NREL/CP-5J00-61240. Posted with permission. Presented

at the 40th IEEE Photovoltaic Specialists Conference.

Performance stabilization of CdTe PV modules using bias and light
 

Timothy J Silverman, Michael G. Deceglie, Bill Marion, Sarah R. Kurtz
 

National Renewable Energy Laboratory, Golden, Colorado, 80401, United States
 

Abstract—Reversible performance changes due to light exposure frustrate repeatable performance measurements on CdTe PV 

modules. It is common to use extended light-exposure to ensure 

that measurements are representative of outdoor performance. 

We quantify the extent to which such a light-exposed state 

depends on module temperature and consider bias in the dark to 

aid in stabilization. We evaluate the use of dark forward bias to 

bring about a performance state equivalent to that obtained with 

light exposure, and to maintain a light-exposed state prior to STC 

performance measurement. Our results indicate that the most 

promising method for measuring a light-exposed state is to use 

light exposure at controlled temperature followed by prompt STC 

measurement with a repeatable time interval between exposure 

and the STC measurement. 

Index Terms—CdTe, thin-film, module, performance, stabilization, light soak, metastability, transient 

I. BACKGROUND AND INTRODUCTION 

Measuring the performance of a PV module under standard 

test conditions (STC) is useful for predicting outdoor performance of new modules and for quantifying performance 

changes due to accelerated testing or outdoor exposure. In 

the dominant thin-film PV technologies, namely CdTe, CIGS 

and a-Si, obtaining an STC performance measurement is 

frustrated by reversible, metastable changes in performance 

on time scales ranging from seconds to several days and 

in magnitudes on the order of 15% [1]. Furthermore, these 

metastable changes can be excited or relaxed by accelerated 

testing, and if not carefully controlled can be confused with 

permanent degradations associated with the accelerated tests. 

Due to the existence of these metastable changes, thinfilm modules often have a range of electrical performance 

states dictated by the time-, temperature-, voltage- and lightdependent chemical and material properties of the solar cells. 

In order to frame our discussion it is useful to define several 

terms associated with these states. We use the term “lightexposed state” to refer to an electrical performance state that is 

representative of that achieved under typical outdoor operation. 

As we will show, this light-exposed state is not necessarily 

unique and can depend on the operating temperature of the 

module. Also note that the light-exposed state could, in 

principle, be brought about with an appropriate procedure in 

the absence of illumination by, for example, applying electrical 

bias to the module. We adopt the term “repeatable state” to 

refer to an electrical performance state that can be repeatably 

achieved through means of some procedure, regardless of how 

representative the state is of operation outdoors. We define 

“stabilization procedure” as any procedure that is intended to 

bring about either a light-exposed state or a repeatable state. 

Stabilization procedures often involve prolonged exposure 

to natural or artificial sunlight, sometimes called light soaking. 

Indeed, the only currently standardized method of stabilization 

is the method in IEC 61646, the type qualification test for thinfilm PV modules [2]. This method requires repeated exposure 

to sunlight in increments of 43 kWh/m2 until successive power 

measurements differ by < 2%. In this work we performed 

a variety of experiments on commercial CdTe modules to 

understand the efficacy of different stabilization procedures, 

paying particular attention to whether they are successful in 

enabling STC measurement of the light-exposed state. The 

experiments are described in detail in Section II, while our 

results and their implications are discussed in Section III. We 

discuss the limitations of the various stabilization procedures, 

including light exposure, and conclude with recommendations 

for making repeatable STC performance measurements representative of outdoor performance. 

In one experiment, described in Sections II-A and III-A, we 

consider how the temperature during light exposure affects the 

stabilized state, highlighting that the light-stabilized state of 

a module depends on exposure conditions. We also consider 

the use of forward bias in the dark to both bring about the 

light-stabilized state and to stabilize performance for the time 

interval between light-exposure and measurement. 

The use of forward bias in the dark has been shown in CIGS 

modules to maintain the light-exposed state [3]. This addresses 

the challenge that the delay between light exposure and indoor 

performance measurement can result in the partial or complete 

relaxation of a module to its unexposed electrical performance 

state. In sections II-B and III-B, we describe experiments 

designed to evaluate this approach for CdTe modules and 

present their results. 

Forward bias in the dark has also been considered as a 

means of bringing about the light-stabilized performance state; 

the use of forward bias at elevated temperature (BET) without 

illumination has been proposed in the CdTe-specific section of 

a draft of the third edition of IEC 61215. In this study we tested 

a version of the proposed BET method, comparing the results 

to those obtained using ordinary light exposure. Interestingly, 

we found that modules which responded differently from each 

other to light exposure responded in the same way to BET, 

suggesting that while it may produce repeatable results, BET 

does not reliably bring about an outdoor-relevant light-exposed 

state. This highlights the challenge that metastable module 

behavior can be dictated by a number of different physical 

mechanisms, and that the particular mix of these mechanisms 

that any one module exhibits may affect its response to 

different techniques. These experiments and their results are 

described in sections II-C and III-C 

II. METHOD 

We used full-size, commercial CdTe modules from two 

manufacturers, referred to as “A-type” specimens and “B­
type” specimens. A-type specimens had been removed from 

outdoor deployment and stored in darkness for > 90 d. B-type 

specimens were previously unexposed. 

For continuous light exposure tests, we used a lightexposure chamber with class BBA light at 1000 W/m2 irradiance. The mean of the measured back-of-module temperatures 

was controlled using forced convection. The instrument measured light I-V characteristics every (2 to 5) min and each 

module was biased at Pmp between measurements. In this 

work, Pmp measurements from this instrument were corrected 

c Inominal using “self-irradiance” by multiplying by sc Isc . No

temperature correction was applied to these measurements. 

For individual light I-V measurements, we used a long-pulse 

solar simulator with class AAA light. Using the measured 

back-of-module temperature, we temperature-corrected measurements from this instrument using the technique described 

in Section II-D. No self-irradiance correction was applied to 

these measurements. 

Elevated-temperature tests were performed in a temperaturecontrolled environmental chamber. Humidity control was not 

used, resulting in relative humidity of < 10% during tests at 

55◦C and < 3% during tests at 85◦C. 

A. Temperature-dependent stabilization 

In order to understand how module temperature affects the 

light-exposed state of the modules, we exposed the modules in 

the light-exposure chamber under a series of different back-ofmodule temperature setpoints. The modules were exposed to 

simulated sunlight at 1000 W/m2 until the Pmp was changing 

< 1%/(24 h). After this condition was met, the chamber 

was turned off for (5-10) min to enable reprogramming, 

then restarted. The first 2 h of the subsequent step were 

carried out at the same temperature as the preceding step, 

then the chamber setpoint changed to the new temperature 

for the remainder of the step. This was carried out for the 

following sequence of temperature setpoints: 55◦C, 62.5◦C, 

70◦C, 62.5◦C, 55◦C. Near the end of the first step of this 

sequence, we carried out the temperature coefficient procedure 

described in Section II-D. 

B. Post-light-exposure stabilization 

We performed a series of experiments to study whether the 

application of forward bias to modules between light exposure 

and indoor light I-V measurements aids in stabilizing the 

module in its light-exposed state for the indoor measurement. 

Modules were exposed in the light-exposure chamber with a 

back-of-module setpoint temperature of 55◦C until their Pmp 

was changing < 1%/(24 h). When the module performance 

was stable, the temperature coefficient measurement described 

in Section II-D was performed. In the case of the A-type modules, part of the stabilizing light-exposure was the experiment 

described in Section II-A, but the last portion was performed 

at 55◦C. 

On a clear day, and once the modules’ performance was 

stable in the light-exposure chamber, they were relocated to an 

outdoor test rack located just outside the long-pulse simulator 

laboratory. On this outdoor rack, the modules were exposed 

to natural sunlight and maximum power-point tracked with 

I-V curves collected at 15-min intervals. The transition time 

between the light-exposure chamber and the outdoor lightexposure was less than 15 min and the modules were exposed 

on the outdoor rack for at least 1.25 h. Immediately following 

this outdoor exposure, we collected a series of pulses on the 

long-pulse simulator. A series of five measurements were made 

at one-minute intervals followed by measurements approximately once every 20 min for a total of 2 h. We then returned 

the modules to the light-exposure chamber at 1000 W/m2 and 

55◦C for (12–48) h before repeating the outdoor-exposure 

and long-pulse simulator measurement procedure. On this 

repetition, the modules were forward biased with a DC power 

supply between light I-V measurements in constant current 

mode. The current setpoint was that which the modules passed 

when biased to 90% of their expected STC Voc in the dark 

immediately upon being brought indoors. 

C. Bias-only stabilization 

We performed two different experiments to investigate the 

efficacy of applying bias to modules in the absence of light 

in order to bring about a light-exposed state. In the “biaslight” experiment the modules were first subjected to forward 

bias at elevated temperature (BET), followed immediately 

by light exposure. In the “light-anneal-bias” experiment, the 

modules were first exposed to light, then annealed at elevated 

temperature in darkness to allow modules to relax back to their 

dark state, and finally subjected to BET. 

The BET procedure used in this work was adapted from 

that proposed in the draft version of edition 3 of IEC 61215. 

Briefly, the method proposed in the draft standard is: 

1) Collect a light I-V curve 

2) Place the test article in a dark test chamber and bring 

the air temperature to 85◦C at ≥ 180◦C/h 

3) Apply a fixed-voltage bias of (0.9 to 1.0)×Voc to the test 

article for 1.5 h 

4) Discontinue bias and return the chamber to room temperature at ≤ −180◦C/h 

5) Collect a light I-V curve 

6) Repeat steps 2 through 5 until three successive measurements result in Pmax−Pmin < 0.02 Pmean 

In this work, we altered the method in two ways: to reduce 

stress on the modules, we changed the bias in step 3 to Vmp, 

which was 60%–75% of Voc. And to complete the experiment Pmax−Pmin in a timely manner we used < 0.04 in step 6. In Pmean

our facility a maximum of three bias steps can be performed 

per workday. We stored the test articles at room temperature 

between shifts of bias testing. The I-V curves in this segment 

of the test were measured on a long-pulse solar simulator at 

25◦C. 

Prior to the bias-light experiment the modules were exposed 

to continuous artificial sunlight in the light-exposure chamber 

at 1000 W/m2 with back-of-module temperature of ∼55◦C 

for > 100 h. The specimens were then annealed in darkness 

at 85◦C for 100 h. To begin the bias-light experiment, the 

modules underwent the BET procedure until the stopping 

criterion was met, at which point we placed it the lightexposure chamber at 1000 W/m2 illumination with back-ofmodule temperature at ∼55◦C. 

For the light-anneal-bias experiment, the modules were 

first exposed in the light-exposure chamber at 1000 W/m2 
with back-of-module temperature of ∼55◦C until the Pmpwas 

changing < 1%/(24 h). Once the modules were stable the 

temperature coefficient measurement described in Section II-D 

was carried out. For the purposes of the post-light-exposure 

stabilization experiment, the modules then underwent the steps 

described in Section II-B. The light-anneal-bias experiment 

then proceeded with a 168-h anneal in the dark at 55◦C. We 

then applied the BET procedure and collected the associated 

I-V curves. 

D. Temperature correction 

We measured module-specific temperature coefficients in 

the light-exposure chamber after ≥ 100 h of light exposure. 

The temperature was perturbed by ±2◦C for ≥1 h per step 

while I-V measurements took place. We used the resulting data 

set to calculate absolute temperature coefficients (for instance 

the Pmp temperature coefficient in W/K) by minimizing the 

standard deviation of the distribution of temperature-corrected 

values for the entire period. These temperature coefficients 

are used to apply additive temperature corrections to measurements made on the long-pulse simulator. 

III. RESULTS AND DISCUSSION 

A. Temperature-dependent stabilization 

The stepped-temperature light-exposure experiment was 

performed to quantify how the light-exposed state of the modules depends on operating temperature. The results are shown 

in Figure 1. In this figure, Pmp is not temperature-corrected 

and its relationship with temperature is superficially ordinary: 

Pmp is lower during the higher-temperature steps. However, at 

the beginning of each step is a transient change in Pmp that 

is many times slower than the change in module temperature 

and is in the opposite direction from the change that would 

be associated with a lag in module temperature. That is, 

the time series appears to be affected by two changes, the 

usual temperature dependence of the power which is captured 

by the temperature coefficient, and also a restabilization as 

the module reaches a new electrical performance state. This 

restabilization reverses in the downward temperature steps, 

supporting the interpretation that the module is reaching a 

different electrical performance state depending on its temperature. This change is approximately (1–2)% for each 7.5◦C 

temperature step. We verified that the apparent restabilization 

was not an artifact of the method by repeating the procedure 

on a crystalline Si module. The Si module exhibited only the 

usual temperature dependence associated with the temperature 

coefficient. 

The temperature shown in Figure 1 is back-of-module 

temperature. The relationship between back-of-module temperature and the temperature of the PV junction depends on 

many factors, including prevailing outdoor conditions or the 

design of light-exposure equipment. These factors must be 

considered when comparing Pmp measurements obtained after 

stabilization outdoors or in different types of light-exposure 

chambers. 

The size of the re-stabilization is of less interest here than 

Fig. 1. Pmp and temperature from the stepped-temperature light exposure test are shown for module A3. Pmp is normalized to the median value for the 

entire test. The transient changes in Pmp last much longer than module temperature transients and are in the opposite direction from Pmp changes due only to 

temperature. Note that the plot starts at 125 h into the test. 
Fig. 2. The light exposure step of the post-light-exposure stabilization test 

caused Pmp changes in both directions and up to 16% in magnitude. These 

data are normalized to the final measurement and are labeled with the module 

identifier. 

its existence, because different modules are expected to exhibit 

different magnitudes of temperature-dependent restabilization. 

This result shows that a module’s light-exposed state is generally dependent on operating temperature during exposure. For 

repeatable stabilization using light exposure, we recommend 

that equipment be used where the module temperature can be 

kept the same for each light exposure. 

B. Post-light-exposure stabilization 

The post-light-exposure relaxation experiments were performed to evaluate whether the application of forward bias 

is an effective means to maintain performance in the lightexposed state between light exposure and STC measurement. 

The first step in these experiments was light exposure, the 

results from which are shown in Figure 2. Module type A 

shows an increase in Pmp of up to 16%. Module type B shows 

two different behaviors upon light exposure: B3 declines in 

performance and B4 improves after a short initial decrease. 

All of the observed Pmp changes were driven by changes in 

Voc and FF. The variety of observed behaviors highlights the 

difficulty of finding one-size-fits-all testing methods for thinfilm modules. 

The effect of room-temperature forward bias between light 

exposure and flash I-V measurements is illustrated in Figure 3. 

Without bias, Pmp changes by up to 3% in the first two hours, 

with some modules improving and some degrading. When bias 

is applied before measurement, all of the modules decline in 

performance approximately 2% to 3%. 

The application of post-light-exposure bias fails to hold 

the modules in their light-exposed state, in contrast to what 

has been observed on CIGS modules [3]. Applying bias 

complicates the test procedure by introducing additional equipment and makes reaching a desired temperature more difficult 

due to self-heating. We recommend that bias not be used 

in an attempt to keep modules in their light-exposed state 

between light exposure and flash testing. To achieve repeatable 

Fig. 3. Results from the unbiased relaxation and biased relaxation tests 

show that room-temperature forward bias fails to stabilize Pmp during the time 

between light exposure and flash testing. These measurements are normalized 

to the initial measurement. Lines are labeled with the module identifier. 

measurements representative of the light-exposed state, the 

delay between light exposure and flash testing should be short 

and kept constant for each specimen. For example, using a 

delay of approximately 1 hour is expected to yield repeatable 

results. A much shorter delay than this introduces the risk of 

taking a measurement while performance is changing rapidly. 

C. Bias-only stabilization 

We performed two tests of the BET stabilization procedure: 

In the “bias-light” test, BET was immediately followed by 

light exposure to determine whether light would cause further 

changes. In the “light-anneal-bias” test, we performed the light 

exposure first, to determine the character of the modules’ 

transients. A subsequent 55◦C anneal step relaxed the modules 

before application of the BET procedure. 

Results from the bias-light test are shown in Figure 4. 

Modules A1 and A2 showed a ∼ 20% increase in Pmp during 

BET. The change was driven mostly by a Voc improvement, 

with a contribution from FF increase. There was virtually no 

further change in performance upon light exposure. In contrast, 

module B2 showed virtually no performance change during 

BET but changed substantially upon light exposure, mostly 

due to FF decrease. This suggests that at the end of the bias 

procedure the module was in a state that was different from 

its light-exposed state under our light-exposure conditions. 

Results from the light-anneal-bias test are shown in Figure 5. The continuous light exposure data in the left pane of 

Figure 5 is the same as that shown in Figure 2. Modules B3 

and B4 have opposite behavior when exposed to light: B4 
Fig. 4. Evolution of Pmp during BET (left) and continuous light exposure (right) as part of the bias-light test. Module B2 undergoes further performance 

changes upon light exposure, suggesting that light exposure excites phenomena that are unaffected by the BET process. Pmp is normalized to the final 

measurement in the left pane and to the initial measurement in the right pane. Data are labeled with the module identifier. 

Fig. 5. Evolution of performance parameters during continuous light exposure (left) and, after annealing at 55◦C, the BET procedure (right) as part of the 

light-anneal-bias test. Although module B3 shows a Pmp decline with light exposure, its Pmp improves with application of the BET procedure. Parameters are 

normalized to the final measurement in the left pane and to a post-light-exposure (pre-anneal) flash test taken approximately 1 h after the light exposure in 

the right pane. The extent to which the modules recovered to their pre-light-exposure state during the anneal is reflected in the bias step 0 points in the right 

panes. Data are labeled with the module identifier. 
improves while B3 degrades. This difference in transient behavior is due to a substantial rise in the FF of module B4 upon 

light exposure, while B3 had a relatively stable FF during 

light exposure. A reversible, light-induced FF improvement 

as seen on module B4 has been observed before [4]. Both 

modules showed a decrease in Voc with light exposure. The 

continuous light exposure data show a fast, ∼ 1 h performance 

change followed by a more gradual change. The anneal step 

returns module performance to that seen during the fast initial 

change, as shown in bias step 0 in Figure 5. However, during 

bias stabilization, the modules both improve. That is, modules 

B3 and B4 move in opposite directions during light exposure 

but both move in the same direction during bias stabilization. 

We attribute this result to the modules’ different FF transient 

behavior. The results show that BET does not consistently 

reproduce the effects of light exposure, and the relationship 

between the BET induced state and the light-induced state 

depends on the behavior of a particular module. 

Both light-exposure and BET are available as stabilization techniques for CdTe modules in the current IEC 61215 

edition 3 draft. The observation that the two methods may 

produce different results needs to be considered. That some 

modules have similar response to light and bias and some 

do not suggests that a variety of mechanisms is present. In 

fact a variety of mechanisms have been proposed to explain 

metastable changes to CdTe cells and modules, including 

light-modulated energetic barriers and changing occupation of 

bulk traps; copper used to optimize the back contact barrier 

height is also widely understood to have an important role 

in determining performance and stability, though it is often 

associated with degradation rather than reversible metastability 

[1], [5]–[9]. 

Given the variety of mechanisms that is likely to be present, 

it is not surprising that BET and light exposure are observed 

to affect the modules in different ways. During BET, electron 

hole quasi-Fermi splitting in the CdTe absorber is similar 

to that achieved with photo-excitation during light exposure. 

However in the buffer and front contact, the quasi-Fermi 

levels are different in dark forward bias and in light exposure. 

Thus a particular product’s mix of metastability mechanisms, 

especially those involving interfacial barrier heights, is expected to determine its responses to the two methods of 

stabilization. While BET may produce a repeatable state (a 

possibility not investigated in this work), our results suggest 

that it does not consistently result in a light-exposed state. 

Where performance measurements need to be representative 

of outdoor performance, we do not recommend the BET 

technique for stabilization. 

IV. CONCLUSION 

We have shown that the performance of CdTe modules 

stabilized using light exposure depends on the temperature 

of the module during the light exposure. We further demonstrated that the application of room-temperature forward bias 

after light exposure fails to keep modules in their lightexposed state. Finally, we showed that the proposed elevatedtemperature dark-bias stabilization method (BET) is not equivalent to light exposure and can produce performance changes 

in the opposite direction of the changes caused by light 

exposure. 

It is important to adopt a stabilization procedure to control 

for metastable changes to thin-film module performance. In 

some cases, it is desirable to reproduce the same electrical performance state that the module would achieve when 

deployed outdoors (the light-exposed state). In other cases 

repeatability alone may be sufficient. In those cases where 

outdoor performance is sought, our results show that lightexposure is preferable to the BET procedure studied here. 

It is important to note that the state achieved with light 

exposure is not necessarily a unique state and can depend on 

module operating temperature. It is thus important that lightexposure be carried out at a repeatable module temperature. 

Furthermore, prompt and repeatable time intervals should 

be adopted between light-exposure and solar simulator I-V 

measurements to maximize repeatability. 

ACKNOWLEDGMENT 

We thank Keith Emery, Steve Rummel and Allan Anderberg 

for performing I-V measurements on the pulsed simulator, 

Kent Terwilliger and Greg Perrin for performing the anneal 

and dark bias testing and Gary Babbitt for chamber assistance. 

Some of the data in this report were obtained using equipment at the Energy Systems Integration Facility (a national 

user facility sponsored by the U.S. DOE Office of Energy 

Efficiency and Renewable Energy) located at the National 

Renewable Energy Laboratory. This work was supported by 

the U.S. Department of Energy under Contract No. DE-AC36­

08GO28308 with the National Renewable Energy Laboratory. 

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