr Results 

Two minority carrier traps labeled E2, E3 and two majority carrier traps labeled H4, H5 were 

detected using CTr; the results are tabulated in table 4.4. The most striking trap H5 was detected 

in the yes-Cu/yes-CdCl2 cell. It has a large density ( ), a high activation energy, 

(deep state), and a small apparent capture cross section . 

0 2 40 −

t = cmnFC

a = 83.0 eVE 217 108 cmx a

− σ =

Data from the other cells suggest that they also possess a high magnitude transient that would be 

detected with higher temperature testing. This can be seen in figure 4.8 that shows the 

temperature dependent amplitude of CTr caused by the forward bias (FB) charging pulse for 

the four cells. With the graph scale set to show H5, the other DES E2, E3, H4 are barely visible. 

ΔC

33
There is insufficient information to reliably assign these signatures to specific defects. E2 with 

eVE is possibly a donor and E3 with is possibly a donor [15]. a = 21.0 2+ Cui a = 43.0 eVE + ClTe

Table 4.4 Summary of detected CTr minority carrier (E2, E3), and majority carrier (H4, H5) trap 

signatures for NREL cells. The applied bias listed indicates whether the CTr was detected during 

the charging pulse or during the subsequent recovery period. 

Ea Trap Cu/CdCl2 eV

σ a

2

cm

0 Ct

−2 cmnF β Applied 

Bias V Temp 

K 

E2 yes/yes .214±.030 1x10-17 0.6 0.70 +0.5 Pulse 103 to 123 

E3 no/yes .427±.019 2x10-14 0.2 1.20 +0.5 Pulse 153 to 173 

H4 no/yes .524±.020 4x10-15 0.2 0.87 0 Recovery 193 to 213 

yes/no .688±.017 9x10-12 0.5 0.46 0 Recovery 203 to 223 

no/no .563±.011 3x10-14 0.2 0.77 0 Recovery 193 to 213 

H5 yes/yes .826±.017 8x10-17 40.0 0.52 +0.5 Pulse 328 to 358 

-200 -150 -100 -50 0 50 100

Temperature (oC)

yes-Cu/yes-CdCl2

no-Cu/yes-CdCl2

yes-Cu/no-CdCl2

no-Cu/no-CdCl2

-5

-4

-3

-2

-1

0

1

E2 E3

H4

H5

∆C (nF cm-2)

-200 -150 -100 -50 0 50 100

Temperature (oC)

yes-Cu/yes-CdCl2

no-Cu/yes-CdCl2

yes-Cu/no-CdCl2

no-Cu/no-CdCl2

-5

-4

-3

-2

-1

0

1

E2 E3

H4

H5

∆C (nF cm-2)

Figure 4.8 The amplitude Δ of capacitance transient caused by the FB charging pulse for NREL 

cells with yes/no Cu and yes/no CdCl

C

2 treatments. Temperature dependence 

34
Figure 4.9 shows the Arrhenius plot for both the AS and CTr detected traps. The AS 

detected traps H1, H2, H3 with τ ranging from microseconds to milliseconds can be seen in the 

upper portion of the graph and the CTr detected traps E2, E3, H4, H5 with τ ranging from 

seconds to 103

s can be seen in the lower portion of the graph. This shows how the CTr in this 

study explores a new region in the Arrhenius space. The unique experimental apparatus and 

custom software expressly designed and built for this purpose have facilitated this. 

There is greater uncertainty in the CTr estimates as indicated by the 75% confidence 

intervals. This is caused in part by the narrow 1000/T range. 

Ea

2 4 6 8 10 12

1000/T 

yes-Cu/yes-CdCl2

no-Cu/yes-CdCl2

yes-Cu/no-CdCl2

no-Cu/no-CdCl2 E2

10-8

10-6

10-4

10-2

102

100

E3

H1

H2

H3

H4

H5

1/(

τ

T2

)

2 4 6 8 10 12

1000/T 

yes-Cu/yes-CdCl2

no-Cu/yes-CdCl2

yes-Cu/no-CdCl2

no-Cu/no-CdCl2 E2

10-8

10-6

10-4

10-2

102

100

E3

H1

H2

H3

H4

H5

1/(

τ

T2

)

Figure 4.9 Arrhenius plot for NREL cells with yes/no Cu and yes/no CdCl2 treatments. H1, H2, 

H3 were detected with AS, and E2, E3, H4, H5 were detected with CTr. 

4.2 IEC Cells 

Room temperature J-V analysis was performed on the four replicates from each of the four 

categories on the cells obtained from Brian McCandless at the Institute for Energy Conversion 

(IEC) in Delaware. A number of the cells had fairly severe shunting indicating that they had 

“shorted out”. For each category, the cell with the best performance was selected for the full 

suite of QE, C-V, J-V-T, AS and CTr testing. 

35
 4.2.1 J-V, QE, C-V Results 

Performance results from each cell category for J-V measurements under illumination are 

tabulated in table 4.5. The performance order relation was the same as for the NREL cells with 

the exception of the yes-Cu/yes-CdCl2 cell which had lower η , , VOC FF and probably 

caused by shunting. The poor 

RSC

η for the no-CdCl2 cells are largely due to low indicating poor 

photocurrent collection. 

SC J

Table 4.5 J-V performance results for IEC cells with yes/no Cu and yes/no CdCl2. 

SC η J VOC

Cu/ CdCl2

% V −2 cmmA

FF

%

RSC

2 Ωcm

ROC

2 Ωcm

yes/yes 6.7 0.66 23.3 44 197 9.2 

no/yes 9.2 0.70 23.6 55 579 9.0 

yes/no 3.4 0.59 12.0 48 330 14.4 

no/no 3.0 0.59 11.5 44 309 24.1 

 

The J-V curves are shown in figure 4.10. Similar to the NREL cells, the rollover effect 

resulting from the back contact Schottky barrier is evident in the no-Cu cells. 

The QE curves, calibrated to the J-V measurement values are shown in figure 4.11. 

The largest contrast is with the overall collection between the yes- and no-CdCl

SC J

2 cells. The yesCdCl2 cells had higher QE values for the shorter wavelength, indicating better photocurrent 

collection near the junction. 

36
-30

-15

0

15

30

-0.5 0 0.5 1 1.5

Voltage

Current density (mA/cm-2) 

yes-Cu/yes-CdCl2

no-Cu/yes-CdCl2

yes-Cu/no-CdCl2

no-Cu/no-CdCl2

Figure 4.10 J-V curves for IEC cells with yes/no Cu and yes/no CdCl2. 

0

20

40

60

80

100

350 450 550 650 750 850

Wavelength (nm)

QE (normalized)

yes-Cu/yes-CdCl2

no-Cu/yes-CdCl2

yes-Cu/no-CdCl2

no-Cu/no-CdCl2

Figure 4.11 QE curves for IEC cells with yes/no Cu and yes/no CdCl2. 

C-V profiles taken at room temperature are shown in figure 4.12. The yes-CdCl2 cells have 

a narrower depletion width indicating a higher space charge density (SCD) near the junction. This 

37
implies a stronger electric field, which improves the electron/hole separation. With most of the 

light being absorbed within the first two μm this can explain why the yes-CdCl2 cells have better 

current collection. The depletion regions of the IEC no-CdCl2 cells are wider (>4μm at zero bias) 

than any of the NREL cells implying that they have the weakest electric field. This is consistent 

with the flat low QE curves indicating weak photocurrent collection throughout the 

photon energy range. For the IEC cells the CdCl

CdTe ν ≤ Eh g

2 treatment increases the apparent doping 

concentration, which contrasts with the NREL cells where it is the Cu treatment that had this 

effect. 

8.2.2 J-V-T Results 

Figure 4.12 Room temperature C-V profiles for IEC cells with yes/no Cu and yes/no CdCl2. 

Figure 4.13 graphs and versus T for the four differently processed cells. For the 

yes-CdCl

VOC SC J

2 cells, carrier freeze-out begins at about -120o

C. With a C-V profile apparent doping 

level , the activation energy for the doping defect is approximately 0.14eV which is 

close to that derived for yes-CdCl

14

a ≈ xN 102

2 NREL cells. This is a further indication that the A-center 

 is largely responsible for the superior current collection in the yes-CdCl +−

Cd − ClV Te

2

2 cells. 

0 123456

Depletion Width (microns)

Na (cm-3)

yes-Cu/yes-CdCl2

no-Cu/yes-CdCl2

yes-Cu/no-CdCl2

no-Cu/no-CdCl2

Zero Bias Rising 1013

1014

1015

1016

0 123456

Depletion Width (microns)

Na (cm-3)

yes-Cu/yes-CdCl2

no-Cu/yes-CdCl2

yes-Cu/no-CdCl2

no-Cu/no-CdCl2

Zero Bias Rising 1013

1014

1015

1016

38
0.0

0.2

0.4

0.6

0.8

1.0

1.2

-190 -140 -90 -40 10

Temperature (o

C)

Voltag

e

yes-Cu/yes-CdCl2

no-Cu/yes-CdCl2

yes-Cu/no-CdCl2

no-Cu/no-CdCl2

0

5

10

15

20

25

-190 -140 -90 -40 10

Temperature (o

C)

mA cm-2

Figure 4.13 and versus T for IEC cells with yes/no Cu and yes/no CdCl VOC SC J 2. 

4.2.2 AS and CTr Results 

Similarly to the NREL cells, AS has detected three distinct majority carrier traps, H1, H2, 

and H3. Capacitance transient measurements have detected two slow traps: labeled H5 for holes 

and E3 for electrons. The trap signatures are presented in table 4. The dependencies are 

shown in figure 4.14, the Arrhenius plots for both the AS and CTr detected traps - in figure 4.15. 

TfC ),( p

Table 4.6 The DES signatures detected by AS ( H1 through H3) and CTr (E3, H5) 

Ea Trap Cu/CdCl2 eVσ a

2

cm

o Ct

2 / cmnF

β Applied 

Bias, V 

Temp 

o

K 

H1 yes/yes 

no/yes 

.160Υ.004 

.157Υ.008 

5x10-13

8x10-13

1.1 

3.1 

0.46 

0.38 - 88 to 118 

83 to 113 

H2 yes/yes 

yes/no 

.358Υ.011 

.353Υ.016 

2x10-13

1x10-13

2.6 

0.6 

0.60 

0.37 - 193 to 248 

203 to 248 

H3 no/yes 

no/no 

.426Υ.003 

.475Υ.005 

2x10-13

7x10-13

3.0 

0.6 

0.68 

0.67 - 223 to 293 

243 to 297 

E3 no/yes .256Υ0.19 2x10-18 0.7 1.10 +0.5 Pulse 133 to 153 

H5 yes/yes 0.637Υ0.17 7x10-19 56.0 0.40 +0.5 Pulse 308 to 353 

39
0

2

4

6

8

10

-190 -140 -90 -40 10 60

0

2

4

6

8

10

-190 -140 -90 -40 10 60

0

2

4

6

8

10

12

-190 -140 -90 -40 10 60

0

2

4

6

8

10

12

-190 -140 -90 -40 10 60

Temperature (o

C)

yes Cu, yes CdCl2 no Cu, yes CdCl2

yes Cu, no CdCl2 no Cu, no CdCl2

12 12

nF cm-2 nF cm-2

0

2

4

6

8

10

-190 -140 -90 -40 10 60

0

2

4

6

8

10

-190 -140 -90 -40 10 60

0

2

4

6

8

10

-190 -140 -90 -40 10 60

0

2

4

6

8

10

-190 -140 -90 -40 10 60

0

2

4

6

8

10

12

-190 -140 -90 -40 10 60

0

2

4

6

8

10

12

-190 -140 -90 -40 10 60

0

2

4

6

8

10

12

-190 -140 -90 -40 10 60

0

2

4

6

8

10

12

-190 -140 -90 -40 10 60

Temperature (o

C)

yes Cu, yes CdCl2 no Cu, yes CdCl2

yes Cu, no CdCl2 no Cu, no CdCl2

12 12

nF cm-2 nF cm-2

Figure 4.14 Cp versus T for IEC cells. Three detected DES are labeled H1, H2, and H3. 

2 4 6 8 10 12

1000/Temperature(K)

yes-Cu/yes-CdCl2

no-Cu/yes-CdCl2

yes-Cu/no-CdCl2

no-Cu/no-CdCl2

10-8

10-6

10-4

10-2

102

100

1/(

τ

T2

)

H1

H2

H3

H5

E3

2 4 6 8 10 12

1000/Temperature(K)

yes-Cu/yes-CdCl2

no-Cu/yes-CdCl2

yes-Cu/no-CdCl2

no-Cu/no-CdCl2

10-8

10-6

10-4

10-2

102

100

1/(

τ

T2

)

H1

H2

H3

H5

E3

Figure 4.15 Arrhenius plots for IEC cells detected with AS (H1, H2, H3) and CTr (E3, H5). 

40
No H1 signature was detectable in the no-CdCl2 cells. Since H1 has been previously 

attributed to both the and the A-center defects we can infer that H1 is dominated 

by in the yes-CdCl

− VCd+−

Cd − ClV Te

2

+−

Cd − ClV Te

2

2 cells and that the concentration is low in all of the cells. The 

IEC cells underwent a bromine methanol (BrM) etch which leaves behind a thinTe layer about 1-

2nm thick, which contrasts with the NREL cells where an NP etch provides a Te layer hundreds 

of nm thick. Since the Te layer serves as a source, this offers an explanation as to why H1 is 

not detectable in the no-CdCl

− VCd

− VCd

2 IEC cells while it is detectable in the no-CdCl2 NREL cells. Also, 

contrary to Albin’s cells, there is no decreasing capacitance with increasing temperature, which 

indicates that the concentration gradient near the back contact is lower than in the NREL 

cells (see Sec.2), which is consistent with the BrM (not NP) etch treatment. 

− VCd

The H2 defect, which has been attributed to , has a higher concentration for the yesCu/yes-CdCl

− CuCd

2 cell than for the yes-Cu/no-CdCl2 cell. Indeed the magnitude of for the former 

is over 4 times larger than for the latter. The H1 concentration in yes-CdCl

0 Ct

2 cells decreases with 

the addition of Cu which implies that CuCd displaces the A center sites. +−

Cd − ClV Te

2

The H3 defect also has a higher concentration for the yes-Cu/yes-CdCl2 cell than for the 

yes-Cu/no-CdCl2 cell implying that CdCl2 contributes to the formation of H3, the attribution of 

which is still uncertain. 

4.3 Cells Fabricated by X. Wu 

This section presents the results of studying six cells: three with intentionally added Cu and 

three without Cu, subjected to three different CdCl2 treatments: (a) 15 min 405o

C anneal, (b) 15 

min 425o

C anneal, and (c) no CdCl2 at all. 

The room temperature J-V measurements for these cells with and without Cu and with the 

three CdCl2 treatments are shown in Table 4.7. The Cu treatment had more impact on cell 

performance. This contrasts with cells received from D. Albin and B. McCandless where the 

CdCl2 treatment had the greater impact on performance. In Wu’s cells the short circuit dynamic 

resistance (“shunt resistance”) increased with the Cu treatment and this has not been previously 

typically observed. The 2nd level metrics VOC, JSC and FF were improved by the Cu treatment with 

41
one exception. With the optimal CdCl2 treatment (15’ @ 405o C) the no-Cu cell had a better JSC. 

This has also been observed in the cells from Albin and McCandless. 

Table 4.7. J-V characterization results for cells from X. Wu. 

Eff Cell Treatment Voc

% mV

sc J

2 / cmmA

FF

%

Rsc

2 Ω − cm

Roc

2 Ω − cm

W 1226-2 yes Cu, 15’ CdCl2, 405 C 14.06 826 23.7 71.8 1360 2.8 

W 1226-8 no Cu, 15’ CdCl2, 405 C 7.20 657 24.6 44.5 191 15.4 

W 1226-4 yes Cu, no CdCl2 10.44 754 20.9 66.3 491 3.3 

W 1226-7 no Cu, no CdCl2 8.47 734 18.9 61.1 369 8.0 

W 1226-3 yes Cu, 15’ CdCl2, 425 C 10.88 801 23.5 57.8 261 4.9 

W 1226-9 no Cu, 15’ CdCl2, 425 C ? 2.24 463 14.8 32.7 83 28.3 

QE data showed higher near band-gap (~840nm) collection as well as higher blue (~450nm) 

collection for the high temperature CdCl2 treated cells (15’ 425C). This indicates increased 

CdS/CdTe intermixing and a thinner effective window layer. For the non-CdCl2 treated cells, the 

QE data showed the least CdS/CdTe intermixing as well as the least collection for wavelengths 

around 550 nm, which is absorbed in the CdTe near the CdS/CdTe interface. 

J-V-T data shows increasing VOC with decreasing temperature. As before a possible 

explanation for this effect is that the VOC limiting DES are freezing out as the temperature 

decreases. This effect is less pronounced on the two cells that had the CdCl2 treatment with no 

Cu. This is consistent with previous observations on cells from Albin, but less so on cells from 

McCandless. The J-V-T data for JSC shows an abrupt decrease in current collection starting at 

 -120o

C for the CdCl2 treated cells. A possible explanation for this is that VCd and related 

complex defects with Ea~0.15eV are the dominant dopants in the CdCl2 treated cells and they 

start freezing out at these temperatures. This is consistent with observations on cells from Albin 

and McCandless. 

A total of twenty detected DES were divided into seven distinct groups based on the DES 

signatures. The Arrhenius plots for these results are shown in Fig. 4.16. The H1, H2, H3, and 

H3B signatures at the top of the graph were detected with AS analysis. The E2, E3, and H5 

signatures at the bottom of the graph were detected with the CTr analysis. These defect signatures 

42
are in general consistent with those detected in cells supplied by Albin and McCandless although 

there are some differences in their signatures. A complete listing of the detected defects is in 

Table 4.8. 

Figure 4.16 Arrhenius plot for DES in the cells produced by Wu and detected with AS (top) and 

CTr (bottom) 

The H1 signature which has been identified as belonging to and related complexes had 

measurable peaks in the cells processed with Cu. For the other cells, while there 

were no peaks, there were indications that the H1 DES is present with a range of values. This 

could be a result of the band bending and the AS distortions analyzed in Sec. 2 or it could be an 

actual band of DES energy levels. Based on the J-V-T data it is possible to conclude that this H1 

defect is the dopant that enables superior current collection in the CdCl

− VCd

p ddC ln/ ω

Ea

2 treated cells. 

H2, which has been previously identified as , is only detected in the Cu treated cells. − CuCd

There are not yet experimental indications for what role, if any, this defect might have in superior 

performance of the Cu treated cells. 

43
The H5 defect is of interest because it has a very strong CTr signature in the cells that have 

both the Cu and CdCl2 treatments. The magnitude of is an order of magnitude larger than that 

from any other detected defect, indicating that it has a high concentration. We have also detected 

H5 with similar magnitudes in the Cu + CdCl

0 Ct

2 treated cells from Albin and McCandless. Because 

of high concentration and position of the level in the bandgap this defect can affect the free 

carrier concentration (Fermi level) and minority carrier lifetime. So far we refrain ourselves from 

identifying the defect responsible for the H5 DES. 

The minority carrier defect E2 occurs with Cu processing and E3 occurs without Cu. The 

H3 and H3B occur mostly without the Cu treatment. This is consistent with observations on cells 

from Albin and McCandless and their impact on cell performance is not known. 

Table 4.8. DES signatures determined with AS and CTr in cells produced by X. Wu 

44
5. IMPROVEMENT OF THE CELL PROCESSING 

At the start of this work we could reliably produce cells with η > %8 and . 

The studies were undertaken aimed at improvement of the cell performance and optimization of 

processing procedures. 

VOC > 650mV

In all these experiments a 1.5x1.5x1/8 inch TEC-15 glass plate was used as a substrate. 

Approximately 200nm of CBD CdS was deposited followed by 3-5 microns gas-jet CdTe (the 

gas-jet system yields a thicker CdTe layer at the center of the plate than at the edges) and CdCl2

anneal. The back contact preparation includes a bromine methanol etch followed by evaporating 

10Å of Cu and 500 Å of ZnTe onto the back surface. Then the rapid thermal anneal (RTA) of a 

back contact structure is applied. The cell is placed in a carbon boat in a bell jar, which is 

evacuated and back-filled with nitrogen. The boat is heated for 3-5 minutes to drive the Cu into 

the cell. When the boat temperature reaches the set temperature, the heating element is turned off. 

Following this, circular gold dots of each in a hexagonal pattern are evaporated onto 

the plate to finish the device. Approximately 50 cells are produced from a single plate. 

2 079.0 cm

The first study was aimed at revealing the optimum temperature for the back contact 

anneal. Following the gold contact deposition, RTA temperatures were applied in succession 

ranging from to in increments of . J-V results averaged from the top 21 

cells out of 26 tested are tabulated in table 5.1. The standard deviations are listed to help gage the 

cell-to-cell non-uniformity within a single plate. The demonstrated the best performance 

as well as the least non-uniformity. This was taken as the default RTA temperature for our 

process. It is evident that the back contact anneal temperature significantly impacts as well 

as , the short circuit dynamic resistance or “shunt” resistance. 

Co 150 Co 400 Co 50

Co 300

VOC

RSC

45
Table 5.1. The effect of the RTA temperature on cell performance

A J-V graph of for a single cell over the range of RTA temperatures is shown in Fig. 5.1. 

For the “under-annealed” cell a voltage dependent current collection is evident. This 

could indicate a weak electric field or lateral non-uniformity with weak diodes. For the “overannealed” cell the rollover is similar to that of severely degraded cells. 

Co 250

Co 350

Figure 5.1. J-V curves for RTA temperatures bracketing the optimum efficiency. 

After that study a number of changes in the CdTe cell growth recipe were tested. One was 

changing the CdS deposition technique from Chemical Bath Deposition (CBD) to thermal 

evaporation. The ability to deposit the CdS layer by thermal evaporation at CSM was recently 

developed. We have suspected that microscopic bubbles or contaminant particles with the CBD 

46
CdS deposition have been leading to the visible pinholes seen later in processing that are believed 

to cause performance degradation. With the thermally evaporated CdS the visible pinholes have 

been virtually eliminated. 

Another performance enhancement was discovered while using nitrogen as the gas-jet 

VTD carrier gas instead of helium. The nitrogen was initially used in place of helium to calibrate 

the gas-jet source and chamber pressures, which are measured with thermocouple gauges that are 

known to give distorted readings with helium. During this process it was found that the nitrogen 

carrier gas improved cell performance. 

The effects of these two changes on the cell performance can be seen in Fig. 5.2. 

Fig. 5.2 Open circuit voltage versus cell efficiency. Dot cells 0.079 cm2

 each, were processed 

with CBD or evaporated CdS, and CdTe deposited in gas-jet system with nitrogen or helium 

carrier gas. 

Each of the four categories in Figure 5.2 consists of approximately 50 dot cells each 0.079 cm2 

on a 1.5”x1.5” panel. The improved with the evaporated CdS layer is evident. The improved 

efficiencies with the N2 carrier gas are due to a combination of better and fill factor. 

VOC

VOC

47
These and other experiments led to the production of a record for the CSM gas-jet VTD 

system cell with η = %6.12 , , , and . It should be 

noticed that the cell had no front contact buffer layer, antireflection coating, etc. As mentioned 

previously, our J-V characterization underestimates cell efficiency due to discrepancy between 

the solar spectrum and that of our light source. 

VOC = 797mV 2 J SC = /3.23 cmmA FF = %68

These highest performing cells were produced with the following processing steps: 

• Began with commercial TEC15 soda lime glass coated with a fluorine doped tin-oxide. 

• Detergent clean that included a UV/ozone step to remove residual organics. 

• 200nm of CdS vacuum evaporated onto the substrate with T C . o substr = 150

• 4~ μm CdTe deposited by gas-jet with a N2 carrier gas, a nominal substrate temperature of 

C , a quartz frit orifice temperature of and a source temperature of . o 375 Co 775 Co 700

• CdCl2 topically applied from an ultrasonic nebulizer mist of 1 mole CdCl2

 solution in a heated chamber. 

• Annealed in a 50/50 oxygen/nitrogen atmosphere at C for 35 minutes. o 400

• 2nm of Cu followed by 50nm ZnTe thermally evaporated onto room temperature substrate. 

• Rapid Thermal Anneal to C to activate the back contact. o 280

• gold back contact dots deposited by thermal evaporation onto room temperature 

substrate. 

2 079.0 cm

• Manual scribe along edge to front contact and application of indium by soldering. 

• The perimeter of the dot cell itself was also scribed in order to minimize edge current 

collection that could artificially inflate the measured current and . J sC

48
6. REFERENCES 

1. “Studies of Basic Electronic Properties of CdTe-Based Solar Cells and Their Evolution 

During Processing and Stress”, V. Kaydanov and T.R. Ohno, Final Technical Report , 

Oct.16, 2001- Aug. 31, 2005, Subcontract No.ADJ-2-30630-05 under Prime Contract No. 

DE-AC36-99-GO10337, Colorado School of Mines, Golden CO, 2005 

2. Blood, P., Orton, J.W., The electrical characterization of semiconductors: majority carriers 

and electron states, Academic Press Limited, London, 1992 

3. Meyer, W., Neldel, H., Z. Tech. Phys. (Leipzig) 12, 588, 1937 

4. Widenhorn, R., Rest, A., Bodegom, E., “The Meyer–Neldel rule for a property determined 

by two transport mechanisms”, J. Appl. Phys. Vol 91, (10), pp. 6524-6528, May 15, 2002 

5. M. Burgelman, P. Nollet and S. Degrave, “Modeling polycrystalline semiconductor solar 

cells”, Thin Solid Films, 361-362, 527-532, 2000 

6. Gloeckler, M., Fahrenbruch. A.L., and Sites, J.R., “Numerical Modeling of CIGS and CdTe 

Solar Cells: Setting the Baseline”, Presented at the World Conference on Photovoltaic 

Energy Conversion, Tokyo, Japan, May, 2003 

7. Fahrenbruch, A., Modeling Results for CdS/CdTe Solar Cells, CSU Report, 

http://www.physics.colostate.edu/groups/photovoltaic/PDFs/ModCdTe.pdf, March 2000 

8. F.H. Seymour, “Studying of Electronic States controlling the Performance of CdTe Solar 

Cells”, Ph.D. Thesis, September 2005, Colorado School of Mines, Golden, CO 

9. McCandless, B., Sites, J. R., in Handbook of Photovoltaic Science and Engineering, edited 

by A. Luque and S. Hegedus, John Wiley & Sons Ltd, West Sussex, England, ch. 14, pp 

617-657, 2003 

10. Castaldini, A., Calvallini, A., Fraboni, B., Fernandez, P., Piqueras, J. , “Deep energy levels 

in CdTe and CdZnTe”, J. Appl. Phys., 83 No 4 p212-2126, 1998 

11. Hofmann, D.M., Omling, P., Grimmeiss, H.G., Meyer, B.K., Benz, K.W., Sinerius, D. 

“Identification of the chlorine A center in CdTe”, Phys. Rev. B 45 11, pp 6247-6250, 1992 

12. Albin, D., Dhere, R., Wu, Z., Gessert, T., Romero, M.J., Yan, Y., Asher, S. “Perturbation 

of Copper Substitutional Defect Concentration in CdS/ CdTe Heterojunction Solar Cell 

Devices”, Proceedings of the Materials Research Society Conference, Symposium F, San 

Francisco, California, USA, April, 2002 

13. Zanio, Kenneth, Semiconductors and Semimetals Volume 13 Cadmium Telluride, 

Academic Press, New York, USA, p162, 1978 

14. Balcioglu, A., Ahrenkiel, R.K., Hasoon, F., “Deep-level impurities in CdTe/CdS thin-film 

solar cells”, J. Appl. Phys., 88 No 12 p7175-7178, 2000 

49
15. Wei, S. H., Zhang, S. B., “Chemical trends of defect formation and doping limit in II-IV 

semiconductors: the case of CdTe”, Phys. Rev. B 66, p155211, 2002 

16. Feldman, S.D., Collins, R.T., Kaydanov, V., Ohno, T.R., “Effects of Cu in CdS/CdTe solar 

cells studied with patterned doping and spatially resolved luminescence”, Appl. Phys. Lett., 

85 (9), 1529-1531, 2004 

17. Xuanzhi Wu, “High-efficiency polycrystalline CdTe thin-film solar cells”, Solar Energy, 77 

(2000), 803-814 

50
7. APPENDICES 

7.1 Personnel

The names, titles, and representative responsibilities of the individuals contributed to this work 

are presented below. 

Timothy R. Ohno, Associate Professor of Physics: Photovoltaic Development, Surface Physics, 

Electroluminescence, Principal Investigator 

Fred H. Seymour, Post Doc. Fellow: Photovoltaic Development; Cell Processing and 

Characterization; Studying of Deep Electronic States by means of Admittance Spectroscopy 

and Transients; Modeling of Cell characteristics with AMPS and SCAPS; Developing of 

Database 

Joe Beach, Assistant Research Professor: Photovoltaic Development, Developing Novel 

Approaches to the Cell Processing; Design and Assembling Processing Equipment; J-V 

Characterization of Cells and the Data Analysis 

Victor I. Kaydanov, Research Professor of Physics: Photovoltaic Development; Studying of the 

Defect Electronic States 

7.2 Laboratory Improvements

 The principal change in the laboratory during this period was modification of the thermal 

evaporation system that had previously been used for back contact preparation. The evaporator 

was modified to permit rapid change-out of evaporation source material, including CdS and CdTe 

for the studies cited above, and sample heating during deposition. As a result of the evaporation 

of Cd-containing compounds a barrier system with venting through a HEPA filter was 

constructed. The system was tested by CSM Environmental Health and Safety to verify that no 

cadmium contamination resulted. 

7.3 Publications/Presentations 

1. “Simulated Admittance Spectroscopy of High Concentration Deep Level Defects in CdTe 

Thin-Film Solar Cells”, F. H. Seymour, V. Kaydanov, T. R. Ohno, J. Appl. Phys., 100, 

033710, August 8, 2006. 

2. “Admittance Spectroscopy Characterization of Deep Electronic States in Cadmium Telluride 

Solar Cells”, F. H. Seymour, V. Kaydanov, T. R. Ohno, Proceedings from 2006 IEEE World 

Conference on Photovoltaics Energy Conversion, Waikoloa, Hawaii, May, 2006. 

51
3. Organized and facilitated VOC mini-workshop at National CdTe team meeting, Golden, 

Colorado, March 10, 2006 (description in Q2 report). 

4. “Performance and defects in CSS and VTD CdTe cells treated with and without Cu and 

CdCl2”, F.H. Seymour, V. Kaydanov, T.R. Ohno, Presentation at National CdTe team 

meeting, Golden, Colorado, March 9, 2006. 

5. “Deep Electronic States in CdTe Solar Cells Detected with Admittance Techniques”, F. H. 

Seymour, V. Kaydanov, T. R. Ohno, DOE Solar Energy Technologies Program 2005 

Review Meeting, Denver, Colorado, November, 2005. 

6. “Studies of Electronic States Controlling the Performance of CdTe Solar Cells”, F. H. 

Seymour, Ph.D. thesis, Colorado School of Mines, October, 2005. 

7. “Cu and CdCl2 Influence on Defects Detected in CdTe Solar Cells with Admittance 

Spectroscopy”, F. H. Seymour, V. Kaydanov, T. R. Ohno, D. Albin, Appl. Phys. Lett., Vol 

87, 153507, October, 2005. 

52
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Studies of Basic Electronic Properties of CdTe-Based Solar Cells 

and Their Evolution During Processing and Stress: Annual 

Technical Report, 01 November 2005 – 31 October 2006 

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14. ABSTRACT (Maximum 200 Words)

We describe the results of our continuing study of deep electronic states controlling open-circuit voltage in CdTe/CdS 

thin-film solar cells (Task 1). The study includes: (1) analysis of factors affecting trap signatures derived from admittance 

spectroscopy and capacitance transients measurements, such as activation-energy capture cross-sections and trapdensity estimates, and (2) comparative studies of cells received from four different sources and prepared with significant 

variations in cell structure and processing procedures. 

15. SUBJECT TERMS

Admittance spectroscopy; capacitance transients; deep electronic states; chemical-bath deposition; post-deposition 

annealing; gas-transport deposition; Gas-Jet system; optimization; 

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