NREL is a national laboratory of the U.S. Department of Energy, Office of Energy 

Efficiency & Renewable Energy, operated by the Alliance for Sustainable Energy, LLC.

Contract No. DE-AC36-08GO28308

Transient Response of 

Cadmium Telluride Modules to 

Light Exposure

Preprint

C. Deline, J. del Cueto, and D. S. Albin

National Renewable Energy Laboratory

C. Petersen, L. Tyler, and G. TamizhMani 

Arizona State University

Presented at the 37th IEEE Photovoltaic Specialists Conference 

(PVSC 37)

Seattle, Washington

June 19-24, 2011

Conference Paper

NREL/CP-5200-50744

July 2011
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1 

TRANSIENT RESPONSE OF CADMIUM TELLURIDE MODULES 

TO LIGHT EXPOSURE

C. Deline1, J. del Cueto1, D.S. Albin1, C. Petersen2, L. Tyler2, and G. TamizhMani2

1

National Renewable Energy Laboratory, Golden, Colorado, USA 2

Arizona State University, Mesa, Arizona, USA 

ABSTRACT

Commercial cadmium telluride (CdTe) photovoltaic (PV) 

modules from three different manufacturers were 

monitored for performance changes during indoor and 

outdoor light-exposure. Short-term transients in Voc were 

recorded on some modules, with characteristic times of 

~1.1 hours. Outdoor performance data shows a similar 

drop in Voc after early morning light exposure. Preliminary 

analysis of FF changes show light-induced changes on 

multiple time scales, including a long time scale. 

Multiple methods of measuring βVoc resulted in a range of 

values on the same CdTe PV module between -0.25%/C

and -0.4%/C, possibly due to concurrent light-induced Voc

transients and temperature changes. This paper highlights 

the need for rapid performance measurement of PV 

samples following light exposure and the possibility of 

incorrect results when using outdoor light exposure to 

collect values of βVoc for CdTe modules. 

INTRODUCTION

Polycrystalline thin-film photovoltaic (PV) modules, 

including those incorporating cadmium telluride (CdTe) 

materials, are rapidly gaining market share [1]. Given the 

growth in thin-film PV installations, it is imperative that 

accurate standards and measurement methods be 

developed to allow accurate performance assessment.

Prior experiments have shown that measurement issues 

arise from the indoor testing of polycrystalline thin-film 

modules, including spectral mismatch, capacitance in

high-speed current versus voltage (I-V) measurement, and 

light exposure effects [2,3,4,5

The light exposure history of a thin-film PV module has 

been shown to affect performance characteristics on two 

time scales: short-term transients, which decay in a matter 

of hours [

]. This last measurement 

effect – light exposure – is the focus of this paper.

6,7], and longer-term effects, which may persist 

longer and may or may not be reversible [8, 9, 10, 11,12]. 

While both effects may influence performance 

measurement, the presence of short time-scale transients 

may affect the repeatability of qualification testing to e.g.

IEC 61646 [13

Experiments were conducted with CdTe PV modules from 

three different manufacturers. Both indoor and outdoor 

exposure experiments were conducted with these 

modules, particularly emphasizing short-term transients in 

measured Voc and fill factor (FF). For all experiments, 

periodic I-V curves are taken with the module maintained 

at its maximum power point voltage (Vmp) in between I-V

curves. For the indoor light/dark exposure experiment, the 

module was alternately light-exposed for 24–200 kWh and 

stored in the dark at room-temperature for 24–100 hours. 

For the outdoor exposure of the modules, the module is 

deployed outdoors in a fixed-tilt configuration.

]. For instance, the IEC 61646 qualification 

standard calls for light-soaking in 43-kW-hour increments 

yet does not specify how soon after exposure the module 

must be measured for power. Transients occurring on the 

order of minutes to hours may or may not be recorded 

during performance measurement, resulting in variability in 

the result. Additionally, the measurement of the 

temperature coefficient of Voc (βvoc) using outdoor

measurement techniques after prolonged storage in the

dark (IEC 61646) may result in light-induced open-circuit 

voltage (Voc) transients being captured along with 

temperature-related effects.

METHOD

A full description of experimental methods is provided in 

[14

Table 1 CdTe modules used for this experiment

]. A total of nine CdTe modules from three 

manufacturers were used in this experiment, with prior 

exposure details shown in Table 1. The modules were 

divided between an indoor light/dark exposure experiment

and an outdoor exposure experiment. Modules from 

manufacturer A had no prior outdoor exposure but were 

allowed to light-soak for a short period of time (~1 kWh) to 

collect initial performance and temperature coefficient 

data, according to IEC 61646. Modules from manufacturer 

B were control modules left over from certification testing 

and had some prior outdoor exposure. The module from 

manufacturer C has been continuously exposed outdoors 

for a period of more than two years.

Mfr. Indoor 

modules

Outdoor 

modules

Pre- experiment condition

CdTe A 2 2 ~1 kWh exposure

CdTe B 2 2 > 1 kWh exposure

CdTe C 0 1 Continuous exposure > 2 yrs

Initial performance characteristics and βvoc were measured 

for each of the CdTe modules. βvoc measurements were 

taken outdoors (after several days of dark/indoor storage) 

in natural sunlight for all modules as the modules warmed 

naturally. βvoc measurements were also taken using a 

Spire 4600 pulsed solar simulator as the modules cooled 

from approximately 60°C indoors. This second method 

provided more accurate βvoc measurements that were

more closely aligned with steady-state outdoor values 
2 

(>24 hours outdoor exposure) given that light-induced Voc

transients are not included with the temperature coefficient 

measurement. The results of these three temperature 

coefficient collection methods are provided in Table 2 and 

labeled as methods 61646 Outdoor, Indoor Flash, and 

Steady-State (outdoor), respectively. 

Table 2 Measured βvoc for the modules under test

Module, βvoc method βvoc measured*

CdTe A, 61646 Outdoor -0.43%/C 

CdTe A, Indoor Flash -0.21%/C 

CdTe A, Steady-State -0.24%/C 

CdTe B, 61646 Outdoor -0.39%/C 

CdTe B, Indoor Flash -0.25%/C 

CdTe B, Steady-State -0.25%/C 

CdTe C, Steady-State -0.25%/C 

*Measurement uncertainty = 0.04%/C 

An illustration of why the IEC 61646 outdoor procedure 

resulted in higher βvoc values is shown in Figure 1. This 

plot shows Voc versus temperature data for module CdTe 

A during its initial light exposure. Data are plotted for three 

different time scales: the initial two hours of exposure, a 

middle period of exposure up to 15 hours, and steadystate conditions up to 100 hours. The Voc value during 

initial light exposure has poor linear dependence on 

temperature, probably due to an initial Voc transient. The 

middle time interval, while showing good linearity with 

temperature, provided a value that is much greater (βvoc = 

-0.34%) than the steady-state value reached after 15 

hours of light exposure (βVoc = -0.24%). 

Figure 1 Module CdTe A- Voc vs. Temperature showing 

βvoc temperature coefficients changing over time. 

Measured βvoc (dVoc/dTmod) stabilizes during lightsoaking to -0.24% following 15 hours light exposure. 

Indoor light exposure of a subset of PV modules was then 

conducted in a Class C Iwasaki large-area Xenon arc 

lamp solar simulator. I-V curves were collected on a fiveminute basis during light exposure using a Daystar RD1200 multi-tracer. Between successive I-V curves, the 

module is kept loaded at its peak power point. Spectral 

mismatch effects and temporal variation of the Class C 

simulator may lead to variation in the recorded short-circuit 

current (Isc) and FF of the modules under test. Spatial and 

spectral mismatch should not affect the measurement of 

Voc reported here, however. 

Temperature measurements are recorded on the front and 

back of the module with T-type thermocouples. Module 

temperature during light exposure remained at 50°–60°C. 

The light intensity was generally constant throughout the 

exposure, but depending on the position of the module, 

the light intensity ranged from 650 W/m2

 to 1,050 W/m2. 

Light exposure of the CdTe modules is provided in 

increments ranging from 24 kWh to 200 kWh. Following 

light exposure, rather than disconnecting the module and 

removing it from the solar simulator, the module remains 

inside the solar simulator, but enshrouded to eliminate 

light exposure. The same temperature and voltage 

monitoring equipment is used during dark exposure to 

eliminate different monitoring equipment as a source of 

systematic error. The Voc and other performance 

parameters of the module are periodically measured by 

quickly removing the dark shroud, taking an I-V sweep, 

and replacing the dark shroud. In a given indoor exposure 

experiment, a module will be exposed to two cycles of light 

and dark exposure of 24–200 hours each. 

Outdoor exposure data was also collected for modules 

from each of the CdTe manufacturers. Modules of a 

similar vintage to those used for indoor light exposure 

tests are used, with similar exposure history. The outdoor 

exposure is conducted at a fixed latitude tilt under natural 

light. A Raydec Multitracer MT-5 unit is used to take I-V

curves every five minutes with the modules loaded at Vmp

in between measurements. Plane-of-array irradiance is 

collected with a Kipp & Zonen CMP-11 pyranometer, and 

backside module temperature is collected with a T-type 

thermocouple. The outdoor data considered here was 

taken following at least two weeks of outdoor exposure, 

allowing the module to achieve some level of stabilization. 

Correction of Voc data back to standard test conditions of 

25°C and 1,000 W/m2

 (STC) is important when comparing 

module performance at different exposure conditions. King 

et al. has proposed the following translation equation for 

Voc [15]: 

ܸ௢௖,଴ ൌ ܸ௢௖,௠௘௔௦ െ ߚ௖௢௏ሺܶ௖௘௟௟ െ ܶ଴ሻ െ ܰ௦

௡௞்

௤ ln ቀ ா

ாబ

ቁ (1) 

where Voc,0 is Voc at STC conditions; Voc,meas and Tcell are 

the measured Voc and cell temperature, respectively; T0 is 

the temperature at STC (25°C); and βVoc is the 

temperature coefficient of Voc, reported for the module in 

units V/C. For glass-glass module construction, the cell 

temperature is generally considered to be 3°C greater 

than module temperature at STC [13]. The second 

subtractive term in Equation 1 corrects the measured Voc

for changes in irradiance. Here, Ns is the number of series 

cells in the module, n is the diode quality factor, k is 

Boltzmann’s constant, and q is the elementary charge. For 

diode quality factor = 1, nkT/q = 26 mV. E/Eo is the 

incident irradiance divided by 1,000 W/m2

. Although 

52 54 56 58 60 62 64 66 68 70 0.62

0.63

0.64

0.65

0.66

0.67

0.68

Temperature (C)

Voc per cell (V)

CdTe A βVoc at different exposure times

βVoc = -0.34%

βVoc = -0.24%

T < 2h

2h < T < 15h

15h < T < 100h
3 

uncertainty exists in the value for diode quality factor in 

polycrystalline thin-film modules, typically reported values 

range from 1.5 to 2 [16

ANALYSIS

]. Note that error due to uncertainty 

in n goes to zero at 1,000 W/m2

. In this analysis, a 

constant value n = 1.5 is assumed for CdTe modules, and 

n = 1.2 is assumed for c-Si modules.

Indoor Voc Transients

With knowledge of the effect of temperature on module 

Voc, it is possible to plot the time series of data to show the 

transient effect of light-soaking on module Voc. 

The temperature-corrected time series for CdTe A is 

shown in Figure 3. Light exposure introduces a 

suppression of Voc that stabilizes after a few hours. 

Removing the module from light restores the initial Voc

value in a similar period of time. This phenomenon was 

repeatable and consistent across all modules tested from 

manufacturer A. A constant temperature coefficient of 

-0.24%/C ± 0.02%/C is determined for both light and dark 

exposure data. Slight adjustments in the assumed βVoc do 

not affect this Voc transient. A close-up view of the Voc

transient shows an exponential decrease with a time 

constant of τ = 1.1 hours. The total magnitude change of 

Voc is 7%–8%.

Figure 2 Module CdTe A: Temperature-corrected Voc

data showing negative voltage transient during light 

exposure with exp(-t/τ) time constant of τ = 1.1 hours. 

The second module type tested, CdTe B, does not show a

strong negative voltage transient when exposed to light. In 

fact, this module type shows a slight improvement in Voc

during light exposure on the order of 2%–3%. Again, this 

behavior was repeatable and consistent over all modules 

tested from this manufacturer. The Voc increase at the 

start of light exposure #2 is also exponential in time with a 

constant of τ = 1.1 hours. However, there is not a similar 

exponential decrease in Voc once the module is removed 

from light exposure; the decay in voltage seems to occur 

gradually over time, at a rate of less than 1% per day.

Figure 3 Module CdTe B: Temperature-corrected Voc

data showing positive voltage transient during light 

exposure.

Indoor Fill Factor Transients

In addition to changes in Voc, the impact of light exposure 

on FF was also monitored during indoor light/dark 

exposure. One complication of assessing any change in 

FF is that it has a dependence on both temperature and 

irradiance (γFF), which is difficult to know a priori [17

For almost all modules considered here, a temperature 

dependence of γFF = -0.16%/C ± 0.05%/C was determined 

by a best linear fit through FF data at constant irradiance. 

The best linearity (R2 = 0.65–0.73) was found during the 

dark exposure portions of the experiment for reasons that 

will be explained shortly. A temperature correction was 

applied to all FF data, which are presented in Figure 4 for 

a CdTe A module and in Figure 5 for a CdTe B module. 

]. 

However, for the indoor light exposure experiments 

considered here, irradiance is kept at a constant value, 

simplifying assessment of the FF coefficient γFF. 

Figure 4 Indoor exposure showing FF changes of 

module CdTe A. Temperature sensitivity analysis 

shows correlation with FF (R2 = 0.73 for γFF = 

-0.16%/C). 

0 50 100 150 200 250 300 350

0.7

0.71

0.72

0.73

0.74

0.75

0.76

0.77

0.78

Indoor light / dark exposure, Module CdTe A

Time (hours)

Voc per cell (temperature compensated)

βVoc = -0.24%

Light exposure

Dark exposure

0 20 40 60 80 100

0.7

0.72

0.74

0.76

0.78

Time (hours)

Temperature corrected Voc per cell (V)

Indoor light / dark exposure, Module CdTe B #2

βVoc = -0.26%

Light exposure

Dark exposure

0 20 40 60 80 100 62

63

64

65

66

Time (hours)

Fill Factor

Indoor exposure, Module CdTe A - Fill Factor over time

γFF = -0.16 %/C

Light exposure

Dark exposure

0 5 10 15

0.72

0.73

0.74

e-t/1.1

~1hr transient
4 

Figure 5 Indoor exposure showing FF changes of 

module CdTe B (#1). Temperature sensitivity analysis 

shows correlation with FF (R2 = 0.65 for γFF = -

0.16%/C).

The plots of FF show several interesting behaviors. First, 

an increase in FF begins with the start of light exposure, 

with module FF increasing by 4%–9% for both indoortested module types. The amount of FF increase is 

relatively unaffected by the value of γFF applied in the 

temperature correction. This increase in FF occurs 

primarily during light exposure. Because the modules had 

little light exposure prior to this experiment, it is possible 

that the FF increase is a one-time transient due to 

improved ohmic contact between the back-side contact 

(possibly incorporating copper) and the high-work-function 

p-CdTe layer [10,11]. 

The strong increase in FF during light exposure occurred 

with no correlation to temperature (R2 < 0.1). This is why 

γFF could only be determined during the dark exposure 

increment of the modules—otherwise the transient in FF

due to light exposure would outweigh any temperature 

dependence of FF. 

In addition to a strong increase in FF that occurs on a time 

scale of 10–24 hours, there is also the possibility of a 

transient decrease in FF with a time scale closer to one 

hour, identified in Figures 4 and 5. The magnitude of this 

decrease is less than 1% for module CdTe A and around 

3% for module CdTe B. However, it is difficult to determine 

any details about this transient due to the action of the 

much stronger FF increase. It is also possible that this is a 

temperature-related transient, which could be related to 

uncertainty in the value of γFF. In this experiment, 

measurements of Rs (dV/dI near Voc) showed similar 

transients to the overall plots of FF. 

One of the modules under test showed a different FF 

response to indoor light/dark exposure. Figure 6 shows 

the change in FF for the second of two CdTe B modules 

tested indoors. Rather than the transient light/dark 

behavior seen in Figures 4 and 5, this CdTe B module 

shows a constant increase in FF even when the module is 

enshrouded in the dark. In addition, the temperature 

dependence of FF is negligible for this module (R2 < 0.05)

during both light and dark exposure. One explanation for 

this different behavior is the possibility that the module 

was insufficiently shrouded during the dark exposure, and 

some light was still able to get through to the module. 

Even a small amount of light would provide a light-induced 

bias that could be responsible for a continuous increase in 

FF. This would further explain the low correlation between 

FF and temperature during dark exposure, as the small 

temperature-dependent FF change is outweighed by a 

much stronger light-induced FF improvement. 

Figure 6 Indoor exposure showing FF changes of 

module CdTe B (#2). Temperature sensitivity analysis 

shows no correlation with FF (R2 < 0.05 for any γFF). 

This module may have been insufficiently shrouded 

during dark exposure.

Outdoor Exposure Experiments

Indoor light exposure experiments show Voc transient 

behavior for different CdTe modules; the next step was to

see if this Voc transient was evident in outdoor exposed 

modules as well. (FF was not assessed due to 

complicated irradiance-dependent γFF). If a module 

experiences Voc reduction due to light exposure, one 

would expect that this behavior would show up as a 

reduction in Voc throughout the day once the sun was high 

enough to begin light-soaking the module. Voc transients 

with a time constant on the order of one hour should be 

visible within the diurnal cycle. Figure 7 shows outdoor 

exposure data for three CdTe module manufacturers and 

also a crystalline Silicon (c-Si) module for comparison (BP 

485J). Note that we have multiplied the c-Si cell voltage by 

1.32x to plot it on the same scale as the higher-cellvoltage CdTe modules. In order to account for differences 

in temperature and irradiance throughout this single, 

cloud-free day, Equation 1 is used to correct Voc data back 

to STC conditions.

As is visible in Figure 7, both modules CdTe B and the 

c-Si reference module show a relatively symmetric 

corrected Voc profile throughout the day. However, CdTe A

and CdTe C both show a noticeable droop in Voc after 

8 AM. The reduction in Voc for CdTe A is around -6% at 

noon relative to both its own peak at 8 AM and relative to 

module CdTe B at noon. This change in Voc is consistent 

both in magnitude and time scale with the indoor Voc

transients for module CdTe A shown in Figure 2. Likewise, 

0 20 40 60 80 100

60

62

64

66

68

Time (hours)

Fill Factor

Indoor exposure, Module CdTe B #1 - Fill Factor over time

γFF = -0.16 %/C

Light exposure

Dark exposure

0 20 40 60 80 100 62

62.5

63

63.5

64

64.5

65

65.5

Time (hours)

Fill Factor

Indoor exposure, Module CdTe B #2 - Fill Factor over time

γ

FF = 0 %/C

Light exposure

Dark exposure

~1hr transient
5 

module CdTe C at noon shows reduced Voc of -1.5% and 

-4% relative to its own maximum and relative to module 

CdTe B at noon, respectively. This is consistent with the 

fact that CdTe A had a much larger Voc transient during 

indoor light/dark exposure than CdTe B.

Figure 7 Outdoor deployed modules, temperature, and 

irradiance corrected by Equation 1. Note a constant 

Voc profile through the day for CdTe B and the cSi 

module, while CdTe A and CdTe C have an early 

morning reduction in Voc, consistent with lightinduced Voc degradation.

The data in Figure 7 depend in part on the assumed value 

of n = 1.5 for CdTe modules. A higher value of n in this 

case was tried, but resulted in an over-correction of the 

data, leading to a greater apparent Voc drop for module 

CdTe A.

SUMMARY

Light exposure experiments were conducted for CdTe 

modules from three different manufacturers. Transients in 

Voc have been identified that occur within the first hour of 

light-soaking, and, if unaccounted for, can cause errors by 

convolution with the measurement of a module’s βVoc

temperature coefficient. This problem is seen during the 

IEC 61646 outdoor βVoc method but not during the indoor 

flash testing βVoc method. The estimation of a module’s 

STC performance can also be affected because 

instantaneous module performance is dependent on its 

light exposure history. Evidence of a diurnal Voc transient 

has been identified in outdoor performance data from 

multiple CdTe manufacturers and is consistent in 

magnitude and time scale with those seen in indoor 

exposure experiments on similar vintage modules. 

ACKNOWLEDGEMENT

The authors would like to thank those companies who 

supplied modules for testing and TUV Rheinland PTL for 

allowing us to perform testing at their facility. This work 

was supported by the U.S. Department of Energy under 

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

Renewable Energy Laboratory.

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0.72

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