Solar Cells, 28 (1990) 121 - 128 121

THIN FILM MODULE FABRICATION AND RELIABILITY ADVANCES

MADELEINE JOHNSON

Chronar Corporation, P.O. Box 1 77, Princeton, NJ 08542 (U.S.A.)

Summary

This paper presents the Chronar experience of deploying 8morphous silicon (a-Si) modules in the field and the resulting efforts to understand and correct observed degradation mechanisms. The data were obtained from several on-site observations, and evaluation and analysis of corrosion effects in a-Si modules. These data are correlated with laboratory tests, and a design for a new a-Si module is proposed.

1. Introduction

One of the most important issues in successful commercialization of thin film amorphous silicon (a-Si) modules is the development of a safe, reliable, and inexpensive encapsulation system. In the past four years, Chronar has combined field experience with laboratory testing to determine the exact requirements for the successful development of such a system.

Corrosion mechanisms on early Chronar modules have been observed and correlated with accelerated laboratory tests that induce the same type of corrosion mechanisms. This development work has resulted in a new design that effectively prevents the corrosion mechanisms as shown by various laboratory tests.

2. Field test data

One of the largest thin film a-Si photovoltaic installations is located at the Alabama Power Company (APCO) test site in Varnons, AL. This 75 kW system was installed in May 1986, and is comprised of 11 264 a.Si modules (Fig. 1). These modules were manufactured by Chronar Corporation at its first 1 MW batch plant, in Port Jervis, NY.

The early observations and performance tests at the APCO test site indicated that electrical performance in the first 2 years had degraded in ex- cess of that which could be readily explained by the Staebler-Wronski [1] Effect (Fig. 2).

0379-6787/90/$3.50 © Elsevier Sequoia/Printed in The Netherlands

122

Field description

Location System Size Operating Voltage Operating Current Number of Modules Performance

Varnons, A L 75 kW design 300 V per string, 14 V per module 32 A per string, 1 A per module 11264 54 kW dc from April 1989

Fig. 1. Alabama Power Company test site data.

ALABAMA POWER PV DEMONSTRATION PROJECT 80

e0

ii 111 MII llHlli J6S6S6 J F M A M J J A S O O N D J 8 F M A M J J A S O N D J 9 F M A

Performance by Month ( Slart 6/19/1986 )

Fig. 2. APCO system performance (d.c.).

The measurements made at the test site in September, 1986 [2] , and October, 1987 [3] showed an 18% decrease in the power output. This de- crease has been largely attributed to corrosion effects in the intercell and the cell-to-frame regions.

Modules have been observed to exhibit "arcing", which occurs when the insulation fails in the cell-to-frame area. The current from the array or the module is then channeled to the failure site, causing arcing or voltage breakdown. This arcing is so intense that it is always accompanied by smoke and flames, which result in glass breakage and charring of the module (Fig. 3).

Fig. 3. Module removed from field.

123

A module that was observed to arc was taken Out of the system at the APCO site and examined for obvious defects. Several small particles were found between the insulation and the aluminum frame and some of these particles were submit ted to Auger analysis. In addition to the expected elements such as carbon, oxygen, and sulfur {rubber gasket by-products of burning), these particles also contained silver, zinc, and aluminum.

The origin of the silver is at tr ibuted to the screen-printed silver bus which runs lengthwise along the module edge, next to the insulating gasket and the frame. It is suspected that the origin of the aluminum is the depos- ited aluminum film of the a-Si cells. The presence of zinc in these particles is less readily explained, and it is suspected to be a componen t of the paint on the aluminum frame.

Work performed at Je t Propulsion Laboratory (JPL) [4, 5] showed that this failure can be readily explained by the electrochemical corrosion of module cell conductor elements and subsequent migration of dissolved metal ions to the frame. The resolution of this failure mechanism has been deter- mined to be one of the most important aspects relevant to long-term power generation.

The electrochemical corrosion is at t r ibuted to the continual leakage current that occurs between the module electrical parts and the frame. The leakage current is composed of charge carriers (such as metallic ions) that move under the influence of voltage reacting with the insulation and the cell frame electrodes to produce corrosion products [6]. Both the temperature and the humidi ty of the surrounding environment influence the rate at which this mass transfer occurs. The arcing is a spectacular manifestation of these leakage currents and results in catastrophic failure of the modules. However, the cont inuous electrochemical corrosion of the module also causes a steady decrease in power output , which does not always result in catastrophic failure [6].

Correlation of the electrochemical corrosion observed on Chronar's a-Si modules in the field with suitable laboratory tests that will induce similar corrosion patterns has proved to be challenging.

3. Correlation of laboratory tests

The standard humidi ty- f reeze exposure test can induce corrosion and electrical degradation when a module is not adequately encapsulated. How- ever, it has been shown that a module which demonstrates adequate encap- sulation by passing the humidi ty- f reeze exposure test may have leakage cu:rents in excess of the allowable values [7] when tested in a " w e t " con- dition.

It is also noted that a dry module can satisfactorily pass the leakage current test, bu t that a module with a wet surface will be more representa- tive of the real field conditions. In an exposed ou tdoor environment, the module will come in contact with liquid water (dew, rain, snow) and, de-

124

pending on the encapsulation system used, moisture sorption will increase with time and the moisture will tend to be retained by the module [8]. The retained moisture contributes to an increase in current leakage and, hence, an increase in electrochemical corrosion. Such a scenario is readily observed at the APCO test site.

Modules representative of those installed at the APCO test site have been tested in the laboratory. The module construction consists of a vinyl- encapsulated back, an edge gasket, an aluminum frame, and an AMP con- nector. The internal bus is silver, and is posit ioned along the positive and negative edges of the module. This construction does not pass the humidi ty- freeze test. A later design, which uses glass/glass encapsulation and which does pass the humidi ty- f reeze test, has been developed at Chronar [9].

The newest module design involves an edge seal that provides an effec- tive moisture barrier and passes both the humidi ty- f reeze test and an elec- trical resistance test [4] developed by JPL specifically for thin film modules. A p ro to type of this module has been tested, together with modules repre- sentative of those at the APCO test site, and the standard glass/glass module. The test is adapted from a submersion test shown by JPL to induce electro- chemical corrosion [ 10].

A diagram of the test as adapted by Chronar is shown in Fig. 4. A diagram of the module constructions that underwent the test is shown in Fig. 5. The special edge t reatment consists of polyisobutylene/urethane or polyisobutylene/si l icone in various application configurations.

The results of the test are encouraging in that corrosion patterns simi- lar to the field observed corrosion occurred on the module representative of those at the APCO test site. Several of the modules made with the special edge treatments did not show evidence of corrosion, and did not have exces- sive leakage currents during the test.

At the complet ion of the test, a set of design criteria for the new module were outlined:

(1) plastic frame; (2) two-component edge seal system; (3) sealed connector; (4) isolated cell string;

20 K'-2

+o 250 V

WATER~ PLATE

Fig. 4. Schematic disl~'~'n of test; the modules are pL~;ially submffcg~l in water at poten- tial charge of 250 V applied for 500 h.

125

~'~,'i ! ~ 1 1 Aluminum Frame "i~i'i'i'G I'as s i"!:!'i"!'!" Rubber Gasket

a-Si Cells / \Vinyl

(a)

~i'~iiiiiii" G ! a ! s i-i',i,i,i,i-i,i,iit, a. s i Ceils //~,.~"/'y;~.'/"/"~;.',~'.~//~/~.~,,~.i~,~ UV Cured Resin

I

(b) / Silicone or Urethanc ,';';';~'~;~'~la~ : " " " " % ; ~,,,,,.,,-,-, , , , , , , , , , , , , , , ~ p,~

• . . . . . . . . . . . . . . . . . . . . . . - . - . . . . .

(c) Fig. 5. Module construction description: (a) type A modules, similar to the APCO site modules; (b ) type B modules, glmm/glass encapsulated with no edge seal; ( c ) type C modules, glass/glass encapsulated with a special edge seal.

(5) capable of maximum system voltage of 600 V; (6) capable of passing all Solar Energy Research Institute (SERI) quali-

fication specifications (DRAFT) [ 11].

4. Description of module design

The module design that was finally chosen as having the most potential for resolution of known failure mechanisms incorporates existing techniques and several new ones. The module is fabricated from 1 ft X 3 ft glass sub- strate that has been processed through a-Si deposition. This segment is border ground, ultrasonically bonded with aluminum foil, glass/glass encap- sulated, cleaned, and then framed in a plastic frame.

The process of applying the plastic frame includes the injection of a two-component sealing system into the frame, which is then wrapped around the module. The connector is applied with two contacts that have had the aluminum foil welded ill place. The connector is potted, a cover is placed on the connector, and the entire system is allowed to cure for several days (Fig. 6).

5. Semi.automated module fabrication

Equipment has been specifically developed to fabricate the a-Si modules in the new design. This semi-automated line has been installed to date in two factories, and will begin manufacture of the new modules in early July. The stages of the semi-automated line are shown in Figs. 7 - 12.

126

P l a s t i c f r a m e

Deta i l x

Fig. 6. New module design.

Fig. 7. Border grinding - - removal of conductive tin oxide.

Fig. 8. Ultrasonic bonding -- application of aluminum foil.

Fig. 9. Glass/glass encapsulation -- application of resin and back glass, and UV cure.

Fig. 10. Framing e q u i p m e n t - notching of plastic frame, injection of sealants, and placing of plastic frame on module.

6. Test objective

Field dep loyment of the new module in a high-voltage system will clearly demonstrate the level of performance that may be expected. The

127

Fig. 11. F r a m i n g e q u i p m e n t - - app l i ca t ion o f p las t ic f l a m e to modu le .

Fig. 12. Spo t we ld ing - - app l i ca t ion of c o n n e c t o r and con tac t s .

best test site for this deployment is the APCO location; 2500 modules will be manufactured on the equipment shown in Figs. 7 - 12 and installed in two of the system rows. These modules will be monitored extensively and addi- tional experience will thus be obtained.

Accelerated tests are continuously performed on the new module according to the SERI Qualification Test Plan (Draft). Current test capa- bility includes the following:

(1) electrical and visual; (2) thermal cycles; (3) humidity-freeze; (4) hot spot endurance; (5) mechanical load; (6) corrosion susceptibility; (7) thermal bypass diode; (8) wet hi-pot; (9) hailstone (planned addition). The completion of accelerated tests simultaneous to the field deploy-

ment of the new module will provide a good baseline for demonstration of adequate module design.

7. Conclusion

Progress in the better understanding of thin film degradation mecha- nisms, which is necessary for the successful commercialization of thin film a-Si modules, can be made through continued field experience, combined with adequate test programs.

Acknowledgments

The author thanks G. McComiskey, J. Kolesar, A. Kovacs, A. Nagy, L. R. Shiue, J. Grez, and F. J. Kampas, for their valuable assistance in the development effort.

128

Special thanks are extended to Jet Propulsion Laboratory, Pasadena, CA; Solar Energy Research Institute, Golden, CO; Florida Solar Energy Center, Cape Canaveral, FL; New Mexico Solar Energy Institute, Las Cruces, NM; and Sandia National Laboratories, Albuquerque, NM, who provided advice and constructive feedback on the performance of Chronar's modules.

References

1 D. L. Staebler and C. R. Wronski, Appl. Phys. Lett. , 31 (1977) 292. 2 Test Report, Alabama Power 100 k W P V System, 1986 (New Mexico Solar Energy

Institute, Las Cruces, NM). 3 Test Report, Alabama Power 100 k W P V System, 1987 (New Mexico Solar Energy

Institute, Las Cruces, NM). 4 R. S. Sugimura, G. R. Mon, L. Wen and R. G. Ross, Jr., Proc. 20th IEEE Photovoltaic

Specialists Conf., Las Vegas, NV, 1988, IEEE, New York, 1989, p. 1103. 5 R. G. Ross, Jr., Proc. 19th IEEE Photovoltaic Specialists Conf., New Orleans, LA,

1987, IEEE, New York, 1987, p. 993. 6 G. R. Mon, Research Forum Proc. -- Reliability and Engineering o f Thin-Film Photo-

voltaic Modules, Washington, DC, 1985, Jet Propulsion Laboratory Publication 85 - 73, Pasadena, CA, 1985, p. 197.

7 Standard for Safety Flat-Plate Photouoltaic Modules and Panels, IL 1703, 1986, 1st edn. (Underwriters Laboratory Inc., Northbrook, IL).

8 G. R. Mon, L. Wen and R. G. Ross, Jr., Proc. 19th IEEE Photovoitaic Specialists Conf., New Orleans, LA, 1987, IEEE, New York, 1987, p. 1090.

9 Test Report, Solar Panel Environmental Tests, Chronar Corporation, Project C87100, 1988 (Southern Electric International, Atlanta, GA).

10 R. G. Ross, Jr., Proc. Photovoltaic Thin-Film Module Reliability Testing and Evalua- tion Workshop, Lakewood, CO, Solar Energy Research Institute, Golden, CO, 1987, p. 179.

11 Interim Qualification Test for Terrestrial Photovoltaic Thin-Film Flat-Plate Modules (Draft), 1989 (Solar Energy Research Institute, Golden, CO).