NREL/CP-5200-57902. Presented at the 39th 

IEEE Photovoltaic Specialists Conference. 

Outdoor Performance of a Thin-Film
 

Gallium-Arsenide Photovoltaic Module
 

Timothy J Silverman∗, Michael G. Deceglie∗, Bill Marion∗, Sam Cowley†, Brendan Kayes†, Sarah Kurtz∗

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

†Alta Devices, Inc., Sunnyvale, California, 94085, United States 

Abstract—We deployed a 855 cm2 thin-film, single-junction 

gallium arsenide (GaAs) photovoltaic (PV) module outdoors. 

Due to its fundamentally different cell technology compared to 

silicon (Si), the module responds differently to outdoor conditions. 

On average during the test, the GaAs module produced more 

power when its temperature was higher. We show that its 

maximum-power temperature coefficient, while actually negative, 

is several times smaller in magnitude than that of a Si module 

used for comparison. The positive correlation of power with 

temperature in GaAs is due to temperature-correlated changes 

in the incident spectrum. We show that a simple correction based 

on precipitable water vapor (PWV) brings the photocurrent 

temperature coefficient into agreement with that measured by 

other methods and predicted by theory. The low operating 

temperature and small temperature coefficient of GaAs give it 

an energy production advantage in warm weather. 

Index Terms—gallium-arsenide, thin-film, outdoor performance 

I. BACKGROUND AND INTRODUCTION 

Thin-film photovoltaic (PV) modules have typically used 

polycrystalline or amorphous materials deposited on inexpensive substrates. In general, the thin-film approach reduces 

the semiconductor material needed and can sometimes be 

made into a flexible, lightweight module. Single-crystal, thinfilm GaAs PV modules provide a complementary thin-film 

approach and have achieved higher efficiencies than their 

polycrystalline and amorphous counterparts [1], [2] and lowcost manufacturing processes are being developed by reusing 

the single-crystal substrates [3], [4]. This approach retains the 

advantage of minimal semiconductor material usage, providing 

the possibility of a high-efficiency, light-weight, thin-film PV 

product for a variety of applications with a cost structure 

that could approach that of CIGS if the process costs can be 

minimized. 

The community has had a strong interest in integrating 

PV directly into roofing products, but experience has shown 

that PV mounted directly into a roof has elevated operating 

temperature, which can result in a reduction in power production on the order of 10%. While it is especially problematic 

for direct-roof mount applications, reduced performance with 

heating is an issue for most PV products since the irradiation 

that drives the photovoltaic conversion also creates waste 

heat. A PV technology with low operating temperature, low 

sensitivity of power to operating temperature, or both, is highly 

desirable. Amorphous silicon products are known to benefit 

from high temperatures because of annealing that reduces 

the defect concentration, but amorphous silicon products have 

usually had efficiencies < 10%. In contrast, it is expected 

that PV technologies that operate near the theoretical limit 

for efficiency should have lower operating temperature and 

smaller performance sensitivity to temperature. 

In this work, we present for the first time an analysis 

of the outdoor behavior of a terrestrial, non-concentrator 

GaAs PV module. Throughout the work, results are illustrated 

by comparison with a monocrystalline Si module. First, we 

consider theoretical and measured variation in performance 

due to changes in temperature and incident spectrum. The 

module operates near the theoretical efficiency limit, so it is 

expected to have low sensitivity to temperature. After applying 

corrections for temperature-correlated changes in spectrum 

outdoors, we show that the power temperature coefficient of 

the GaAs module power is substantially lower than that of the 

Si module. In addition, we find that the GaAs module operates at a substantially lower temperature. We show that this 

difference is caused primarily by the high operating voltage 

and large bandgap of the GaAs module. The differences in 

operating temperature and in temperature coefficient lead to a 

difference in energy production that gives the GaAs module 

an advantage in hot weather. We also describe the low light 

performance of the GaAs module, and show that, in cloudy 

conditions, it performs at or above the same efficiency as in 

high-irradiance conditions. 

II. METHOD 

The GaAs module under test was fabricated by Alta Devices 

Inc., and is similar to a module described previously [2]. Alta 

Devices modules are made of thin-film single-junction GaAs 

solar cells. These devices are grown on reusable single-crystal 

GaAs substrates by metalorganic chemical vapor deposition 

(MOCVD), and then lifted off using the epitaxial lift-off 

(ELO) process [3] to create single-crystal thin-film solar cells 

on flexible plastic substrates. To make a module, the cells 

are interconnected in an overlapping “shingled” matrix to 

minimize shading losses. The interconnected cell matrix is 

then laminated to glass using standard processes. 

This technology has achieved an NREL-certified worldrecord cell efficiency for single-junction devices under a nonconcentrated AM1.5G spectrum [5] of 28.8%, and a worldrecord module efficiency of 24.1% under the same conditions 

[1]. The thin-film design allows for fundamentally higher 

performance than is possible for wafer-based designs, due 
to improved utilization of recycled bandedge photons. The 

highest efficiency that we are aware of for wafer-based singlejunction cells under a non-concentrated AM1.5 global spectrum is 26.4% [6]. 

The GaAs module (aperture area 855 cm2) and a conventional monocrystalline silicon module (6,100 cm2 aperture 

area) were deployed outdoors on the Performance and Energy 

Rating Testbed (PERT) at the National Renewable Energy 

Laboratory (NREL) Outdoor Test Facility (OTF). PERT is 

part of an ISO 17025-accredited program. This facility is 

located at 39.7404◦N, 105.1778◦W at 1,798 m elevation. The 

modules are mounted at 40◦ tilt, facing due south. For each of 

several modules, the PERT system collects a current-voltage 

characteristic (I-V curve), back-of-module temperature and 

certain meteorological measurements every fifteen minutes. 

Irradiance is measured using two thermopile pyranometers 

and a silicon photodiode. Approximately 200 m away at the 

Solar Radiation Research Laboratory (SRRL), a GPS-based 

instrument measures precipitable water vapor (PWV) every 

fifteen minutes. 

Open-circuit voltage (Voc), short-circuit current (Isc), maximum power (Pmp) and fill factor (FF) were derived from I-V 

curve data using ASTM E1036-08 [7]. Throughout this work, 

Pmp and Isc were proportionally corrected to 1,000 W/m2 irradiance using the irradiance measurement from the thermopile 

pyranometer. Only I-V curves meeting numerous qualityassurance criteria were considered. Among these criteria are 

that the irradiance measured by the fast-responding silicon 

photodiode pyranometer changes by less than 5 W/m2 during 

the measurement. The data used in this work were collected 

from 24 July 2012 through 30 September 2012, with the 

exception of near-STC reference measurements collected on 

17 October 2012. 

Where noted, baseline performance parameters for comparing GaAs and Si performance were taken from outdoor 

I-V measurements on the PERT system on a cold clear day 

(October 17, 2012). In these cases, irradiance was between 

950 W m−2 and 1000 W m−2, and module temperature 

was less than 1.5◦C from 25◦C. The performance parameters 

from these IV curves were corrected for incident spectrum, 

irradiance, and temperature. 

A. Effects of operating temperature on performance

To determine the Isc temperature coefficient that should be 

expected from PV cells of different types, we computed the 

theoretical temperature coefficients for the two materials. This 

was done by calculating the integral of the product of the 

cell’s temperature-dependent quantum efficiency (QE) with the 

AM1.5G reference spectrum. The temperature dependence of 

the QE was approximated by shifting the band-edge region of 

the QE curve for a 1 K rise above standard test conditions 

temperature (298.15 K) according to a simple approximation 

of band gap dependence on temperature [8], [9]. 

To characterize the modules’ response to various ambient 

conditions, we first computed “apparent” temperature coefficients. A least-squares linear fit was made to Voc, Isc, Pmp, or 

FF versus temperature using data collected when irradiance 

was between (950 and 1050) W/m2. The slope of this fit, 

relative to the value of the fit at 25◦C, is the apparent 

temperature coefficient. This is not an invariant property of the 

module and is determined without accounting for changes in 

spectrum or angle of incidence, both of which have changes 

that are correlated with temperature and may have a larger 

effect than that of changes in temperature. 

To decouple spectral effects from temperature effects, we 

investigated the relationship between Isc and PWV. PWV has 

been shown to be a good proxy for spectrum in analysis 

of CdTe modules [10]. Using the same irradiance range as 

before, we selected data within ±2.5◦C of the median highirradiance module operating temperature. The result was a 

set of data taken at nearly constant irradiance and module 

temperature but with variable PWV. We then fitted a line to 

the Isc versus PWV plot and used it to correct Isc values 

to 14.2 mm PWV, the value used for the standard AM1.5 

spectrum. The resulting set of PWV-corrected Isc data retains a 

dependence on temperature that is less dependent on spectrum. 

To directly measure the dependence of the I-V parameters 

on temperature, we performed an outdoor temperature-rise test 

in clear conditions at solar noon. We covered the module 

and allowed it to reach ambient temperature, then removed 

the cover to collect I-V curves as the module temperature 

increased. When possible, we also subsequently covered the 

module, allowing it to partially return to ambient temperature, 

periodically uncovering it briefly for additional I-V curves. 

The current and power measurements were then proportionally 

corrected for minor variations in irradiance. 

B. Differences in operating temperature

To investigate differences in operating temperature, we 

calculated the heating power dissipated in the module from 

two sources: the energy from photons absorbed in the module 

which do not contribute to the operating point photocurrent 

and the excess energy of the photons that do contribute to 

photocurrent. These calculations are made for the Si module 

and the GaAs module operating under maximum power conditions for AM1.5G illumination using total spectral reflectance 

measurements made on a spectrophotometer with integrating 

sphere and measured QE. The GaAs calculation is based on 

an indoor I-V measurement at standard test condition (STC, 

1000 W/m2, AM1.5G spectrum, 25◦C). The Si calculation is 

based on an outdoor measurement taken on a cold clear day 

which was irradiance, temperature, and spectrally corrected to 

STC. 

The power dissipated in the module from non-photocurrentproducing photons, Pabs, is calculated as a function of photon 

energy, E, according to 

 Pabs(E) = Φ(E) 1 −R(E)−Qmp(E) E, (1) 

where Φ is the the photon flux, R is the measured reflectance, 

and Qmp is the external quantum efficiency (EQE) scaled such 

that the maximum power current density is given by the inte­

 gral over the AM1.5G spectrum, Jmp = qQmp(E)Φ(E)dE, 
 

where q is the elementary charge. The excess energy of 

photocurrent-generating photons also contributes to heating in 

photovoltaic modules. While any photon with energy greater 

than the bandgap can produce photocurrent, only qVmp is 

extracted from the module under maximum power operation. 

The difference between the photon energy and the energy 

extracted from the module is dissipated as heat from the 

thermalization of hot-carriers to the band edge, Joule heating, 

and entropic losses [11]. The power dissipated as heat in the 

module due to excess energy of converted photons is given by 

Pexcess(E) = Φ(E)Qmp(E) E −qVmp . (2) 

Following the method described in [12], with the parameters 

suggested therein for the baseline conditions, we used calculated values for the power dissipated in each module, along 

with the average ambient temperature during the experiment, 

to calculate predicted operating temperatures for the modules. 

C. Temperature effect on energy production

We investigated the expected difference in energy production due only to the differences in operating temperature and in 

temperature coefficient between Si and GaAs. For each technology, the module temperature rise above ambient measured 

during our experiment was fitted to a simple relationship, 

Tmod −Tamb = aGPOA +T0G, (3) 

where Tmod is module temperature, Tamb is ambient temperature, a is the slope of the temperature rise-irradiance line, GPOA 

is plane-of-array (POA) irradiance and T0G is the temperature 

rise at zero irradiance. This fit enables the prediction of the 

approximate module temperature using only POA irradiance 

and ambient temperature. 

We calculated module temperature for each hour of the year 

using POA irradiance and ambient temperature derived from 

TMY2 data for Boulder, Colorado and for Phoenix, Arizona 

in the United States. A temperature correction factor FT was 

then calculated using 

FT = 1 +γ(Tmod −25◦C), (4) 

where γ is the temperature coefficient of Pmp, independent 

of spectrum, calculated from the outdoor cover-uncover test. 

We then calculated the energy-weighted average temperature 

correction factor for the entire year, Fy

T , which indicates the 

factor by which module efficiency should be multiplied to 

correct for the effects of operating temperature alone. If Fy

T

is < 1, operating at elevated temperature has reduced the 

module’s energy-weighted average efficiency for the year, and 

thus the annual amount of energy produced, below the STC 

value. 

D. Irradiance effect on performance

The irradiance dependence of the GaAs module’s performance was investigated with the aid of the clearness index 

(CI), which is the ratio of module-plane irradiance to extraterrestrial irradiance in the same plane. This gives a metric of how 

clear or cloudy the sky conditions were during a particular 

measurement. The power measurements for this comparison 

were temperature- and irradiance-corrected. 

III. RESULTS AND DISCUSSION 

A. Effects of operating temperature on performance

The calculation of the theoretical temperature coefficient is 

illustrated in Fig. 1. The colored areas show the integrands for 

calculating photocurrent, which increases as the bandgap side 

of the QE curve shifts to lower energy with increasing temperature, due to bandgap narrowing. The theoretical temperature 

coefficients for short circuit current are shown on the plots and 

are based on the calculation at 298.15 K and 299.15 K (the 

curve at 400 K is shown for illustrative purposes). Based on 

this calculation the positive temperature coefficient of Isc for 

GaAs is expected to be more than twice as large as for Si. 

1.0 1.5 2.0 2.5 3.0 3.5

0

100

200

300

400

500

600

1.0 1.5 2.0 2.5 3.0 3.5

0

100

200

300

400

500

600

(a)

(b)

GaAs at 300 K

400 K

Isc 0.059 %/K

Si at 300 K

400 K

Isc 0.027 %/K

1.0 1.5 2.0 2.5 3.0 3.5

0

100

200

300

400

500

600

photon energy HeVL

current density HA m-2 eV-1

L

photon energy (eV)

Fig. 1. Theoretical calculation of the spectral current density for Si (a) 

and GaAs (b) cells operating at different temperatures. The gray curve is 

the standard AM1.5G spectrum. Each other curve shows the product of a 

QE curve and the spectrum. This product is the integrand for calculating 

temperature-dependent photocurrent. The calculated values for the Isc temperature coefficient, based on calculations at 298.15 K and 299.15 K, are 

given in each plot. 

Current has higher relative sensitivity to temperature in 

GaAs cells than in Si cells for two reasons. First, the bandgap 

of GaAs shifts more with increasing temperature than the 

bandgap of Si. Second, the larger bandgap of GaAs cells 

means less of the solar spectrum can be collected in the first 

place. It is also apparent from Fig. 1 that the two technologies 

respond differently to changes in spectrum. In particular, the Si 

QE overlaps with more of the near-infrared water absorption 

bands than the GaAs QE. 

The Pmp of the two modules, measured outdoors and corrected to 1,000 W/m2, is plotted against temperature in Fig. 2a. 

In this work, plots of the I-V curve parameters are normalized 

to the value of the linear fit at 25◦C. The slope of the GaAs 

fit is unexpectedly positive. Note also that although the data 

were collected on the two modules at the same time and under 

the same ambient conditions, the module temperature of the 

GaAs module is noticeably lower than that of the Si module. 

The dependence of Voc on temperature is illustrated in Fig. 3. 
The loss of Voc for a given increase in temperature is more 

than twice as large in the Si module. 

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GaAs æ 0.04%/K

Si à –0.38%/K

(a)

40 45 50 55 60

0.85

0.90

0.95

1.00

module temperature (oC)

Pmp (norm., corr.)

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GaAs æ –0.08%/K à

Si à –0.41%/K

(b)

40 45 50 55 60

0.85

0.90

0.95

1.00

module temperature (oC)

Pmp (norm., corr.)

Fig. 2. Pmp, measured between (950 and 1050) W/m2, normalized to Pmp,0 

and corrected to 1,000 W/m2 is plotted against temperature. The data in (b) 

are corrected to PWV of 14.2 mm using the relationship in Fig. 4. The linear 

fits in (b) provide a spectrum-independent Pmp temperature coefficient. 

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GaAs æ –0.14%/K

Si à –0.32%/K

40 45 50 55 60

0.86

0.88

0.90

0.92

0.94

0.96

0.98

module temperature (oC)

Voc (norm.)

Fig. 3. Voc, filtered between (950 and 1050) W/m2 and normalized to Voc,0 

is plotted against temperature. Linear fits, single-prediction 95% confidence 

bands and slopes relative to Voc at 25◦C are shown. 

The positive Pmp trend in GaAs is due to the positive 

Isc temperature dependence exceeding the small, negative Voc 

temperature dependence. The FF of the GaAs module is nearly 

independent of temperature. 

We propose that the apparent sensitivity of Isc to temperature is due to its dependence on spectral variables that are 

correlated with temperature. PWV was chosen as a simple 

scalar to represent these spectral changes. The fit shown in 

Fig. 4 was used to correct Isc and Pmp for spectral influence. 

Isc increases with increasing PWV because water absorbs 

mainly portions of the solar spectrum that are unavailable 

to the solar cell. So an increase in PWV causes a moderate 

decrease in total irradiance but only a small decrease in the 

energy available to the cell. This causes the positive slope in 

irradiance-corrected Isc with PWV. Fig. 1 shows that the small 

decrease in photocurrent due to the water-absorption bands is 

expected to be even smaller in GaAs than in Si, explaining 

the order of the slopes in Fig. 4 

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GaAs æ 0.21%/mm

Si à 0.08%/mm

5 10 15 20 25

0.97

0.98

0.99

1.00

1.01

1.02

1.03

PWV (mm)

Isc (norm., corr.)

Fig. 4. Isc collected with irradiance between (950 and 1050) W/m2, module 

temperature within ±2.5◦C of median high-irradiance operating temperature, 

corrected to 1000 W/m2 and normalized to its value at 14.2 mm PWV is 

plotted against PWV. Linear fits, single-prediction 95% confidence bands and 

slopes relative to Isc values at 14.2 mm PWV are shown. 

By applying the Isc PWV correction to Pmp we found 

the temperature coefficients to be −0.08%/K for GaAs and 

−0.41%/K for Si, as shown in Fig. 2b. Thus the Pmp temperature coefficient for GaAs is not actually positive, but it 

is substantially smaller than for Si. Because of its relative 

insensitivity to temperature, temperature-correlated spectral 

effects caused the GaAs module to perform better in conditions 

where it was also operating at higher temperature. 

Each outdoor temperature-rise test resulted in multiple I-V 

curves collected at irradiance near 1000 W/m2 and covering 

temperature ranges of ≥ 9 K. After correcting to 1,000 W/m2, 

the Isc and Pmp temperature coefficients were determined and 

are compared to the temperature coefficients determined by the 

other methods in Table I. It appears that the PWV correction 

eliminates most, but not all, of the spectral effect on the 

temperature coefficients. 

TABLE I 

Isc and Pmp temperature coefficients determined using three methods. 

method 

G

Isc (%/K) 

aAs 

Pmp (%/K) 

Si 

Isc (%/K) Pmp (%/K) 

apparent (uncorr.) 0.20 ±0.03 0.04 ±0.03 0.10 ±0.01 −0.38 ±0.01 

PWV correction 0.07 ±0.04 −0.08 ±0.05 0.05 ±0.03 −0.41 ±0.05 

uncover test 0.08 ±0.02 −0.10 ±0.02 0.03 ±0.01 −0.44 ±0.05 

calculated 0.06 0.03 

The low temperature coefficient of the GaAs modules means 

that, on warm days, its loss in performance compared to STC 

is smaller than for the Si module. An example time series 

of irradiance-corrected Pmp, normalized to Pmp,0, is shown in 

Fig. 5b. Here, Pmp,0, is taken from a corrected cold clear day 

measurement for the Si modules and an indoor measurement 

for the GaAs module The plot shows that in warm weather, 

the GaAs module’s low power temperature coefficient enables 

it to operate at higher relative efficiency for most of the day. 

At noon on the example day, the GaAs module’s normalized 

Pmp is 96% and the Si module’s is 87%. 
06:00 09:00 12:00 15:00 18:00

-5

0

5

10

15

20

25

30

Tmod–Tamb (K)

GaAs

Si

(a)

06:00 09:00 12:00 15:00 18:00

0.6

0.7

0.8

0.9

1.0

Pmp (normalized)

GaAs

Si

(b)

Fig. 5. (a) Module temperature rise above ambient and (b) irradiancecorrected Pmp, normalized to Pmp,0, are plotted versus time. The light lines 

show data from five additional Si modules from various manufacturers. The 

data were collected on 9 September 2012, a completely clear-sky day with 

an average PWV of 7.8 mm and peak ambient temperature of 34◦C. 

B. Differences in operating temperature

We observed that the GaAs module operated at a lower 

temperature than typical Si modules. The temperature rise 

above ambient is shown for a clear example day in Fig. 5a. 

Considering only high-irradiance conditions, the Si module 

deployed for the same time period as the GaAs module 

operated an average of 25.0◦C above ambient temperature 

and the GaAs module operated an average of 18.4◦C above 

ambient. Pabs(E) and Pexcess(E), calculated according to Eqs. 1 

and 2, are plotted for a Si module and the GaAs module in 

Fig. 6. For the Si module, the total power dissipated by nonphotocurrent-generating absorption is 307 W m−2, and the 

total power dissipated by excess photon energy of converted 

photons is 447 W m−2. For the GaAs module, the total 

power dissipated by non-photocurrent-generating absorption is 

340 W m−2, and the total power dissipated by excess photon 

energy of converted photons is 278 W m−2. Thus the total 

heating power dissipated in Si module is calculated to be 

753 W m−2, and the total power dissipated as heat in the 

GaAs module is calculated to be 618 W m−2. Following the 

method described in [12], we calculated predicted operating 

temperatures of 25.5 ◦C above ambient for the Si module and 

20.1 ◦C above ambient for the GaAs module. 

Based on these calculations, we attribute the reduced operating temperature of the GaAs module primarily to the 

reduction in heating from the excess energy of converted 

photons. This advantage is likely due both to the higher 

bandgap of GaAs (reducing thermalization loss) and the high 

quality of the absorber material, reducing non-radiative carrier 

recombination and enabling a high operating voltage. While 

the GaAs module has a higher reflectance than the Si module 

in the IR, its higher bandgap leads to an increase in the 

total sub-bandgap energy available to heat the module, and 

ultimately a higher overall total power dissipated by nonphotocurrent-generating absorption. 

0 1 2 3 4

0

100

200

300

400

500

600

Non-photocurrent absorption

0 1 2 3 4

0

100

200

300

400

500

600

Converted photon

excess energy

GaAs

Si

(a)

(b)

photon energy (eV)

power lost to heat (W m-2 eV-1)

Fig. 6. Sources of heat dissipation in the GaAs and Si modules under 

maximum power operation calculated as functions of incident photon energy 

according to Eqs. 1 and 2 from indoor measurements of STC performance, 

EQE, and reflectance. (a) Heating from photons which are absorbed, but do 

not contribute to photocurrent (Pabs(E)). (b) Heating from the excess energy 

of photons which do contribute to photocurrent (Pexcess(E)) 

Jan Apr Jul Oct Jan

0.8

0.9

1.0

1.1

daily avg. FT

Boulder

GaAs

Si

Jan Apr Jul Oct Jan

0.8

0.9

1.0

1.1

daily avg. FT

Phoenix

GaAs

Si

Fig. 7. Energy-weighted daily average temperature correction factor FT is 

plotted for TMY2 data in Boulder and Phoenix. 

C. Temperature effect on energy production

Fd

T , the energy-weighted daily average value for FT , is 

plotted versus time in Fig. 7. The Si module shows a larger 

difference in performance between the warm and cold seasons 

in both locations and lower overall performance in Phoenix, 

where the ambient temperature is normally much higher. The 

effects of operating temperature and temperature coefficient 

only are thus expected to result in the GaAs module producing 

(3±1)% more energy per year in Boulder and (8±2)% more 

energy per year in Phoenix. This analysis does not consider the 

modules’ different spectral responses or the different incident 

spectra in the two example locations. 

D. Irradiance effect on performance

For the GaAs module, temperature- and irradiance-corrected 

power, normalized to power at STC, is plotted against irradiance in Fig. 8. The normalized power is >90% of the one-sun 

value above 500 W/m2. At lower irradiance, the data spread 

according to clearness index: low irradiance in a cloudy sky 
(low CI) produces better performance (≥ 90% of the highirradiance performance) than low irradiance in a clear sky 

(high CI). This is because low irradiance in a clear sky occurs 

at sunrise and sunset, when the spectrum is red-rich. 

0 200 400 600 800 1000 1200

0.0

0.2

0.4

0.6

0.8

1.0

1.2

irradiance (W m–2)

Pmp (normalized)

CI: 0.0–0.2–0.4–0.6–0.8–1.0

Fig. 8. Pmp temperature- and irradiance-corrected and normalized to power 

at STC is plotted against irradiance. The color indicates the clearness index 

(CI) during each measurement. 

IV. CONCLUSION 

In the GaAs module, temperature-correlated spectral 

changes influence performance more strongly than temperature 

changes themselves. We showed that changes in spectrum 

and temperature are theoretically expected to affect different 

PV technologies to different degrees. This was borne out by 

analysis of outdoor measurements on a Si and a GaAs module. 

Under our test conditions, for the GaAs module, irradiancecorrected power output is higher when module temperature is 

higher. We showed that this is due to the GaAs module’s relatively small Voc temperature coefficient and that temperaturecorrelated spectral effects dominate the Isc-temperature relationship. PWV was used to correct Isc data for spectral changes 

and to derive spectrum-independent temperature coefficients 

that agree well with theoretical values and with a separate 

outdoor experiment. With the spectral correction, the Pmp 

temperature coefficient for the GaAs module is negative, 

but its magnitude is several times smaller than for the Si 

module. The reduced operating temperature and small Pmp 

temperature coefficient give the GaAs a performance advantage in warm weather or in adverse mounting conditions, 

such as in building-integrated applications. Considering only 

the differences in operating temperature and in temperature 

coefficient, compared to the Si module, the GaAs module is 

expected to produce annually (3±1)% more energy in Boulder 

and (8 ±2)% more energy in Phoenix when in an open-rack 

configuration. The performance of the GaAs module has a 

strong dependence on sky conditions at low irradiance, where 

for low values of CI, performance is similar to the highirradiance performance. 

The operating temperature of the GaAs module was substantially lower than that of the Si module because of a reduction 

in the heat generated by the excess energy of photocurrentgenerating photons. Further reductions in operating temperature may be achieved through improvements in infrared 

reflectivity. 

V. ACKNOWLEDGMENT 

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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