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Metastable Changes to the Temperature Coefficients of Thin-film
 

Photovoltaic Modules
 

Michael G. Deceglie, Timothy J Silverman, Bill Marion, Sarah R. Kurtz
 

National Renewable Energy Laboratory, Golden, Colorado, 80401, United States
 

Abstract—Transient changes in the performance of thin-film 

modules with light exposure are a well-known and widely 

reported phenomenon. These changes are often the result of 

reversible metastabilities rather than irreversible changes. Here 

we consider how these metastable changes affect the temperature 

dependence of photovoltaic performance. We find that in CIGS 

modules exhibiting a metastable increase in performance with 

light exposure, the light exposure also induces an increase in 

the magnitude of the temperature coefficient. It is important 

to understand such changes when characterizing temperature 

coefficients and when analyzing the outdoor performance of 

newly installed modules. 

Index Terms—CIGS, photovoltaic, metastability, transient, 

light-soak, temperature coefficient 

I. INTRODUCTION 

Thin-film photovoltaic (PV) modules are known to exhibit 

a variety of transient and reversible changes in performance 

upon exposure to light [1]. In the case of CIGS devices, it is 

widely reported that both the fill factor (FF) and open-circuit 

voltage (Voc) increase with periods of exposure to sunlight, and 

that these changes reverse in the dark. The exact nature of such 

changes is known to depend on the particular formulation of 

the device layers. Here, we investigate how metastable changes 

to CIGS modules also affect the temperature coefficients of 

the performance parameters, i.e., the decrease in performance 

observed at elevated temperatures. 

We observe that the slope of the power vs. temperature curve 

becomes steeper after prolonged light exposure. A variety 

of physical mechanisms are generally thought to contribute 

to transient reversible changes in CIGS modules, including 

interface and bulk trap states that change state under illumination [1]–[6]. In Section III, we discuss how our results 

are consistent with light-induced interfacial barrier lowering. 

Our interpretation is based on the observation that change 

in the maximum power point (Pmp) temperature coefficient is 

driven by a change in the FF. It is also supported by a larger 

light-induced decrease in the apparent series resistance of the 

modules at low operating temperature compared to that at high 

operating temperature because electronic transport through 

or over an interfacial energy barrier is highly temperature 

dependent. 

In general, the physics dominating different CIGS cells 

and modules can cause the devices to exhibit different lightinduced transient behavior. Previous work has investigated 

the possibility of light-induced changes to temperature coefficients, but no substantial effect was observed [7]. This highlights the importance of considering each product and CIGS 

formulation separately, because different devices can have 

different dominant physics, and thus, different performance 

changes due to illumination. It is critical to understand and account for light-induced changes when measuring temperature 

coefficients on new modules and when performing analysis of 

outdoor performance data for newly installed arrays to ensure 

that the outdoor performance of the module is appropriately 

captured. 

II. METHOD 

We conducted this study with two commercially available 

CIGS modules of the same model. Prior to this study the 

modules had not been exposed to light other than for initial 

characterization on a long-pulse solar simulator. The apparatus 

for the study was a light-soak chamber that uses an array of 

metal-halide lamps to illuminate the test plane at 1000 W m−2. 

The spatial variation in irradiance in the test plane was less 

than 5% and the irradiance was controlled with closed-loop 

feedback from an intensity sensor. The temperature of the 

modules was monitored with a thermocouple on the center of 

the back surface of the modules. We estimate the temperature 

measurement to be accurate within ±2◦C. Temperature was 

controlled with forced-air cooling based on closed-loop feedModule 1

Module 2

Fig. 1. Transient improvement in the Pmp, FF, and Voc. The data are filtered 

for module temperatures between 53◦C and 57◦C to exclude the temperature 

ramps. Both FF and Voc contribute to the improvement in Pmp. 
back from the module thermocouples. For measurements taken 

in the light-soak chamber, we estimate a ±0.25% uncertainty 

in the current and a ±0.19% uncertainty in voltage. 

We mounted the modules in the chamber at room temperature. The first stage of the test was a 30-minute warmup in which the lamps were struck and allowed to stabilize. 

The lamps remained on and controlled to 1000 W m−2 for 

the remainder of the test. During the warm-up, the module 

temperature stabilized at 55◦C. The module temperature was 

then ramped at a rate of 1 K min−1 from 55◦C to 80◦C, then 

back to 55◦C. The module temperature was then maintained 

at 55◦C for 163 hours. This 163-hour phase is referred to as 

the light-soak phase for the purposes of this study. Data collection was lost for two periods during the 163-hour segment 

(apparent in Figure 1), but the illumination remained on, and 

the temperature controlled, during these intervals. After the 

163-hour light soak, the same 1 K min−1 ramp from 55◦C 

to 80◦C was performed. We collected current-voltage (I-V) 

curves every five minutes during the test. The modules were 

biased at Pmp in the intervals between I-V curves. 

To investigate the temperature coefficients of the performance parameters, namely Voc, FF, and Pmp, we considered 

I-V curves collected during the ramps from 55◦C to 80◦C at 

the beginning and end of the test. To correct for the change 

in performance due to light exposure during the temperature 

ramps, we fit a line to the time series of 55◦C data points just 

before and just after the ramp steps and use this fit to correct 

the slope of the performance parameter vs. temperature plots 

extracted from the ramp. We performed a linear fit to each of 

the performance vs. temperature data series, and normalized 

the plots to the the standard test condition parameter values 

provided on the manufacturer specification sheet. To eliminate 

noise in time-series Pmp data due to fluctuations in irradiance 

and uncertainty in the current measurement, the Pmp time series 

was linearly corrected by the ratio of the median short circuit 

current (Isc) observed during the test to the measured Isc. The 

measured Isc fluctuated less than ± 0.75%. 

To test whether the transient changes were reversible, we 

repeated the light-soak and temperature-coefficient measurement procedure after a period of dark storage, part of which 

was at elevated temperature. After 330.3 hours of total light 

exposure, the modules were stored at room temperature for 

approximately one week and then at 55◦C for 165 hours. After 

an additional four days of dark storage at room temperature, 

the light-soak and temperature-coefficient measurement process was then repeated. 

We also fit I-V curves with the single-diode model [8] to 

quantify the change in the shape of the characteristic. The fit 

was performed using a nonlinear optimization algorithm to 

first optimize the photocurrent (Iph), saturation current (Iph), 

series resistance (Rs), and diode ideality factor (n) while 

assuming a very high shunt resistance (Rsh). These preliminary 

parameters were then used as input to a second-stage fit where 

the shunt resistance was allowed to vary along with the other 

parameters. For these I-V curves, we found that this approach 

was very robust to choice of starting parameters. To make the 

fit physically relevant, the measured voltages were divided by 

the number of cells in the series-connected module. 

III. RESULTS AND DISCUSSION 

The modules used in this study exhibited a substantial 

improvement in performance with extended periods of light 

exposure. Time-series data showing the transient changes in 

Pmp, Voc, and FF at constant temperature (55◦C) are shown 

in Figure 1. This transient improvement occurred over more 

than 100 hours of continuous illumination. Both the FF and 

Voc show monotonic improvement but with different profiles 

in time. 

TABLE 1
 

Temperature coefficients for Pmp and FF measured at different stages of the
 

experiment.
 

Stage Pmp Temp. Co. (%K−1) FF Temp. Co. (%K−1) 

Initial 

Light soak 

Dark storage 

Additional light soak 

−0.42 

−0.46 

−0.44 

−0.47 

−0.12 

−0.16 

−0.14 

−0.16 

Figure 2 compares the pre- and post-light-soak temperature 

dependence of the performance parameters. The associated 

temperature coefficients are summarized in Table 1. The values 

in Table 1 are the mean from the two modules, and are 

reported as a percentage of the performance parameter at 

25◦C. For both modules, we observe a steeper slope for the 

post-light-soak normalized Pmp vs. temperature, indicating a 

change in the maximum-power temperature coefficient. This 

change corresponds to a relative increase in magnitude of 

the mean Pmp temperature coefficient of 9%. As shown in 

Figure 2, the Voc temperature coefficient does not change 

substantially upon light-soaking; the mean for the two modules 

remains relatively constant at 0.31%K−1. We observed the 

largest relative change in the FF temperature coefficient, 

which increased in magnitude by 29%. 

Further insight can be gained from our experimental results 

by considering how the light soak changes the I-V characteristic at different temperatures. I-V curves collected at different 

temperatures before and after the light-soak phase are shown 

for module 1 in Figure 3. The current and voltage values 

of the curves are normalized to the maximum and minimum 

values from the post-light-soak 55◦C curve. The single-diodemodel fit parameters for the curves from Figure 3 are shown 

in Table 2. The results in Figure 3 show that the light soak 

improves the FF more at low operating temperature than at 

high temperature. This result is also apparent in Figure 2. The 

parameters in Table 2 show that this change can be described 

by a larger light-induced decrease in series resistance at low 

temperature than at high temperature. 

Both the FF and Voc light-induced transients observed 

here have been widely observed. For a discussion regarding 

the physical origin of the changes in temperature-dependent 

performance, we focus on the change in FF because the 

relative light-induced change in temperature coefficient is 

largest in FF. One mechanism frequently cited as contributing 
Module 1

Module 2

Pre-light-soak

Post-light-soak

Module 1

Module 2

Module 1

Module 2

(a) (b) (c) 

–0.46%/K

–0.45%/K

–0.41%/K

–0.43%/K

–0.31%/K

–0.31%/K

–0.30%/K

–0.32%/K

–0.16%/K

–0.16%/K

–0.12%/K

–0.13%/K

Fig. 2. The temperature dependence of Pmp, Voc, and FF measured on both modules by ramping the module temperature before (“pre-light-soak”) and after

(“post-light-soak”) the first 163-hour light exposure. The magnitude of the FF temperature coefficient increased by 29% upon light soaking. The parameters

are normalized to the standard test condition values provided on the manufacturer specification sheet. The best fit-lines for each data set are also plotted and

their slopes are noted on the plots.

TABLE 2

Single-diode-model fit parameters for the curves shown in Figure 3

State Temp. ◦C Iph (A) Io (μA) Rs (Ω) n Rsh (Ω)

Pre-light-soak 53 2.24 0.2 0.035 1.34 6.84

Pre-light-soak 78 2.23 3.1 0.030 1.38 6.93

Post-light-soak 55 2.24 0.2 0.026 1.35 6.45

Post-light-soak 79 2.22 2.4 0.025 1.36 6.37

to the transient behavior of CIGS modules is the energy

barrier caused by the conduction-band offset between the

buffer and the absorber [2]–[4]. Such a barrier can inhibit the

collection of photo-excited carriers and thus may increase the

apparent series resistance of the module. When the module

is exposed to light, the electron and hole quasi-Fermi levels

split, which drives deep level traps to change their charge

state resulting in a reduction in the conduction band barrier.

The conduction-band barrier inhibits the collection of photogenerated electrons, which must either tunnel through the

barrier or undergo thermionic emission. When the barrier is

reduced through light exposure, the transfer of electrons is

enhanced, which improves the FF.

Our results are consistent with this microscopic picture of

transient light-induced performance changes. Charge transfer

across the conduction-band barrier, by tunneling or thermionic

emission, is enhanced at elevated temperature. Therefore, a

reduction in this barrier through light-soaking would be expected to benefit lower-temperature operation more. At lower

operating temperature, the charge transfer across the barrier is

less efficient; thus, performance benefits more from a reduction

in this barrier. This explanation is consistent with our results.

As shown in Figures 2 and 3, we observed more improvement

in the FF at low temperature than at high temperature. The fit

parameters in Table 2 show that this change can be described

by changes in the series resistance.

We found that the observed transient, including the change

in temperature coefficient, is at least partially reversible.

Storage of the module in the dark at room temperature for

approximately one week and then at 55◦C for 165 hours

resulted in a 5% decrease in Pmp from the light-soaked state,

as measured with a flash solar simulator at 25.9 ± 0.3

◦C.

Upon light-soaking, the mean temperature coefficient for Pmp

increased in magnitude from −0.44%K−1to −0.47%K−1.

Pre-light-soak 53°C

Pre-light-soak 78°C

Post-light-soak 55°C

Post-light-soak 79°C

Fig. 3. Current-voltage curves collected for module 1 at the beginning

and end of the first light exposure. The light exposure improves the lowtemperature FF more than the high-temperature FF.
This was driven by a change in the FF temperature coefficient, 

with the mean temperature coefficient of FF increasing in 

magnitude from −0.14%K−1 to −0.16%K−1, a 15% relative 

increase in the magnitude of the slope. The change in the 

temperature dependence of FF is shown in Figure 4. These 

changes, summarized in Table 1, suggest that the changes in 

temperature coefficient are reversible, but we note that these 

changes are similar to the uncertainty of the measurement. 

Module 1

Module 2

–0.17%/K

–0.16%/K

–0.14%/K

–0.14%/K

Pre-light-soak

Post-light-soak

Fig. 4. The temperature dependence of FF at the beginning and end of 

the final light-exposure. On average between the two modules, we observe a 

15% increase in the magnitude of the slope upon light exposure. The FF is 

normalized to that from the manufacture specification sheet. The best-fit lines 

for each data set are also shown and their slopes are indicated on the plot. 

IV. CONCLUSION 

Here we have considered how metastable changes in CIGS 

modules affect not only the performance, but the degree to 

which performance changes with operating temperature. In 

particular, we observed a 29% increase in the magnitude 

of the FF temperature coefficient upon light soaking, which 

results in a 9% increase in the magnitude of the the Pmp 

temperature coefficient. We suggest that this change is due 

to a light-induced reduction in the conduction-band barrier, 

which tends to benefit low-temperature performance more than 

high-temperature performance due to the thermally activated 

charge-transfer mechanisms governing forward-bias collection 

of photo-generated electrons. This suggests that the effect 

observed in this study would be sensitive to the electrically 

active interfaces in the device and would be expected to 

vary under different processing conditions. Ultimately, it is 

the final light-soaked temperature coefficient that will govern 

the photovoltaic output of an installation. Thus, it is critical 

to characterize and understand changes in the temperaturedependent performance due to light exposure when measuring 

temperature coefficients or applying temperature corrections 

to performance data from newly installed modules. 

ACKNOWLEDGMENT 

We gratefully acknowledge Steve Rummel, Allan Anderberg, and Keith Emery from NREL for I-V measurements 

that supported this work. We thank Kannan Ramanathan from 

NREL for insightful conversations. Some of the data in this 

report were obtained using equipment at the Energy Systems 

Integration Facility (a national user facility sponsored by 

the US DOE Office of Energy Efficiency and Renewable 

Energy) located at the National Renewable Energy Laboratory. 

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

under Contract No. DE-AC36-08GO28308 with the National 

Renewable Energy Laboratory. 

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