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An Unlikely Combination of 

Experiments With a Novel HighVoltage CIGS Photovoltaic Array 

Preprint 

J.A. del Cueto and B.R. Sekulic 

Presented at the 2006 IEEE 4th World Conference on 

Photovoltaic Energy Conversion (WCPEC-4) 

Waikoloa, Hawaii 

May 7–12, 2006 

Conference Paper 

NREL/CP-520-39877 

May 2006 
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AN UNLIKELY COMBINATION OF EXPERIMENTS WITH A NOVEL HIGH-VOLTAGE 

CIGS PHOTOVOLTAIC ARRAY * 

J.A. del Cueto, B.R. Sekulic 

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

ABSTRACT 

A new high-voltage array comprising bipolar strings of 

copper indium gallium diselenide (CIGS) photovoltaic (PV) 

modules was inaugurated in 2005. It is equipped with a 

unique combination of tests, which likely have never before been deployed simultaneously within a single array: 

full current-voltage (I-V) traces, high-voltage leakage current measurements, and peak-power tracking or temporal 

stepped-bias profiling. The array nominally produces 1 kW 

power at 1 sun. The array’s electrical characteristics are 

continuously monitored and controlled with a programmable electronic load interfaced to a data acquisition system 

(DAS), that also records solar and meteorological data. 

The modules are mounted with their frames electrically 

isolated from earth ground, in order to facilitate measurement of the leakage currents that arise between the high 

voltage bias developed in the series-connected cells and 

modules and their mounting frames. Because the DAS 

can perform stepped biasing of the array as a function of 

time, synchronous detection of the leakage current data 

with alternating bias is available. Leakage current data 

and their dependence on temperature and voltage are 

investigated. Array power data are analyzed across a wide 

range of varying illuminations and temperatures from the IV traces. Array performance is also analyzed from an energy output perspective using peak-power tracking data. 

INTRODUCTION 

A major goal of the U.S. Solar Program is the commercialization of PV modules with 30-year lifetimes or 

more, sustaining less than 0.5% annual performance degradation rate, and at costs consistent with market rates of 

electricity. To meet this goal, module reliability and performance stability need to be tested and insulated from 

stress mechanisms that precipitate module degradation—

such as excessive exposure to moisture, temperature, and 

high voltage. In the 1980's, the Jet Propulsion Laboratory 

(JPL) investigated the connection between high-voltage 

leakage currents and contact corrosion in PV cells and 

modules, establishing key thresholds of accumulated 

charge—the cumulative integral of the leakage currents—

that would result in 50% failures [1]. The thresholds for 

crystalline-silicon (c-Si) modules ranged 1–10 coulombs 

per centimeter length (C/cm) of module perimeter, and 

0.1–1 C/cm for amorphous-silicon (a-Si) modules. At these 

thresholds, JPL found that the prime failure mechanism 

induced by high-voltage stress was electrochemical corrosion and degradation of the electrical contacts resulting in 

series-resistance increases. At NREL, we investigated 

high-voltage stress on c-Si and a-Si modules, and identified the dominant leakage current pathways as a function 

of temperature and humidity: conduction through the glass 

cover sheet at high relative humidity, and interface conduction under dry conditions [2]. In 2005, we installed a 

new CIGS PV array to investigate the effects of long-term 

exposure of this technology to high voltage stress and 

assess how well it can meet program goals. 

EXPERIMENTAL PROCEDURE 

The 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. The modules are segregated into four 

groups of six modules, two groups for each string. Prior to 

deployment, all the modules were baseline tested at standard test conditions (STC). All the modules tested between 8.6% and 10.8% efficiency at STC. We performed 

both the dry and wet hi-pot tests to ascertain that the 

modules met the high-voltage leakage current criteria: the 

leakage currents were well below 1 micro-amp. We set up 

three separate measurement capabilities for the array: full 

current-voltage (I-V) traces, peak-power tracking and/or 

stepped bias loading, and high-voltage leakage current 

measurements. The array is deployed at fixed latitude tilt 

(~40°) with respect to horizontal, and mounted on a fiberglass structure, electrically isolated from earth ground with 

ceramic insulators. The module frames are electrically 

connected to each other on the support structure and then 

routed through a resistive network before connection to 

earth ground. Leakage currents driven by the high-voltage 

built up by the series-connected modules will pass to 

ground via the frames, and across the resistive network, 

where they are sensed by differential voltage inputs on a 

data-logger. The electrical characteristics and bias of each 

string are controlled with a dual-channel programmable 

electronic load (PEL), interfaced to a DAS. The DAS controls the PEL and executes the I-V traces, peak power 

tracking, and/or stepped-bias voltage loading on a regular 

schedule. Also, module temperatures in each string, and 

irradiance data, sensed by a pyranometer, plus other meteorological elements are monitored by the DAS. 

*This work has been authored by an employee or employees of the Midwest Research Institute under Contract No. DE-AC36-99GO10337 with the U.S. Department of Energy. The 

United States Government retains and the publisher, by accepting the article for publication, acknowledges that the United States Government retains a non-exclusive, paid-up, irrevocable, worldwide license to publish or reproduce the published form of this work, or allow others to do so, for United States Government purposes. 

1
ANALYSIS AND RESULTS 

 Analysis of the data is realized by: 1) integration of 

peak-power tracking and irradiance data with time, yielding daily energy outputs; 2) accumulation of all I-V trace 

records, statistically filtering for predominantly clear-sky 

conditions, followed by analysis and segregation into irradiance bins; and 3) integration of the leakage current 

data out through the module frames, resulting in accumulated daily leakage charge. We arrive at array efficiency 

quotients from two distinct angles: one is from the more 

usual I-V trace data representing power measurements 

from each string, divided by the incident power on each 

string (area of the string times the irradiance), and the 

second is from the ratio of the daily energy output from 

each string divided by the daily insolation energy (integral 

over time of the irradiance), yielding an effective efficiency 

(ηEFF) defined by Eq. 1, analogous to the realistic reporting 

conditions (RRC) efficiency previously reported [3]. The IV trace data are further characterized by the usual power 

parameters: open-circuit voltage (Voc), short-circuit current (Isc), and fill factor (FF). We note the average efficiency at STC (ηSTC) for the positive (+) and negative (-) 

strings of the array: respectively, 9.64% and 9.53%. The 

total aperture area for each string is 4.877 m2

, equally split 

between groups in the string. The total perimeter length for 

each group of six modules is 1.915 m. 

[AreadtP dtIrr )1(]

Daily

String

Daily

EFF MAX ⋅= ⋅⋅ ∫ ∫ η

Performance and Stability Analysis ± String Module Temperatures (°C) or Performance Ratio (%)

1-Apr 31-May 31-Jul 30-Sep 30-Nov 30-Jan 1-Apr

0

10

20

30

40

50

60

70

80

90

100

+ Str PR - Str PR

+Str Tmod -Str Tmod

Fig. 1. Daily PR data and module temperatures for each of 

the ± strings, plotted against time starting April 2005. 

In Fig. 1, we show the daily performance ratio (PR) 

data of the (+) and (-) strings of the array, derived from 

energy output analysis. The performance ratio are calculated as the ratio of the effective efficiency (ηEFF), derived 

by Eq. 1, divided by the average efficiency of all the modules in each of the respective strings as measured at STC 

(ηSTC). In this graph, we also plot the average daytime 

module temperatures (Tmod) sampled from each of the 

strings. For convenience, we plot both PR and Tmod 

along the same abscissa, and note that the units of the PR 

are in %, while those of Tmod are °C. The data are plotted 

against time from April 2005 to April 2006. Figure 1 portrays the inverse dependence of the PR with average daytime module temperatures, ranging from 93%–99% in the 

winter down to about 70% in the summertime. We illustrate the usefulness of quantifying energy production performance via the PR with an example: on 13 April 2006, 

the integrated solar insolation was 8.53 kW-h/m2

, and the 

(+) and (-) strings attained 78.5% and 76.8% PR, respectively. So taking the product of the PR times the respective 

average ηSTC and area for each string, times the insolation 

yields the energy output for each string, respectively, 3.14 

and 3.05 kW-h. 

We now present the array data derived from power 

analysis of the full I-V traces executed by the PEL and 

DAS, across varying illumination and temperatures. In Fig. 

2, we depict the Voc and FF data reduced into irradiance 

bins 50W/m2

 wide and corrected to reference module 

temperature (25°C), plotted against irradiance, for each 

string. From Fig. 2, we observe that Voc for the (+) string 

is larger by ~3–5 volts than for the (-) string, and that for 

both, Voc goes from 285–290 volts at high irradiance, and 

drops down to ~225 volts at 50 W/m2

 irradiance. The FF 

data for both strings appear to be 59%–61% at high irradiance. Going from high irradiance down to 300 W/m2

, the 

FF for both strings increase by ~2% absolute. This increasing dependence of the FF with decreasing irradiance 

is consistent with series-resistance limited PV performance and is observed for single modules as well [4]. Below 

300 W/m2

, the FF data diminish for both strings down to 

~57%. Our capability to measure the array’s full I-V characteristics enables us to dissect and identify potential failure modes in future deterioration of performance to 

greater degree than from mere optimum-power point data. Voc(volts)

220

230

240

250

260

270

280

290

300

+ Str Voc - Str Voc

+ Str FF -Str FF

FF(%)

55

57

59

61

63

65

Irradiance (W/m2)

0 200 400 600 800 1000 1200

Fig. 2. Array Voc and FF for both strings corrected to 25°C 

module temperatures, respectively, read from left- and 

right-hand abscissae, plotted against irradiance. 

In Fig. 3, we depict the efficiency data of each string 

plotted versus time, derived from power analysis of the full 

I-V traces executed by DAS and corrected to reference 

(25°C) module temperature, for a random sampling of 

cases with irradiance values within a window of 1000 ± 50 

W/m2

. These data represent string power conversion efficiency taken under predominantly clear-sky conditions. 

The data for the (+) and (-) strings are read, respectively, 

along the left- and right-hand abscissae. The temperature 

coefficients of the efficiency (derived from power analysis) 

2
were calculated piecewise in irradiance bins 50 W/m2 

wide, by linear regression against module temperatures: 

above 750 W/m2

 irradiance, these coefficients are an average of –0.19%/°C and –0.26%/°C, respectively, for the 

(+) and (-) strings, with 1 standard deviation of ~0.04%/°C. 

Fig. 3 shows that the average (+) string efficiency is about 

9.1%, and just below that, 8.9%, for the (-) string. When 

we perform linear regression of the efficiency versus time 

for these data, we obtain no statistically significant degradation of performance from either string: the losses per 

year are at or less than 1 part in 103

, and are about the 

same size as the statistical error in the coefficients. 

5/1/05 7/31/05 10/30/05 1/29/06 5/1/06

(+) String Efficiency

(-) String Efficiency

7

8

9

10

7

8

9

10 -Str Eff

+Str Eff

Fig. 3. Temperature-corrected array efficiency data at 

1000 W/m2

, plotted versus time, for the (+) and (-) strings, 

respectively, read from left- and right-hand abscissae. 

High-Voltage Leakage Currents 

 Previously, we had [2] analyzed the leakage currents 

from modules exposed to high-voltage stress and concluded that: leakage occurs along different pathways depending on the temperature and relative humidity, and is 

thermally activated with an activation energy that depends 

on humidity. In Fig. 4, we present the daily integrated 

leakage charge, expressed in milliCoulombs (mC), from 

each of the four groups of modules in the array portrayed 

on a semilog plot against the inverse of the average daily 

absolute module temperatures. For clarity, we’ve restricted 

the data to show only those corresponding to days where 

the energy out from each string was at or over 2.1 kW-h. Daily Leakage Charge (milliCoulombs)

1/Tmod (°K-1) * 1000

0.1

1

10 + Str Group 1

- Str Group 1

+ Str Group 2

- Str Group 2

3.1 3.2 3.3 3.4 3.5 3.6 3.7

Fig. 4. Integrated daily leakage charge plotted against 

inverse absolute module temperature for each of the four 

module groups in the array. 

We emphasize that each leakage charge datum depicted in Fig. 4 represents the combined leakage from the 

six modules all from within the same group. Because all 

the modules are connected in series, for the groups biased furthest from ground potential, the average cumulative bias on that group will be offset to its largest positive 

or negative value. These high-voltage groups are labeled 

as group 1 for both (+) and (-) strings in Fig. 4, while the 

groups of six modules with lower average bias are labeled 

as group 2. This is the reason that the leakage currents 

and charge for both group-1 modules in each string are 

about a factor of 2.8 to 2.9 larger than those of the group2 modules—namely, it reflects the nearly linear partition of 

the string voltages across each module in each group and 

the fairly ohmic dependence of the leakage currents with 

bias. Aside from variations in leakage due to biasdependence as discussed, there is significant scatter 

within each group’s leakage data, which is predominantly 

due to the leakage dependence on humidity that is unaccounted for and is convoluted in the daily average data. 

The thermally activated behavior of the charge leaked 

by the modules emanating from high-voltage bias is made 

evident in Fig. 4. From the slope of the characteristic exponential thermal dependence shown, one can derive the 

activation energy of the leakage charge for the data 

shown: the activation energies are 0.72 electron volts 

(eV), 0.64 eV, 0.78 eV, and 0.71 eV, respectively, for each 

of the string groups—(+) group 1, (+) group 2, and (-) 

group 1, (-) group 2—with a standard deviation of ~0.05 

eV. The sizes of all the activation energies of the leaked 

charge strongly suggest that the leakage currents are 

dominated by conduction through the soda-lime silicate 

glass, whose electrical conductivity is thermally activated 

with activation energies between 0.6 and 0.8 eV [2, 5]. 

At this stage, it is worthwhile to estimate the potential 

lifetime crisis point emanating from high-voltage stress for 

these modules by using the data in Fig. 4, and by recalling 

the damage threshold of accumulated charge—resulting in 

50% failure rate, as determined by JPL—was on the order 

of 0.1–1 Coulombs per centimeter of perimeter (C/cm) for 

a-Si thin-film modules. Although there is no guarantee that 

those damage thresholds are strictly applicable to the 

CIGS modules, it is still worthwhile to compare them with 

a-Si because of the similar encapsulation and thin-film 

construction of the devices. For the hottest days depicted 

in Fig. 4, the daily leakage charge for the higher-offset 

biased modules is 10 mC, which occurs when the average 

daytime module and air temperatures are, respectively, in 

the ranges of 40°–45°C and 23°–30°C. Although we don’t 

get too many days like that in Colorado, days with similar 

or even hotter (~40°C) average daytime temperatures are 

common throughout the world. So we take the case of 10 

mC leakage charge occurring on 200 days out of the year: 

over the course of 10 years, similar modules exposed to 

high voltage as the group 1 modules will accumulate 20 C 

of charge, that when spread evenly over 1.915 m of module perimeter, yields ~0.1 C/cm—which is significantly 

close to the lower threshold for damage determined by 

JPL. Hence, the exposure of this CIGS technology to highvoltage stress in hot climates merits further scrutiny. 

3
We scrutinize the response of the leakage currents to 

changes in bias voltage. We portray this response in Fig. 

5, which is a composite graph depicting the leakage currents—from the two higher offset bias groups in the array 

(± string groups 1)—on two different days, 18 April 2006 

and 14 March 2006, respectively, at top and bottom portions of the graph, plotted against time of day. The corresponding string bias voltages vs. time are also shown in 

this graph and are read from the left-hand abscissae. The 

± group-1 leakage currents are read along the right-hand 

abscissae. On these two days, the array was controlled in 

distinctly different modes: 1) tracking the optimum-power 

point (POPP) on Apr. 18 at top, and 2) in discreet voltage 

steps vs. time profile on Mar. 14 at bottom. The steppedvoltage vs. time profiles are executed with the PEL controlling the array in constant-voltage mode, while the DAS 

sends varying voltage setpoints vs. time to the PEL. Then, 

the bias the array actually develops is whatever it can deliver given the incident irradiance, either up to the setpoint 

or Voc, whichever one is less. 

3:00 6:00 9:00 12:00 15:00 18:00 21:00

18 Apr. 2006

14 Mar. 2006

± Vbias

ILeak (μAmps) ILeak (μAmps)

-0.20

-0.10

0.00

+0.10

+0.20

-0.15

-0.10

-0.05

0.00

+0.05

+0.10

+ Str ILeak 1

- Str ILeak 1

Vbias (+)

Vbias (-)

Vbias (±volts)

0

50

100

150

200

250

+ Str ILeak1

- Str ILeak1

Vbias (±volts)

0

50

100

150

200

250

Fig. 5. Array bias and leakage currents vs. time profiles on 

two different days, with the array controlled in distinct 

ways: optimum power at top, or in discreet voltage steps 

bottom. Bias voltages and leakage currents are read, respectively, from left-hand or right-hand abscissae. 

 Note that when controlling the array in POPP mode, the 

bias voltages vs. time profiles are smooth, going from zero 

up to about ±210 volts for most of the day, and both 

strings’ leakage currents exhibit smoothly varying behavior, obtaining maximum values, typically 0.05 to 0.1 microamps, in the early afternoon when the temperatures are at 

their highest. In contrast, when stepping the array bias in 

large fixed increments, the leakage currents appear to be 

larger by a factor two immediately after the bias is 

stepped, when incrementing to high voltage. Interestingly, 

when decrementing the bias in large discreet steps at bottom, the leakage currents actually flip to opposite polarity: 

the positive string leaks negative current, and the negative 

string vice-versa, for intervals that last on the order of 

minutes. Because there is no power supply in this circuit, 

other than the array itself, which is incapable of switching 

to net opposite polarity, one of the few ways this may be 

explained seems via a relaxation of a dipole layer of ion 

charges induced in the soda-lime glass adjacent to and by 

the high-voltage bias of the PV cells. 

CONCLUSIONS 

 We have deployed a new array with unique capabilities: peak-power tracking, full I-V characteristic traces, and 

leakage current measurements, using CIGS thin-film PV 

modules in a high-voltage configuration. We have scrutinized array performance in two ways: from the I-V traces 

and energy output considerations. We present this performance in Table 1 below, derived both ways, with the 

caveat that we may not be weighing all days equally 

throughout the year due to array downtime. We emphasize the significance of the energy-based efficiency and 

performance ratio (PR) products in the 2nd and 4th rows: 

given an energy budget—like 6.1 kW-h/m2

/day—and when 

the average daytime air temperatures are 13°–14°C, the 

array will deliver 8.1% or 7.95% of this as electrical energy, respectively, from (+) and (-) strings. We give the 

energy-based relative temperature coefficient: –0.38%/°C 

versus average daytime air temperature. Note that the 

energy-based efficiencies are less than those derived at 

one irradiance and temperature listed in rows 3 and 5. 

Table 1. Array performance. 

String

Average 

Daytime 

Air Temp. 

Average

Module 

Temp. 

Irradiance or 

Insolation 

Eff (%) 

Method 

PR 

(%) 

+ 14 °C 28 °C 6.12 

kW-h/m2

/day 

8.12 

Energy 84.3

+ Temp. 

Corrected 25 °C 1001.5 W/m2 9.11 Power 94.5

- 13 °C 27 °C 6.18 

kW-h/m2

/day 

7.95 

Energy 83.5

- Temp. 

Corrected 25 °C 1001.4 W/m2 8.87 Power 93.1

ACKNOWLEDGEMENTS 

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

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

REFERENCES 

[1] GR Mon, RG Ross. “Electrochemical Degradation of 

Amorphous-Silicon Photovoltaic Modules”, Proc. 18th 

IEEE PVSC, Las Vegas, NV, p. 1142 (1985). 

[2] JA del Cueto, T.J. McMahon, “Analysis of Leakage 

Currents in Photovoltaic Modules Under High Voltage 

Bias in the Field”, Progress in PV: Research & Applications 10, 15–28 (2002). 

[3] K Bucher, G Kleiss, D Batzner, “RRC Module Energy 

Rating: A Module Survey”, Proc. 26th IEEE PVSC, pp. 

1187–1191 (1997). 

[4] JA del Cueto, “Review of the Field Performance of 

One Cadmium Telluride Module”, Progress in PV: 

Research & Applications 6, 433-446 (1998). 

[5] FV Tooley (ed.) Handbook of Glass Manufacture, Vol. 

II, Section 17: Physical Properties of Glass, Ashlee 

Pub. Co., New York, NY, 3rd edn. 1984, pp. 937–947. 

4