Conference Paper

NREL/CP-520-47748

August 2010

Progress Toward a Stabilization 

and Preconditioning Protocol for 

Polycrystalline Thin-Film 

Photovoltaic Modules

Joseph A. del Cueto, Chris A. Deline, 

Steve R. Rummel, and Allan Anderberg

Presented at the 35th IEEE Photovoltaics Specialists Conference

Honolulu, Hawaii

June 20-25, 2010
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1

PROGRESS TOWARD A STABILIZATION AND PRECONDITIONING PROTOCOL FOR 

POLYCRYSTALLINE THIN-FILM PHOTOVOLTAIC MODULES

Joseph A. del Cueto, Chris A. Deline, Steve R. Rummel, Allan Anderberg

National Renewable Energy Laboratory, Golden, CO, United States

ABSTRACT

Cadmium telluride (CdTe) and copper indium gallium 

diselenide (CIGS) thin-film photovoltaic (PV) modules can 

exhibit substantial variation in measured performance 

depending on prior exposure history. We studied the

metastable performance changes in these PV modules 

with the goal of establishing standard preconditioning or 

stabilization exposure procedures to mitigate measured 

variations prior to current-voltage (IV) measurements. We 

present the results of our case studies of module

performance vs. exposure: light-soaked at 65°C, exposed 

in the dark under forward bias at 65°C; and finally longerterm outdoor exposure. We find that stabilization can be 

achieved using either light or dark bias methods. 

Additionally, we performed and present capacitancevoltage profile measurements on the modules to examine 

the changes in depletion widths or its hysteresis plus 

electronic carrier concentrations as a function of exposure.

INTRODUCTION

Short-term transient increases in the open-circuit voltage 

(VOC) of CdTe and CIGS devices have been observed 

after illumination or forward bias [1]. These persist on time 

scales spanning seconds to hours. Longer-term 

metastable changes in the electrical characteristics of 

CdTe modules have also been observed under 

illumination [2], occurring over time scales of a thousand 

hours (h) of exposure at 1-sun. These effects can reverse 

subsequently when the modules are put into dark 

storage—defined as low light level conditions at room 

temperature—in periods of days to weeks. Transient 

changes in CdTe devices have also been observed to 

occur on time scales of days, attributed to changes in 

electronic states and recombination [3]. In CIGS devices, 

reversible changes in fill factor (FF) and VOC are observed 

when devices are measured after voltage bias or lightsoaking [4]. These or other metastable changes have 

been attributed to persistent photoconductivity due to 

copper migration [5], charging-discharging of donors at the 

CdS/CIGS interface and deep acceptors in CIGS [6], and 

selenium-copper vacancy complexes [7]. In CIGS devices, 

thermal exposure such as during damp heat stress (85°C)

appears to generate changes in both the absorber and 

interface, which diminish the FF and VOC [8, 9], although in 

these references, exposures were conducted in 

unencapsulated devices. Most of the losses appear to be 

reversible with subsequent light-soaking in CIGS modules 

[10]. In CdTe devices, when copper (Cu) is added to the 

back contact, device performance improves, but it 

introduces metastability issues [11, 12].

These transient or metastable effects pose a challenge 

when trying to accurately gauge PV module performance, 

such as during sampling from the production stream or in 

module certification tests. The current certification 

standard for thin-film PV modules—IEC 61646 [13]—

prescribes 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 more toward amorphous silicon, and is quite 

likely not optimal for stabilizing polycrystalline CdTe or 

CIGS PV devices. Moreover, it appears very probable that 

in CdTe and CIGS, there are multiple ranges of time 

scales over which transitory effects in electrical 

characteristics may take place—short times on the order 

of minutes to hours, and much longer time scales like 

weeks or more. The different time scales probably emerge 

via disparate mechanisms. For example, carrier traps are 

likely responsible for short-term transients, whereas

species migration may be responsible for metastability. In 

this paper, we refer to the short- or longer-time scale

effects as transient or metastable, respectively.

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 evolution of measured performance, 

it may be necessary or 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. We undertook the latter task and 

considered alternate stabilization paths: light-soaking at 

1-sun or forward biasing in the dark, as suggested in the 

literature [1, 3, 4], carried out at elevated temperatures. 

EXPERIMENT 

A flow chart of our experimental study plan is shown in 

Fig. 1, indicating baseline, preconditioning, and then the 

main sequence of either light-soaking or forward bias in 

the dark, both at nominally 65°±5°C module temperature,

as alternate exposure paths carried out in weatherometer 

chambers indoors. Prior to the main stabilization 

sequence, we exposed modules under natural outdoor 

lighting at open-circuit for two short-time increments—

1 kWh/m2 and 26 kWh/m2

—to study if that produces

adequate, feasible preconditioning, as indicated at the top

portion of the figure. Afterward, if individual modules 

passed minimum performance quality, they underwent 

dark, thermal anneal at 90°C for 48 h, nominal RH 
2

(~30%), and open-circuit, in one of the chambers, prior to 

indoor stabilization. This dark thermal anneal is expected 

to ‘relax’ some of the changes caused by stress or 

exposure. At baseline and after each exposure, dark and 

light I-V measurements were conducted. Moreover, 

because it’s been proposed that hysteresis in the 

depletion width vs. voltage bias profile derived from 

capacitance-voltage (C-V) profile measurements may 

correlate with CdTe device stability [12], we measured C-V 

profiles at various stages of exposure.

Figure 1 Stabilization study plan flow chart.

After dark, thermal anneal, we began phase I of the study. 

It’s worth pointing out that in phase I, to prevent module 

damage, a conservative value of forward bias voltage was 

applied halfway between Voc and the maximum-powerpoint voltage, which generates substantially less than the 

maximum-power-point current (IMPP). In phase II, we 

applied larger voltages to get up to the IMPP, during dark 

exposure. This probably made a substantial difference in 

the rate at which we drove module stabilization. Also in 

phase I, we ran four voltage-biased, dark exposures but 

five light-soak ones. In phase II, we ran three 48-h 

increments each of light and dark exposures. Exposures 

carried out indoors were performed in 24- or 48-h 

increments, after which I-V and C-V measurements were 

conducted. After accumulating ~120 h of total exposure, 

phase I ended and the modules lay in dark storage for 2 to 

3 months, after which they were re-tested (I-V, C-V). Then 

phase II began with most of the same modules, except 

that the modules were swapped with respect to indoor 

exposure: those that had been light-soaking in phase I

then went into dark forward bias exposure in phase II, and 

vice-versa. After about 120 h of cumulative exposure, the 

modules were put again into dark storage for 4 months, 

after which we began phase III: outdoor deployment for 

~6–8 weeks, for a total accumulated light exposure of 295 

kW-h/m2

, before bringing the modules in for retesting. By 

performing this step, we hope to capture the nominal 

outdoor stabilized performance data and how they 

compare with the indoor stabilization values.

The starting module set is enumerated in Table 1. The first 

column lists module type, consisting of three types of 

CdTe modules—A, B, C—plus two types of CIGS 

modules—A and B; second column lists number of 

modules; and the third column notes any pre-existing 

exposure. Not all of the modules finished all three phases 

in this study. The two CdTe B modules failed during 

phase II (open-circuit); the one pre-exposed CIGS type A 

failed after dark 90°C anneal before phase I, due to low 

FF. At the end of phases II and III, 8 modules remained.

Module No. Pre-existing exposure

CdTe A 2 Yes, outdoors, hot-humid, 3 years

CdTe B 2 No, nascent

CdTe C 1 Yes, indoor light-soak 2002,1130 kWh/m2

CIGS A 4 No, 3 nascent controls from 2003; 

1 exposed in hot-humid outdoors 3 years

CIGS B 2 No, nascent

Table 1 List of study plan modules at start.

Dark and light I-V data were measured on the large-area 

continuous solar simulator (LACSS) test bed by the Cell 

and Module Measurements Team at NREL. Light I-V 

measurements conform to standard test conditions (STC):

25 °C, 1000 W/m2 total irradiance, using IEC 60904-3 2nd

ed. global reference spectrum. Dark I-V data were 

measured after light I-V, up to 20–30 volts higher than VOC

to attain forward current values above the short-circuit

current (ISC). C-V profiles were measured with a precision 

LCR meter (HP-4284 A), with frequency range 20 Hz–

1 MHz. The 4284 A supplies both the AC excitation signal 

and the DC bias voltage (VB) for the sweep. A Keithley 

digital multimeter was used to measure the applied DC 

bias, VB, across the module accurately, as the applied VB

can be appreciably loaded down by either shunt or diode 

conductance. The excitation frequencies initially were 

varied over a wide range to obtain the location where the 

best (lowest dissipation) high-frequency capacitance 

signals reside. In spite of popular misconception that

module capacitance is too large to measure, we point out 

that its feasibility is based on the large number of seriesconnected cells in modules, plus the fact that capacitors in 

series add up as the reciprocal of each capacitor. Also the 

magnitude of the area of each cell is similar to the number 

of cells, and so the total module capacitance is equivalent 

to that of a ~0.5–2 cm2 size cell, provided that cells are 

uniform enough and there are no pathologically bad cells.

b) dark-soak a) light-soak

*Note: light-soak

1 CIS B module in

Phase Ι outdoors

Dark thermal anneal 90°C, RH ~30%, 48 h

light / dark

soak?

Yes No

exposure

t>120 h?

Phase ΙΙ:

swap modules

a) light -> dark

b) dark -> light

Phase ΙΙΙ: Expose outdoors ~6 weeks,

load ~ Pmax, total =295 kWh/m2

Ι

ΙΙ

Phase Ι

or ΙΙ?

Measure IV, CV

Phase ΙΙ

t=0

Dark storage ~ 2-3 months

Indoor ~1-sun*See note

Tmod ~65±5°C, load~Pmax

Measure IV, CV

dark, Vbias ~Vmax-Voc,

Tmod ~ 65±5°C

Dark storage ~ 4 months

Main Sequence

Phase Ι, t=0

No

Yes

Evaluate

η >Min? Discard

Begin

Measure Baseline 'storage state' IV, CV

Precondition outdoors: natural light, open circuit

Measure IV, CV

Measure IV, CV
3

RESULTS: PERFORMANCE CHANGES

Figure 2 is a composite-panel graph depicting the relative 

changes in CdTe modules’ power parameters, (ηSTC), FF, 

VOC, and ISC, plotted vs. exposure category, respectively, 

going from top to bottom panes, in all phases of our study; 

changes are expressed as percent change relative to 

baseline data measured in May 2009. The left-most 

category represents any pre-existing measurements,

typically years prior to baseline data; baseline is the 

second category from left. At baseline, all relative changes 

are by definition zero. The next two columns represent 

changes incurred during preconditioning outdoors; after 

that comes phase I data, which in turn is followed by dark 

storage; then phase II data, followed by another period of 

dark storage; and finally phase III data at right. In phase I, 

A1, B1, and C1 underwent biased dark exposure, while A2 

and B2 went through light soaking; these were swapped in 

phase II with A1, B1, and C1 in light soak, while A2 and 

B2 went into dark exposure. The middle two legends of 

symbols in Fig. 2 indicate which module went into either 

light or dark exposures in phases I or II.

Figure 2 Changes to module efficiency, FF, VOC and ISC, respectively going from top to bottom panes, plotted vs. 

exposure category for CdTe modules A1, A2, B1, B2, and C1, as percent relative to values at baseline, May 2009.

Initially with 1 kW-h/m2 outdoor exposure after baseline, 

there are slight drops in all performance data, from 1% to 

3%. Subsequently, after 26 kW-h/m2 outdoor exposure, 

the A and B modules improve, while module C1 loses 

about 9% of its performance. After the 48 h dark, thermal 

(90°C) anneal, the A and C1 modules improve while the B 

modules’ performance degrades. Most improvements are 

due to increases in VOC in dark 90°C anneal for all the 

modules, ranging 1% to 7%. The A1 module’s efficiency 

improves the most, ~17% after 90°C anneal, to the level it 

had been years prior to baseline before any exposure, due 

to increases in both FF and VOC. After 90°C anneal, the B 

modules’ performance data decline, which is due

predominantly to large FF losses ranging 15%–30%. 

Afterward, in phase I, biased dark exposure ameliorated 

the performance (up from that after 90°C anneal) of the B1 

module to about 19% loss relative to baseline; but light 

soaking ended up degrading the performance of the B2 

module close to ~22% loss relative to baseline. The A1

module performance declines in two subsequent intervals 

in biased dark exposure, largely as a result of ISC losses in 

the second interval, which reverse for the third biased-dark 

interval, ending about 12% higher in performance than at 

baseline. The A2 module performance data hardly change 

after dark thermal anneal to the end of phase I during 

which it was light-soaked. At the end of phase I, the C1 

module performance—exposed in dark forward bias—is 

nearly identical to that after the outdoor pre-conditioning.

In the dark storage intermission between phases I and II, 

both the A and B2 modules’ performance data degrade 

slightly, with losses in the A modules’ VOC data 

responsible for this drop. During the dark storage, the C1 

and B1 modules’ performance data improve due to 

amelioration of their VOC or FF, respectively. Subsequently 

during phase II, the A1 and A2 modules’ performance data 

increase, respectively, while in biased-dark and light-soak 

exposures due to increases in their VOC; whereas the C1 

modules’ data varies slightly 1%–2% throughout phase II, 

exhibiting slight increases (~1%) in FF data, and 1.5% 

drop concurrently in the VOC data. Noteworthy, both B 

modules fail in the open-circuit condition during phase II, 

indicating high series resistance. Infrared images reveal 

hot spots at the bus bar contacts, and may thus reflect 

problems not intrinsic to the absorber. In phase II following 

dark storage, the ISC data appear to increase ~1% and 

then decrease 1%–2%, respectively, in the first or next two 

exposure intervals.

 ∆η STC (%)

-30

-20

-10

0

10

20

∆ FF (%)

-30

-20

-10

0

10

∆ VOC (%)

-5

0

5

10

∆ ISC (%)

-6

-4

-2

0

2

A1

A2

B1

B2

C1

A1 light

A2 dark

B1 light

B2 dark

C1 light

Phase III

A1 dark

A2 light

B1 dark

B2 light

C1 dark

Precondition Phase I Phase II

PreExisting

Baseline

1 kWh/m2

outdoor

26 kWh/

m2

outdoor

Dark 90°C

anneal

24kWh/m2

24 h dark

48kWh/m2

48 h dark

48kWh/m2

48 h dark

48kWh/m248kWh/m

48 h dark

2

48kWh/m

48 h dark

2

48 h dark

48kWh/m2

Storage

Dark

State

Storage

Storage

State

295 kWh/ m2

outdoors

Dark

Storage

CdTe Modules
4

At the beginning of phase III following dark storage, both A 

modules’ performance seem to degrade largely as a result 

of decline in VOC data. The C1 module data are largely 

unchanged during dark storage. The 295 kWh/m2 of 

outdoor exposure in phase III brings slight improvement in 

A1 performance ~3%, via increase in VOC by about 6%. 

Module A2 exhibits no change in performance after phase 

III, while C1 shows ~2% degradation. During phase III, all 

the FF data exhibit slight degradation of ~1% to 4%, while 

all the modules’ ISC data show about 4% improvement. In 

terms of where performance data end up after phase III, 

that of A1 and A2 are close to what they were at the end 

of phase I, respectively, in dark exposure or light-soak.

Module C1’s performance data end up ~5% lower than at 

baseline, and ~5% higher than at the end of phase I 

biased dark exposure—close to midway between the ends 

of phase I and II exposures, respectively, in dark or light 

exposures.

Figure 3 Changes to module efficiency, FF, VOC and ISC, respectively, going from top to bottom panes, plotted vs. 

exposure category for CIGS modules A1, A2, A3, and B1, B2, as percent relative to values at baseline, May 2009.

The relative changes to CIGS modules’ power parameters 

are portrayed in Fig. 3, which similarly to Fig. 2 is a multipane graph showing the CIGS modules’ ηSTC, FF, VOC,

and ISC data, respectively, going from top to bottom 

panels, plotted vs. exposure category. The changes are 

expressed as percent from baseline, the second category 

from left. The A1, A3, and B2 modules were in biased dark 

exposure in the first phase, and the A2 and B1 modules 

were in light exposure in phase I. In phase II, A1, A3, and 

B2 were light-soaked, while A2 and B2 were in biased 

dark exposure. In phase II, as mentioned, the forward bias 

voltages for dark exposure were increased to maintain 

~IMPP at STC current flowing at 65°C. Any pre-existing

data are shown in the left-most category. After outdoor

preconditioning under natural light: the A modules’ ηSTC

data improve between 1% and 6%, largely due to FF 

increases; while the B modules’ performance degrade 

slightly ~1%–3% predominantly as a result of loss in VOC. 

For the A and B modules, during preconditioning, the VOC

and ISC data exhibit ambiguous trends, albeit declining for 

most except for A1. After the dark thermal anneal, both A 

and B module performance degrade, due to a combination 

of losses in FF and VOC data, respectively, ranging 3%–

12% and 2%–3%; while the ISC data remain largely flat 

(except for A1). This degradation behavior of VOC data for 

the CIGS after dark 90°C anneal is contrasted against that 

of the CdTe modules, which show improved VOC during 

the same exposure. 

In phase I, the A modules in forward bias dark exposure 

improve more than the A2 module in light-soaking: A1 and 

A3 recover most of the losses in FF and VOC incurred in 

dark 90°C anneal, while A2 does not recover as much. For 

the B modules, the recovery of sustained losses in FF 

occurs for both dark and light exposures, but their VOC

data do not recover as much as for the A modules. At the 

end of phase I, the B1 module performance data (dark 

exposure) are close to that recorded after outdoor 

preconditioning. All of the other modules ηSTC data are 

visibly lower, with that for A2 (light-soak) experiencing the 

largest net decline, 10%, relative to that after outdoor 

preconditioning, mostly as a result of loss of FF. The dark 

storage intermission between phases I and II, induces 

much smaller metastable changes for the CIGS modules, 

about 2% or less, than for the CdTe A an B modules 

during the same intermission. In phase II, A1 and A3 went 

into light-soaking, which produced a small (≤ 2%), change 

in performance; A2 underwent biased dark exposure,

which had a more profound change dropping about 5%, 

largely a result of VOC and ISC losses. Although there 

∆ Isc (%) ∆ Voc (%) ∆ FF (%)

-15

-10

-5

0

5

10

-4

-3

-2

-1

0

1

2

-6

-4

-2

0

2

4

Precondition

Phase I Phase II Phase III

A1

A2

A3

B1

B2

A1 dark expo

A2 light soak

A3 dark expo

B1 dark expo

B2 light soak

A1 light soak

A2 dark expo

A3 light soak

B1 light soak

B2 dark expo

PreExisting

Baseline

1 kWh/m2

outdoor

26 kWh/ m2

outdoor

90°C

Dark

anneal

24kWh/m2

24 h dark

48kWh/m2

48 h dark

48kWh/m2

48 h dark

48kWh/m2

48 h dark

48kWh/m2

48kWh/m

48 h dark

2

48 h dark

48kWh/m2

Storage

Dark

State

Storage

Storage

State

295 kWh/ m2

outdoor

Dark

Storage

CIGS Modules

∆ η STC (%)

-15

-10

-5

0

5

10
5

seems to have been slightly larger metastable change in 

dark storage for the B modules, performance data for the 

B modules after the end of phase II are nearly identical to 

those at the end of phase I, dropping ~5% relative to 

baseline, and about 2% lower than after outdoor 

preconditioning. 

In the dark-storage state at the beginning of phase III, the 

A and B modules exhibited small (≤ 2%) changes in 

performance from their respective values at the end of 

phase II. We note that the largest metastable changes in 

any of the power parameters during dark storage between 

phases II and III comes via 1% to 2% improvement in VOC

data, and that most of these VOC increases are lost after 

295 kW-h/m2 outdoor exposure in phase III. By the end of 

phase III, the A1 and A3 module performance data 

improved by 2% over that at dark storage, while that of the 

A2 and B modules degraded by about 2%. During phase 

III, we note the slight changes in FF data: ranging 1%–2% 

increases or 2% decreases, respectively, for A and B 

modules. ISC data increase by 2% for A and B modules, 

except for A2, which loses 2% of its ISC. Comparing the 

modules’ performance data at the end of phase III relative 

to that after phase I: the performance of A1 is nearly 

identical, that of A3 is about 3% higher, A2’s is about 5% 

lower, and that of the B modules are about 3%–4% lower. 

C-V PROFILING

From C-V measurements, we deduced the depletion width 

(WD) vs. bias voltage, VB, (at the cell level) profiles, shown

in Fig. 4 for CIGS modules A1 and A2, respectively, at left 

and right panes, at various states of exposure starting 

from baseline, proceeding to preconditioning, dark thermal 

(90°C) anneal, and the ends of phases I, II, and III. The 

baseline data are represented by dark blue crosses in 

both figures, while dark black circles represent Wd vs. VB

data after dark thermal anneal. For A1, the Wd vs. VB

profile at baseline extends from 0.33 microns at 0.7 V in 

reverse bias down to 0.1 microns at 0.3 V in forward bias. 

For CIGS A2, the WD data are about 0.1 microns higher 

throughout this range. At baseline and VB=0 V, the 

hysteresis in Wd for the A1 and A2 sweeping from reverse 

into forward bias are, respectively, 0.067 and 0.058 

microns. After dark thermal anneal, both A1 and A2 Wd 

vs. VB profiles are significantly shifted upward relative to 

all other profiles after every other exposure, and though 

not shown, the carrier densities have dropped by about a 

factor of 5. Perhaps more significantly, after 90°C anneal, 

the size of the hysteresis in the depletion width for the A1 

module doubles to ~ 0.13 microns at VB=0, while that of 

A2 increases only to about 0.05 microns. The larger WD

hysteresis generated after dark thermal anneal decrease 

in size with subsequent light or biased-dark exposures, but 

henceforth always remain larger for the A1 module than 

for A2. The significance of these variations with exposure 

and ability to move the edge of the depletion width around 

with bias may portend higher stability as well as higher 

efficiency after exposure for A1, even though both 

modules began this study with similar PV efficiency. It’s 

valuable to point out that either light or biased-dark 

exposures at 65°C brought about somewhat similar 

changes in WD profiles after dark 90°C anneal.

Figure 4 WD vs. Vb after various exposures for CIGS 

A1 and A2, at baseline, after dark thermal anneal and 

after each of phases I, II, and III.

C-V profiles were also measured for the CdTe modules.

For brevity, we do not portray these results graphically, but

just note their general features: the dark thermal anneal 

step displaces the WD profile downward to lower values in 

contrast to the CIGS modules; the hysteresis in WD

profiles appears largely in forward bias in contrast to the 

CIGS modules where it’s largely seen in reverse bias. 

Either light or biased dark exposures subsequently return 

the WD profile close to WD values observed near baseline. 

And larger hysteresis correlates with outdoor exposures.

DISCUSSION

One of the hypotheses tested in our study is that either 

light-soaking or forward bias exposure in the dark, at 

65°C, can stabilize performance to within 2%, as called 

out by IEC 61646. It appears that at the end of phase I, all 

the CIGS modules plus the CdTe A2 and B1 modules 

achieved stability by this definition, which comprises both 

light and biased-dark exposures. Many of the CdTe 

modules exhibited large changes in performance (>2% 

relative) while in dark storage between phases I and II. 

However at the end of phase II, the 3 surviving CdTe 

modules stabilized equally well in either light or biaseddark exposure. Both of the CdTe B modules failed in the 

open-circuit mode in phase II, so it’s hard to know whether 

they would have stabilized. In contrast, all the CIGS 

modules’ performance data—except for B2—were largely 

unchanged during dark storage between any of the 

phases. Yet at the end of phase II, it also appears that 

either type of light or dark exposures brought about 

performance stability, except for CIGS A2. The CIGS A2

module’s performance data dropped about 4% in biaseddark exposure in phase II, plus another ~1% more upon 

295 kWh/m2 outdoor exposure in phase III. This module 

might actually not be stable, instead of metastable. 

However well the modules stabilized in indoor light or dark 

exposures, it is instructive to compare how close their 

performance data are when stabilized by either indoor or 

outdoor exposure at the end of phase III, as the latter is 

the best measure we have relating to their field Wd (microns)

Vb (V/cell) -0.8 -0.6 -0.4 -0.2 0.0 0.2 0.4 0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

-0.8 -0.6 -0.4 -0.2 0.0 0.2 0.4

Vb (V/cell)

End Phase I dark

End Phase II light

End Phase III out

End Phase I light

End Phase II dark

End Phase III out

CIGS A1 CIGS A2 Baseline

Precon 27 kWh/m2

Dark 90° C anneal
6

performance. At the end of phase III, the performance of 

CdTe A2 and C1 modules lie closest— about 2% away or 

less—to their respective values at the end of phase I or 

the end of phase II, respectively, both of which were in 

light soaking in their respective phases. The performance 

of CdTe A1 at the end of phase I, in biased dark exposure, 

is closer to that at the end of phase III. For the CIGS A1 or 

A3 modules, in biased-dark exposure in phase I, light-soak 

in phase II: the performance data of A1 at the end of 

phase I is closer to, and that of A3 is closer, that at the 

end of phase III. CIGS A2 performance data continues the 

same trend in phase III as that in phase II, where it is in 

biased dark exposure, implying the module is unstable. 

For the CIGS B2 module, its performance data after either 

phases I or II are similar to that at the end of phase III, 

although that after phase II—biased dark exposure—were 

slightly closer. For CIGS B1, the performance after lightsoaking in phase II is closer. An important consideration 

is how the performance data compare between that just 

after initial outdoor preconditioning and the final outdoor 

exposure in phase III. Here, only the CdTe A2 and CIGS 

A3 modules’ performance data are within 2% of their 

respective values. 

CONCLUSIONS

A study plan was devised for testing stabilization of CdTe 

and CIGS modules indoors and outdoors The data are 

somewhat inconclusive about whether indoor light-soaking 

or forward-biased dark exposure brings about faster 

stabilization, and how close these indoor procedures 

approach outdoor stabilization. It appears either method is 

capable of achieving nearly equal results in terms of 

stability, as after either method was used, many of the 

modules appear to be stabilized, defined by metric of 2% 

or less change in performance. However, compared with 

longer-term outdoor deployment, neither method seems to 

achieve better or more accurate results. Yet, it also 

appears that if forward-bias dark exposure is used, one 

should favor higher currents—close to IMPP—in order to 

stabilize faster. Furthermore, the results obtained with 

initial outdoor preconditioning are not better at predicting 

outdoor performance than the indoor exposure methods. It 

appears that there may be two time scales (or more) 

involved in the metastable effects: one that occurs over 

short times on the order of minutes to hours, that may be 

saturated by outdoor preconditioning, and another that 

occurs over a hundred hours or more and that requires 

elevated temperature and bias to stabilize. Exactly how 

much remains to be determined, but we have begun to 

study a second group of CIGS and CdTe modules. Also, 

there are modules that are unstable and will decay further 

over much longer time scales. We tested C-V profiling on 

modules successfully, and the data—WD vs. VB—correlate 

with the changes observed during exposure, albeit it 

remains to be seen whether this may yield predictive 

information about potential stability issues. For the CIGS A 

modules, larger displacement or hysteresis of WD vs. VB

profile with exposure correlated to higher performance, 

whereas for CdTe the opposite may be the case.

ACKNOWLEDGEMENT

This work was supported by the U.S. Department of 

Energy under Contract No. DOE-AC3608GO28308 with 

the National Renewable Energy Laboratory.

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[3] S. Hegedus, D. Desai, D. Ryan, B. McCandless, 

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WCPEC, Vienna, 1998, 428-433.

[6] P. Zabierowski, U. Rau, M. Igalson, “Classification of 

metastabilities in the electrical characteristics of ZnO/CdS/ 

Cu (In,Ga)Se2 solar cells,” Thin Solid Films 387, 2001, 

147-150 

[7] M. Igalson, M. Cwil, M. Edoff, “Metastabilities in the 

electrical characteristics of CIGS devices: Experimental 

results vs theoretical predictions,” Thin Solid Films 515, 

2007, 6142-6146.

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Films 403-404, 2002, 320-324.

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Contract Report, NREL/TP-413-8121, 1995, pp. 62-72.

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and type approval,” Sec. 10.19; International 

Electrotechnical Commission Central Office 3, rue de 

Varembé, CH-1211 Geneva 20 Switzerland,2nd ed., 2008.
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4. TITLE AND SUBTITLE

Progress Toward a Stabilization and Preconditioning Protocol for 

Polycrystalline Thin-Film Photovoltaic Modules

5a. CONTRACT NUMBER

DE-AC36-08-GO28308

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J.A. del Cueto, C.A. Deline, S.R. Rummel, and A. Anderberg

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NREL/CP-520-47748

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PVD9.1430

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14. ABSTRACT (Maximum 200 Words)

Cadmium telluride (CdTe) and copper indium gallium diselenide (CIGS) thin-film photovoltaic (PV) modules can 

exhibit substantial variation in measured performance depending on prior exposure history. This study examines the 

metastable performance changes in these PV modules with the goal of establishing standard preconditioning or 

stabilization exposure procedures to mitigate measured variations prior to current-voltage (IV) measurements.

15. SUBJECT TERMS

photovoltaic; performance; thin-film modules; CdTe; CIGS; stability; stabilization; preconditioning; metastable 

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