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Reliability studies of Eureka modules
Solar Cells, 30 (1991) 559--562
559
Reliability studies of Eureka modules J. Grez, J. K o l e s a r and F. Kampas Princeton, N J 08542 (U.S.A.)
(Received October 25, 1990)
Abstract Leakage current from gl~Lqs/photovoltaic(PV) thin film/ethylene vinyl acetate/glass photovoltaic modules can be reduced to acceptable levels by removing the PV thin films at the border of the module. Sandblasting is a workable method to accomplish this. However, the resulting increase in the surface conductivity of the glass in the sandblasted region increases the width of the border required.
1. I n t r o d u c t i o n Electrochemical corrosion is one of the mechanisms which reduce photovoltaic module lifetime [1 ]. It is a t roubl esom e problem for thin-film photovoltaic modules because the conductive transparent oxide and back met~lliT.ation films ext end out to the edge of the substrate u p o n which t hey are deposited, and may actually be " w r a p p e d a r o u n d " the edge. Electrical currents which flow from the thin films to a grounded frame or grounding point result is dissolution or delamination of the I~hotovoltaic thin films. E x p e r i m e n t s have shown that the transfer of 0.1 C of ionic charge p er centimeter of pe r i m e t e r from the module to the frame of a thin,film module results in large efficiency losses [2]. The qualification test designed to address this probl em is the 'T~ret Insulation--Resistance" test [3]. The rationale behind this test is described in Sugimura et al. [1 ]. It requires that the resistance between each edge of the module and a surfactant solution be over 100 M~, when m e a s u r e d at a voltage o f 500 V d.c. between the solution and the leads of the module. Also, the resistance should be over 100 M ~ when the surface of the module, including electrical terminations, has been w et t ed by spraying on the surfactant solution. This test is p e r f o r m e d on modules before and after two different accelerated aging s e que nc e s which consists of either 200 thermal cycles or 50 thermal cycles plus 20 humidity/freeze cycles.
2. R e s u l t s It is Chronar's experience that humidity/freeze cycling substantially increases the leakage current observed in the Wet Insulation Test and the Elsevier Sequoia/Printed in The Netherlands
560
related Wet High-Potential Test. The combination of elevated temperature (85 °C) and elevated humidity (85%) results in saturation of the module border. Presumably the elevated t em pe rat ure increases the diffusion rate of water into the module. An isolating interior bor de r region which contains no conductive films is used in Chronar's Eureka module to reduce the magnitude of leakage current (see Fig. 1). This bor de r is p r o d u c e d by sandblasting away the photovoltaic thin films. Calculations were p e r f o r m e d to estimate the width of the border isolation region required to pass the Wet Insulation--Resistance and the Wet HighPotential tests, using data on the conductivity of ethylene vinyl acetate (EVA) and EVA-soda lime glass interfaces from the JPL group [2]. At 25 °C and 100% relative humidity, they give the bulk conductivity of EVA as being about 10 -13 (12cm) -1 and the interface conductivity of EVA-soda lime glass as 10-12~ -1. The second n u m b e r is actually the surface conductivity of the soda lime glass, which dominates the interface conductivity. It is clear from the two numbers that the interface conductivity dominates for EVA thickness less than 1 cm. Therefore the interface resistivity should be 106M~K:]. The Wet Insulation-Resistance test requires a resistance of over 100 M~, whereas the Wet High-Potential test requires a current of less than 50 /zmA at a voltage of 1000 V plus twice the system voltage. Since the system voltage for a string of ten Eureka modules in series is about 450 V, the Wet High-Potential test is pe r f or m e d at 1800 V and requires a resistance of over 36 MI2. We have found that the conduction is non-ohmic over 500 V, and that the two tests are approximately equivalent. Since the surface resistivity of the EVA-soda lime glass at 25 °C and 100% relative humidity is given as 106 M ~ / ~ and a resistance of over 100 M~ is required, the isolated bor de r must have a width of more than 10 -4 times its length. For the 5 ft length of a Eureka module, this is only 0.006 in. Measurements on Eureka modules after humidity/freeze cycling, however, have demonstrated that a b o r d e r width of approximately ½ in is required to pass the two tests reliably. In order to understand the discrepancy between
SUPERSTRATE I
II I I I BUSBAR
I IJ
I [I III I JI PV THIN FILMS
I ij
III
EVA
SUBSTRATE Fig. 1. Cross-sectionalview of the border of a long side of a Eureka Module. Vertical dimensions not to scale.
561 t h e o r y and experiment, 4 in square test samples were p r e p a r e d which consisted of glass/aluminum foil/EVA/glass. The aluminum foil was 3½ in square, giving an isolated b o r d e r ¼ in wide. These borders were sandblasted in half of the samples, as it was believed that sandblasting might increase the surface conductivity of moisture saturated soda lime glass. The results of the e x p e r i m e n t confirmed the supposition. Before humidity/ freeze cycling, all the samples passed currents in the nei ghborhood of 0.1 pA at 1800 V. The leakage currents of the samples with sandblasted borders were about a factor of 2 higher than the currents from the plain glass borders. After 10 humidity/freeze cycles, the currents from the samples with plain glass b o r d er s were several tenths of a microampere. Currents from the samples with sandblasted bor de r s were over 50 pA, failing the test. These results are summarized in Table 1. The r eas o n for the ver y different p e r f o r m a n c e of the sandblasted b o r d e r after humidity/freeze cycling can be u n d e r s t o o d in t erm s of the chemical reactions that o c c u r on a wetted soda lime glass surface [4]. The glass reacts with the water via monovalent cation exchange, producing a dilute sodium hydroxide solution, which is highly conductive. This reaction is diffusion controlled, so that the increase in surface area caused by sandblasting increases the rate of the reaction. Also, the sandblasting removes the surface of the glass, which is typically depleted of sodium oxide. It is also quite possible that the diffusion rate of water along the glass/EVA interface is higher for the sandblasted surface. Another c o n s e q u e n c e of the chemical reactions which o c c u r on a wetted glass surface is the fact that the surface conductivity is time dependent. This may explain why the surface conductivity of soda lime glass given by the JPL group [6] is different from data obtained from Coming Glass varying f r o m 10 -3 to 10 -12 12-1 as the relative humidity changes from 0 to 100%, at r o o m temperature. The C om i ng data indicates that the surface conductivity is ar o u n d 10 -12 12-1 for relative humidities bet w een 0 and 50%. Above 50% relative humidity, the conductivity increases rapidly, reaching 10 - s 12-1 at 90% relative humidity. The fact that a ¼ in b o r d e r isolation width is not wide enough for a 4 in x 4 in test sample to pass the Wet High-Potential test after humidity/ freeze cycling was originally a source of concern. However, experiments TABLE 1 Average leakage currents at 1800 V from 4 in×4 in samples Current (pA)
Plain borders Sandblasted borders
aHumidity/freeze.
Initial
10 H/F" cycles
0.07 0.12
0.14 > 50
562 h a v e s h o w n t h a t t h e 5 ft long side of a E u r e k a m o d u l e h a s a l e a k a g e c u r r e n t o f a r o u n d 4 ~ m A b e f o r e h u m i d i t y / f r e e z e cycling, a n d less t h a n t w i c e t h a t a f t e r h u m i d i t y / f r e e z e cycling. A p p a r e n t l y , the entire ½ in b o r d e r d o e s n o t b e c o m e w a t e r s a t u r a t e d , e v e n a f t e r 20 h u m i d i t y / f r e e z e cycles.
3. C o n c l u s i o n s An interior s a n d b l a s t e d isolation r e g i o n r e d u c e s l e a k a g e c u r r e n t s in E u r e k a M o d u l e s to a c c e p t a b l e levels. H o w e v e r , the d a m a g e d o n e t o the glass s u r f a c e r e q u i r e s t h a t t h e i s o l a t e d r e g i o n h a v e a w i d t h o f ½ in. Masking, or less d a m a g i n g m e t h o d s o f thin film r e m o v a l , w o u l d like r e q u i r e a t h i n n e r i s o l a t e d r e g i o n a n d give f u r t h e r r e d u c t i o n in l e a k a g e c u r r e n t s values.
References 1 R. S. Sugimura, G. R. Mon, L. Wen and R. G. Ross, Jr., Proc. 20th IEEE Photovoltaic Specialists' Conf., Las Vegas, NV, September 26--30, 1988. 2 G. R. Mon, L. C. Wen, R. S. Sugimura and R. G. Ross, Jr., Proc. lPthElectvical/Electrenics Insulation Confl., Chicago, IL, September 25-28, 1989. 3 R. DeBlasio, L. Mrig and D. Waddington, Interim Qualification Tests and Procedures f o r Terrestrial Photovoltaic Thin-Film Flat-Plate Modules, SERI/TR-213o3624, 1990. 4 R. Doremus, Glass Science, John V~rfley& Sons, New York, 1973, p. 225.
559
Reliability studies of Eureka modules J. Grez, J. K o l e s a r and F. Kampas Princeton, N J 08542 (U.S.A.)
(Received October 25, 1990)
Abstract Leakage current from gl~Lqs/photovoltaic(PV) thin film/ethylene vinyl acetate/glass photovoltaic modules can be reduced to acceptable levels by removing the PV thin films at the border of the module. Sandblasting is a workable method to accomplish this. However, the resulting increase in the surface conductivity of the glass in the sandblasted region increases the width of the border required.
1. I n t r o d u c t i o n Electrochemical corrosion is one of the mechanisms which reduce photovoltaic module lifetime [1 ]. It is a t roubl esom e problem for thin-film photovoltaic modules because the conductive transparent oxide and back met~lliT.ation films ext end out to the edge of the substrate u p o n which t hey are deposited, and may actually be " w r a p p e d a r o u n d " the edge. Electrical currents which flow from the thin films to a grounded frame or grounding point result is dissolution or delamination of the I~hotovoltaic thin films. E x p e r i m e n t s have shown that the transfer of 0.1 C of ionic charge p er centimeter of pe r i m e t e r from the module to the frame of a thin,film module results in large efficiency losses [2]. The qualification test designed to address this probl em is the 'T~ret Insulation--Resistance" test [3]. The rationale behind this test is described in Sugimura et al. [1 ]. It requires that the resistance between each edge of the module and a surfactant solution be over 100 M~, when m e a s u r e d at a voltage o f 500 V d.c. between the solution and the leads of the module. Also, the resistance should be over 100 M ~ when the surface of the module, including electrical terminations, has been w et t ed by spraying on the surfactant solution. This test is p e r f o r m e d on modules before and after two different accelerated aging s e que nc e s which consists of either 200 thermal cycles or 50 thermal cycles plus 20 humidity/freeze cycles.
2. R e s u l t s It is Chronar's experience that humidity/freeze cycling substantially increases the leakage current observed in the Wet Insulation Test and the Elsevier Sequoia/Printed in The Netherlands
560
related Wet High-Potential Test. The combination of elevated temperature (85 °C) and elevated humidity (85%) results in saturation of the module border. Presumably the elevated t em pe rat ure increases the diffusion rate of water into the module. An isolating interior bor de r region which contains no conductive films is used in Chronar's Eureka module to reduce the magnitude of leakage current (see Fig. 1). This bor de r is p r o d u c e d by sandblasting away the photovoltaic thin films. Calculations were p e r f o r m e d to estimate the width of the border isolation region required to pass the Wet Insulation--Resistance and the Wet HighPotential tests, using data on the conductivity of ethylene vinyl acetate (EVA) and EVA-soda lime glass interfaces from the JPL group [2]. At 25 °C and 100% relative humidity, they give the bulk conductivity of EVA as being about 10 -13 (12cm) -1 and the interface conductivity of EVA-soda lime glass as 10-12~ -1. The second n u m b e r is actually the surface conductivity of the soda lime glass, which dominates the interface conductivity. It is clear from the two numbers that the interface conductivity dominates for EVA thickness less than 1 cm. Therefore the interface resistivity should be 106M~K:]. The Wet Insulation-Resistance test requires a resistance of over 100 M~, whereas the Wet High-Potential test requires a current of less than 50 /zmA at a voltage of 1000 V plus twice the system voltage. Since the system voltage for a string of ten Eureka modules in series is about 450 V, the Wet High-Potential test is pe r f or m e d at 1800 V and requires a resistance of over 36 MI2. We have found that the conduction is non-ohmic over 500 V, and that the two tests are approximately equivalent. Since the surface resistivity of the EVA-soda lime glass at 25 °C and 100% relative humidity is given as 106 M ~ / ~ and a resistance of over 100 M~ is required, the isolated bor de r must have a width of more than 10 -4 times its length. For the 5 ft length of a Eureka module, this is only 0.006 in. Measurements on Eureka modules after humidity/freeze cycling, however, have demonstrated that a b o r d e r width of approximately ½ in is required to pass the two tests reliably. In order to understand the discrepancy between
SUPERSTRATE I
II I I I BUSBAR
I IJ
I [I III I JI PV THIN FILMS
I ij
III
EVA
SUBSTRATE Fig. 1. Cross-sectionalview of the border of a long side of a Eureka Module. Vertical dimensions not to scale.
561 t h e o r y and experiment, 4 in square test samples were p r e p a r e d which consisted of glass/aluminum foil/EVA/glass. The aluminum foil was 3½ in square, giving an isolated b o r d e r ¼ in wide. These borders were sandblasted in half of the samples, as it was believed that sandblasting might increase the surface conductivity of moisture saturated soda lime glass. The results of the e x p e r i m e n t confirmed the supposition. Before humidity/ freeze cycling, all the samples passed currents in the nei ghborhood of 0.1 pA at 1800 V. The leakage currents of the samples with sandblasted borders were about a factor of 2 higher than the currents from the plain glass borders. After 10 humidity/freeze cycles, the currents from the samples with plain glass b o r d er s were several tenths of a microampere. Currents from the samples with sandblasted bor de r s were over 50 pA, failing the test. These results are summarized in Table 1. The r eas o n for the ver y different p e r f o r m a n c e of the sandblasted b o r d e r after humidity/freeze cycling can be u n d e r s t o o d in t erm s of the chemical reactions that o c c u r on a wetted soda lime glass surface [4]. The glass reacts with the water via monovalent cation exchange, producing a dilute sodium hydroxide solution, which is highly conductive. This reaction is diffusion controlled, so that the increase in surface area caused by sandblasting increases the rate of the reaction. Also, the sandblasting removes the surface of the glass, which is typically depleted of sodium oxide. It is also quite possible that the diffusion rate of water along the glass/EVA interface is higher for the sandblasted surface. Another c o n s e q u e n c e of the chemical reactions which o c c u r on a wetted glass surface is the fact that the surface conductivity is time dependent. This may explain why the surface conductivity of soda lime glass given by the JPL group [6] is different from data obtained from Coming Glass varying f r o m 10 -3 to 10 -12 12-1 as the relative humidity changes from 0 to 100%, at r o o m temperature. The C om i ng data indicates that the surface conductivity is ar o u n d 10 -12 12-1 for relative humidities bet w een 0 and 50%. Above 50% relative humidity, the conductivity increases rapidly, reaching 10 - s 12-1 at 90% relative humidity. The fact that a ¼ in b o r d e r isolation width is not wide enough for a 4 in x 4 in test sample to pass the Wet High-Potential test after humidity/ freeze cycling was originally a source of concern. However, experiments TABLE 1 Average leakage currents at 1800 V from 4 in×4 in samples Current (pA)
Plain borders Sandblasted borders
aHumidity/freeze.
Initial
10 H/F" cycles
0.07 0.12
0.14 > 50
562 h a v e s h o w n t h a t t h e 5 ft long side of a E u r e k a m o d u l e h a s a l e a k a g e c u r r e n t o f a r o u n d 4 ~ m A b e f o r e h u m i d i t y / f r e e z e cycling, a n d less t h a n t w i c e t h a t a f t e r h u m i d i t y / f r e e z e cycling. A p p a r e n t l y , the entire ½ in b o r d e r d o e s n o t b e c o m e w a t e r s a t u r a t e d , e v e n a f t e r 20 h u m i d i t y / f r e e z e cycles.
3. C o n c l u s i o n s An interior s a n d b l a s t e d isolation r e g i o n r e d u c e s l e a k a g e c u r r e n t s in E u r e k a M o d u l e s to a c c e p t a b l e levels. H o w e v e r , the d a m a g e d o n e t o the glass s u r f a c e r e q u i r e s t h a t t h e i s o l a t e d r e g i o n h a v e a w i d t h o f ½ in. Masking, or less d a m a g i n g m e t h o d s o f thin film r e m o v a l , w o u l d like r e q u i r e a t h i n n e r i s o l a t e d r e g i o n a n d give f u r t h e r r e d u c t i o n in l e a k a g e c u r r e n t s values.
References 1 R. S. Sugimura, G. R. Mon, L. Wen and R. G. Ross, Jr., Proc. 20th IEEE Photovoltaic Specialists' Conf., Las Vegas, NV, September 26--30, 1988. 2 G. R. Mon, L. C. Wen, R. S. Sugimura and R. G. Ross, Jr., Proc. lPthElectvical/Electrenics Insulation Confl., Chicago, IL, September 25-28, 1989. 3 R. DeBlasio, L. Mrig and D. Waddington, Interim Qualification Tests and Procedures f o r Terrestrial Photovoltaic Thin-Film Flat-Plate Modules, SERI/TR-213o3624, 1990. 4 R. Doremus, Glass Science, John V~rfley& Sons, New York, 1973, p. 225.