.

National Renewable Energy Laboratory

1617 Cole Boulevard, Golden, Colorado 80401-3393 

303-275-3000 • www.nrel.gov

Operated for the U.S. Department of Energy 

Office of Energy Efficiency and Renewable Energy 

by Midwest Research Institute • Battelle 

Contract No. DE-AC36-99-GO10337 

C.R. Osterwald, J. Pruett, and T. Moriarty 

Prepared for the 31st IEEE Photovoltaics Specialists 

Conference and Exhibition 

Lake Buena Vista, Florida 

January 3–7, 2005

February 2005 • NREL/CP-520-37357 

Crystalline Silicon Short-Circuit 

Current Degradation Study: 

Initial Results 
NOTICE 

The submitted manuscript has been offered by an employee of the Midwest Research Institute (MRI), a 

contractor of the US Government under Contract No. DE-AC36-99GO10337. Accordingly, the US 

Government and MRI retain a nonexclusive royalty-free license to publish or reproduce the published 

form of this contribution, or allow others to do so, for US Government purposes. 

This report was prepared as an account of work sponsored by an agency of the United States

government. Neither the United States government nor any agency thereof, nor any of their employees, 

makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, 

completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents 

that its use would not infringe privately owned rights. Reference herein to any specific commercial 

product, process, or service by trade name, trademark, manufacturer, or otherwise does not necessarily 

constitute or imply its endorsement, recommendation, or favoring by the United States government or any 

agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect 

those of the United States government or any agency thereof. 

Available electronically at http://www.osti.gov/bridge

Available for a processing fee to U.S. Department of Energy 

and its contractors, in paper, from: 

U.S. Department of Energy

Office of Scientific and Technical Information 

P.O. Box 62

Oak Ridge, TN 37831-0062

phone: 865.576.8401 

fax: 865.576.5728 

email: mailto:reports@adonis.osti.gov

Available for sale to the public, in paper, from: 

U.S. Department of Commerce 

National Technical Information Service

5285 Port Royal Road 

Springfield, VA 22161 

phone: 800.553.6847 

fax: 703.605.6900 

email: orders@ntis.fedworld.gov

online ordering: http://www.ntis.gov/ordering.htm

Printed on paper containing at least 50% wastepaper, including 20% postconsumer waste 
CRYSTALLINE SILICON SHORT-CIRCUIT CURRENT 

DEGRADATION STUDY: INITIAL RESULTS 

C.R. Osterwald, J. Pruett, and T. Moriarty

National Renewable Energy Laboratory (NREL), Golden, CO 80401 

ABSTRACT 

Following our observation of slow degradation of 

short-circuit current (Isc) in crystalline silicon (x-Si) modules 

that was correlated with ultraviolet (UV) exposure dose, we

initiated a new study of individual x-Si cells designed to 

determine the degradation cause. In this paper, we report 

the initial results of this study, which has accumulated 

1056 MJ/m2 of UV dose from 1-sun metal-halide irradiance, equivalent to 3.8 years at our test site. At this time, 

the control samples are unchanged, the unencapsulated 

samples have lost about 2% of Isc, and the samples encapsulated in module-style packages have declined from 

1% to 3%, depending on the cell technology. 

INTRODUCTION 

Previously, we reported slow (0.25% to 0.60% per 

year) degradation of Isc in x-Si modules that were stressed 

with real-time outdoor, accelerated outdoor, and accelerated indoor exposure techniques [1-3]. The degradation 

rates were linear with the total UV radiation exposure 

doses (our outdoor test site averages 274 MJ/m2

 of total 

UV per year). Several possible causes of the slow degradation were identified: 1) obscuration or absorption in the 

glass superstrate, 2) obscuration or absorption in the ethylene vinyl acetate (EVA) encapsulant, 3) degradation of 

the antireflection (AR) coatings, or 4) degradation of the pn junctions. Isolating these possible causes in modules is 

not an easy task. Because modules contain a large number of cells connected in series, nondestructive currentvoltage (I-V) and quantum efficiency (QE) measurements 

on individual cells are quite difficult or impossible. Nondestructive measurements of the superstrate glass and encapsulate materials are also impossible. Consideration of 

all the available evidence seemed to rule out obscuration 

or absorption (the first two causes), but we were unable to 

determine if the losses were caused by changes to the AR 

coatings or to the p-n junctions. The QE data from these 

modules showed that about 75% of the Isc losses occurred 

at wavelengths longer than 800 nm, which was inconsistent with browning of EVA [4]. 

Because of the correlation with the total UV dose for 

all exposure methods, and because one indoor method 

used only UVA-340 fluorescent radiation, we hypothesized 

that UV radiation was responsible for the observed slow

degradation. Although we suspected that a simple thermal 

process was not responsible, we were not able to conclusively eliminate this cause. Therefore, in 2003, we initiated a new experiment using a sample set consisting of 

individual x-Si, cells instead of modules, that was designed 

to isolate the exact cause of the observed Isc degradation, 

if possible. If a thermal process is responsible for the degradation, cells stressed at elevated temperatures in the 

dark should degrade. On the other hand, if UV damage is 

responsible, unencapsulated samples should degrade 

faster than conventional module packaged samples, and 

cells that have UV blocking should show no degradation. 

The experiment will be run until at least ten years of 

equivalent outdoor exposure is accumulated. 

Three different types of x-Si cells were obtained from 

the same two manufacturers that produced the modules 

used in the previous study. These are: 1) cast poly-Si with 

TiO2 antireflection coating, 2) single-crystal Czochralski 

(Cz) Si with TiO2 AR coating, and 3) single-crystal Cz-Si 

with textured front surface and no AR coating. All cells 

have factory-soldered ribbons for contacts, and the cells 

were assembled into three different package types: 1) 

unencapsulated (bonded to glass substrates with silicone 

rubber), 2) AFG Solatex/EVA encapsulation, and 3) Schott 

GG 495/EVA (UV-blocking colored glass) encapsulation. 

The encapsulated samples all have conventional Tedlarpolyester-Tedlar (TPT) backsheets. There is a total of 

eight cells of each type. Six of each type are currently

being exposed to metal-halide 1-sun illumination inside an 

environmental chamber with the ambient temperature controlled to 18°C, which results in cell temperatures in the 

range 53°-63°C. No humidity is introduced into the chamber. UV irradiance is measured with an Eppley Total UV 

Radiometer (TUVR) calibrated by the NREL Solar Radiation Research Laboratory. One unencapsulated sample of 

each type is held in an oven at 55°-60°C without light, and 

three unencapsulated samples are retained as controls 

and are not stressed. The test protocol called for careful 

light I-V, dark I-V, and QE measurements of all cells initially and at each intermediate step when the exposure is 

interrupted. Light I-V measurements are made in accordance with ASTM E 948 [5]. 

RESULTS TO DATE 

At the time of this writing, the samples have accumulated a total UV dose of 1056 MJ/m2

. There have been no 

measurable changes to the open-circuit voltages (±2 mV 

1 
out of 600 mV), and one unencapsulated cell has a 10% 

loss of fill factor due to an increased series resistance. 

We note that none of the cells showed the rapid initial 

light-induced degradation of Isc due to the boron-oxygen 

metastability in p-type Si solar cells [6]. It is not known if 

the cells were light soaked by the manufacturers prior to 

delivery.

Control and oven samples

Figures 1 and 2 show the measured Isc values of the 

control and oven samples as a function of the total UV 

exposure dose, normalized to the initial baseline measurements. Although these samples were not exposed in 

the environmental chamber, they were retested at the 

same times with the exposed samples. These two plots 

demonstrate the excellent repeatability of the NREL standard cell measurement facility for Isc. Comparison of the 

oven-exposed samples with the controls indicates there 

are no detectable changes. 

Fig. 1. Normalized Isc versus total UV exposure dose for 

the unexposed control samples. 

Fig. 2. Normalized Isc versus total UV exposure dose for 

the samples held at 53°-63°C in the dark. 

Solatex/EVA-encapsulated samples

The EVA-encapsulated samples are beginning to 

show losses of Isc, which are listed in Table 1. With correlation coefficients near zero, it cannot be concluded that 

the cast-Si TiO2 samples have changed. The Cz-Si TiO2

samples, on the other hand, have linear correlation coefficients higher than 0.9; these trends are plotted in Fig. 3. 

Figure 4 shows the changes in the absolute QE for one of 

these samples (#2); note that all losses have occurred at 

wavelengths shorter than 700 nm. 

Table 1. Isc degradation rates and linear-fit correlation 

coefficients for the samples encapsulated with EVA in 

module-style packages under Solatex superstrates. 

Sample Rate 

(%/GJ/m2

) r

2

Cast-Si TiO2 AR #1 +0.15 0.024 

Cast-Si TiO2 AR #2 -0.51 0.173 

Cz-Si TiO2 AR #1 -2.41 0.908 

Cz-Si TiO2 AR #2 -2.70 0.906 

Cz-Si no AR #1 -0.98 0.532 

Cz-Si no AR #2 -1.38 0.605 

Fig. 3. Normalized Isc versus total UV exposure dose for 

the Cz-Si samples with TiO2 AR coating and encapsulated 

with EVA in module-style packages under Solatex superstrates. 

2 
Fig. 4. Absolute external QE versus wavelength and 

total UV exposure dose for Cz-Si sample #2 with TiO2 AR 

coating and encapsulated with EVA under Solatex superstrates. 

Unencapsulated samples

Table 2 lists the Isc linear degradation rates with corresponding correlation coefficients for the unencapsulated 

samples. The linear fits are illustrated in Fig. 5 for the 

cast-Si TiO2 samples. Although the degradation in Fig. 5 

is not large, it appears to be greater than the ±1% Isc

measurement repeatability, as seen in Figs. 1 and 2. The 

medium values of the correlation coefficients could indicate that UV dose is not the only cause of the degradation. 

It is interesting to note that there does not appear to be 

any differentiation between the cell types, unlike the EVAencapsulated samples. There are no clear trends in the 

QE data that might indicate specific wavelength regions in 

which the losses are occurring. 

Sample Rate 

(%/GJ/m2

) r

2

Cast-Si TiO2 AR #1 -1.16 0.428 

Cast-Si TiO2 AR #2 -1.39 0.558 

Cz-Si TiO2 AR #1 -1.11 0.672 

Cz-Si TiO2 AR #2 -1.36 0.505 

Cz-Si no AR #1 -1.93 0.849 

Cz-Si no AR #2 -0.72 0.276 

Table 2. Isc degradation rates and linear-fit correlation 

coefficients for the unencapsulated samples. 

Fig.5. Normalized Isc versus total UV exposure dose for 

the unencapsulated cast-Si samples. 

UV-blocking superstrate samples

At this point in the test program, it is apparent that the 

Schott GG495 UV-blocking superstrates are not stable 

enough for studying Isc degradation. The instability is evident in the large swings in Fig. 6, which plots the normalized Isc results for the cast-Si with TiO2 AR coating samples. Including the results for the Cz-Si samples (Fig. 7), 

the plate-to-plate instabilities differ and are large enough 

to mask any changes in cell performance. It would be 

informative to study transmittance changes on a sample of 

GG495 glass as a function of UV exposure. 

Fig. 6. Normalized Isc versus total UV exposure dose for 

the cast-Si samples encapsulated under UV-blocking 

GG495 superstrates with EVA. 

3 
Lastly, Isc losses are beginning to be detectable in the 

unencapsulated cells (Table 2). These losses differ from 

those of the Solatex/EVA samples because so far they are 

all of similar magnitude, plus they do not appear to be 

wavelength-dependent. Soiling could explain these 

losses, but the appearance of these samples is still similar 

to those of the control and oven samples. Also, a simple 

cleaning step at the end of the long-term testing should 

indicate if soiling is responsible. 

ACKNOWLEDGEMENTS

The authors thank Spire Corp. and Steve Hogan for 

performing all the laminations on the cell samples. This 

work is supported by the U.S. Department of Energy under 

Contract No. DE-AC36-99G010337. 

REFERENCES

Fig. 7. Normalized Isc versus total UV exposure dose for 

the Cz-Si samples encapsulated under UV-blocking 

GG495 superstrates with EVA. [1] C.R. Osterwald, J.P. Benner, J. Pruett, A. Anderberg, 

S. Rummel, and L. Ottoson, “Degradation in Weathered 

Crystalline-Silicon PV Modules Apparently Caused by UV 

Radiation,” Proc. 3rd World Conf. on PV Energy Conversion, Osaka, Japan, 2003, ISBN 4-9901819-3-8. 

DISCUSSION AND CONCLUSIONS

Undoubtedly the most disappointing results have to 

be with the UV-blocking superstrates. A way was needed 

to expose cells to light and at the same time prevent any

UV radiation; colored-glass filters seemed to be the only

solution. In hindsight, however, the transmittance instabilities of colored-glass filters are well known. It was hoped 

that the filters would stabilize after an initial exposure period of some length, but this has not yet occurred. Even if 

they do eventually stabilize, it would likely be impossible to 

draw any conclusions about changes in the cells. 

[2] C.R. Osterwald, A. Anderberg, S. Rummel, and L. 

Ottoson, “Degradation Analysis of Weathered CrystallineSilicon PV Modules,” Proc. 29th IEEE PV Spec. Conf., 

Piscataway, NJ, 2002, pp. 1392-1395. 

[3] C.R. Osterwald, J. Pruett, D.R. Myers, S. Rummel, A. 

Anderberg, L. Ottoson, and T. Basso, “Real-Time and 

Acclerated Solar Weathering of Commercial PV Modules,” 

Proc. 2001 NCPV Prog. Rev. Meeting, NREL, Golden, 

CO, 2001, p. 309. The results from the unencapsulated and the Solatex/EVA encapsulated samples, on the other hand, are 

interesting and trends are becoming evident. First, the 

unencapsulated samples exposed only in the oven are 

unchanged, which would indicate that a solely thermal 

process is not responsible for losses of Isc. This result is 

contrasted with the cells encapsulated under Solatex/EVA, 

which seem to be degrading linearly with respect to total 

UV dose in at least one case (Table 1 and Fig. 3). These 

Isc losses are occurring at wavelengths shorter than 

700 nm (Fig. 4), which is consistent with EVA browning [4]. 

The interesting point so far is that the degradation rates 

vary with the cell type (Table 1), which is consistent with 

the previous results on modules [1]. If EVA browning is 

the only process occurring, one would expect the rates to 

be independent of cell type. 

[4] F.J. Pern and S.H. Glick, “Photothermal Stability of 

Encapsulated Si Solar Cells and Encapsulation Materials 

Upon Accelerated Exposures,” Solar Energy Materials & 

Solar Cells, 61 (2000), pp. 153-188. 

[5] ASTM International Standard E 948, “Standard Test 

Method for Electrical Performance of Photovoltaic Cells 

Using Reference Cells Under Simulated Sunlight,” Annual 

Book of ASTM Standards, Vol. 12.02, 2004. 

[6] T. Saitoh, H. Hasigami, S. Rein, and S. Glunz, “Overview of Light Degradation Research on Crystalline Silicon 

Solar Cells,” Prog. in PV: Research and Applications, 8, 

537 (2000) 

4 
F1147-E(12/2004) 

REPORT DOCUMENTATION PAGE Form Approved 

OMB No. 0704-0188

The public reporting burden for this collection of information is estimated to average 1 hour per response, including the time for reviewing instructions, searching existing data sources, 

gathering and maintaining the data needed, and completing and reviewing the collection of information. Send comments regarding this burden estimate or any other aspect of this 

collection of information, including suggestions for reducing the burden, to Department of Defense, Executive Services and Communications Directorate (0704-0188). Respondents 

should be aware that notwithstanding any other provision of law, no person shall be subject to any penalty for failing to comply with a collection of information if it does not display a 

currently valid OMB control number. 

PLEASE DO NOT RETURN YOUR FORM TO THE ABOVE ORGANIZATION.

1. REPORT DATE (DD-MM-YYYY)

February 2005 

2. REPORT TYPE

Conference Paper 

3. DATES COVERED (From - To)

3-7 January 2005 

5a. CONTRACT NUMBER

DE-AC36-99-GO10337 

5b. GRANT NUMBER

4. TITLE AND SUBTITLE

Crystalline Silicon Short-Circuit Current Degradation Study: Initial 

Results 

5c. PROGRAM ELEMENT NUMBER

5d. PROJECT NUMBER

NREL/CP-520-37357 

5e. TASK NUMBER

PVB57101 

6. AUTHOR(S) 

C.R. Osterwald, J. Pruett, and T. Moriarty 

5f. WORK UNIT NUMBER

7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES)

 National Renewable Energy Laboratory 

 1617 Cole Blvd. 

 Golden, CO 80401-3393 

8. PERFORMING ORGANIZATION 

REPORT NUMBER

NREL/CP-520-37357 

10. SPONSOR/MONITOR'S ACRONYM(S)

NREL 

9. SPONSORING/MONITORING AGENCY NAME(S) AND ADDRESS(ES)

11. SPONSORING/MONITORING 

AGENCY REPORT NUMBER

12. DISTRIBUTION AVAILABILITY STATEMENT

National Technical Information Service 

U.S. Department of Commerce 

5285 Port Royal Road 

Springfield, VA 22161 

13. SUPPLEMENTARY NOTES

14. ABSTRACT (Maximum 200 Words)

Following our observation of slow degradation of short-circuit current (Isc) in crystalline silicon (x-Si) modules that was 

correlated with ultraviolet (UV) exposure dose, we initiated a new study of individual x-Si cells designed to determine the 

degradation cause. In this paper, we report the initial results of this study, which has accumulated 1056 MJ/m2

 of UV 

dose from 1-sun metal-halide irradiance, equivalent to 3.8 years at our test site. At this time, the control samples are 

unchanged, the unencapsulated samples have lost about 2% of Isc, and the samples encapsulated in module-style 

packages have declined from 1% to 3%, depending on the cell technology. 

15. SUBJECT TERMS

PV; crystalline silicon (x-Si); short-circuit current (Isc); module; ultraviolet (UV); metal-halide irradiance; 

16. SECURITY CLASSIFICATION OF: 19a. NAME OF RESPONSIBLE PERSON 

a. REPORT 

Unclassified 

b. ABSTRACT 

Unclassified

c. THIS PAGE 

Unclassified

17. LIMITATION 

OF ABSTRACT

UL 

18. NUMBER 

OF PAGES 

19b. TELEPHONE NUMBER (Include area code)

Standard Form 298 (Rev. 8/98) 

Prescribed by ANSI Std. Z39.18