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Title: Development of economical improved thick film solar cell contact. Quarterly report No. 1

Abstract

Materials were surveyed to provide candidates for an all metal electrode paste system. These consisted of a major constituent metal powder, a low melting metal powder suitable for a liquid phase sintering medium, and a powder material suitable as an etchant for silicon dioxide at sintering temperatures. By means of thermal gravimetric analysis a suitable binder was identified for low temperature fired inks. The all metal ink concept was first demonstrated with the silver system to avoid the problems of limited process windows encountered with base metal systems. A number of solid materials capable of selectively etching silicon dioxide at modest temperatures were identified. One of these, silver fluoride, has yielded very good results, wetting the silicon surface after removing a thick (3000 A) silica layer. Silver pastes containing the above materials have been prepared. A paste with silver fluoride was screened onto N-type silicon with 5..cap omega..cm resistivity. The resulting contact pads had excellent adhesion but were not electrically ohmic. Another metal ink paste, incorporating boric acid, gave substantially lower resistances; however, it would not pass a 1-hour boiling DI water immersion without losing scratch resistance.

Authors:
;
Publication Date:
Research Org.:
Ross (Bernd) Associates, San Diego, CA (USA)
OSTI Identifier:
5949645
Report Number(s):
DOE/JPL/955164-1
DOE Contract Number:
NAS-7-100-955164
Resource Type:
Technical Report
Country of Publication:
United States
Language:
English
Subject:
14 SOLAR ENERGY; SILICON OXIDES; ETCHING; SILICON SOLAR CELLS; ELECTRIC CONTACTS; SILVER FLUORIDES; SCREEN PRINTING; BORIC ACID; ELECTRICAL PROPERTIES; INKS; METALS; POWDERS; SINTERING; SOLUBILITY; CHALCOGENIDES; DEPOSITION; DIRECT ENERGY CONVERTERS; ELECTRICAL EQUIPMENT; ELEMENTS; EQUIPMENT; FABRICATION; FLUORIDES; FLUORINE COMPOUNDS; HALIDES; HALOGEN COMPOUNDS; HYDROGEN COMPOUNDS; INORGANIC ACIDS; OXIDES; OXYGEN COMPOUNDS; PHOTOELECTRIC CELLS; PHOTOVOLTAIC CELLS; PHYSICAL PROPERTIES; SILICON COMPOUNDS; SILVER COMPOUNDS; SOLAR CELLS; SURFACE COATING; SURFACE FINISHING; TRANSITION ELEMENT COMPOUNDS; 140501* - Solar Energy Conversion- Photovoltaic Conversion

Citation Formats

Ross, B., and Mentley, D. Development of economical improved thick film solar cell contact. Quarterly report No. 1. United States: N. p., 1979. Web. doi:10.2172/5949645.
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Ross, B., & Mentley, D. Development of economical improved thick film solar cell contact. Quarterly report No. 1. United States. doi:10.2172/5949645.
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Ross, B., and Mentley, D. Mon . "Development of economical improved thick film solar cell contact. Quarterly report No. 1". United States. doi:10.2172/5949645. https://www.osti.gov/servlets/purl/5949645.
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@article{osti_5949645,
title = {Development of economical improved thick film solar cell contact. Quarterly report No. 1},
author = {Ross, B. and Mentley, D.},
abstractNote = {Materials were surveyed to provide candidates for an all metal electrode paste system. These consisted of a major constituent metal powder, a low melting metal powder suitable for a liquid phase sintering medium, and a powder material suitable as an etchant for silicon dioxide at sintering temperatures. By means of thermal gravimetric analysis a suitable binder was identified for low temperature fired inks. The all metal ink concept was first demonstrated with the silver system to avoid the problems of limited process windows encountered with base metal systems. A number of solid materials capable of selectively etching silicon dioxide at modest temperatures were identified. One of these, silver fluoride, has yielded very good results, wetting the silicon surface after removing a thick (3000 A) silica layer. Silver pastes containing the above materials have been prepared. A paste with silver fluoride was screened onto N-type silicon with 5..cap omega..cm resistivity. The resulting contact pads had excellent adhesion but were not electrically ohmic. Another metal ink paste, incorporating boric acid, gave substantially lower resistances; however, it would not pass a 1-hour boiling DI water immersion without losing scratch resistance.},
doi = {10.2172/5949645},
journal = {},
number = ,
volume = ,
place = {United States},
year = {Mon Jan 01 00:00:00 EST 1979},
month = {Mon Jan 01 00:00:00 EST 1979}
}
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Technical Report:
View Technical Report (5.56 MB)
DOI:  10.2172/5949645
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  • The potential for economy and efficiency has been demonstrated for the thick film metallization process using screen printing for solar cell electrodes. However, process reliability and materials economy remain deficient. It is believed that these deficiencies can be removed by the use of ink formulations designed specifically for silicon solar cells, departing from ceramic technology tradition and utilizing all metal systems. Various measurement methods of series resistance and specific contact resistance were considered. Several attempts were made to produce eutectic alloys in air and in neutral (nitrogen) atmospheres, but the resulting product was not considered satisfactory. A commercial source ofmore » eutectic aluminum-silicon and aluminum-germanium was located. Fifteen all metal pastes were fabricated during this reporting period including 12 base metal pastes. Silver inks fired in hydrogen were non-adherent. Electrical results of screened solar cell tests were poor, indicating diffused layer penetration. Similar results occurred with titanium-palladium-silver controls. A survey was made of alloys containing nickel and copper. Solubilities of nickel, silver and copper in various low melting metals were calculated from the phase diagrams and plotted. An ink milling procedure was written for all metal inks. Base metal powders were ordered and partially received. A tube furnace was purchased and installed, along with a gas handling system, designed for hydrogen and nitrogen firing.« less

  • In the second half of the investigation of all metal screened electrodes, the focus was on base metal pastes in addition to further work with the silver systems. Contact resistance measurements were refined. A facility allowing firing in hydrogen and other atmospheres was acquired. Several experiments were made applying screenable pastes to solar cells. Doping investigations emphasized eutectic alloys reduced to powders. Metal systems were reviewed. A previously published vapor pressure curve for silver fluoride was corrected. Base metal experiments were done with nickel and copper using lead and tin as the frit metals. Severe adhesion problems were experienced withmore » hydrogen atmospheres in all metal systems. A two step firing schedule was devised based upon experimentation which gave evidence that the silver fluoride-silicon dioxide reaction was modified by the presence of hydrogen. It was found that nitrogen prefiring allowed the silver fluoride dissociation and oxide removal without causing catastrophic oxidation of the base metal powders. The subsequent hydrogen firing step reduced oxides tht had formed and gave the proper sintered structure. Electrodes were coherent, adherent, and solderable in both nickel lead and copper lead systems. Towards the end of the contractual period aluminum-silicon and aluminum-germanium eutectic doping additions to copper pastes were tried on 2 1/4'' diameter solar cell back contacts, both with good results (eta = 9.4% AM1 uncoated).« less

  • The potential for economy and efficiency has been demonstrated for the thick film metallization process using screen printing for solar cell electrodes. However, process reliability and materials economy remain deficient. It is believed that these deficiencies can be removed by the use of ink formulations designed specifically for silicon solar cells, departing from ceramic technology tradition and utilizing all metal systems. The objectives of this investigation are as follows: 1) eliminate the glass frit which has been the conventional liquid phase sintering medium and adhesive for metallization inks; 2) provide an appropriate metal which can serve as the liquid phasemore » sintering medium; 3) find a chemical constituent which effectively removes the native oxide from the silicon during the firing step, which can be made part of the ink, and which either becomes fugitive or remains an inert part of the matured metallization; and 4) maintain cognizance of the cost objectives of the LSA Project in selecting materials and processes. Progress is reported. (WHK)« less

  • Films of CdS were prepared by screen printing and were sintered using a variety of firing conditions and flux concentrations. At the higher temperatures, longer firing temperatures, and lower flux concentrations crystals of cadmium oxide formed on the surface of the CdS film. Preparation of the copper sulfide barrier via the aqueous solution treatment was found to produce suitable barriers only when the films were pre-etched in HCl and when the dip solution containing Cu/sub 2/Cl/sub 2/ was hot. CdS films formed on indium oxide coated glass showed no evidence of indium penetration into the CdS layer. Transmission studies ofmore » the CdS layer failed because of the opacity of the films relative to the equipment available.« less

  • The objective of this program is to evaluate the use of screen printing as a technique for producing large area solar cells. The work presented deals primarily with the preparation and improvement in performance of screen printed CdS cells. Thermal gravimetric analysis of the CdS inks used to print CdS films confirm that all the fugitive binders and flux are removed under firing conditions used to prepare the CdS films. Warpage of the Nesatron glass substrates makes their use questionable. Multiple layers of CdS appear to resolve a pin hole problem previously encountered. (WHK)