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

Electrical Bias as an Alternate 

Method for Reproducible 

Measurement of Copper Indium 

Gallium Diselenide (CIGS) 

Photovoltaic Modules

Preprint

Chris Deline, Adam Stokes, Timothy J.

Silverman, Steve Rummel, Dirk Jordan, 

and Sarah Kurtz

Presented at SPIE Optics + Photonics 2012

San Diego, California

August 12–16, 2012

Conference Paper

NREL/CP-5200-56078

August 2012
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1

Electrical bias as an alternate method for reproducible 

measurement of copper indium gallium diselenide (CIGS) photovoltaic 

modules

Chris Deline*, Adam Stokes, Timothy J. Silverman, Steve Rummel, Dirk Jordan, Sarah Kurtz

National Renewable Energy Laboratory, 16253 Denver West Blvd, Golden, CO, USA 80401

ABSTRACT

Light-to-dark metastable changes in thin-film photovoltaic (PV) modules can introduce uncertainty when measuring 

module performance on indoor flash testing equipment. This study describes a method to stabilize module performance 

through forward-bias current injection rather than light exposure. Measurements of five pairs of thin-film copper indium 

gallium diselenide (CIGS) PV modules indicate that forward-bias exposure maintained the PV modules at a stable 

condition (within 1%) while the unbiased modules degraded in performance by up to 12%. It was also found that 

modules exposed to forward bias exhibited stable performance within about 3% of their long-term outdoor exposed 

performance. This carrier-injection method provides a way to reduce uncertainty arising from fast transients in thin-film 

module performance between the time a module is removed from light exposure and when it is measured indoors, 

effectively simulating continuous light exposure by injecting minority carriers that behave much as photocarriers do. 

This investigation also provides insight into the initial light-induced transients of thin-film modules upon outdoor 

deployment.

Keywords: Thin film, Photovoltaic, Transients, Metastability, CIGS, Forward Bias

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 peak-power voltage (Vmp) in the dark resulted in the same reversible increase in Voc and fill factor 

(FF) as light exposure at Vmp bias, it was suggested that these Voc and FF changes are driven by the bias history of the 

thin-film sample rather than its light exposure history. In these cases, dark storage returns the device’s performance to its 

original value.

Investigation into CIGS device metastability (reversible performance change) has indicated that performance changes 

can be ascribed in part to intrinsic defect centers functioning as hole traps4,5

. Biasing the junction shifts the Fermi level, 

allowing defect states to populate or depopulate. Long characteristic charging times may be due both to the high 

activation energy of the defect state, and to local atomic displacement and lattice relaxation upon acceptance of charge6

. 

First-principles modeling of intrinsic defect states by Lany and Zunger has identified one main complex that could 

explain the majority of CIGS device metastability: selenium / copper vacancies (VSe – VCu) working in tandem as an 

amphoteric divacancy complex7

. This complex behaves as a compensating donor (VSe – VCu)

+ for Fermi levels near the 

valence band, and as an acceptor (VSe – VCu)

- for higher Fermi levels8

. In the acceptor configuration, the defect 

contributes to the net hole density of the absorber4

, which directly increases the Voc and reduces the absorber depletion 

width, providing additional improvement in Voc and FF from reduced recombination6

. Removal of bias (either photoinduced or current injected) causes the complex to return to its ground state (VSe – VCu)

+ in a matter of minutes or hours. 

Capacitance measurements and other experimental results strongly support the theoretical role of (VSe – VCu) in CIGS 

device short-term reversible changes9

.

Theoretical calculations have identified a second defect complex that may also contribute to metastable performance of 

CIGS: indium-on-copper antisite donors (InCu). Although the defect state is primarily implicated in limiting the Voc of 

practical CIGS devices due to Fermi-level pinning10, the deep electron trap state may also contribute to reversible 

performance changes under specific reverse-bias and red-light exposure conditions8

.
2

Additional theories on the microscopic processes leading to metastable changes in CIGS performance include ionic drift 

models, which suggest that the motion of ionic species including indium and copper may be responsible for light-dark 

performance differences of CIGS devices11. It is possible that multiple mechanisms are responsible for device 

performance changes on different time scales, with longer-term performance change driven by ionic motion rather than 

reversible population and depopulation of electronic states. However, the evidence of low annual degradation rates on 

par with that of crystalline silicon modules12 suggests that long-term degradation of polycrystalline thin-film modules 

from ionic motion may not be an inherent reliability issue13. In fact, the CIGS absorber has been calculated to be quite 

robust to ionic diffusion / drift, with the electrochemical potential tending to concentrate mobile Cu in the ternary 

material away from the heterojunction6

.

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 6164614 — requires successive light-soaking increments of 43 kWh/m2 integrated irradiance until the incremental 

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) exposure15. For instance, high-temperature exposure typically 

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

The challenge of short-term transients is in measuring the performance of thin-film PV devices repeatably under 

standard test conditions (STC, 1 kW/m2 irradiance and 25oC module temperature). A typical approach is to light-expose

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 exposure.

Our goal is to devise a procedure to maintain the performance of thin-film modules at their outdoor state, so that 

measurements made either after dark storage, manufacture, or certification will be reproducible, within measurement 

uncertainty. This procedure is intended to minimize fast transient performance changes that may arise in the few minutes 

to hours elapsing between light exposure and indoor measurement of a typical CIGS module. Prior reports have dealt 

with the longer-term performance implications of one-sun light exposure or forward bias in the dark16,17, but this method 

deals mainly with performance changes visible in the first several hours of exposure2,9

. Five pairs of CIGS modules are 

measured after various sequences of exposures, and the transients are documented and compared with the outdoormeasured performance.

2. METHOD

An investigation of forward-bias exposure on CIGS module performance was conducted with three complementary 

experiments, some of which shared common exposure steps. The first two studies involved outdoor-deployed CIGS 

modules, subsequently brought indoors and monitored during dark storage. In Experiment I, five pairs of CIGS modules 

(commercially available) were allowed to relax from an outdoor-exposed, pseudo-stable condition to an indoor, roomtemperature dark state. One of each pair of modules was maintained at Imp forward bias by a power supply, while the 

other module was maintained unbiased in the dark. The forward-bias operating point of the module was determined by 

placing a benchtop power supply in constant-current mode, set to the nameplate Imp of the module under test. Both 

modules were cooled using fans and room-temperature air, which maintained module temperature within several degrees 

of STC (25 oC). 

Experiment II investigated a single CIGS module as it was repeatedly deployed outdoors with full-sun exposure, then 

brought indoors, and maintained either at Imp forward bias or unbiased, in the dark at room temperature. Sequentially 

exposing the same module both with and without bias removes uncertainty from any difference in module response, 

focusing purely on the difference of one module between forward-bias and no-bias exposure.

Experiment III focused on the transient performance changes of the CIGS modules as they were deployed outdoors and 

light-exposed under load. The outdoor exposures studied under Experiment III were simply the initial outdoor 

deployments of Experiment I and II, investigated in more detail. Within the first week of outdoor deployment, 

normalized I-V curves indicated changes of several percent in Voc, FF, and Isc.
3

The CIGS modules used in these experiments were obtained from four different manufacturers in five separate batches, 

labeled here as CIGS A through E. Four of the five pairs of modules (CIGS A, CIGS B, CIGS C, and CIGS E) were of 

glass-glass monolithic construction, using CIGS active material on a glass substrate construction with a transparent 

conductive oxide (TCO) layer, an ethylene vinyl acetate (EVA) encapsulant, a glass front sheet, and an aluminum frame. 

One module group included a weatherproof back sheet as well. CIGS D modules are of a different construction: flexible 

CIGS cells, tabbed and mechanically integrated into a module with an EVA encapsulant, a backsheet, and a front glass. 

The ages of the modules fall into two different groupings. CIGS A modules are of an older vintage constructed in the 

early 2000s, while CIGS B through CIGS E modules are more modern, obtained after 2009. Additional details are given 

in Table 1:

Table 1: Module construction and efficiency details

Module group name Qty Acquisition year Construction Initial area efficiency

CIGS A 2 2003 Framed, monolithic 7.4%

CIGS B 2 2010 Framed, monolithic 11%

CIGS C 2 2011 Framed, monolithic 13%

CIGS D 2 2009 Framed, interconnected cells 8.6%

CIGS E 2 2012 Framed, monolithic 14.5%

2.1 Details for Experiments I and II

Table 2 shows the sequence of Experiment I. A set of two modules from each batch was exposed outdoors at the 

Outdoor Test Facility (OTF) at the National Renewable Energy Laboratory. The modules were rack mounted at latitude 

tilt and held at the maximum power point. Temperature- and irradiance-corrected I-V curves were taken every 15 

minutes. Outdoor performance curves were collected by a Daystar I-V Multitracer, with plane-of-array irradiance 

monitored with Kipp & Zonen CMP-11 pyranometers, and back-of-module temperature monitored with a T-type 

thermocouple. When the maximum power of the modules appeared stable (±2%) the modules were removed from the 

rack and brought indoors. All modules were light-exposed for at least 30 kWh/m2

.

Within 20 minutes of being brought indoors, an I-V curve was measured for each module on an indoor long-pulse solar 

simulator (Spi-Sun 4600 SLP). Within 30 seconds, one module was then forward-biased at its nameplate maximumpower current (Imp). The other module was left indoors at open-circuit (unbiased). Both modules were placed in front of 

fans circulating 25oC air. I-V sweeps were taken for both modules using the long-pulse solar simulator under Standard 

Test Conditions (STC) 0.5, 1, 2, 4, 7, 24, and 96 hours after being brought indoors. The biased module was kept under 

forward bias between measurements and removed from the power supply less than a minute prior to each measurement.

The small temperature rise of the forward-biased module (2°–3°C) is accounted for by a temperature correction in the 

STC measurement. The unbiased module was allowed to relax in the dark between measurements, and is maintained at 

25°C.

Table 2: Experiment I sequence

Step CIGS X module 1 CIGS X module 2

1 Light-exposed 30 kWh/m2 at MPP

I-V curves every 15 minutes

2 Modules brought indoors

Immediate indoor light I-V curve

3 Dark storage unbiased Dark storage forward-biased at Imp

Indoor light I-V curves after 0.5, 1, 2, 4, 7, 24 and 96 hours

In Experiment II, a single CIGS A module was monitored both outdoors and indoors to determine the reversibility of 

performance changes in response to light and dark exposure. Table 2 shows the sequence of Experiment II. The 

experiment is essentially identical to Experiment I, except that rather than testing two modules simultaneously in either a 

forward-bias or unbiased condition, a single module is tested sequentially in unbiased, then in forward-biased conditions.
4

Table 3: Experiment II sequence

Step CIGS A module

1 Light-exposed 30 kWh/m2 at MPP

I-V curves every 15 minutes

2 Module brought indoors

Immediate indoor light I-V curve

3 Dark storage unbiased

Indoor light I-V curves after 0.5, 1, 2, 4, 7, 24 and 96 hours

4 Light-exposed 30 kWh/m2 at MPP

I-V curves every 15 minutes

5 Module brought indoors

Immediate indoor light I-V curve

6 Dark storage forward-biased at Imp

Indoor light I-V curves after 0.5, 1, 2, 4, 7, 24 and 96 hours

The CIGS A module was first exposed outdoors for > 30 kWh/m2 (step 1) to stabilize its electrical performance. The 

module was held at Pmp during outdoor deployment and characterized by I-V sweeps taken every 15 minutes using the 

Daystar I-V multitracer, as above. The module was then brought indoors and characterized at STC with a long-pulse 

solar simulator for up to 96 hours, as above. In steps 4–6 the module was placed back outside for another 30-kWh/m2

light exposure, then brought back indoors for I-V characterization, again up to 96 hours. This time, the module was 

placed at Imp forward bias between measurements rather than allowing the module to remain unbiased between 

measurements. 

3. RESULTS AND DISCUSSION

3.1 Experiment I

I-V curves measured after modules were brought indoors indicate a reduction in Pmp as the module is stored in the dark 

without applied bias. Figure 1 shows a series of I-V curves taken for the CIGS A module, which was allowed to relax in 

the dark without bias. Over the four days of indoor measurement, Pmp decreased by ~12%, primarily through loss of Voc

and FF. 

Figure 1: Indoor I-V curves of a CIGS A module, resting in the dark following outdoor exposure. Performance decreased with time 

as the module rested in the dark at room temperature, particularly through reductions in FF and Voc.
5

Figure 2: Pmp degradation of modules stored in the dark at open-circuit after light exposure. Pmp change is calculated as a percentage, 

relative to the first indoor I-V measurement. CIGS D in particular displayed no measurable change with dark storage. The remaining 

modules displayed an exponential decay in Pmp. Four of the five thin-film models had degraded ~1% or more within one hour.

The reduction in Pmp for the other unbiased modules is shown in Figure 2 as a function of time. The performance 

reduction is displayed as a percentage of the initial indoor I-V measurement point. As was the case in Figure 1, most of 

the performance change comes from a reduction in Voc and FF, both of which decrease exponentially. In a four-hour 

span the maximum power degraded 0%–7% depending on the manufacturer. CIGS D, the only module that is not 

monolithically integrated, was the only module demonstrating little or no change in Pmp in response to dark storage. The 

remaining modules continued to decline in performance up to the end of the measurement period. CIGS A, B, and C did 

not show an indication of having completely stabilized after 100 hours. However, at the end of the test the reduction in 

Pmp had slowed to approximately 1% every 48 hours.

The early time data in Figure 2 indicate that performance loss due to dark exposure is rapid, with four of the five CIGS 

modules losing over 1% of Pmp within the first hour. It is also evident that the module experiencing the greatest 

magnitude loss also has the fastest rate of Pmp loss (CIGS A in particular). Overall power change after 100 hours in the 

dark following outdoor light exposure was 0% for CIGS D, –3.7% for CIGS E, –4.7% for CIGS C, –6.6% for CIGS B, 

and –12.4% for CIGS A. Details are provided in Table 4 showing the Pmp loss of each unbiased module following 100 

hours of indoor dark storage, along with the rate of each exponential Pmp decrease.

Table 4: Performance change and time constant for modules stored in the dark

Module group name Pmp change after 100 hours Time for half maximum loss

CIGS A –12.4% 2.5 hours

CIGS B –6.6% 22 hours

CIGS C –4.7% 15 hours

CIGS D 0% N/A (no change)

CIGS E –3.7% 10 hours

Additional detail on the performance parameters responsible for Pmp change for the five unbiased modules is shown in 

Figure 3, showing that Pmp loss is mainly due to FF, with slight reductions in Voc. Again, the CIGS D module showed 

little change in either Voc or FF. The remaining modules exhibited a fast drop in Voc within the first several hours, 

quickly stabilizing, except for CIGS B, which sustained Voc loss throughout the entire exposure. Fill factor loss tended 

to be a more prolonged and sustained decline through the 100-hour dark exposure. Isc (not shown) was relatively 

constant, increasing slightly (< 0.5%) for most modules.
6

Figure 3: Change in Voc (left) and FF (right) for the five unbiased modules during dark relaxation, relative to the initial indoor 

measurement point.

Because of the rapid performance loss experienced by CIGS modules immediately following light exposure, we 

investigated the possibility of using forward bias to maintain modules at a known performance condition. As the 

modules shown in Figure 2 were allowed to relax in the dark unbiased, a second module was connected to a power 

supply at Imp forward bias.

Figure 4: Forward-biased module’s Pmp relative to final outdoor performance. In all cases, forward bias results in stabilization of 

electrical performance to 1%, and final performance within 4% of the outdoor performance of the module.

Figure 4 shows the performance of the second module of each module pair, maintained at Imp bias after being brought 

indoors. The Pmp of each module is measured at the time intervals given in Table 2, and plotted relative to the final 

outdoor-measured Pmp value. Rather than allowing the module’s Pmp to decline over time, as in Figure 2, the application 

of forward bias maintains performance within a narrower operating window, especially for module A. Four of the five 

modules (B through E) have a variation of less than 1% after being brought indoors and measured for the first time. 
7

CIGS C, D, and E are initially measured 2%–3% below their outdoor stabilized condition, and the modules maintain that 

performance for the next 150 hours. CIGS B is initially measured at 2.5% above its outdoor-stabilized condition, and 

remains near 1% of that value.

The CIGS A module varied quite rapidly with application of forward bias, increasing in Pmp by ~5% within the first hour 

of biased exposure. In the first hour, CIGS A increased in Pmp from 4% below its outdoor-deployed Pmp value to 1% 

above. Additional bias exposure of CIGS A continued a slow Pmp increase to a final value 3% above its outdoor 

deployed Pmp value. What is possibly occurring here is that an initial power loss of ~4% might have happened between 

the final outdoor measurement and the initial indoor I-V measurement as the module was brought indoors. Application 

of forward bias recovered this transient loss, bringing the final stabilized measurement within a few percent of its longterm outdoor deployed condition.

The reason for CIGS A’s large and fast forward-bias response may be connected to the way its twin module performed 

unbiased. The unbiased module’s magnitude of Pmp change is the greatest, and the time constant of change is the 

shortest of all the unbiased modules. A large performance change might occur in both of the CIGS A modules during 

the short ~20-minute gap between outdoor light exposure and indoor flash testing. This would also indicate that the 

performance loss shown in Figure 2 for the unbiased CIGS A module may be under-predicting the amount of loss 

experienced during dark storage. When measuring these CIGS modules, a smaller delay between outdoor light exposure

and indoor I-V curve measurement will result in a closer match between the indoor measurement and the long-term 

outdoor performance.

It should be noted that while four modules stabilized within 1% of their initial indoor-measured condition, the value 

relative to their final outdoor Pmp was either 3% high or 3% low. These modules exhibited stable performance once 

forward bias was applied, with less than a 1% change throughout the indoor I-V curve measurement. This is notable 

since the first indoor I-V data point was taken before the application of forward bias. Therefore we can say that the bias 

itself is probably not responsible for the +/– 3% offset value. It is therefore possible that the offset between the indoor 

and outdoor measured Pmp values is a measurement difference between the indoor and outdoor measurement test 

platforms. A 3% difference between indoor and outdoor STC measurement is within the 5% stated uncertainty in 

absolute Pmp value given for the two testing platforms. Additional uncertainty also arises from the translation equations 

required to adjust outdoor performance of modules operating at ambient conditions to STC conditions, as will be

discussed below.

A clear difference is visible between the modules forward-biased in Figure 4, and their unbiased twins in Figure 2. 

Except for CIGS D, which did not degrade when left unbiased, the application of bias has clear benefits in terms of the 

stability of a module’s performance, particularly if a module is allowed to wait over an hour between light-soak and 

measurement. Without bias, the modules experienced between a 1% and 2.5% drop in Pmp after an hour in the dark, 

compared with the forward-biased modules, which maintained stability between 0.5% and 1%, out to 100 hours. Even 

after 100 hours of resting in the dark, some modules continued to show performance changes, while forward bias 

appeared to completely stabilize all the modules within a matter of hours.

3.2 Experiment II

The second experiment more clearly shows the advantage of forward-biasing modules prior to indoor measurement by 

directly comparing results from the same CIGS A module. The module was first light-soaked, then allowed to rest 

unbiased in the dark between indoor I-V measurements. Then the module was re-deployed outdoors and re-measured 

indoors, now with Imp bias applied between measurements. Figure 5 shows both the forward-bias and no-bias results for 

the same CIGS A module.

As in Figure 2 the unbiased CIGS A module rapidly degraded in Pmp, losing performance of approximately 14% within 

the first 24 hours. This performance loss is entirely reversible, with outdoor performance recovering to the same starting 

value following another 30-kWh/m2 outdoor light exposure. This time, when the module is placed under Imp forward 

bias indoors, the performance has an initial drop of 4%–5%, which is recovered within an hour of forward bias. This 

behavior strongly mirrors the initial loss and subsequent recovery of the forward-biased CIGS A module in Experiment 

I, as shown in Figure 4. Indeed, the application of forward bias maintains the module at a Pmp operating condition within 

a few percent of the outdoor-deployed value, compared with leaving the module in the dark, which resulted in 

substantial loss in Pmp.
8

Figure 5: Comparison of the application of forward bias and no bias for the same CIGS A module following light soak. Indoor data is 

plotted along with Pmp from the final stabilized outdoor data.

3.3 Outdoor deployment of modules

I-V data were collected during the outdoor light exposure of the modules and corrected for temperature and irradiance 

back to STC conditions18 using manufacturer-provided temperature coefficients. Prior to being deployed outdoors, all 

modules had been stored in the dark for a period greater than one week.

The outdoor data were first filtered for plane-of-array irradiance between 900 and 1100 W/m2

. Variable conditions were 

identified that can cause considerable noise in the data. Plane-of-array irradiance was measured before and after the 

sweep of the I-V curve (taken on the order of several seconds). A change in irradiance of more than 10 W/m2 during this 

time period resulted in the rejection of that data point. In addition, it was required that module temperature would 

change less than 5oC and irradiance less than 100 W/m2 in a 5-minute interval. This further reduced the incidence of 

variable conditions in the outdoor measurements. Additional outliers (caused by partial shading, etc.) can be detected by 

investigating the ratios of Pmp to plane-of-array irradiance and Isc to Imp, which were given acceptance criteria depending 

on the individual module characteristics.

The data were analyzed for evidence of a recovery in Pmp during light exposure, which would indicate the reversibility of 

Pmp losses seen in dark-exposed modules. Figure 6 shows results for one module chosen from each of two module 

groups: CIGS A and CIGS E. (Outdoor data for CIGS B, C, and D are given in Appendix A.) Individual I-V parameters 

(Pmp, Isc, Voc, and FF) are shown over time after being corrected to initial measured conditions and normalized by module 

nameplate values.

The largest changes in outdoor performance values were seen in Pmp and FF, as was the case with the indoor results. A 

visible increase in Pmp and FF is seen for all modules except CIGS D, which did not experience any change, also in 

keeping with indoor performance results. To better visualize the changes in Pmp and FF, an exponential best-fit line is 

plotted through the data in Figure 6, according to Eq. 1:

( ) θ [1 θ exp( /τ )] 1 F t = − − t 2 (1)

where F(t) is the best-fit function for Pmp or FF, θ1 ,θ2 and τ are unconstrained fitting variables, and t is time. This 

model describes exponential growth with a final value of θ1. The exponential best-fit is also used to determine the final 

outdoor-deployed Pmp value for the different modules; its value at the conclusion of light soaking is used as an initial 

reference for the indoor Pmp data in Figure 4 and Figure 5.
9

Figure 6: Outdoor I-V parameters for one of the CIGS A modules and one of the CIGS E modules. All performance data are 

temperature and irradiance corrected and plotted here relative to the module’s initial measured value. Uncertainty in outdoor 

measurement (as indicated by the vertical lines on the right) is on the order of several percent due, in part, to temperature and 

irradiance corrections. Exponential best-fit to Pmp and FF (solid lines) follow Eq. 1.

In the case of the CIGS A and CIGS B modules, outdoor performance stabilizes within the first 2–3 days. For CIGS C 

and CIGS E modules, stabilization requires more time, roughly one week of outdoor deployment. This is in keeping 

with the general findings of the indoor measurements that the larger magnitude changes of CIGS A proceeded more 

rapidly than the smaller magnitude changes of CIGS C and CIGS E. Although these modules were all deployed 

outdoors during initially sunny conditions, several gaps exist in the plots of Figure 6, generally indicating variable or 

cloudy conditions. Also, ambient temperature was different for the exposure of various modules, with CIGS A-C 

deployed in December and February, and CIGS D and CIGS E deployed in March and May. It is unclear how different 

irradiance or ambient temperature conditions affected the rate of outdoor stabilization of these modules.
10

The rate of change for outdoor light-exposed modules was different from the indoor decay of Pmp in the dark. For CIGS 

A and B, the outdoor-exposed modules reached a stable condition after being deployed outdoors for 1–3 days. However, 

no such stabilization was seen in a similar time interval of indoor dark storage. Both CIGS A and CIGS B maintained a 

downward trend in performance over 100 hours of indoor storage. Conversely, for CIGS C and CIGS E modules, the 

outdoor performance data showed that stabilization—if achieved at all— required over a week of outdoor exposure, but 

the indoor dark-exposed modules reached a Pmp minimum within several days of dark storage.

3.4 Comparison with theory

As mentioned in the introduction, reversible performance loss in CIGS modules has been studied previously, with

several possible mechanisms identified to explain the dark/light metastability. In this experiment, the primary I-V 

parameter lost during indoor dark exposure was FF, followed by Voc. Timescales for this performance loss range from 

an hour to > 100 hours, depending on module vintage and construction. It is possible that the performance loss is 

consistent with the Lany-Zunger defect mechanism, which primarily affects Voc, and secondarily FF. However, not all 

CIGS modules exhibited loss in Voc and FF. Notably, the CIGS D module exhibited a small Voc increase in response to 

dark exposure instead of a decrease. More work must be done to identify the reasons for this difference in behavior, and 

to determine whether it is related to the tabbed-cell module construction of CIGS D (as opposed to monolithic 

construction), or if the difference is attributable to the active layer of the CIGS D modules. Regardless of the cause of 

the reversible Pmp loss in the dark, it has been shown here that the application of Imp forward bias to CIGS modules 

stored in the dark will maintain CIGS module performance near outdoor light-exposed performance. The impact of dark 

storage Pmp loss can be mitigated by this procedure, which may allow for more accurate indoor I-V measurements of 

thin-film modules without the need for costly or bulky indoor light-soaking equipment.

4. CONCLUSION

We have documented the reversible performance change of five pairs of CIGS PV modules in response to outdoor light 

exposure and indoor dark storage. We identified that in four of the five modules, dark exposure led to a loss in Pmp

between 3% and 12%, driven largely by FF and Voc reduction, consistent with the metastability mechanism proposed by 

Lany and Zunger. Time scales for this performance loss are on the order of hours to tens of hours, highlighting the 

importance of quick measurement of CIGS modules following light exposure, if thin-film modules are to be measured 

on indoor flash-test equipment.

An alternative to light exposure of CIGS modules prior to indoor measurement was investigated by applying forward 

bias to each module. The constant current of this forward bias was set to the nameplate rated Imp of each module. 

Stabilization of each module was accomplished by less than an hour of forward bias, after which the module’s 

performance varied less than 1%, for the entire 100-hour experiment. The stabilized Pmp of the modules under forward 

bias was within 3% of the final outdoor-deployed Pmp measurement. More work is needed to determine whether the 3% 

difference reflects the uncertainties from outdoor measurement and translation to STC or a difference in the mechanism 

underlying the observed transients. 

Application of forward bias to CIGS modules brought indoors enables the collection of indoor I-V measurements that 

may be closer to the long-term outdoor measurements than when modules are stored unbiased. This relieves the time 

pressure of staging the modules in the solar simulator immediately after they are brought indoors. Furthermore, the 

method reduces variability in measurements that are taken on the same module or type of module, but with different 

delays between the light exposure and the moment the measurement is made.

ACKNOWLEDGEMENT

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. DOE-AC36-08GO28308 with the 

National Renewable Energy Laboratory.
11

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APPENDIX A: OUTDOOR DATA FOR CIGS B-D