Research on polycrystalline thin-film submodules based on CuInSe{sub 2} materials. Final subcontract report, 11 November 1990--30 June 1995

Abstract

Ken Zweibel for helpfhl suggestions and coordination of efforts on our behalf. Executive SummaryRecent progress in photovoltaics has been particularly notable in several materials systems, especially in copper'indium diselenide (CIS). Small area cells using this material have recently exhibited conversion efficiency in the range of 17% [1,2]. This material system exhibits a number of important advantages. It has a very high optical absorption coefficient throughout a major portion of the solar spectrum. It can be deposited in thin f i l m s having suitable electronic qualities on low cost substrates by a variety of methods. Modification of electronic properties such as band gap using ternary alloys has been accomplished, thus making the system versatile as well as robust.This program marks the entry of Solarex into the development of CIS based photovoltaic (PV) product. This initial effort began with the development of manufacturable deposition methods for all required thin ilm layers and the development and understanding of processes using those methods. It necessarily included demonstration of the potential for high conversion efficiency, evidenced by the achievement of 14.4% conversion efficiency (total area) in small cells and followed with the development of viable methods for module segment formation and interconnection. Finally; these process steps were integrated to fabricate monolithic CIS based submodules which exhibited aperture area efficiencies exceeding 11%.A more important result of this program is the basis of understanding that has been established in developing this material for PV applications. This basis of understanding is absolutely necessary to address issues of manufacturability and cost which are of paramount importance to the goal of commercialization. Early in the program, it was recognized that manL&acturability would be determined by successful solutions to issues of yield, reproducibility and control as much as by material and energy costs, conversion efficiency and process speed.Yield is strongly affected by shunt formation in modules, and shunt formation is in turn a strong function of the method used for absorber layer deposition. Issues of control and reproducibility are also strongly related to the absorber formation process. Accordingly, a significant effort was undertaken during this program to explore several alternative methods for absorber layer formation with attention to these issues. SpecZcally, the absorber layer formation techniques which were evaluated included: sputtering elemental precursors at low temperature followed by reaction at 'high temperature, a hybrid process using sputtered metallic precursors followed by reaction at high temperatures in an environment of elemental selenium, co-evaporation and concurrent reaction using elemental sources, and evaporation and/or sputtering of binary selenide precursors followed by reaction at high temperature with selenium. As a result, Solarex has identified at least one absorber formation process which is very robust ...