Conference Paper 

NREL/CP-520-42811 

November 2008 

Accelerated Stress Testing 

and Diagnostic Analysis of 

Degradation in CdTe Solar Cells 

D.S. Albin 

National Renewable Energy Laboratory

Presented at the 2008 SPIE Optics+Photonics Meeting 

Reliability of Photovoltaic Cells, Modules, Components and Systems 

San Diego, California 

August 10–14, 2008 
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Accelerated Stress Testing and Diagnostic Analysis of Degradation in 

CdTe Solar Cells 

David S. Albin

National Renewable Energy Laboratory, 1617 Cole Blvd., Denver, CO, USA 80401 

ABSTRACT 

Solar cell module reliability is inextricably linked to cell-level reliability. This is particularly so with thin-film 

technologies. In CdTe, reliability issues historically associate with back contact stability and the use of Cu as an 

extrinsic dopant. Using a simple approach by which identical cells are heated under open-circuit bias and 1-sun 

illumination, degradation activation energies of 0.63 and 2.94 eV in laboratory-scale CdS/CdTe devices were identified 

in the accelerated stress temperature range of 60 to 120 °C. At lower stress temperatures, cell performance changes were 

linearly correlated with changes in both fill factor (FF) and short-circuit current (Jsc). At higher stress temperatures, 

changes in efficiency were correlated with changes in FF and open-circuit voltage (Voc). The measured activation 

energy of 0.63 is associated with Cu-diffusion. During the early stage of stress testing, which may provide additional 

back contact annealing, improvements in FF were due to Cu-diffusion. Decreased performance observed at longer stress 

times (decreased FF and Voc), according to a two-diode Pspice model, were due to both increased space-charge 

recombination (near the junction) and decreased recombination in the bulk. Kirkendall void formation (S-outdiffusion) at 

the CdS/CdTe interface is given as responsible for the 2.9 eV degradation mechanism. 

Keywords: CdTe solar cell, degradation mechanism, recombination, activation energy, Kirkendall effect 

1. INTRODUCTION 

Polycrystalline CdS/CdTe devices show good solar cell potential with laboratory efficiencies of 16.5% having 

been demonstrated [1]. In addition to considerable research in maximizing performance, recent work has also focused 

on cell stability due to concerns surrounding the use of Cu as a dopant in these structures [2,3,4]. Stress testing typically 

involves elevated temperature as a means to accelerate degradation [5]. To date, temperatures ranging from 65 to 200 °C 

have been used in similar CdTe stability studies. Though most use fixed stress temperatures, others have considered 

temperature itself as a variable [6,7]. Recently, it has been reported that processing can impart strong differences in 

stability, and thus potentially different degradation mechanisms [8,9]. Viable stress protocols require acceleration 

temperatures that only invoke mechanisms expected under actual use conditions. The primary goal of this study was to 

ascertain the presence and types of mechanisms affecting CdS/CdTe device stability in the temperature range of 60 to 

120 °C. It should be noted that the results presented here are specific to cells made using the specific growth conditions 

described here. 

2. EXPERIMENTAL PROCEDURE 

Polycrystalline CdS and CdTe films were deposited by chemical bath deposition (CBD) and close-spaced 

sublimation (CSS) respectively. These layers were deposited on 1.5" x 1.5" tin-oxide coated Corning 7059 glass 

superstrates. The CBD solution consisted of 550 ml DI water, 8 ml of 0.033 M cadmium acetate, 15 ml of reagent-grade 

ammonium hydroxide (28-30%), and 4.7 ml of 1.0 M ammonium acetate heated to 92 °C where it was then titrated (2 ml 

at 10 m intervals) with 8 ml of 0.067M thiourea solution for 37 m. The CSS process was subsequently performed at a 

substrate growth temperature of 620 °C and CdTe source temperature of 660 °C in an ambient of 1 torr O2 and 15 torr 

He for 5 m. This processing resulted in CdS and CdTe layers of approximately 80 nm and 9-10 microns respectively. 

Resulting glass/SnO2/CdS/CdTe layers were then annealed at 400 °C suspended in a CSS configuration over a pelletized, 

anhydrous CdCl2 source in an ambient of 100 torr O2 and 400 torr helium for 8 m. Prior to contact application, a 1:88:35 

volume mixture of HNO3:H2PO4:H2O acid etch was used to remove surface oxides and produce a Te-rich layer to 

1 
improve initial performance [10] and stability [9]. Contacts consisted of a Cu-doped, graphite paste mixture brushed 

onto the etched CdTe surface and subsequently annealed in He at 280 °C for 25 m followed by the final application of a 

Ag-paste (Acheson) conductor. The average efficiency of 24 similarly made superstrate CdTe solar cells was measured 

using a calibrated, xenon-arc, Oriel solar simulator to be 12.6%. A total of 6 individual cells were tested at each of 4 

different stress temperatures for times exceeding 2000 hours. 

The stress station consisted of two parts: an illumination source supported over individually heated sample 

holders. Illumination was supplied using a xenon-arc, Atlas CPS+ Suntest station calibrated to deliver 1-sun at the 

sample test plane over extended periods of time. A borosilicate filter was used to eliminate UV radiation below ~300 

nm. Approximately 0.75 cm2

 cell samples were placed in aluminum fixtures designed to hold the cells facing this 

illumination (glass-side up) in an open-circuit configuration (no backcontact-to-tin oxide shorting). These fixtures were 

attached to aluminum blocks independently heated by 300 W cartridge heaters. Thermocouples were inserted into these 

blocks to provide feedback for individual RKC-CB100 temperature-controllers. Calibration curves correlating 

aluminum block temperature with true junction temperatures (using thermocouples mounted to dummy cell back 

contacts) were performed prior to stress testing. Cell stress temperature was monitored continuously during tests and 

was fixed constant at 60, 80, 100, and 120 °C during the test duration within ± 1°C. Stress testing was performed in a 

laboratory environment where relative humidity (RH) was controlled at ~30% at 25 °C. At stress temperatures of 120 to 

60 °C, this corresponds to ~ 0.5 to 4.8 % RH respectively. 

Cell current and voltage (JV) measurements were performed on all cells initially (t = 0) and after stress (t > 0). 

A previous [7] study of cell performance as a function of stress temperature reported an activated degradation energy of 

~1 eV. Using this value, stress measurement times were targeted in order to best compare device performance stressed 

at different temperatures. At these pre-determined times, devices were removed as a group, stored in the dark at 25 °C 

for 12-24 hrs, and then had their dark and light JV characteristics measured. These measurements were used to 

determine Voc, FF, Jsc, and efficiency (η%). In order to extract information regarding recombination, series and shunt 

resistance, as well as back contact related parameters, measured JV curves were fit to a discrete element circuit model 

shown in Figure 1 using a freeware version of PSpice (Orcad PSpice Student Ver. 9.1). 

Figure 1. Circuit model used to extract device parameters.

By default, PSpice uses Si diodes where forward current equals the sum of the normal and recombination 

currents and the diode quality factor, N, varies between 1 and 2 depending upon the particular default Si diode used. In 

this model, a parallel combination of forward-biased diodes was used to independently simulate recombination currents 

in the quasi-neutral (JQNR) and space-charge (JSCR), i.e., depletion regions by effectively setting the recombination term in 

the default diode template to zero (e.g., Irec = recombination current = 0), and N to either 1 (QNR) or 2 (SCR). The 

theoretical basis for this two-diode model as a general expression for the current produced in a solar cell can be found in 

ref.[11]. Though useful for deconvoluting individual forward-current terms, this model assumes Shockley-Read-Hall 

recombination via a single, mid-gap trap. Such ideal conditions are not expected. The presence of impurities and 

structural defects (e.g., grain boundaries, and interface states introduced by the relatively large lattice-mismatch 

(~11.7%) between CdS/CdTe) complicate recombination beyond this single-level approximation. Unfortunately, more 

sophisticated models (e.g., the use of AMPS-1D [12]) in which recombination can be more accurately studied were 

deemed as having too many "curve-fitting parameters" for this study. The back contact behavior was modeled using a 

combination reverse-biased diode (Jb) and shunt conductance (1/Rb) that was previously shown capable of fitting JV 

characteristics in the first quadrant [13]. 

2 
3. RESULTS AND DISCUSSION 

JV data for two representative devices tested at 60 and 120 °C immediately after fabrication (t=0), and after 1 

and 723 hrs of stress are shown in Figure 2. Initially, most devices showed cross-over of the dark and light curves which 

was removed after the first hour of light soak. Cross-over is attributed to photoconductivity introduced by Cu diffusion 

resulting from the final 280 °C back contact anneal [14]. Since cross-over is metastable, affects performance primarily 

in the first quadrant, and is effectively removed by an initial light soak, following discussions regarding changes in 

device performance sometimes use t=1 hr measurements as approximating the initial performance. 

Figure 2. JV data for representative cells stressed at extreme values of time and temperature. 

 As shown in Figure 2, devices stressed at 60 and 120 °C show roll-over in the first quadrant due to increasing 

back contact barrier height. The latter was observed to increase with temperature and time indicating a gradual 

degradation of the back contact during stress. Figure 3 plots the average values of Voc, Jsc, FF, and η% for the 6 devices 

tested at each stress temperature as a function of stress time out to 723 hrs. 

0.84

0.83

0.82

0.81

0.80

0.79

0.78

Voc (volts)

0 200 400 600

Stress Time (hrs)

 T = 60 °C

 T = 80 °C

 T = 100 °C

 T = 120 °C

(a)

24

23

22

21

20

Jsc (mA/cm2

)

0 200 400 600

Stress Time (hrs)

 T = 60 °C

 T = 80 °C

 T = 100 °C

 T = 120 °C

(b)

70

65

60

55

50

FF (%)

0 200 400 600

Stress Time (hrs)

 T = 60 °C

 T = 80 °C

 T = 100 °C

 T = 120 °C

(c)

13

12

11

10

9

Efficiency (%)

0 200 400 600

Stress Time (hrs)

 T = 60 °C

 T = 80 °C

 T = 100 °C

 T = 120 °C

(d)

Figure 3. Changes in Voc (a), Jsc (b), FF (c), and η% (d) as a function of time and stress temperature 

3 
 By visual observation, it appears that changes in cell performance (η%) mimic similar changes in both Voc and 

FF. These modal observations, i.e., how degradation manifests itself, provide insight regarding cell degradation 

mechanisms. To better quantify this effect, linear regression fits of the %change (t > 0) relative to the initial 

measurement (t = 0) of η% (deltaEff) vs. %changes in Voc (deltaVoc), Jsc (deltaJsc), and FF (deltaFF) were made from 

each combination of η%, Voc, Jsc, and FF measured during stress testing. (Note that these are not true differentials and 

thus, not derivable in a purely analytical fashion.) These plots for measurements performed at T=60 °C and 120 °C are 

shown in Figure 4. The horizontal line indicates the overall response mean for each parameter, while the off-horizontal 

lines indicate the regression (solid-line) bracketed by the 95% confidence intervals (dashed-lines) for the fit. Also 

shown in this figure are values of R2

 which equals the ratio of the linear model sum of squares to the total sum of 

squares. In other words, R2

 measures the proportion of the total data variance around the mean explained by the linear 

fit. A R2

 = 1.0 indicates that all data variation can be explained by the fit, while R2=0 indicates that the model serves no 

better predictor than the overall response (horizontal line) mean. 

R2

=0.038 R2

=0.781

R2

=0.004

R2

=0.973

R2

=0.354 

R2

=0.617 

Figure 4. Cell Performance correlations of Performance change with changes in Voc (top row), Jsc (middle row), and FF 

(bottom row) measured during stress testing at T=60 °C (left column) and 120 °C (right column). 

4 
It is quite obvious that changes in cell performance, deltaEff, can largely be attributed to changes in FF at both 

T=60 and 120 °C. What is not obvious from a casual observation of Figure 3 however, is the correlation between 

deltaEff and deltaJsc observed at T=60 °C. This latter mode appears to be temperature dependent, and is no longer 

statistically significant at T=120 °C. This same temperature dependence is also observed for Voc. Repeating this 

procedure at all stress temperatures, values of R2

 for deltaEff vs. deltaVoc, deltaJsc, and deltaFF were calculated and 

plotted as a function of temperature in Figure 5. 

1.0

0.8

0.6

0.4

0.2

0.0

R2 (0 to 1)

60 80 100 120

Stress Temperature (C)

 deltaEff vs deltaX

 X = Voc

 X = Jsc

 X = FF

Figure 5. Correlation of cell performance changes with changes in Voc, Jsc, and FF. 

As seen in Figure 5, there are systematic variations in how well changes in cell performance (deltaEff) correlate 

with changes in Voc, Jsc, and FF. At the lowest temperature of stress, 60 °C, changes in η% can be associated with 

changes in both FF, and Jsc. As temperature increases, the correlation of deltaEff with deltaFF increases to almost a 

perfect correlation of 1.0. At the same time, the correlation of deltaEff with deltaJsc gradually decreases and is replaced 

by a very strong correlation with deltaVoc. It must be noted however, that at all temperatures, changes in FF remain the 

most dominant predictor of overall cell efficiency during stress, and thus, is an important parameter to monitor. 

During early times of stress, some interesting, non-monotonic changes in cell parameters were observed. A 

magnified view of FF as a function of temperature and time shows this in Figure 6. The time axis has been adjusted to 

emphasize these subtle changes. 

Figure 6. Non-monotonic changes in cell FF observed during initial stress. 

70

68

66

64

62

60

0 5 10 15 20 25 30

t=18 hr

t=4 hr

t=1 hr

70 fig. inset 

65

60

55

50

FF (%)

100 200 300

t=18 hr

t=285 hr

t=4 hr T = 60 °C

 T = 80 °C

t=1 hr T = 100 °C

 T = 120 °C

0

Stress Time (hrs)

5 
 

As seen in Figure 6, FF was observed to undergo a slight improvement in value (after an initial decrease) at 

different times dispersed by what appears to be temperature. The maximum value in FF occur at t = 285, 18, 4, and 1 hrs 

for cells stressed at 60, 80, 100, and 120 °C, respectively. These short term improvements represent possible benefits of 

continued annealing of the cells immediately after fabrication. It is unclear whether these benefits are due to thermal 

effects alone, or a combination of light and heat. With longer periods of stress, as seen in Figure 3, FF once again begins 

to decrease monotonically. In order to gain some insight into why FF decreased at longer stress times, JV curves were 

reproduced using Pspice and the discrete circuit model shown in Figure 1. A compilation of model parameters extracted 

for devices stressed at 60 and 120 °C after 1 and 723 hrs of stress is given in Table 1. 

unstressed (t=1 hr) stressed (t=723 hr)

Parameter T (C) Dark Light Dark Light

JQNR 60 1.5E-16 4.0E-17 2.7E-16 3.0E-18

JSCR 60 7.0E-10 2.0E-09 6.0E-10 2.3E-09

Rs 60 3.0 1.6 0.8 0.5

Rsh 60 1.0E+05 400 1.0E+08 600

Jb 60 none none 0.027 0.038

Rb 60 none none 15.0 14.7

JQNR 120 5.0E-16 1.0E-16 8.2E-16 1.0E-17

JSCR 120 4.4E-10 2.0E-09 5.8E-10 4.0E-09

Rs 120 2.2 1.4 3.0 1.0

Rsh 120 1.0E+08 500 1.0E+08 325

Jb 120 0.900 0.990 0.013 0.015

Rb 120 9.0 9.0 45.0 29.0

Table 1. Two-diode fit parameters for unstressed (t=1 hr) and stressed (t=723 hrs) cells (J and R terms shown in units of 

A/cm2

 and ohms⋅cm2 respectively) 

 Back contact behavior is modeled in Pspice using a diode leakage current term Jb that is inversely proportional 

to barrier height at the CdTe/backcontact interface. The values JQNR and JSCR represent the forward leakage currents 

associated with recombination in the quasi-neutral and space-charge (depletion) regions respectively. Though JSCR has an 

exp(qV/2kT) voltage dependence, the large magnitude of this forward current dominates recombination, and thus FF, in 

the cell power quadrant. It is only at voltages near and above Voc that JQNR makes a contribution to forward current. As 

seen in Table 1, space-charge recombination (JSCR) measured for the cells under illumination increased slightly with 

increasing stress temperature (2.0⋅10-09 increased slightly to 2.3⋅10-09 A/cm2

 at T=60 °C and 2.0⋅10-09 increased to 

4.0⋅10-09 A/cm2

 at T=120 °C). At the same time, quasi-neutral region recombination (JQNR) for the same cells actually 

decreased with increasing stress temperature (4.0⋅10-17 down to 3.0⋅10-18 A/cm2

 at T=60 °C and 1.0⋅10-16 down to 

1.0⋅10-17 A/cm2

 at T=120 °C). The improvement in JQNR, and smaller increase in JSCR helps to explain why Voc

degradation stabilizes at stress temperatures of 60 to 80 °C in Figure 3 relative to higher stress temperatures. 

Cell performance change as a function of temperature and time was used to determine activation energies of 

degradation. Typical determinations of this type involve plotting ln(time to failure) as a function of inverse absolute 

temperature [5]. The resulting Arrhenius plot can then be used to determine activation energies as a function of 

temperature. A definition of failure equal to a decrease of 10% total efficiency (relative to the initial measured 

performance) was used to determine time to failure. This was not an altogether arbitrary value. A 1 eV degradation 

mechanism tested at an acceleration temperature of 100 °C for ~700 hrs corresponds to 10 years of service a product 

use-temperature of 50 °C. Thus, a 10% degradation in this test is somewhat based upon an approximate degradation rate 

of ~1%/year. 

 Figure 7 shows the variation of average deltaEff versus stress time as a function of temperature and the 

resulting Arrhenius plot obtained from this data assuming a 10% drop as the definition of failure. Since the %change in 

efficiency for devices tested at 60 °C did not "fail" over the course of nearly 2000 hrs of stress time, a power law fit of 

the data was required to extrapolate to this level. Curved line fits of the data shown in Figure 7 are based upon the 

equation: 

%change in Efficiency (deltaEff) = a ⋅ (time)b

 (1) 

6 
where a and b were adjusted with an intentional bias towards fitting data where performance was observed to decrease 

monotonically (i.e., non-increasing) in order to remove transient effects seen at shorter stress times (e.g., Figure 6). The 

regression coefficients a and b were determined by the method of least squares for power-law equations [15]. 

 

Figure 7. Percent change in cell performance (deltaEff) plotted as a function of stress time (left). Resulting Arrhenius 

plot (right) showing activation energies calculated from deltaEff plot using the power-law relationship. 

 

 The activation energy of 0.63 eV determined from the data at 100 and 120 °C is a close match to the literature 

value of Cu diffusion (0.67 eV) in CdTe [16]. Cu diffusion is thus implicated as being the mechanism responsible for 

degradation observed at higher stress temperatures. Cu diffusion also appears to be the mechanism for the short term 

improvements in FF shown in Figure 6. The FF maxima at 1, 4, 18, and 285 hrs of stress at T=120, 100, 80, and 60 °C 

when combined with the Cu in CdTe diffusion data from ref.[16] (Ea = 0.67 eV, Do = 3.7⋅10-4 cm2

/s) result in similar 

characteristic diffusion distances of between 1.2 and 3.3 microns as shown in Figure 8. 

0.25

0.5

1

2

4

8

16

Distance (um) = 2(Dt)1/2

1 4 16 64 256

Time (hrs)

 T=60 C

 T=80 C

 T=100 C

 T=120 C

t=285 hr

L = 3.3 um

t=18 hr

L =1.6 um

t=4 hr

L = 1.4 um

t=1 hr

L = 1.2 um

Figure 8. Characteristic diffusion distance corresponding to each FF maxima observed in Figure 6. 

 The Cu-doped graphite paste back contact in these relatively thick (9-10 micron) cells acts as a semi-infinite 

source of Cu. Cu diffusion distances of 1-3 microns from the back contact into CdTe could correspond to a motion of 

Cu from the quasi-neutral region into the space-charge (depletion) region. As Cu migrates from the back contact 

through the quasi-neutral region, FF increases. Further diffusion into the depletion region results in a decrease in FF. 

 The higher activation energy of 2.94 eV observed at lower stress temperatures was initially perplexing. It was 

not quite clear why a mechanism with higher energy requirements would dominate over one with lower activation (e.g., 

Cu diffusion at 0.63 eV). A review of the literature however revealed an interesting possibility. McCandless, et al. 

reported the grain boundary and bulk diffusion activation energies for S (from CdS) into CdTe as 2.0 and 2.8 eV 

7 
respectively [17]. The close agreement between these values and the activation degradation energies reported in this 

study was corroborated further by high-resolution scanning electron microscopy (SEM) images shown in Figure 9 of the 

CdS/CdTe interface of cells before and after stress. 

500 nm (a) 

CdTe 

CdS 

SnO2 60 °C, 2700 hrs

(b) 500 nm 

120 °C, 870 hrs

500 nm (c) 

as-grown, no stress 

Figure 9. SEM images of the CdS/CdTe interface in stressed (T=60 (a) and 120 °C (b)) and unstressed (c) cells 

8 
SEM images shown in Figure 9 are intentional, conservative representations of these interfaces. For example, 

the relatively void-free interface shown for the as-grown, no stress cell was observed along the entire cross-section of the 

cell. No delamination of the CdTe from the CdS layer was observed. In contrast, the CdTe film for the 120 °C case had 

delaminated entirely from the CdS/TCO superstrate in numerous regions not shown in Figure 9. Diffusion of S during 

stress testing is not unlikely. The open-circuit bias of these cells under stress establishes a field across the cell which 

aids the diffusion of Cu+

 from the backcontact and in effect accelerates the degradation of the cell. This same field can 

be expected to assist the oppositely sensed diffusion of S ions from the CdS into the CdTe film. It is a well accepted fact 

that interdiffusion at the CdS/CdTe interface is not balanced during high temperature processing. At high growth 

temperatures, S-outdiffusion exceeds the in-diffusion of Te [18]. The Kirkendall effect predicts that such interdiffusion 

involving asymmetric atomic fluxes will result in a bulk movement of the interface into the CdS, leading to a general 

"consumption" of that layer, i.e., a 'thinning" of the CdS. This is indeed what is typically observed with CdS/CdTe 

interfaces joined at high temperature [19]. The Kirkendall effect also suggests that if such bulk movement cannot occur 

(perhaps at lower temperatures), voids will be introduced in the CdS layer as a consequence of nonequal diffusion rates. 

4. CONCLUSIONS 

The temperature-dependent degradation of 9-10 micron thick CdS/CdTe solar cell devices made using CBDdeposited CdS, and CSS-grown CdTe under open-circuit bias and 1 sun illumination was measured using 6 replicate 

device structures at four different temperatures in the range 60 to 120 °C. Changes in performance under stress at lower 

stress temperatures were linearly correlated with changes in both FF and Jsc. With increasing stress temperature, the 

correlation between performance change and FF increases (approaches 1.0) while the correlation with Jsc is replaced by 

an even stronger correlation with Voc change. At high stress temperatures of 100-120 °C, degradation is characterized by 

an activation energy of 0.63 eV which is attributed to Cu diffusion. The diffusion of Cu results in both increased spacecharge and decreased quasi-neutral region recombination. At lower stress temperatures of 60-80 °C, degradation is 

characterized by a much higher activation energy of 2.94 eV. Outdiffusion of S from the CdS into the CdTe is proposed 

as the responsible mechanism here. This belief is supported by the observation of Kirkendall-related voids in the CdS 

layer of stressed cells relative to unstressed cells. 

5. ACKNOWLEDGEMENTS 

This work was supported by the U.S. Department of Energy under Contract No. DE-AC36-99G010337 with the 

National Renewable Energy Laboratory (NREL). The author also wishes to thank Bobby To for providing the SEM 

measurements and Wyatt Metzger for discussions concerning device physics.

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[19] Albin, D., Yan, Y., and Al-Jassim, M., "The Effect of Oxygen on Interface Microstructure Evolution in CdS/CdTe 

Solar Cells," Prog. Photovolt: Res. Appl. 10, 309-322 (2002). 
F1147-E(10/2008) 

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November 2008 

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4. TITLE AND SUBTITLE

Accelerated Stress Testing and Diagnostic Analysis of Degradation 

in CdTe Solar Cells 

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DE-AC36-08-GO28308 

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6. AUTHOR(S)

D.S. Albin 

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NREL/CP-520-42811 

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PVB76701 

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The primary goal of this study was to ascertain the presence and types of mechanisms affecting CdS/CdTe device 

stability in the temperature range of 60 to 120 °C. It should be noted that the results presented were specific to cells 

made using the specific growth conditions described. 

15. SUBJECT TERMS

PV; CdTe solar cell, degradation mechanism, recombination, activation energy, Kirkendall effect 

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