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 

Conference Paper 

NREL/CP-520-42523 

May 2008 

Performance Test of Amorphous 

Silicon Modules in Different 

Climates — Year Four: Progress 

in Understanding Exposure 

History Stabilization Effects 

Preprint 

R. Rüther and A.A. Montenegro 

LABSOLAR – Laboratório de Enegia Solar 

J. del Cueto, S. Rummel, A. Anderberg, 

and B. Von Roedern 

National Renewable Energy Laboratory 

G. Tamizh-Mani 

Arizona State University East

Presented at the 33rd IEEE Photovoltaic Specialists Conference 

San Diego, California 

May 11–16, 2008 

NREL is operated by Midwest Research Institute ● Battelle Contract No. DE-AC36-99-GO10337 
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PERFORMANCE TEST OF AMORPHOUS SILICON MODULES IN DIFFERENT 

CLIMATES – YEAR FOUR: PROGRESS IN UNDERSTANDING EXPOSURE HISTORY 

STABILIZATION EFFECTS 

R. Rüther1,2, J. del Cueto3, G. Tamizh-Mani4, A.A. Montenegro1, S. Rummel3, A. Anderberg3 & B. von Roedern3

1

LABSOLAR – Laboratório de Energia Solar 2

LabEEE – Laboratório de Eficiência Energética em Edificações, 

Universidade Federal de Santa Catarina / UFSC, Caixa Postal 476, Florianopolis - SC, 88040-900 BRAZIL 

Tel.: +55 48 3721 5174 FAX: +55 48 3721 7615 Email: ruther@mbox1.ufsc.br 3

National Renewable Energy Laboratory, Thin-Film Partnership, 1617 Cole Boulevard, Golden CO 80401-3393, USA 

Tel.: +1 303 384 6480 FAX: +1 303 384 6790 Email: Joseph_DelCueto@nrel.gov 4

Photovoltaic Testing Laboratory, Arizona State University East, 7349 East Unity Ave., Mesa AZ 85212 USA 

Tel: +1 480 727 1241 FAX: +1 480 727 1223 Email: manit@asu.edu 

ABSTRACT 

In a round robin outdoor exposure experiment carried 

out in three different climates, we have previously 

demonstrated that amorphous silicon (a-Si) PV modules 

reach higher stabilized performance levels in warmer 

climates. The four-year experiment involved three 

identical sets of thin-film a-Si modules from various 

manufacturers deployed outdoors simultaneously in three 

sites with distinct climates. Each PV module set spent a 

one-year period at each site before a final period at the 

original site where it was first deployed. The experiment 

aimed to determine the light-induced degradation and 

stabilization characteristics of a-Si regarding specific 

history of exposure, and to compare degradation rates in 

different climates. We propose that after the initial sharp 

degradation associated with the Stabler-Wronski effect 

(SWE) has passed, the subsequent stabilized 

performance levels attained will depend largely on light 

exposure and a characteristic temperature associated 

within a coherent time-scale. PV modules which were first 

deployed at the lowest-temperature site for one year, 

reaching a stabilized state, and were then further 

deployed at higher temperature sites for two more years, 

experienced considerable recovery in output parameters 

(Pmax and FF). However, when further deployed back at 

the original, lowest-temperature site, performance 

degraded back to the first year, original level. 

INTRODUCTION 

It is estimated that by 2010 the total annual 

production capacity for thin-film photovoltaic (PV) 

technologies worldwide will be more than 3.7 GWp [1]. 

There has been renewed interest in amorphous silicon (aSi) thin-film PV [2], spurred mostly by the current shortage 

of crystalline silicon (c-Si) modules on the market, due to 

the present favorable scenario for PV in many countries. 

Nearly 35% of the projected thin-film production capacity 

for 2010 is expected to be related to a-Si, amounting to 

some 1.3 GWp per year [1]. 

The reversible light-induced degradation of 

amorphous silicon (a-Si) thin-film solar cells, also known 

as the Staebler-Wronski effect (SWE [3]) results in a 

decrease in output performance after prolonged exposure 

to sunlight. The SWE leads to module performance 

reaching a stabilized state after the first year outdoors; 

most of this degradation can be reversed by thermal 

annealing at 150o

C for 2 hours in the dark. It has been 

suggested that the stabilization of the SWE does not 

occur because an equilibrium between light-induced 

degradation and thermal or light-induced annealing is 

reached, but rather that it occurs because the degradation 

phenomenon itself is self-limiting, and that once enough 

degradation has been introduced (at a given minimum 

temperature), the degradation process fades away [4,5]. 

From this model, we derive the hypothesis that a-Si PV 

module long-term power output might depend not only on 

the operating conditions, but also on the temperature 

history of light exposure, and that a-Si devices operating 

at a year-round high temperature site, with higher 

minimum operating temperatures, will stabilize at higher 

performance levels than identical modules deployed at 

sites where minimum operating temperatures are lower, 

even if at both sites modules experience the same 

maximum operating temperatures. This aspect, 

associated with the negligible temperature coefficient of 

power of stabilized a-Si PV generators [6], gives this 

material a competitive edge for applications in warm 

climates [7-9]. 

In a four-year experiment designed to assess the 

performance of a-Si devices operating at three sites with 

different climates, we have previously shown that after the 

first year of outdoor exposure, modules deployed at the 

site with the highest minimum operating temperature 

experienced the highest stabilized output level [10]. 

1
EXPERIMENTAL

As previously reported [10-12] the experimental 

design involves three identical sets of seven multi-junction 

a-Si modules commercially available from five different 

manufacturers, which were simultaneously exposed 

outdoors in three sites with distinct climates, as described 

below, in a round robin exposure experiment. The fouryear experiment aims to determine the light-induced 

degradation and stabilization characteristics of multijunction a-Si PV modules with regards to geographic 

location- and climate-specific dependent effects, specific 

history of exposure effects, and to monitor and compare 

degradation rates in different climates. The three locations 

where the round-robin took place can have their climate 

conditions described as follows: 

Site AZ: Mesa, Arizona - USA (Arizona State University 

Photovoltaic Testing Laboratory – PTL, Lat.: 33o

N); 

Climate: dry desert, with cool winters and hot summers; 

Site CO: Golden, Colorado - USA (National Renewable 

Energy Laboratory – NREL, Lat.: 40o

N); Climate: dry 

continental, with cold winters and warm summers; 

Site BR: Florianopolis, Santa Catarina - Brazil 

(Laboratorio de Energia Solar – LABSOLAR / UFSC, Lat.: 

27o

S); Climate: moist maritime, with warm winters and hot 

summers. 

An experimental procedure was designed, in which 

each PV module set would be deployed outdoors at one 

site for approximately 12 months, be shipped back to 

NREL for measurements under a SPIRE 240A simulator 

at Standard Testing Conditions (STC = 1000 W/m2

; AM 

1.5 spectrum, operating cell temperature 25o

C); be sent to 

the next (second) site in the second year; and to the 

remaining site (third) in the third year, before being sent 

back to the original site where it was first deployed 

outdoors for a final deployment period in the fourth year. 

Every permutation of sites included STC measurements 

at NREL. During outdoor deployment, modules were 

individually connected to fixed resistive loads, in order to 

operate close to their maximum power point. 

Initially, all samples from the three identical sets of 

double- and triple-junction, a-Si PV modules were 

measured for a baseline I-V trace at NREL at STC before 

shipment to each site for deployment outdoors. 

In early 2001, within a one-week time window, each 

of the three sites started deploying one of the identical 

sets of modules outdoors, at fixed latitude tilt as per each 

site, facing toward the equator, and I-V characteristics 

under real operating conditions were measured on a 

monthly basis and normalized to STC. 

RESULTS AND DISCUSSION

We follow up on results presented previously for the 

first, second, and third years of outdoor deployment [10-

12], adding our latest data in Figures 1 and 2. 

Additionally, in Figure 3 we present real-time 

measurements of the fill factors obtained from I-V curves 

on single- tandem- and triple-junction modules deployed 

outdoors in Colorado, on the Performance and Energy 

Ratings Testbed (PERT), situated at the outdoor test 

facility at NREL, from June 2000 up to May 2006, 

comprising measurements in two irradiance ranges: 1000 

±25 and 250 ±12.5 W/m2

. 

Set A - SPIRE

Analysis #1 - Pnormaliz / Pnominal

60

65

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- CO BR AZ CO

place - in chronological order of exposure

Pnormaliz / Pnominal [ % ]

Manufacturer #1

Manufacturer #2

Manufacturer #3

Manufacturer #3

Manufacturer #4

Manufacturer #4

Manufacturer #5

Set B - SPIRE

Analysis #1 - Pnormaliz / Pnominal

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- BR AZ CO BR

place - in chronological order of exposure

Pnormaliz / Pnominal [ % ]

Manufacturer #1

Manufacturer #2

Manufacturer #3

Manufacturer #3

Manufacturer #4

Manufacturer #4

Manufacturer #5

Set C - SPIRE

Analysis #1 - Pnormaliz / Pnominal

60

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125

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- AZ CO BR AZ

place - in chronological order of exposure

Pnormaliz / Pnominal [ % ]

Manufacturer #1

Manufacturer #2

Manufacturer #3

Manufacturer #3

Manufacturer #4

Manufacturer #4

Manufacturer #5

Figure 1: Evolution of the output power, normalized to nominal 

(nameplate) power, of the three sets of identical a-Si modules 

deployed outdoors in three different climates (the abscissa shows 

deployment sequence) over the four-year round-robin period. Set 

A (top pane) was first deployed in CO, Set B (middle pane) in BR, 

and Set C (bottom pane) in AZ. All measurements were carried 

out at NREL under a SPIRE 240A solar simulator at STC.

2
Figure 1 shows the evolution of the output power at 

STC, normalized to nominal (nameplate) power, for the 

three sets of a-Si modules deployed outdoors over the 

four-year round-robin period. Set A (top pane) was first 

deployed in CO, Set B (mid pane) in BR, and Set C 

(bottom pane) in AZ. In all cases, PV modules which were 

on a stabilized state at one particular site, experienced 

either partial recovery or further decline in power output, 

depending on whether the next outdoor deployment 

period was finalized in a warmer or cooler weather at the 

time the modules were removed from the test rigs and 

sent to NREL for STC measurements. 

Set A - SPIRE

Analysis #2 - Pnormaliz / Pnormaliz0

60

65

70

75

80

85

90

95

100

105

- CO BR AZ CO

place - in chronological order of exposure

Pnormaliz / Pnormaliz0 [ % ]

Manufacturer #1

Manufacturer #2

Manufacturer #3

Manufacturer #3

Manufacturer #4

Manufacturer #4

Manufacturer #5

Set B - SPIRE

Analysis #2 - Pnormaliz / Pnormaliz0

60

65

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105

- BR AZ CO BR

place - in chronological order of exposure

Pnormaliz / Pnormaliz0 [ % ]

Manufacturer #1

Manufacturer #2

Manufacturer #3

Manufacturer #3

Manufacturer #4

Manufacturer #4

Manufacturer #5

Set C - SPIRE

Analysis #2 - Pnormaliz / Pnormaliz0

60

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- AZ CO BR AZ

place - in chronological order of exposure

Pnormaliz / Pnormaliz0 [ % ]

Manufacturer #1

Manufacturer #2

Manufacturer #3

Manufacturer #3

Manufacturer #4

Manufacturer #4

Manufacturer #5

Figure 2: Evolution of the output power, normalized to original, 

unstabilized power, of the three sets of identical a-Si modules 

deployed outdoors in three different climates (the abscissa shows 

deployment sequence) over the four-year round-robin period. Set A 

(top pane) was first deployed in CO, Set B (middle pane) in BR, and 

Set C (bottom pane) in AZ. All measurements were carried out at 

NREL under a SPIRE 240A solar simulator at STC.

The recovery seen in all cases when modules were 

sent from BR to AZ (especially for Sets A and C) can be 

traced back to higher temperatures (varying from 10 to 

20o

C) at the AZ site during the season when modules 

were removed from the respective outdoor deployment 

rigs. Also noteworthy is the final decline experienced by 

Set A when going back to its original site CO. 

Figure 1 also shows that most module 

manufacturers, except manufacturer #5 in our experiment, 

underrate their product by 20 to 25% to account for the 

SWE, aiming at meeting their specification sheet 

performance at the stabilized state. For manufacturers #1 

to #4 our results demonstrate that under a +/- 10% 

tolerance, module rating will meet specifications after four 

years of outdoor exposure in most cases. However, for 

module Set A, which was originally, and in the fourth 

year, deployed in the coolest site CO, a number of 

samples reached the stabilized state at performance 

levels that would not meet specifications. Also, for Set C, 

when modules stabilized at site AZ were further deployed 

at site CO, most of them reached a new stabilized state 

with performance levels below specifications. Modules 

from manufacturer #5 showed the worst rating of all PV 

modules measured; eventhough they would all meet a +/- 

5% STC tolerance before outdoor exposure, none of the 

samples measured delivered their nameplate rating, not 

even before stabilization. 

Figure 2 shows the same data presented in Figure 

1, but normalized to original, unstabilized power, 

translating, therefore, the absolute amount of degradation 

in output experienced by all modules when deployed 

outdoors at the different site conditions. Here we see that 

modules from manufacturer #5, which in Figure 1

presented the poorest rating, turned out to be the most 

stable of all manufacturers models in terms of StaeblerWronski degradation, especially in cooler climates. This 

figure also shows that most modules degraded from 20 to 

30% during the first year of outdoor exposure, and that 

the spread in degradation levels among the different 

module types is relatively constant. 

After the final round of outdoor deployment, we have 

submitted a selected sample of our PV modules to SPIRE 

measurements at different temperatures to determine 

their stabilized temperature coefficients. Table I shows 

the temperature coefficients, along with the adjusted 

square correlation coefficient of the fit (TmAve) for each 

power parameter fit. There were two temperatures 

measured on the module, front and back, as a result of 

temperature adjustments using a backside heater for 

varying temperature while the PV module was measured 

on the SPIRE. The power parameters temperature 

coefficients are defined as Tcoeff[Parm] = (1/Parm(25o

) * 

dParm/dTm, where Tm is average of front and back 

temperature measurements. These temperature 

coefficients are considerably lower than what is typically 

seen in these a-Si PV modules before stabilization, which 

are the temperature coefficients that manufacturers print 

on their products specification sheets. These lower 

temperature coefficients also confirm our previous field 

3
results [6], showing that at the system level, stabilized aSi PV arrays are virtually inert to (warmer than STC) 

operating temperatures. 

Table I: Temperature coefficients of efficiency (Eff), open circuit 

voltage (Voc) and fill factor (FF) of a-Si modules after four years 

of outdoor deployment. 

Manufacturer Eff(%) Voc(%) FF(%) 

4

#2 (Tcoeff) -0.105 -0.316 0.120 

#2 (adjusted R square) 0.8760 0.9980 0.8793 

#3 (Tcoeff) -0.087 -0.388 0.219 

#3 (adjusted R square) 0.6601 0.9962 0.9491 

#3 (Tcoeff) -0.079 -0.363 0.186 

#3 (adjusted R square) 0.9252 0.9979 0.9825 

#4 (Tcoeff) -0.101 -0.321 0.121 

#4 (adjusted R square) 0.9277 0.9981 0.9886 

#4 (Tcoeff) -0.053 -0.294 0.128 

#4 (adjusted R square) 0.8150 0.9990 0.9878 

We propose that after the initial sharp degradation 

associated with the SWE in a-Si modules has passed, 

that subsequent stabilized performance levels attained will 

depend largely on light exposure and a characteristic 

temperature associated within a coherent time-scale. That 

stabilized level will exhibit either higher or lower values 

proportional to an exposure factor that scales as the 

product of the light intensity – i.e. exposure hours at fixed 

intensity – times the module temperature, and which 

reflects an accumulation history of this factor occurring for 

time intervals ranging 12-16 weeks prior. Figure 3 shows 

FF (%) data plotted against time for single- (1-J), tandem- 

(2-J) and triple-junction (3-J) modules. The FF data exhibit 

seasonal oscillations vs. time, with amplitudes of ~±3% for 

all the modules in both high and low intensity ranges. The 

maxima of the fluctuations in FF occur about the end of 

August and the minima occur at the end of March. The 

fact that the amplitudes in FF fluctuations are all 

comparable in both high and low irradiance ranges, 

implies that these likely do not result via degradation in 

slope of the I-V characteristics at open and closed circuit, 

but instead are likely a result of variations in diode quality. 

We support our claim that a-Si stabilization levels 

result from a mechanism reflecting the product of light 

exposure times temperature with data shown in Figure 4. 

This is a composite graph showing the cumulative integral 

of the irradiance times air temperature in the left pane, 

and the average of the product of the irradiance times air 

temperature on the right pane, both computed assuming 

four distinct summing intervals of time, 12, 16, 20 and 24 

weeks prior, plotted vs. time. Note that the fluctuations 

shown plus peaks and troughs in both panes suggest a 

close correlation with that of the FF data in Figure 3. The 

data on the left of Figure 4 depict sums of integrals over 

time and we suspect it more closely represents exposure 

than just the sums of the products on the right. The peaks 

and troughs are essentially shifted leftward by 4-6 weeks 

earlier for the integral data. Note also that the effect of 

longer summing time (weeks) intervals computed result in 

extrema that shift toward later times. We also point out 

that if instead of these products of irradiance times air 

temperature, one were just to plot the cumulative sums of 

irradiance integral (kWhr/m2

), those sums do not exhibit 

similar structure – periodic fluctuations or amplitudes – 

because irradiance sums over prior 12-24 weeks actually 

exhibit larger variations due to weather. By comparing the 

extrema in both Figures 3 and 4, we then propose that 

after the initial SWE onset has passed, that a-Si 

stabilization behaves as if it has a memory of its light and 

temperature exposure for a time span of 12-16 weeks. FF (%) single-junction

44

46

48

50

52

54

FF (%) tandem-Junction FF (%) triple-junction

53

55

57

59

61

63

65

67

6/00 12/00 6/0112/01 6/02 12/02 6/0312/03 6/0412/04 6/0512/05 6/06

53

55

57

59

61

63

65

3-J 250

3-J 1000

2-J 1000

2-J 250

1-J 250

1-J 1000

Figure 3: FF data for single- (1-J), tandem- (2-J) and triplejunction (3-J) a-Si modules, depicted, respectively, in bottom, mid 

and top panes, plotted vs. date, in two irradiance ranges: 1000 

and 250 W/m2

. 

CONCLUSIONS 

We have shown that after the initial sharp 

degradation associated with the SWE has passed, the 

subsequent stabilized performance levels attained will 

depend largely on light exposure and a characteristic 

temperature associated within a coherent time-scale. PV 

modules which were first deployed at the lowesttemperature site for one year, reaching a stabilized state, 

and were then further deployed at higher temperature 

sites for two more years, experienced considerable 

recovery in output parameters. However, when further 

deployed back at the original, lowest-temperature site, 

performance degraded back to the first year, original level. 

We have presented results that corroborate our original 

hypothesis that a site where PV modules experience 

higher operating temperatures year-round, will lead to 

higher stabilized performance levels. 
12/5/00 12/5/01 12/5/02 12/5/03 12/4/04 12/4/05

0

5

10

15

20

25

0

50

100

150

200

250

300

12/5/00 12/5/01 12/5/02 12/5/03 12/4/04 12/4/05

Week Cnt: 16

Week Cnt: 24

Week Cnt: 12

Week Cnt: 20

][ Tair Irr

weeks

Ave ⋅ ∑ )/( m2 C kW °−

{ ∑ } ∑

∫ (MW-hr-°C / m2 ) weeks days

⋅ Irr Tair dt

Figure 4: Cumulative sums over prior 12, 16, 20 and 24 weeks of: the integral of air temperature times irradiance with time in left pane, 

and product of air temperature times irradiance on the right, for Golden, CO, plotted vs. date. 

REFERENCES

[1] H.S.Ullal & B. von Roedern, „Thin-Film CIGS and 

CdTe Photovoltaic Technologies: Commercialization, 

Critical Issues, and Applications, 22nd European PV Solar 

Energy Conference, Milan - Italy, 2007, pp. 1-4. 

[2] K.Zweibel, B. von Roedern & H. Ullal, “Finally: Thinfilm PV!”, Photon International, October, 2004, pp.48-54. 

[3] D.L.Staebler & C.R.Wronski, “Reversible Conductivity 

Changes in Discharge-Produced Amorphous Si”, Appl. 

Phys. Lett. 31, 1977, pp. 292-294. 

[4] B. von Roedern & B. Kroposki, “Can the StaeblerWronski Effect Account for the Long-Term Performance of 

a-Si PV Arrays?”, 14th NREL/SNL PV Program Review 

Meeting - AIP Conf. Proc, AIP, Lakewood, 1996, pp. 313-

316. 

[5] J.del Cueto & B.von Roedern, “Temperature-Induced 

Changes in the Performance of Amorphous Silicon MultiJunction Modules in Controlled Light-Soaking”, Prog. 

Photovolt: Res. Appl. 7, 1999, pp. 101-112. 

[6] R.Rüther, H.G.Beyer, A.A.Montenegro, 

M.M.Dacoregio. I.T. Salamoni & P.Knob, “ Performance 

Results of the First Grid-Connected, Thin-Film PV 

Installation in Brazil: Temperature Behaviour and 

Performance Ratios over Six Years of Continuous 

Operation”, 19th European Photovoltaic Solar Energy 

Conference, Paris - France, 2004, pp. 1487-1490. 

[7] R.Rüther, P.Knob, H.G.Beyer, M.M.Dacoregio & 

A.A.Montenegro, “High Performance Ratios of a DoubleJunction a-Si BIPV Grid-Connected Installation After Five 

Years of Continuous Operation in Brazil”, 3rd World 

Conference on PV Energy Conversion, WCPEC3, Osaka 

- Japan, 2003, pp. 3091-3094. 

[8] R.Rüther, “Demonstrating the Superior Performance of 

Thin-Film, Amorphous Silicon for Building-Integrated PV 

Systems in Warm Climates”, International Solar Energy 

Society’s 1999 Solar World Congress, ISES, Jerusalem, 

1999, pp. 221-224. 

[9] R. Rüther & J. Livingstone, ”Seasonal variations in 

amorphous silicon solar module outputs and thin film 

characteristics”, Sol. Energy Mater. and Solar Cells 36, 

1994, pp. 29-41. 

[10] R.Rüther, G.Mani, J.del Cueto, J.Adelstein, 

A.Montenegro & B.von Roedern, “Performance Test of 

Amorphous Silicon Modules in Different Climates: Higher 

Minimum Operating Temperatures Lead to Higher 

Performance Levels”, 3rd World Conference on PV Energy 

Conversion, WCPEC3, Osaka - Japan, 2003, pp. 501-

504. 

[11] R.Rüther, G.Mani, J.del Cueto, J.Adelstein, 

A.Montenegro & B.von Roedern, “Performance Test of 

Amorphous Silicon Modules in Different Climates – Year 

Two: Higher Minimum Operating Temperatures Lead to 

Higher Performance Levels”, 19th European PV Solar 

Energy Conference, Paris - France, 2004, pp. 1617-1620. 

[12] R.Rüther, G.Mani, J.del Cueto, J.Adelstein, 

M.Dacoregio & B.von Roedern, “Performance Test of 

Amorphous Silicon Modules in Different Climates – Year 

Three: Higher Minimum Operating Temperatures Lead to 

Higher Performance Levels”, 31st IEEE Photovoltaic 

Specialists Conference, Orlando - USA, 2005, pp. 1-4. 

5
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Performance Test of Amorphous Silicon Modules in Different 

Climates - Year Four: Progress in Understanding Exposure History 

Stabilization Effects 

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R. Rüther, J. del Cueto, G. Tamizh-Mani, A.A. Montenegro; 

S. Rummel, A. Anderberg, and B. von Roedern 

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14. ABSTRACT (Maximum 200 Words)

In a round robin outdoor exposure experiment carried out in three different climates, we have previously 

demonstrated that amorphous silicon (a-Si) PV modules reach higher stabilized performance levels in warmer 

climates. The four-year experiment involved three identical sets of thin-film a-Si modules from various manufacturers 

deployed outdoors simultaneously in three sites with distinct climates. Each PV module set spent a one-year period 

at each site before a final period at the original site where it was first deployed. The experiment aimed to determine 

the light-induced degradation and stabilization characteristics of a-Si regarding specific history of exposure, and to 

compare degradation rates in different climates. We propose that after the initial sharp degradation associated with 

the Stabler-Wronski effect (SWE) has passed, the subsequent stabilized performance levels attained will depend 

largely on light exposure and a characteristic temperature associated within a coherent time-scale. PV modules 

which were first deployed at the lowest-temperature site for one year, reaching a stabilized state, and were then 

further deployed at higher temperature sites for two more years, experienced considerable recovery in output 

parameters (Pmax and FF). However, when further deployed back at the original, lowest-temperature site, 

performance degraded back to the first year, original level. 

15. SUBJECT TERMS

PV: amorphous silicon; modules; thin film; Staebler-Wronski effect; light-induced degradation; coefficients of 

efficiency; open-circuit voltage; fill factor 

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