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

Efficiency & Renewable Energy, operated by the Alliance for Sustainable Energy, LLC.

Contract No. DE-AC36-08GO28308

Metastable Electrical 

Characteristics of 

Polycrystalline Thin-Film 

Photovoltaic Modules upon

Exposure and Stabilization

Preprint

Chris A. Deline, David S. Albin, 

and Steve R. Rummel 

National Renewable Energy Laboratory

Joseph A. del Cueto 

National Renewable Energy Laboratory 

and Abound Solar

Presented at SPIE Optics + Photonics 2011

San Diego, California

August 21-25, 2011

Conference Paper

NREL/CP-5200-52441

September 2011
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1 

Metastable electrical characteristics of polycrystalline thin-film 

photovoltaic modules upon exposure and stabilization

Chris A. Deline*a, Joseph A. del Cueto†

a

National Renewable Energy Laboratory, 1617 Cole Blvd, Golden, CO 80401 

a,b, David S. Albina

, and Steve R. Rummela 

b

Abound Solar, 3755 Precision Drive, Suite 180, Loveland, CO 80538 

ABSTRACT

The significant features of a series of stabilization experiments conducted at the National Renewable Energy Laboratory 

(NREL) between May 2009 and the present are reported. These experiments evaluated a procedure to stabilize the 

measured performance of thin-film polycrystalline cadmium telluride (CdTe) and copper indium gallium diselenide 

(CIGS) thin-film photovoltaic (PV) modules. The current-voltage (I-V) characteristics of CdTe and CIGS thin-film PV 

devices and modules exhibit transitory changes in electrical performance after thermal exposure in the dark and/or bias 

and light exposures. We present the results of our case studies of module performance versus exposure: light-soaked at 

65°C; exposed in the dark under forward bias at 65°C; and, finally, longer-term outdoor exposure. We find that 

stabilization can be achieved to varying degrees using either light-soaking or dark bias methods and that the existing IEC 

61646 light-soaking interval may be appropriate for CdTe and CIGS modules with one caveat: it is likely that at least 

three exposure intervals are required for stabilization. 

Keywords: Thin film, Photovoltaic, Transients, Metastability, CdTe, CIGS, IEC 61646

1. INTRODUCTION

Changes in the I-V parameters of polycrystalline thin-film PV devices have long been studied, particularly the reversible 

effect of light exposure and voltage bias on device efficiency1

. Early on, it was recognized that the open-circuit voltage 

(Voc) of CIGS and CdTe devices was affected by the prior voltage- and light-exposure history of the device2,3. Because 

voltage exposure near Vmp in the dark resulted in the same reversible increase in Voc as light exposure at Vmp bias, it was 

suggested that these Voc changes are driven by the bias history of the thin-film sample rather than its light exposure 

history. In all cases, dark storage returns the device’s Voc back to its original value. The shift has been ascribed to a 

continuum of trap states in the absorber junction, which depopulate when the cell is forward biased2

. The time constant 

of this Voc transient was on the order of minutes to hours, depending on the length of exposure to the voltage bias. We 

therefore categorize this Voc effect as a short-term transient. An additional contributor to short-term reversible device 

performance change has been identified as persistent photoconductivity, yielding voltage and series resistance changes in 

response to light exposure4,5. The challenge of short-term transients is in measuring the performance of thin-film PV 

devices repeatably under standard test conditions (STC). A typical approach is to light-soak a particular module that has 

been stored in the dark prior to measurement. However, to ensure the module is at the requisite 25°C for the performance 

measurement, the module is typically allowed to cool in the dark for up to 1 hour, which may undo some of the 

performance changes produced by light-soaking.

Longer-term changes in the electrical characteristics of CIGS and CdTe modules have also been observed under 

illumination6-10, occurring over time scales of 1000 hours of exposure at one-sun. Some of these longer-term exposure 

changes can be reversed when the modules are put into dark storage—defined as low-light-level conditions at room 

temperature—in periods of days to weeks. These reversible performance changes that arise after longer-term exposure 

(more than tens of hours) are termed metastable changes here. In CIGS modules, metastable changes have been 

attributed to persistent photoconductivity in the bulk CIGS11, and charging-discharging of defect states at the CdS/CIGS 

 

* chris.deline@nrel.gov; phone 303-384-6359

† Joseph del Cueto is now with Abound Solar: jdelcueto@abound.com; phone 970-619-5378
2 

interface12,13. In CdTe modules, an additional factor implicated in metastable performance changes is motion of ion

species including copper. Copper inclusion in the back-side contact of CdTe modules can improve the ohmic connection 

with the high-work-function p-CdTe layer14. However, stability can be compromised in the process7,15, possibly due to 

Cu diffusion from the back contact16. 

These longer-term metastable changes pose a challenge when trying to accurately gauge PV module performance 

following stress testing in module certification tests. The current certification standard for thin-film PV modules—IEC 

6164617 — requires successive light-soaking increments of 43 kWh/m2 integrated irradiance until the relative changes in 

measured power are 2% or less. This procedure was designed to account for Staebler-Wronski changes in amorphous 

silicon modules and is quite likely not optimal for stabilizing polycrystalline CdTe or CIGS PV devices following stress 

test (damp heat under bias) exposure. For instance, high-temperature exposure typically produces temporary efficiency 

improvements in a-Si modules that are not replicated in CIGS or CdTe modules.

Our goal is to devise a set of procedures suitable for stabilizing the measured performance in CIGS and CdTe PV 

modules so that measurements made either after dark storage, manufacture, or certification will be reproducible to within 

allowable tolerance. Because there seem to be short- and long-term effects on measured performance, it may be useful to 

consider separate procedures: one for preconditioning, or removing the transient effects, plus a second that addresses 

stabilizing the module to its long-term performance measured when deployed outdoors. The latter task is described here 

(and in prior reports18,19), specifically as two possible stabilization paths: light-soaking at one-sun or forward-biasing in 

the dark, carried out at elevated temperatures as suggested in the literature2,9,10. 

2. EXPERIMENTAL STUDY PLAN

Our study plan to understand stabilization is based in part on procedures already available for standard performance 

testing. The details of this plan are depicted in Figure 1 and consist of a series of indoor and outdoor exposure sequences, 

with performance measurements (I-V and C-V) at STC following each exposure. The experimental procedure can be 

split into four phases: initial characterization and exposure, indoor light stabilization, indoor dark stabilization, and longterm outdoor light exposure.

Figure 1. Study plan used to probe transitory electrical behavior in CdTe and CIGS modules. Only 

certain modules went through Phase 2, where light and dark indoor exposures were swapped.

Performance measurements were taken after each interval in the process, including C-V measurement 

and dark and light I-V curves taken at STC.

The initial characterization and exposure phase (Phase 0) begins with natural outdoor light exposure for all modules, 

conducted at open circuit (OC) without temperature control. This exposure is conducted in two intervals totaling 28–

37 kWh/m2

. The next step consists of a dark thermal (85°–90°C) anneal for 48 hours, nominal relative humidity (RH) 

(~30%), at OC (no bias), in an indoor thermal chamber. The importance and significance of the dark thermal anneal is 
3 

that it emulates a shorter version of thermal stress tests encountered by modules in IEC certification (e.g., damp heat) 

which is known to alter performance—partly in a reversible manner. Indeed, in this experiment the dark thermal anneal 

exposure resulted in a large reduction in module performance that was partly recovered during subsequent stabilization 

exposures.

The next two phases of the experiment (Phases 1 and 2) consist of indoor stabilization through either light exposure 

under load or forward-biased dark exposure at elevated temperature. These two options were selected to identify 

metastable changes that are driven exclusively by light exposure as opposed to those driven equally by light or electrical 

bias. Making this distinction is important because dark current soaking has a number of advantages over light-soaking, 

including a lower cost, ease of use, and the ability to stabilize modules right up to the time of taking an STC 

measurement, since the bias could in theory be applied at room temperature. Additional details of these exposures are as 

follows: during exposure, module temperatures are maintained at a nominal 65°±10°C, and module bias is maintained 

between Vmp and Voc using resistive loads (light-soaking) or power supplies (dark current soaking). Dark exposure 

durations range from 24 to 48 hours, and light exposures range from 24–48 kWh/m2 at ~ one-sun intensity.

Following three or four such dark or light exposures, the modules were stored in the dark for 2 to 3 months, after which 

time Phase 2 began (for certain modules only). Details of Phase 2 are identical to Phase 1 except that modules which had 

previously been light-soaking are instead exposed in dark forward-bias, and vice versa. After about 120 hours of 

additional exposure, the modules are again put into dark storage for 4 months. The final exposure phase (Phase 3) 

consists of outdoor deployment for 6 to 8 weeks, for a total additional light exposure of ~300 kW-h/m2

. During the 

outdoor deployment phase, the modules are maintained at Vmp by fixed load resistors and peak-power-tracking power 

electronics. The outdoor exposure data are used to correlate and validate indoor versus outdoor behavior.

2.1 Modules used in the study

The modules used to probe transitory electrical behavior are described in Table 1 below, with details on module type by 

manufacturer or model number, whether dark- and light-exposed modules were swapped during Phase 2, quantity of 

each type, module construction, and previous exposures, if any.

Table 1. List of CdTe and CIGS modules investigated.

Module 

Type

Phase 2 

Swap? Quantity Module Construction Pre-Existing Exposure Conditions

CIGS A Yes 3 Glass-substrate-glass laminate Unexposed, three controls from 

2003

CIGS B Yes 2 Glass-flexible-substrate-glass 

laminate, solder bond interconnect Unexposed, new

CIGS C No 6 Glass-substrate-glass laminate Unexposed, new

CdTe A Yes 2 Glass-superstrate-glass laminate Yes, outdoors 3 years

CdTe B Yes 2 Glass-superstrate-glass laminate Unexposed

CdTe C Yes 1 Glass-superstrate-glass laminate Yes, indoor light-soak, 1130 

kWh in 2002

CdTe D No 6 Glass-superstrate-glass laminate Unexposed, new

CdTe E No 6 Glass-superstrate-glass laminate Unexposed, new

Not all of the modules finished all three phases in this study. Only half of the module types (CIGS A, B and CdTe A, B, 

and C) were intended to be swapped during Phase 2. For the remaining modules (CIGS C, CdTe D and E), sufficient 

quantities were available to dedicate half of the modules to indoor light exposure and the other half to indoor dark bias 

exposure. Additionally, some other modules failed to make it through the entire test: the two CdTe B modules failed 

during Phase 2 (open circuit), and one CdTe D and one CdTe E module were damaged by handling during Phase 1.
4 

The pre-existing condition of the modules varied slightly, with some of the module types being brand new, some being 

older but unexposed, and some having been previously deployed outdoors. In particular, the CdTe A and CdTe C 

modules had prior light exposure history, which may have introduced variability in the results of this experiment, as will 

be discussed below.

3. RESULTS - CIGS MODULES

Changes in STC efficiency (∆ηSTC) for the first two types of CIGS modules (A and B) are shown below in Figure 2. 

Figure 3 describes the ηSTC changes for the third type of CIGS module (CIGS C). The ηSTC efficiency values are plotted 

relative to their initial as-received “dark storage state,” and show the ∆ηSTC as a function of the cumulative exposure in 

kWh/m2

. Since both light and dark exposures are plotted in the same chart, a 1:1 relationship is assumed between hours 

of dark exposure and kWh/m2 of light exposure for the purpose of these charts. Modules receiving indoor light-soaking 

in Phase 1 are plotted with a dashed line, while modules receiving dark forward-bias are plotted with a solid line. During 

Phase 2, the light-soaked and dark-soaked modules of CIGS A and CIGS B are switched, which is reflected in the chart.

0 100 200 300 400 500 600 700

-15

-10

-5

0

5

10

kWh/m2 exposure

ηSTC % change

CIGS A and B with exposure time

CIGS A1

CIGS A2

CIGS A3

CIGS B1

CIGS B2

90C Anneal step Ph1 / Ph2 Outdoor

exposure

Dashed lines

= light soaked

Figure 2. Efficiency changes for CIGS A and CIGS B modules, relative to initial baseline exposure. 

0 100 200 300 400 500 600

-10

-5

0

5

kWh/m2 exposure

ηSTC % change

CIGS C relative to initial baseline CIGS C1

CIGS C2

CIGS C3

CIGS C4

CIGS C5

CIGS C6

85C Anneal step

<- Indoor / Outdoor ->

exposure exposure

Dashed lines

= light soaked

Figure 3. Efficiency changes for CIGS C modules, relative to initial baseline exposure. 
5 

The salient features of the exposures are described here: During the initial preconditioning light-soak from 0 to 36 

kWh/m2 cumulative exposure, all of the CIGS A modules’ performance improves by roughly 2%–6%, while that for 

CIGS B modules declines by 1%–2%. These changes are predominantly driven by changes in fill factor (FF), followed 

by changes in short-circuit current (Isc) in order of importance. CIGS C module ηSTC remains roughly the same (within 

1%–2%). The following dark thermal anneal executed at 90°C depresses ηSTC of all CIGS modules by 5%–12% relative 

to baseline, driven largely by FF losses and then Voc losses in importance, while the Isc values remain relatively 

unchanged. Again, the purpose of this thermal anneal is to begin our stabilization test with metastable changes that may 

be encountered in a longer-duration damp heat stress test through IEC 61646.

An immediate recovery of some or all of the efficiency loss of the dark thermal anneal is produced by the first 24-hour 

stabilization exposure (concluding at 100 kWh of total exposure) regardless of whether the exposure involved lightsoaking or dark current soaking. According to the technical criteria of the IEC 61646 standard, all CIGS modules shown 

in Figures 2 and 3 would be considered “stable” following the second exposure (concluding with 150 kWh total 

exposure) because two successive exposures had a ∆ηSTC less than 2%. Indeed, aside from module CIGS A2, which 

continued a slow decline throughout all exposure steps, the other CIGS modules remained within 3% of their first postanneal measurement throughout the entire remaining 500–600 kWh/m2 exposure. It can be stated generally that for these 

CIGS modules, indoor light-soaking and current-biasing in the dark are equally effective in delivering performance 

stability, and a majority of the post-anneal stabilization occurs within the first 24 hours of light or dark exposure.

Following indoor exposure, the modules were all exposed to ~300 kWh of outdoor light-soaking at Pmp to insure that the 

indoor stabilized conditions are representative of outdoor, field-stabilized conditions. The effect of outdoor exposure was 

different on different module types, with CIGS A modules all experiencing an efficiency improvement (+2.5%) and 

module types CIGS B and CIGS C experiencing a reduction in ηSTC following outdoor exposure (-3% for CIGS B, 

-1.5% for CIGS C).

3.1 I-V parameter details

Changes in ηSTC are driven by the three I-V parameters: Voc, Jsc, and FF. The trends described here were consistent 

across all CIGS modules, with typical behavior shown for CIGS C modules in Figure 4. Jsc was the smallest contributor 

to ∆ηSTC, with a slight increase in Jsc (3%) seen during the dark thermal anneal. After this increase, Jsc remains relatively 

constant throughout indoor exposure, regardless of whether the module is light- or dark-soaked. A final increase in Jsc of 

~1% was seen in most modules during the outdoor exposure increment. In general, Jsc was negatively correlated with 

ηSTC changes. The main drivers of ∆ηSTC were FF and Voc, which trended after each other. In the case of the CIGS C 

modules, Voc changes were of the largest magnitude, while ∆FF dominated efficiency changes in CIGS A and CIGS B 

modules. In particular, ηSTC improvements in the CIGS A modules during the initial light preconditioning (prior to dark 

thermal anneal, visible in Figure 2) were driven by FF increases. In all CIGS modules, FF and Voc decreased during dark 

anneal, slowly recovering during subsequent exposures, regardless of light or dark exposure path. During the final 

outdoor exposure increment, each module type behaved differently, but again, no difference was apparent between 

modules on the dark or light exposure path. In CIGS A modules, all I-V parameters improved by ~1% during outdoor 

light-soaking. In CIGS B modules, ∆ηSTC was dominated by a 3% drop in FF. In CIGS C modules, Voc and FF each drop 

by 1%–1.5%. Additional details on the I-V parameters of each of the module types is shown below in Table 2 at four 

points in the exposure experiment: following the dark thermal anneal, at the conclusion of indoor exposure in Phases 1 

and 2, and after the ~300 kWh/m2 outdoor exposure in Phase 3. 
6 

0 100 200 300 400 500 600

-10

-5

0

5

kWh/m2 exposure

STC value % change

CIGS C: IV parameters relative to initial baseline

Jsc

Fill Factor

Voc

85C Anneal step

<- Indoor / Outdoor ->

exposure exposure

Figure 4. Changes in I-V parameters for CIGS C modules, relative to the first post-anneal exposure. 

Table 2. I-V parameter STC values (relative to initial baseline) at four exposure conditions. 

Module/Value Anneal (%) Last Ph1 (%) Last Ph2 (%) Outdoor (%)

CIGS A Jsc * 2.9 2.0 0.8 1.9

CIGS A FF* -9.3 -2.3 -1.6 -0.8

CIGS A Voc * -2.0 -0.3 -0.3 0.2

CIGS B Jsc 0.5 0.7 -1.1 -0.5

CIGS B FF -3.7 -1.6 -0.9 -4.1

CIGS B Voc -2.7 -2.0 -2.3 -2.4

CIGS C Jsc 2.8 2.9 NA 3.9

CIGS C FF -3.8 -0.2 NA -1.3

CIGS C Voc -8.6 -1.7 NA -3.2

*Excluding CIGS A2, which had irregular stability following thermal anneal

Additional investigation was made into FF changes to determine the major driver for these changes: series resistance 

(Rs), shunt conductance (Gsh), or diode quality factor (a). In all of the CIGS modules, the best correlation to ∆FF could 

be traced back to diode quality factor a changes. Correlation coefficients of -0.85 or better were found for all modules 

between ∆FF and ∆a. The only other significant correlations were found in CIGS module A2, which experienced 

significant FF loss after the dark anneal step, primarily in a (-0.99 correlation) and Rs (-0.73 correlation). Additionally, 

CIGS C modules showed a correlation (-0.5±0.2) between ∆FF and ∆Gsh. 

4. RESULTS - CDTE MODULES

4.1 CdTe A – CdTe C modules

Changes in STC efficiency (∆ηSTC) for the first three types of CdTe modules (A through C) are shown in Figure 5. The 

ηSTC efficiency values are plotted relative to their “dark storage state” at the beginning of the experiment and show their 

∆ηSTC as a function of the cumulative exposure in hours of dark exposure or kWh/m2 of light exposure. As above, 

modules receiving indoor light-soaking in Phase 1 are plotted with a dashed line, while modules receiving dark forwardbias are plotted with a solid line. During Phase 2, the light-soaked and dark-soaked modules of CdTe A and CdTe C are 

switched, which is reflected in the chart.
7 

During the exposure process, module CdTe B1 developed an unexpected loss in FF following the dark anneal step. 

Subsequent exposures brought back some of its performance, but module CdTe B2 meanwhile began to experience 

reduced FF as well. The FF losses of both modules are correlated with increased Rs and subsequently attributed to 

contact problems with the bus-bars as evidenced by IR images. Both CdTe B modules eventually failed in an OC

condition during Phase 2—an irreversible change. The CdTe B modules will therefore not be considered further for this 

experiment. 

0 100 200 300 400 500 600 700

-40

-30

-20

-10

0

10

20

kWh/m2 exposure

ηSTC % change

CdTe modules A-C with exposure time

CdTe A1

CdTe A2

CdTe B1

CdTe B2

90C Anneal step CdTe C1

Ph1 / Ph2 Outdoor

exposure

Figure 5. Efficiency changes for CdTe A, CdTe B, and CdTe C modules, relative to initial baseline 

exposure. Both CdTe B modules failed midway through Phase 2. 

With the failure of the CdTe B modules, the sample pool was greatly reduced for the remaining CdTe modules in this 

exposure sequence. However, some general comments can be made about the behavior of these modules during 

exposure. Unlike the CIGS modules discussed above, the CdTe A and C modules responded to the dark thermal anneal 

exposure by an increase in ηSTC, to values in some cases 10% above the starting baseline condition. A simple explanation 

may be suggested for this apparent anomalous behavior. As noted in Table 1, the CdTe A modules had a prior exposure 

history and when new had ηSTC 5%–17% higher than their starting value in this experiment. As part of these modules’

prior exposure history, they were deployed in a hot, humid climate, leading to substantial decline in ηSTC, close to the 

baseline value measured here in this experiment. Table 1 also notes that the C1 module had been light-soaked (~1130 

kW-h/m2

) several years earlier and was stored in the dark since. Module CdTe C1’s prior ηSTC was about 5% higher than 

what was measured for its reference baseline here. The improvement in ηSTC for these CdTe modules following the 90°C 

dark anneal exposure can thus be thought of as returning some amount of pre-existing metastable performance to these 

modules as well as undoing some of the metastable ηSTC loss that was experienced from prior exposure. 

With the small sample size, it is difficult to determine whether performance changes are driven differently by lightsoaking versus current-soaking after the dark anneal step. It is certain that by the end of Phase 2, after accumulating 

~200 hours of either indoor light or current exposure, each of the modules (A1, A2, and C1) have stabilized within 2% 

∆ηSTC. It is also true that module A2 stabilized earlier in the exposure sequence, following several dark current soaks by 

the end of Phase 1, and modules A1 and C1 only stabilized in Phase 2 after they had gone through several dark current 

exposure increments. However, this does not necessarily indicate that dark current exposure will stabilize any given 

CdTe module any better or faster than indoor light-soaking.

Following the indoor light and dark exposure increments, the CdTe modules were deployed outdoors for ~300 kWh/m2

light exposure, as with the CIGS modules above. Also like the CIGS B and CIGS C modules, a subsequent reduction in 

ηSTC resulted from the outdoor exposure. Relative to the final indoor exposure in Phase 2, ηSTC for A1 and A2 drops by 

5%, while that for C1 drops by 2%. 

A comment on the I-V parameters driving ηSTC changes in CdTe A and C modules can also be made here. For all three 

modules, Voc remained the most constant, reaching a stabilized value early in the experiment after the initial 30 kWh/m2

light-exposure preconditioning. The other two parameters—FF and Jsc—varied widely through the first half of the 
8 

experiment and were responsible for most of the variations in ηSTC for CdTe A and C modules. The values did not 

always move in the same direction from exposure to exposure, either; a decrease in one exposure step could be followed 

by an increase in the following step. By the end of Phase 2 exposure, FF in all three modules had stabilized at a value 

2%–5% above the initial baseline value. Jsc had also stabilized back around the original baseline level by the end of 

Phase 2 for CdTe A1 and C1, and much earlier for CdTe A2. Following the final 300 kWh/m2 outdoor exposure, Jsc

increased by 1% in all modules, while FF declined by 3%–5%. Voc also declined slightly. A full summary of these I-V 

parameter changes is available in a prior report18. 

4.2 CdTe D and CdTe E modules

Figures 6 and 7 display the ηSTC changes for CdTe D and CdTe E modules. The main features of ηSTC changes are similar 

across these two module types, so they will be discussed together. Unlike the CdTe A and CdTe C modules, these 

modules have had no prior exposure history, so they are all starting from the same nascent condition. As a result, most of 

the stabilization changes are monotonic—there are less of the random up and down movements that were seen in the 

CdTe A and CdTe C data, presumably caused by differences in their initial starting condition. It should be noted that one 

of the modules in each of group CdTe D and CdTe E were lost due to breakage in mishandling. The damage had nothing 

to do with the exposure steps, so their data up to the point of failure are included in this analysis.

The ηSTC changes are plotted separately for the modules exposed to indoor light-soaking and those exposed to dark bias. 

Although there are slight variations between the two exposure tracks, it is difficult to say that the variations are caused 

by anything beyond module-to-module variation as opposed to the exposure type itself. For instance, the spread in 

module response within each exposure type is greater than the difference between the two different types of exposure. 

The one possible area where there may be a discernable difference between light-exposed and dark-exposed ηSTC is at 

the final outdoor exposure. For CdTe D, the indoor light-exposed modules did not experience additional power loss 

following outdoor deployment, while the indoor dark-current modules had ∆ηSTC = -7%. A full description of exposure 

results are provided below.

0 100 200 300 400 500 600

-15

-10

-5

0

5

10

kWh/m2 exposure

ηSTC % change

CdTe D relative to initial baseline

Current soak

Light soak

85C dark anneal Outdoor ->

exposure

CdTe D

0 100 200 300 400 500 600

-20

-15

-10

-5

0

5

kWh/m2 exposure

ηSTC % change

CdTe E relative to initial baseline

Current soak

Light soak

85C dark anneal Outdoor ->

exposure

CdTe E

Figure 6 (left). Efficiency changes for CdTe D modules, relative to initial baseline exposure. The 

current-soaked modules experienced an efficiency drop during final outdoor exposure, but the lightsoaked modules did not.

Figure 7 (right). Efficiency changes for CdTe E modules, relative to initial baseline exposure. Little 

apparent difference is visible between the light-soaked and current-soaked modules.

During the first two pre-conditioning steps (1.2 and 36 kWh/m2 outdoor exposure) there is an initial ~5% decrease in 

ηSTC after the first hour that is largely recovered during the remaining light-soak. A look at the I-V parameter changes in 

Figures 8 and 9 can provide some insight. The initial 5% drop coincides with loss in FF and Voc in all modules, followed 
9 

by a recovery of FF and an increase in Jsc through the second outdoor light-soak increment. The dark thermal anneal step 

causes degradation in ηSTC for all modules, to varying degrees. The FF is primarily affected, being reduced by an average 

of -6% for both CdTe D and CdTe E modules. Jsc is also similarly affected, with a similar -6% change on average for D 

and E modules during the dark anneal step. This situation is different from that of CdTe A and CdTe C modules, where 

Jsc remained unchanged during the dark anneal step. The changes seen in Voc during dark anneal differ slightly between 

CdTe D and CdTe E modules. For the CdTe D modules, there is a uniform increase in Voc of ~2% on average. This 

follows similar behavior of CdTe A and CdTe C modules, where Voc increases slightly during dark anneal. With the 

CdTe E modules, the response varies from module to module, with roughly half showing a 2% increase in Voc and the 

other half showing a 2% decrease in Voc. Interestingly, in subsequent exposure steps, those modules that had an increase 

in Voc gave up some of those increases, while modules that had lost Voc during dark anneal had a subsequent 

improvement. It appears that the dark anneal is driving metastable Voc changes in opposite directions for different 

modules, which are subsequently stabilized during indoor light or current exposure. More detailed plots of these modules 

are given in Appendix A.

The losses sustained during the dark anneal step are reversed during subsequent indoor exposures. After 24 hours of 

either light or current exposures, most CdTe D and CdTe E modules’ ηSTC were within 4% of baseline values. Although 

module efficiency trended lower for all modules throughout the remaining exposure steps, on an individual module 

basis, all of the modules had stabilized by the end of the indoor exposure sequence, according to the IEC 61646 criterion, 

to within 2%. However, while there were 48-hour increments in which ∆ηSTC was less than 2% for each of the modules, 

there were also many increments in which the module ∆ηSTC was equal or greater than 2% in 48 hours, sometimes after 

the module would have been deemed “stable” based on IEC 61646. The CdTe D and CdTe E modules never achieved 

the same level of stability as the CIGS C modules or other CdTe modules through this exposure procedure. Some 

additional short-term transients that were not controlled for in the experiment may be contributing to this apparent 

instability.

0 100 200 300 400 500 600

-10

-5

0

5

10

kWh/m2 exposure

STC value % change

Jsc

Fill Factor

Voc Light

Voc Dark

85C dark anneal Outdoor ->

exposure

CdTe D

0 100 200 300 400 500 600

-8

-6

-4

-2

0

2

4

kWh/m2 exposure

STC value % change

Jsc

Fill Factor

Voc

85C dark anneal Outdoor ->

exposure

CdTe E

Figure 8 (left). I-V parameter changes for CdTe D modules, relative to initial baseline exposure. There 

was an apparent difference between Voc in current-soaked modules and light-soaked modules, so these 

values are plotted separately.

Figure 9 (right). I-V parameter changes for CdTe E modules, relative to initial baseline exposure.

Following the indoor light and dark exposure increments, the CdTe D and CdTe E modules were deployed outdoors for 

~300 kWh/m2 light exposure. A slight difference was found between the light-soaked and dark-soaked CdTe modules. In 

particular, modules that had previously been exposed to indoor dark bias experienced a greater loss in ηSTC during

outdoor exposure. In the case of CdTe D, the dark-soaked modules lost around -7% ηSTC while the light-soaked modules 
10 

lost only -0.5% ηSTC. For CdTe E modules, the difference is less stark, with dark-soaked modules having ∆ηSTC of -5% 

and light-soaked modules having ∆ηSTC ~ -3.5%. Examination of Figure 8 indicates that the difference between lightexposed and current-exposed modules rests primarily in the difference in Voc, which decreased significantly in the darkexposed modules following final outdoor exposure, but remained the same in light-exposed modules.

5. DISCUSSION

5.1 IEC 61646 light-soaking recommendations

A primary purpose of this exposure experiment was to investigate the IEC 61646 qualification procedure as written and 

to determine if it contains an appropriate-duration light-soaking regimen following a CdTe or CIGS module’s exposure 

to indoor accelerated stress. For almost every module tested here, whatever ηSTC changes were produced by our “dark 

anneal” indoor exposure were largely reversed by the very first 24-hour increment of exposure. The reversal occurred 

whether either light-soaking or dark bias at 65°C was employed as the exposure condition. Aside from a slight difference 

in Voc for CdTe D modules following outdoor deployment, there was hardly any discernable difference between an 

indoor light-soaked module and a module exposed to 65°C in the dark with module bias at Imp in this experiment. And 

the largest ∆ηSTC recovery following the dark anneal step occurs in the first 24-hour exposure.

However, the metastable performance changes in this experiment were not completely realized in the first 24-hour 

exposure. Additional stabilization intervals resulted in additional incremental changes—within the 2% IEC 61646 limit 

for some modules, and greater than 2% for other modules. It is difficult to determine to what extent these changes are 

metastable effects relaxing to their “true” value through additional stabilization, as compared to long-term irreversible 

changes that may be occurring simultaneously. For obtaining stabilized performance at the 2% level or better, our data 

show that the CIGS modules in some cases required greater than 100 hours of exposure using either light-soaked or 

biased exposures at 65°C indoors. There did not appear to be any additional simultaneous instability or long-term 

degradation in these modules (aside from CIGS A2, which experienced Rs degradation).

For CdTe modules, our tests involved a total of 200 hours of exposure, and it is difficult to say that module stability 

improved to wtihin 2% during this exposure interval. In particular, CdTe D and E modules experienced Voc and FF loss 

throughout the exposure sequence, possibly as a result of the additional “stabilization” exposures. There is also the issue 

of inherent instability in some of the CdTe modules. As opposed to the CIGS modules, which showed relatively little 

change from stabilization step to stabilization step, the CdTe modules could vary widely from step to step—sometimes 

losing >5% ηSTC in one exposure step to regain it in the next. This inherent variation coupled with a decreasing trend in 

ηSTC suggests that such modules with inherent variation may never meet the IEC requirement of ∆ηSTC < 2%—and may 

also be exposed to unrelated stress through the stabilization process.

Two points are therefore identified for the light-soaking requirement as stated in IEC 61646, Section 10.19. First of all, it 

is pointed out that while some modules may have reached a “stable” condition (ηSTC within 2% between subsequent 

measurements) after two 43 kWh/m2 exposure increments, we have identified continued monotonic increases that may

result in a final stabilized value several percentage points higher (or lower) than the value at 86 kWh if the module were 

to be further light-soaked. Depending on the accuracy required for the measurement, a minimum light exposure of 

100–120 kWh/m2 may be required to completely drive out metastable changes in the module. 

Additionally, it is acknowledged that certain modules may display apparent instability from measurement to 

measurement in a non-monotonic fashion. This may be caused by short-term transient behavior that is not accounted for 

in the exposure or measurement process and may prevent the module from being declared stable when in fact most longterm reversible changes have already occurred. To account for this possible measurement issue, it is suggested that two 

adjacent 43kWh/m2 measurements be averaged if they can be considered non-monotonic and then compared with the 

third 43kWh/m2 measurement. To illustrate, consider P1, P2, and P3, each a module power measurement following a 43 

kWh/m2 exposure increment. If P2 < P1 but P3 > P2, then P2 and P3 would be averaged, and P1 would be compared 

with the average of (P2,P3) in the equation: 2*(Pmax –Pmin)/(Pmax+Pmin) < 2%. This should provide some allowance for 

transient effects in ηSTC measurements likely tied to extrinsic factors rather than effects intrinsic to the semiconductor.
11 

5.2 Difference between indoor and outdoor stabilized condition, and dark and light bias

Regarding the effective difference between indoor light-soaking at Vmp and dark exposure at 65°C with module bias 

between Vmp and Voc, this experiment has shown little difference between the two conditions in recovering initial dark 

anneal ηSTC loss. Particularly in regards to recovering losses from the dark anneal step, both 24 hours of light-soaking

and 24 hours of dark bias exposure appeared equally effective. It may therefore be appropriate to suggest dark exposure 

at Imp forward bias and module temperature between 40°–60°C (to mirror existing IEC 61646 rules) as an appropriate 

alternative to indoor light exposure for CdTe and CIGS modules. One possible mitigating factor with this suggestion is 

the fact that differences in Voc were evident in light- versus dark-exposed modules following outdoor exposure. 

Particularly for CdTe D and E modules, the indoor light-exposed modules stabilized more closely to outdoor final values 

of ηSTC. There may therefore be some value in indoor light exposure, especially if the module has a known sensitivity to 

light exposure in particular (versus a metastability equally driven by light or by forward bias).

5.3 Instability of polycrystalline thin-film measurements

Transient changes in CIGS modules appear more reversible and predictable than in CdTe modules. Our C-V data 

showed that for CIGS modules, carrier concentrations decline by factors of 5 to 20 after dark thermal anneal. but these 

are reversed in subsequent exposures lasting ~100 kWh. The Voc and FF losses in CIGS observed after dark thermal 

anneal or exposures also appear consistent with theoretical models that indicate a correlation between larger depletion 

width and a lowering of Voc and FF20.

More variation is apparent when some of the CdTe modules’ ηSTC is measured. It may be the case that CdTe modules 

have more intrinsic variability, and a larger sample set is required. Some of the variation may also be related to shortterm Voc and FF transients, which were found in some CdTe modules on time scales of ~1 hour following lightsoaking21. If this is the case, short-term transients may introduce random uncertainty in STC measurements as modules 

are allowed to relax in the dark to different extents following light-soaking or dark bias exposure. Some suggestions may 

be given to possibly address this issue. Applying forward bias to the module right up to the point of I-V measurement 

could possibly reduce measurement uncertainty for these CdTe modules; so could applying strict requirements on the 

amount of time lag allowed between the indoor light-soaking increment and the indoor STC performance measurement. 

A third option is low-level light-soaking while the module cools to STC temperature. Additional research on this point 

may provide verification of these recommendations.

5.4 Module pre-conditioning

Our test plan also included simple outdoor pre-conditioning—similar to that currently used at NREL and other test 

labs—but ours comprised two steps of short and longer durations. Our pre-conditioning tests and data suggest that 

transients are still occurring and/or not saturated until well after 1 kW-h/m2

. This is shown in both CIGS and CdTe 

modules by the surprisingly large difference (up to 8%) in ηSTC and I-V parameters between data points at 1 kWh/m2 and 

30 kWh/m2

. A more satisfactory amount of pre-conditioning would accrue up to 20–30 kW-h/m2 to remove these 

metastable effects, but this is likely not feasible in a majority of test laboratories. This further highlights the difficulty of 

accurate ηSTC measurements for thin-film modules.

6. CONCLUSION

We developed and executed a test plan to probe transitory electrical behavior and performance in CdTe and CIGS 

modules and applied it to several groups of modules, numbering a total of 28. Our initial goal as stated was to develop a 

set of procedures that will stabilize the measured performance of polycrystalline CdTe and CIGS PV modules, 

specifically targeting methods that improve upon the current thin-film stabilization standard—IEC 61646—as noted in 

the introduction.

Our data indicate that thermal exposures temporarily degraded performance by substantial amounts, but most of these 

losses were truly metastable phenomena and largely recoverable after ~24–72 hours in most cases, using either light-
12 

soaking or dark exposure with forward-bias currents near Imp, while at elevated temperatures (65°±10°C). This represents 

a very positive result with regard to reversing metastable losses.

As a result of our series of tests, it became clear that certain modules (notably CIGS and certain CdTe) stabilized 

quickly, within the first two exposure steps. Other CdTe modules required additional time to be considered “stabilized” 

as per the IEC 61646 guidelines. A persistent loss in FF and Voc coupled with possible short-term transient effects made 

measurement within 2% difficult for some CdTe modules. This difficulty could possibly be mitigated by further 

investigation and control of short-term Voc changes, along with a shorter IEC 61646 light-soaking interval for CIGS and 

CdTe modules. An alternative stabilization method involving indoor dark exposure at 65°C with module bias between 

Vmp and Voc was compared with the traditional indoor light-soaking. The indoor dark exposure was found to cause 

similar metastable changes in all I-V parameters for both CIGS and CdTe modules. Some difference was evident in Voc

changes in CdTe D modules between the light and dark exposure segment, but this was not the case for other modules.

7. ACKNOWLEDGEMENTS

We thank Alan Anderberg from the NREL Cell and Module Characterization Team for measurements performed at 

STC. This work was supported by the U.S. Department of Energy under Contract No. DE-AC36-08-GO28308 with the 

National Renewable Energy Laboratory.

APPENDIX: ADDITIONAL RAW DATA PLOTS

0 100 200 300 400 500 600

-15

-10

-5

0

5

kWh/m2 exposure

ηSTC % change

CdTe D - Efficiency

CdTe D1

CdTe D2

CdTe D3

CdTe D4

CdTe D5

CdTe D6

85C dark anneal

Outdoor ->

exposure

0 100 200 300 400 500 600

-15

-10

-5

0

5

kWh/m2 exposure

ηSTC % change

CdTe E - Efficiency

CdTe E1

CdTe E2

CdTe E3

CdTe E4

CdTe E5

CdTe E6

85C dark anneal

Outdoor ->

exposure
13 

0 100 200 300 400 500 600

-7

-6

-5

-4

-3

-2

-1

0

1

kWh/m2 exposure

Voc % change

CdTe E - Voc

CdTe E1

CdTe E2

CdTe E3

CdTe E4

CdTe E5

CdTe E6

85C dark anneal

Outdoor ->

exposure

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