Solar Cells, 12 (1984) 257 - 261 257 POLYCRYSTALLINE THIN FILM SOLAR CELLS: SPECULATIONS ON FUTURE RESEARCH AND DEVELOPMENT KEN ZWEIBEL, ALLEN HERMANN and RICHARD MITCHELL Solar Electric Conversion Research Division, Solar Energy Research Institute, Golden, CO 80401 (U.S.A.) 1. Introduction The most successful polycrystalline thin film solar cells are based on CuInSe2 and on CdTe, both types having reported efficiencies of over 10%. Boeing makes (Cd, Zn)S/CuInS% devices under subcontract from the Solar Energy Research Institute (SERI) and has demonstrated 11% efficiency (ELH lamp; air mass (AM) 1 solar simulation; 25 °C). Boeing recently (Sep- tember 1982) began a joint venture (Sovolco) with Reading and Bates to commercialize their high efficiency cells. Industrial research on CdTe has been pursued by several companies including ARCO Solar Inc., Kodak, Ametek, Monosolar and Matsushita. Polycrystalline thin films have a number of advantages when compared with other solar cell materials. (1) There are many possible polycrystalline thin films, some with exceptional photovoltaic properties (e.g. CuInSe2 has an absorption coef- ficient nearly an order of magnitude greater than that of amorphous silicon). (2) They can be deposited onto inexpensive substrates (steel or glass). (3) Less material is used, thus reducing the overall demand for relatively scarce elements, the materials cost and the purification cost. (4) It is not necessary to grow, cut and polish crystals. (5) Continuous fabrication can be used to raise throughput speed and to allow in situ module fabrication (electronic interconnects). There are also a number of disadvantages or problems with thin films including (a) possible grain boundary and other problems associated with materials and depositions, (b) lower efficiency than analogous single-crystal devices, (c) unproven large-scale high throughput fabrication systems (effi- ciency of materials use and process yield) and (d) possible stability problems with new materials. As a photovoltaics technology, thin films competes with several other strategies that also aim to produce electric power on a large scale. Table 1 compares these strategies. To reach their potential, polycrystalline thin film solar cells must be (1) made more efficient, especially in modules, and (2) made inexpensively, in terms of substrates and fabrication systems. In what follows, we shall discuss the research directions needed to meet these goals. 0379-6787/84/$3.00 © Elsevier Sequoia/Printed in The Netherlands