A national laboratory of the U.S. Department of Energy

Office of Energy Efficiency & Renewable Energy

National Renewable Energy Laboratory 

Innovation for Our Energy Future 

Stability of CIS/CIGS Modules at 

the Outdoor Test Facility over 

Two Decades 

Preprint 

J.A. del Cueto, S. Rummel, B. Kroposki, 

C. Osterwald, and A. Anderberg 

National Renewable Energy Laboratory 

Presented at the 33rd IEEE Photovoltaic Specialists Conference 

San Diego, California 

May 11–16, 2008 

Conference Paper 

NREL/CP-520-42541 

May 2008 

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STABILITY OF CIS/CIGS MODULES AT THE OUTDOOR TEST FACILITY 

OVER TWO DECADES 

J.A. del Cueto, S. Rummel, B. Kroposki, C. Osterwald, A. Anderberg 

National Renewable Energy Laboratory (NREL), 1617 Cole Boulevard, Golden, CO 80401, USA 

ABSTRACT 

We examine the status and question of long-term stability of copper indium diselenide (CIS) photovoltaic (PV) 

module performance for numerous modules that are deployed in the array field, or on the roof of, the outdoor test 

facility (OTF) at NREL, acquired from two manufacturers. 

Performance is characterized with current-voltage (I-V) 

measurements obtained either at standard test conditions 

(STC) or under real-time monitoring conditions, taken over 

the course of many years. We present and scrutinize I-V 

characteristics for degradation modes. Analysis yields that 

CIS PV modules can exhibit either moderate (2% to 4% 

per year) to negligible or small (less than 1% per year) 

degradation rates, and that the predominant loss mode 

appears to be fill factor diminution, often associated with 

increases in the series resistance in some of the modules. 

A secondary mode of degradation observed comprises 

metastable changes to the open-circuit voltage. The featured modules are deployed on three separate testbeds. 

INTRODUCTION 

Currently, the highest performing thin-film PV devices 

are cells comprised of copper indium-gallium diselenide 

(CIGS) at approximately 19.9% [1]. The best efficiencies 

obtained from large area PV modules of this technology 

are about 11%–12%, as measured at standard test conditions (STC). However, the long-term stability of CIS/CIGS 

modules has become a key issue with this technology. 

Performances changes are observed in CIGS devices 

when subjected to damp-heat stress and/or illumination or 

bias as noted: decreases in open-circuit voltage(Voc) via 

changes in doping density and conduction band offsets 

between CIS absorber and cadmium sulfide (CdS) window 

layers [2]; losses in Voc due to defect-induced recombination and fill factor (FF) loss by type conversion at the 

CdS/CIGS interface brought about by degradation of the 

zinc oxide (ZnO) transparent conductive oxide (TCO) [3]; 

defects in and degradation of the ZnO and CdS layers, 

leading to poor conductivity or damage at the CdS/CIGS 

interface [4]; degradation of the top ZnO or CdS/CIGS 

interface [5] leading to series resistance increases and FF 

losses. Most of the studies cited have focused on unencapsulated CIGS devices. 

EXPERIMENTAL PROCEDURE 

Beginning in 1988 and continuing up until 2005, CIS 

modules from one manufacturer (A) were deployed at the 

OTF; and starting 2002, we began deploying CIS modules 

from a second manufacturer (B). We emphasize that the A 

modules are no longer manufactured or available, but we 

still retain many of them deployed at the OTF. All modules 

were baseline tested under STC using one or more of 

three solar simulator testbeds: the SPIRE pulsed-light 

source, the large-area continuous (LACSS) and the standard outdoor measurement (SOMS) systems. Afterwards, 

modules were deployed out in the field as: 1) single freestanding modules (A and B) on the long-term exposure 

rack, loaded with a fixed resistor across their terminals 

(whose value approximates the optimum load at STC); 2) 

single modules (A and B) deployed on the Performance 

and Energy Ratings Testbed (PERT) system on the roof of 

the OTF starting in 1997; and 3) an array of type A modules on the high-voltage stress testbed (HVST2) starting in 

2005. Periodically, the long-term exposure and PERT 

modules were taken indoors for further measurements of 

I-V characteristics at STC. Module I-V power parameters 

(Voc, FF, etc.) measured at STC are subsequently scrutinized for changes against time. 

PERT modules are connected to a data acquisition 

system (DAS) that performs I-V traces in situ once every 

15 minutes, and at other times actively maintains the 

modules loaded at their optimum power point appropriate 

for the instantaneous lighting and temperature conditions. 

The HVST2 array comprises 24 thin-film CIGS modules 

deployed in two, bipolar strings of 12 modules connected 

in series, with nominally ±300 VDC open-circuit voltage 

and 1 kW total power. For this array, another DAS also 

performs I-V traces in situ on each of the strings periodically, or actively loads each string at instantaneous optimum-power-point conditions at other times. Additionally, 

the HVST2 DAS can be programmed to, and performs 

cyclic biasing of each string to fixed-voltage levels that are 

stepped over time to produce square-wave bias vs. time. 

PERT and HVST2 modules are deployed at fixed, latitude tilt (40°) and face due south: these data comprise 

real-time outdoor measurements, executed with programmable electronic loads that trace the I-V characteristics. 

Also, we monitor and record irradiance with broadband 

pyranometers, plus air and module temperatures using 

type ‘T’ thermocouples. More detailed description of and 

analysis of PERT system and outdoor data were described elsewhere [6]. In this paper, we narrow our focus 

of real-time PERT and HVST2 data to that obtained within 

fixed irradiance windows, and perform corrections of the IV power parameters to reference temperature (25°C) by 

least-squares fitting thereof to linear temperature dependence scrutinized and calculated within 30-day intervals. 

Furthermore, for the HVST2 array, we present the power 

performance rating calculated monthly according to 

PVUSA Test Conditions (PTC) regression analysis [7]. 

1
The STC data measured on the LACSS were further 

analyzed by normalizing the module voltages and currents 

to the unit cell level, achieved by: dividing the module voltage (V) by the number of series-connected cells, and current to current density (J) by dividing by the area of each 

unit cell (taken as the module aperture area divided by the 

number of cells). These cell areas are approximately 

10%–15% larger than the active area —so J data are correspondingly smaller—due to interconnect and border 

area losses extant in modules. Device level parameters 

like the series resistances, diode quality factors (A), reverse saturation current density (J0), and shunt conductance (Gsh) values are derived via standard diode analysis as shown in the literature [8]: e.g., series resistance 

data are derived from the intercept of the locus of slopes 

of the J-V data (dV/dJ) vs. J, extrapolated to the origin with 

1/J (for dark) or 1/(J+JLight) (for light) characteristics. 

DATA AND ANALYSIS 

Long-Term Outdoor Testbed 

Figure 1 is a composite graph depicting the I-V power 

parameters measured predominantly on the SPIRE at 

STC for modules on the long-term exposure rack: the efficiency, FF, Voc and Isc data are shown, respectively, at 

bottom, second-from-bottom, second-from-top, and the top 

panes in the graph, plotted vs. time; for the A modules 

deployed beginning in 1988, and starting in 2002 for the B 

modules. The abscissae for the Isc and Voc panes in Fig. 

1 comprise changes in scale in order to depict disparate 

current and voltages. Also depicted are: an A module (orange) deployed on PERT and tested indoors occasionally 

beginning in 1997, and a B module also deployed on 

PERT. The A modules represent five separate vintages of 

CIS technology ranging 1988 to 1998. The initial efficiency 

values for the A modules are about 8% in 1988, improving 

over time up to 11%–12%; initial B module efficiency are 

about 11%. The efficiency data of A modules from 1990–

1992 vintages appear stable, but later-vintage A module 

data are less stable. One of the 1988 A modules was removed and returned to manufacturer in 1998 after dropping to ~6%, but a second one has held up better, undergoing a cumulative change in efficiency from about 8% to 

5.9% over a 20-year period, or equivalently 1.3% relative 

loss per year. Furthermore, Fig. 1 depicts some of the 

difficulties encountered in accurate testing methodology at 

STC over the long-term: in 1990 a hardware revision altered the data abruptly for the 1988 A modules, resulting 

in a sharp drop of 0.5% in efficiency, without which the 

loss rate would be only 1.0% per year (%/yr). The relative 

performance loss rates for the modules shown in Fig. 1 

range from negligible for 1990 A modules, up to -2.2%/yr 

for the 1998 A modules. For the 1997 A module depicted 

in Fig. 1—resident of the PERT—there appears transient 

improvement, about 10%-relative between tests conducted in 2006 and 2007: during this time the module lay 

indoors under low light levels and room temperature, while 

the PERT system was re-wired. Some of this transient 

behavior also appears in the PERT data shown later. 

0.5

0.6

1997 A

1998 A

2002 B

1988 A

1990 A

1992 A

1994 A

1997 A

1998 A

2002 B

1988 A

1990 A

1992 A

1994 A

Taken out of serv ice

Change in SPIRE

5

6

7

8

9

10

11

12

45

50

55

60

65

70

1.9

2.0

2.1

2.2

2.3

2.4

2.5

2.6

22

23

24

25

26

27

50

51

52

53

Efficiency (%) FF (%) Isc (A) A modulesVoc (V) B modules

1/88 1/90 1/92 1/94 1/96 1/98 1/00 1/02 1/04 1/06 1/08

Change

in SPIRE

Figure 1. CIS/CIGS A and B module efficiency, FF, Voc, 

and Isc data, respectively, at bottom, second-from-bottom, 

second-from-top, and top panes, measured indoor at STC, 

plotted against time from 1988 up to 2008. 

Although it may be early to conclude definitively 

based on six years of data, the B modules in Fig. 1 appear 

more stable, exhibiting a cumulative loss of less than 0.5% 

absolute, less than 1%/yr relative loss rate. Fig. 1 shows 

the primary mode of degradation for A modules is FF loss, 

followed by some Voc decline. However, note that Voc 

appears to exhibit slight improvement after the initial few 

years of deployment for the A modules of vintages made 

on or before 1992, followed by subsequent loss afterwards. Similarly for B modules, there is some FF loss with 

time, plus transient and slight improvement in Voc initially 

after deployment. 

Losses in FF are scrutinized for select A and B modules, by deriving and analyzing the slopes (dV/dJ, 3-point) 

of the dark and light J-V characteristics, for information on 

series resistance and shunt conductance. These data are 

depicted in composite, multi-pane graphs, Figs. 2 and 3, 

respectively, for 1988 A and 1994 A modules, and 1998 A 

and 2002 B modules. In both Figs. 2 and 3, the slopes 

(dV/dJ) are read along the left-hand abscissae, their dependence plotted against 1/J along the ordinate at the 

bottom in the case of the dark J-V data, or plotted against 

1/(J + JLight) along the upper ordinate (in reverse order) for 

2
the light J-V data. The units of the ordinate axes are inverse amps per square centimeter (cm2

/A); for the abscissae they are ohm-cm2

. The values of JLight used are the 

measured short-circuit current density (Jsc). The top and 

bottom panes in each graph portray: the data for 1988 A 

and 1994 A modules, respectively, in Fig. 2; and in Fig. 3, 

the data for 2002 B and 1998 A modules, respectively. 

The series resistance are derived from extrapolation of the 

locus of slopes to the origin 1/J or 1/(J+JLight). In the figure 

legends, ‘Drk’ or ‘Lit’ refer, respectively, to dark or light J-V 

data; and the two-digit numbers (e.g., 99, 02, 05, 07, 08) 

denote the year the measurements were taken to show 

how the series resistance values have evolved with time. dV/dJ (

Ω-cm2

)

0

5

10

15

20

25

CIS A: 1988

Drk-99

Drk-07

Drk-08

Lit-99

Lit-07

Lit-08

400 300 200 100 0

1/[J + JL] (cm 2/A)

1/J (cm 2/A)

0 100 200 300 400

dV/dJ (

Ω-cm2

)

0

5

10

15

20

25

0 100 200 300 400

1/J (cm 2

/A)

CIS A: 1994

Lit-99

Lit-02

Lit-08

Drk-99

Drk-02

Drk-08

400 300 200 100 0

1/[J + J L] (cm 2/A)

Figure 2. Slopes dV/dJ for 1988 A and 1994 A modules, 

respectively, in top and bottom panes, plotted vs. 1/J for 

dark J-V data read along lower ordinate, and 1/(J+JL) for 

light J-V data, read along upper ordinate axes. 

From the top pane in Fig. 2, we observe that the series resistance of the 1988 A module has changed only 

slightly between 1999 and 2008, by about 1 ohm-cm2

 for 

dark data and somewhat less for the light data. Conversely, for the 1994 A module in the lower pane, the dark 

J-V slopes are more appreciably shifted upwards, indicating an increase of about 3 ohm-cm2

, somewhat less than 

~1 ohm-cm2

 for light J-V slopes, evolving over time for 

curves taken in 1999, 2002 and 2008. Also note that for 

the 1994 A module, the dV/dJ data exhibit non-linear behavior in far forward bias in the dark, that may be indicative of the development and appearance of a reverse-bias 

barrier [8] for the device in the dark, whose effect however, diminishes in the light. Similarly, the top-most pane 

in Fig. 3 indicates non-linear characteristics for the 2002 B 

module, an effect that appears to exacerbate with time 

(2002 to 2008) for the dark dV/dJ data, but to lesser extent 

in the light. This curvature for the 2002 B data complicates 

the analysis, albeit, note that even though the slope of the 

characteristics (dV/dJ–1/J) get steeper, the intercept values with the origin do not appear to alter significantly with 

time, indicating that the changes are not likely due to series resistance. For the 1998 A module in the bottom pane 

of Fig. 3, we observe substantial increases in series resistance values over time, about 4 ohm-cm2

 and 1–2 ohmcm2

, respectively, for dark and light data. We emphasize 

that because Jsc values are notably larger for the A modules (30 mA/cm2

) than for the B modules (24 mA/cm2), 

that even with comparable series resistance, the A module 

performance will be impacted more adversely. 

1/J (cm 2/A)

dV/dJ (

Ω-cm2)

0

5

10

15

20

25

CIS B: 2002

Lit-08

Lit-02

Drk-05 Lit-05

Drk-02

Drk-08

0 100 200 300 400

400 300 200 100 0 1 /[J + J L] (cm 2/A)

CIS A: 1998

 dV/dJ (

Ω-cm2)

0

5

10

15

20

25

1/[J + JL] (cm 2/A)

0 100 200 300 400

1/J (cm 2/A)

Lit-99

Lit-02

Lit-07

Drk-9 9

Drk-02

Drk-0 7

400 300 200 100 0

Figure 3. Slopes dV/dJ for 2002 B and 1998 A modules, 

respectively, in top and at bottom panes, plotted vs. 1/J for 

dark J-V data read along lower ordinate, and 1/(J+JL) for 

light J-V data, read along upper ordinate axes. 

Determination of Gsh values from typical J-V curves 

is illustrated in Fig. 4: in upper and lower panes, respectively, depicting the slopes dJ/dV near zero bias, for the 

2002 B and 1998 A modules, measured over the course of 

several years (e.g., 1999, 2002, 2008, etc.) in the dark at 

25°C temperature. The units shown for the slopes are 

milliSiemens per cm2

 (mS/cm2), reduced to unit area cell 

level. We take the value of Gsh to be the minimum of the 

locus of the slopes against bias, which typically occurs at 

3
zero or in slight reverse bias. We point out that we tend to 

minimize the reverse bias voltage employed in the J-V 

measurements in order to minimize the risk of damaging 

the modules. From Fig. 4, we see that the shunt has been 

slowly increasing for the 2002 B module, from 0.2 mS/cm2 

in 2002 up to ~ 0.55 mS/cm2 in 2008; whereas it has remained fairly low for the 1998 A module, at or less than 

0.2 mS/cm2

. Although the increase for the 2002 B appears 

significant, even at the 2008 level (0.55 mS/cm2

), it has a 

small impact on the reduction of the optimum-point current 

and FF at 1-sun illumination, albeit it is just above the 1% 

level. Where it has a larger impact on relative loss is at low 

light levels. For the 1998 A module, the shunt has negligible effect on performance loss. We point out that the derived series resistance and shunt values can be and are 

subsequently used to correct the apparent measured voltages for series resistance effects and observed current for 

parasitic shunt conductance in order to extract diode quality (A) and J0 data from the J-V curves [8]. 

 2002 B

Drk-02

Drk-05

Drk-08

Volts per cell

 1998 A

Drk-99

Drk-02

Drk-06

Drk-08

-0.1 0.0 0.1 0.2

dJ/dVcell (mS/cm2

) 0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

dJ/dVcell (mS/cm2

)

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

Figure 4. Shunt conductance (Gsh) determination from the 

slopes (dJ/dV) near zero bias for 1998 A and 2002 B 

modules, respectively, shown in lower and upper panes, 

vs. voltage per cell between –0.1 V and +0.2 V. 

High Voltage Stress Testbed Array 

 In Fig. 5, we show the monthly rated power for each 

of the positive (+) and negative (–) strings of the HVST2 

array, using the PTC ratings method, plotted against time 

from Feb. 2005 up till March 2008. Both strings appear to 

be experiencing a moderate decline in rated power. The PTC Regression Rated Power (W)

Jan'05 Jul'05 Jan'06 Jul'06 Jan'07 Jul'07 Jan'08

(+) String

(−) String -3.80%/yr

-2.55%/yr

350

400

450

Figure 5. HVST2 CIGS A module bipolar string array 

power at rated PVUSA Test Conditions (PTC) vs. time. 

relative power loss rate for the negative string appears 

significantly larger ~ -3.8%/yr than that of the positive 

string at -2.55%/yr. Each string’s losses are further scrutinized via the I-V power parameters illustrated in Fig. 6. 

(+) String

(-) String

Efficiency (%)

7.0

7.5

8.0

8.5

9.0

9.5

10.0

50

55

60

65

70

FF(%)

270

275

280

285

290

295

Voc (V)

2.2

2.3

2.4

2.5

2.6

2.7

Isc (A)

2.8

(+) String

(-) String

4/2/05 10/2/05 4/2/06 10/2/06 4/2/07 10/2/07 4/1/08

Figure 6. HVST2 array strings’ Isc, Voc, FF, and efficiency 

data, respectively, at top, second-from-top, second-frombottom, and bottom panes, within 1000 ±25 W/m2

 irradiance, corrected to 25°C temperature, plotted vs. time. 

Figure 6 is a composite graph depicting the I-V power 

parameters for each string of the array, taken from I-V 

traces measured outdoors within a narrow band of irradiance levels, 1000 ±25 W/m2

, under predominantly clearsky conditions, corrected to reference temperature (25°C). 

The efficiency, FF, Voc and Isc data are shown, respectively, in the bottom, second-from-bottom, second-fromtop, and top panes of the graph, plotted vs. time from April 

2005 to March 2008. The efficiency are calculated by dividing the power from each string by the irradiance incident on the sum aperture areas of the 12 modules in each 

string. Fig. 6 indicates that FF losses are largely responsible for the performance losses in each of the array’s 

strings, dropping from about 62.5% to 58.5%, and 63% 

down to 55.6%, respectively, for the positive and negative 

strings, over the three-year period; or equivalently a loss 

rate of -2.1%/yr and -4.0%/yr, respectively, for positive and 

negative strings. The Voc data for the negative string are 

overall lower by about 7 volts than for the positive string; 

and exhibit slight degradation loss rates, -0.3%/yr and -

0.2%/yr, respectively, for the negative and positive strings, 

whose sizes are larger than the 95% statistical significance level (~ ±0.04%/yr). The overall loss rates in performance are -2.9%/yr and -4.7%/yr, respectively, for the 

4
positive and negative strings at 1000 W/m2

, rates that are 

slightly larger than the power loss rates obtained from the 

PTC regression analysis portrayed in Fig. 5. 

PERT System 

 The I-V power parameters for a 1997 A and 2002 B 

module are portrayed, respectively, in Figures 7 and 8, 

using data taken outdoors under predominantly clear-sky 

conditions with a DAS, focusing on two narrow bands of 

irradiance at 1000 ±25 W/m2

 and 500 ±25 W/m2. The data 

are corrected to 25°C temperature. Additionally, the Isc 

data are normalized to the center of each respective illumination level, but are not spectrally corrected. Each figure comprises a multi-pane graph depicting the Isc, Voc, 

FF, and efficiency data, respectively, in the top, secondfrom-top, second-from-bottom, and bottom panes, plotted 

vs. time from Jan. 2002 to Dec. 2007. There are two notable gaps in these data: during 2005 when the modules 

were deployed and biased at peak-power but no I-V traces 

were taken, and from mid-2006 to mid-2007, when the 

PERT was re-wired, during which time the modules were 

stored indoors. 

Jan'02 Jan'03 Jan'04 Jan'05 Jan'06 Jan'07 Jan'08

Efficiency (%)7

8

9

10

57

59

61

63

65

FF(%)

24

25

26

Voc (V)

500 ± 25 W/m2

1000 ± 25 W/m2

Isc(A)

1.0

1.1

1.2

1.3

2.2

2.3

2.4

1000 ± 25 W/m 2

500 ± 25 W/m2

1997 A

Figure 7. 1997 A module Isc, Voc, FF, and efficiency data, 

respectively, at top, second-from-top, second-from-bottom, 

and bottom panes, at 1000 W/m2

 and 500 W/m2 irradiance, corrected to 25°C temperature plotted vs. time. 

A salient feature derived from these two figures is that 

the primary source of performance decline for each module appears dominated by FF degradation. Another notable feature is the appearance of transient behavior: in FF 

data for the 1997 A module, and increases in Voc for the 

2002 B module (~2 volts at 1000 W/m2

), both observed 

just after re-deployment in 2007. The 2002 B module Voc 

data show transient increase just after initial deployment in 

2002 as well. For the 1997 A module, the FF data appear 

elevated above the long-term trend line by 4%-absolute 

after re-deployment in Aug. 2007, to nearly identical values as it had in Jan. 2002. None of the transitory changes 

in the 1997 A FF data are temperature related, as these 

data are temperature corrected, and if anything, the 

changes in uncorrected FF data are even more prominent. 

Jan'02 Jan'03 Jan'04 Jan'05 Jan'06 Jan'07 Jan'08 Efficiency (%)

9

10

11

12

FF(%)

65

67

69

71

73

50

51

52

53

54

55

Voc (V)

0.8

0.9

1.0

1.1

Isc(A) 1.9

2.0

2.1

1000±25 W/m2

 500±25 W/m2

1000±25 W/m2

 500±25 W/m2

 500±25 W/m2

2002 B

1000±25 W/m2

Figure 8. 2002 B module Isc, Voc, FF, and efficiency data, 

respectively, at top, second-from-top, second-from-bottom, 

and bottom panes, at 1000 W/m2

 and 500 W/m2 irradiance, corrected to 25°C temperature plotted vs. time. 

 We note the contrasting behavior in FF data as time 

progresses: for the 1997 A module, data at 1000 W/m2

 are 

offset to lower values compared to data at 500 W/m2

 by 

~3%-absolute, degradation in both irradiance bands appears to occur in tandem at a rate of about -0.7%/yr; for 

the 2002 B module, data in each illumination window start 

out at comparable values, but subsequently, the data at 

500 W/m2

 decline more rapidly (-0.9%/yr) than that at 

1000 W/m2 (-0.5%/yr). The FF degradation rates at the 

two illumination levels appear consistent with seriesresistance dominated losses for the 1997 A module, and 

shunt-related losses in the 2002 B module. Finally, we 

note the largest uncompensated variations in efficiency 

depicted in Figs. 7 and 8 are driven by seasonal fluctuations in Isc, that albeit are temperature-corrected, are larger for data at the lower irradiance than at high irradiance; 

and surmise these are driven by a combination of cyclic 

cosine response to angle-of-incidence and air mass oscillations, both of which attain their respective high values 

during the winter, lows in summertime, and whose variations are larger for data at 500 W/m2

 irradiance. 

5
SUMMARY AND CONCLUSIONS 

The degradation rates for the various CIS/CIGS modules studied under the test conditions referenced are listed 

in table 1, with mean loss rates and their 95% statistical 

confidence levels listed, respectively, in the second and 

third columns from the left. The first 10 rows, below the 

top, comprise modules strictly tested at STC, residents of 

the long-term exposure testbed. The next populated six 

rows comprise the PERT system modules; and last four 

rows represent the HVST2 array positive and negative 

string module data. For the PERT modules, tests at STC 

as well as real-time data measured in situ within the 500 

W/m2

 and 1000 W/m2 irradiance bands are listed. For the 

HVST2 array, we present the degradation measured in 

situ at 1000 W/m2

, and from the PTC-regression-rating 

analysis, respectively, in the last four rows. For example: 

for the 1988 A module, the calculated loss rate is -0.90 

±0.13 %/yr, with 95% confidence, during the time between 

November 1990 and March 2008, as gauged at STC indoors; for the 2002 B module on PERT, the loss rate 

measured at STC indoors between 2002 and 2006, is -

0.65 ±3.3 %/yr, which does not appear significant, but 

however, for the same module, the in-situ PERT data 

measured outdoor between 2002 and 2007 at 1000 W/m2

indicates a loss rate of -0.89 ±0.04 %/yr, which is statistically significant. Some of this ambiguity is due to FF declines that are partly offset by increases in Voc or Isc. 

Table 1. Summary of Performance Degradation Rates 

Module 

Type 

ΔEff/Eff0

(%/yr) 

95% 

(%/yr) 

TEST 

CONDITION TIMELINE 

1988 A -0.90 0.13 STC Nov-90 –Mar-08 

1990 A -0.21 0.10 STC Oct-91–Mar-08 

1990 A -0.32 0.12 STC Oct-91–Mar-08 

1992 A -0.43 0.22 STC Aug-92–Mar-08 

1992 A -0.43 0.18 STC Aug-92–Mar-08 

1994 A -1.08 0.14 STC Mar-95–Mar-08 

1994 A -0.95 0.17 STC Mar-95–Mar-08 

1998 A -2.25 0.23 STC Jan-99–Mar-08 

1998 A -2.14 0.56 STC Jan-99–Nov-02 

2002 B -0.69 0.96 STC Aug-02–Mar-08 

1997 A -2.10 1.06 STC (PERT) Aug-97–May-07 

1997 A -1.35 0.14 500 PERT Jan-02–Dec-07 

1997 A -1.27 0.04 1000 PERT Jan-02–Dec-07 

2002 B -0.65 3.30 STC (PERT) Aug-02–Oct-06 

2002 B -1.80 0.16 500 PERT Aug-02–Dec-07 

2002 B -0.89 0.14 1000 PERT Aug-02–Dec-07 

2004 A -2.87 0.15 1000 HVST2 

POS. STR. Apr-05–Mar-08 

2004 A -4.68 0.15 1000 HVST2 

NEG. STR. Apr-05 –Mar-08 

2004 A -2.55 0.86 PTC HVST2 

POS. STR. Apr-05–Mar-08 

2004 A -3.77 0.82 PTC HVST2 

NEG. STR. Apr-05 –Mar-08 

We assert that for some modules, notably the 1990 A 

and 1992 A modules, the degradation rates are quite 

low—ranging -0.2%/yr to -0.4%/yr—even though the individual FF loss rates are higher, because they’re offset by 

positive changes in Voc. This happens to some degree as 

well for the 2002 B modules measured at STC. Furthermore, from STC data, the primary loss mechanism for the 

A modules appears to be dominated by series resistance 

increases, further exacerbated by their high Jsc. We also 

note that moderately higher degradation rates are observed for the 2004 A modules on the high-voltage array, 

between -2.5%/yr and -4.7%/yr, with the highest loss rate 

incurred by the negatively biased string. 

For the B modules measured indoors at STC, the 

overall loss rates appear statistically insignificant, albeit 

there exists a substantial FF loss mode—3% to 5%—

whose source is complicated: predicated by a combination 

of shunt conductance and/or apparent diode quality factor 

increases. The shunt conductance increases in these by a 

factor of 3–5, but can only be partly responsible for the FF 

loss, that may appear as increases in diode quality factor, 

but under closer scrutiny looks more like changes in a twodiode device model, with the primary diode weakening, as 

indicated by increases in J0. 

ACKNOWLEDGEMENTS 

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

Energy under Contract No. DE-AC36-99GO10337. I thank 

Ingrid Repins and Keith Emery at NREL for their timely 

and valuable editorial comments on this paper 

REFERENCES

[1] I Repins, M A Contreras, B Egass C DeHart, J Scharf, 

C L Perkins, B To, R Noufi, “19.9%-efficient 

ZnO/CdS/CuInGaSe2 Solar Cell with 81.2% Fill Factor,” 

Prog. in PV: Res. & Appl. 16, 2008, pp. 235–239. 

[2] C Deibel, V Dyakonov, J Parisi, J Palm, S Zweigart, F 

Karg, “Influence of damp heat testing on the electrical 

characteristics of Cu(In,Ga)(S,Se)2 solar cells,” Thin Solid 

Films, 403-404, 2002, pp. 325–330 

[3] M Schmidt, D Braunger, R Schaffler, H W Schock, U 

Rau, “Influence of damp heat on the electrical properties 

of Cu(In,Ga)Se2 solar cells,” Thin Solid Films 361-362, 

2000, 283–287 

[4] G A Medvedkin, E I Terukov, Y Hasegawa, K Hirose, 

K Sato, “Microdefects and point defects optically detected 

in Cu(In,Ga)Se2 thin film solar cells exposed to damp and 

heating,” Sol. Ener. Mats & Sol. Cells 75, 2003, pp. 127–

133. 

[5] J Wennerberg, J Kessler, M Bodegard, L Stolt, “Damp 

Heat Testing of High Performance CIGS Thin Film Solar 

Cells,” Proc. 2nd WCPEC (1998), Vienna, pp. 1161–1164. 

[6] J A del Cueto, “Review of the Field Performance of 

One Cadmium Telluride Module,” Prog. In PV: Res. & 

Appl., 6, 1998, pp. 433–446. 

[7] C M Whitaker, T U Townsend, “Application and Validation of a New PV Performance CharacterizationMethod,” Proc. 26th IEEE PVSC, 1997, p.1253–1256. 

[8] Hegedus SS, Shafarman WN. Thin-Film Solar Cells: 

Device Measurements and Analysis. Progress in PV: Res. 

& Appl. 12: 2004; pp. 155–176. 

6
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4. TITLE AND SUBTITLE

Stability of CIS/CIGS Modules at the Outdoor Test Facility over Two 

Decades: Preprint 

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NREL/CP-520-42541 

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J.A. del Cueto, S. Rummel, B. Kroposki, C. Osterwald, and 

A. Anderberg 

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14. ABSTRACT (Maximum 200 Words)

We examine the status and question of long-term stability of copper indium diselenide (CIS) photovoltaic (PV) 

module performance for numerous modules that are deployed in the array field, or on the roof of, the outdoor test 

facility (OTF) at NREL, acquired from two manufacturers. Performance is characterized with current-voltage (I-V) 

measurements obtained either at standard test conditions (STC) or under real-time monitoring conditions, taken over 

the course of many years. We present and scrutinize I-V characteristics for degradation modes. Analysis yields that 

CIS PV modules can exhibit either moderate (2% to 4% per year) to negligible or small (less than 1% per year) 

degradation rates, and that the predominant loss mode appears to be fill factor diminution, often associated with 

increases in the series resistance in some of the modules. A secondary mode of degradation observed comprises 

metastable changes to the open-circuit voltage. The featured modules are deployed on three separate testbeds. 

15. SUBJECT TERMS

PV; copper indium diselenide; photovoltaic; module; current-voltage; open-circuit voltage; short-circuit current; 

electronic loads. 

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