RESEARCH OPPORTUNITIES IN POLYCRYSTALLINE COMPOUND SOLAR-CELLS

Polycrystalline photovoltaic (PV) materials show substantial promise for achieving the U.S. Department of Energy PV costs and performance goals. Cadmium telluride (CdTe) and CuInSe2 (CIS) both have recently demonstrated device efficiencies in the 15 percent range with promise of achieving 20 percent efficiencies. Large area thin film CIS and CdTe modules in the 0.4 to 0.7 m2 size have also been fabricated with aperture efficiencies approaching ten percent. In spite of these results, polycrystalline thin film materials are relatively unexplored. Fundamental research opportunities in materials for thin film solar cells can be categorized under three major headings, from the most general to the most specific: areas with long-range potentialities, areas with a larger exploratory content, and areas with specific need. Each of these issues can play a vital role in the development of improved solar cells. The discussion of research opportunities in this paper starts with the more general opportunities and works its way to the most specific ones: (i) updating the basic investigation of defect properties and compensation in Group II-VI and related materials; exploring the basic materials science of the growth process for synthesis from layers of the elements, and exploring the potentials of bandgap engineering; (ii) search for new materials, interactions between defects and grain boundaries in polycrystalline materials, and exploration of the p-i-n structure for solar cells in a more general way; and (iii) doping of and contacts to p-CdTe, junction transport and effects of heat treatments on CdTe and CuInSe2, and development of Group II-VI ternaries.