Device Physics of Thin-Film Polycrystalline Cells and Modules: Annual Subcontract Report, 6 December 1995 - 5 December 1996

During 1996, a number of projects were carried out at Colorado State University (CSU) on Cu(In,Ga)Se2 (CIGS) and CdTe solar cells and small modules. CSU participated directly in the deposition of CIGS at NREL for the first time. Five separate substrates were used, and sodium was both deliberately introduced and deliberately blocked from exiting soda-lime substrates. In general, sodium in the CIGSled to better junction properties and higher efficiency. In other CIGS measurements, CSU showed that electrodeposited absorber material made at NREL produced competitive cells. Voltages, normalized to bandgap, were about 50 mV less than the best evaporated CIGS cells. CSU also showed, in collaboration with Solarex, that the existence of a high-resistivity ZnO layer is probably not critical forcells with relatively thick CdS window layers. In collaboration with seven CdTe fabrication laboratories, CSU measured the effect of CdS thickness on cell parameters. Although voltage and fill-factor generally degrade for CdS thickness below 100 nm, the exceptions suggest that with at least some fabrication techniques, CdS thickness can be reduced to the point that high quantum efficiency in theblue and a good diode junction are not mutually exclusive. A number of artifacts were investigated that appear in module measurement and analysis, but that are generally negligible for small test cells. These include effects due to module-cell geometry and misleading conclusions from selective illumination experiments. NREL data from the highest-efficiency CIGS and CdTe cells were analyzed toprovide direct comparisons of different fabrication techniques. The three commonly used NREL deposition systems have produced CIGS cells with very similar junction properties. Similarly, after adjustment for bandgap, the best South Florida junctions from several years ago are remarkably similar to those produced by Golden Photon in 1996. CSU continues to play a significant role in both the CIGSThin-Film and the CdTe projects summarized above.