Wide-bandgap CIAS thin-film photovoltaics with transparent back contacts for next generation single and multi-junction devices

This paper discusses the performance, testing, and problems of copper indium aluminum diselenide (CIAS) thin-film devices with CIAS co-evaporated in a large-area moving substrate deposition system on transparent back contact technology. The CIAS alloy is being developed for use as a wide-bandgap radiation-resistant semiconductor for application as both a high-voltage single-junction photovoltaic (PV) cell using low-cost thin-films on lightweight flexible substrates, and as a top cell in efficient two-terminal monolithic tandem (multi-junction) PV cells. After significant development, it appears that conventionally formed CIAS devices experience a drop-off in performance with CIAS bandgaps above 1.5 eV, in addition to decreased zero bias depletion widths, and increased JL(V) effects with increasing CIAS bandgap. However, the performance drop-off is above the single-junction optimum bandgap, and higher-bandgap devices have not been tested with better-matched buffer layers. This paper also discusses CIAS devices with transparent back contacts. A wide-bandgap CIAS device with infra-red (IR) transparent back contacts and substrates can offer better performance over standard CIGS devices by being better tuned to the optimum bandgap for AM0 solar spectrum, reduced ohmic losses, and temperature dependencies compared to that typical of low-bandgap CIS alloys, reduced operating temperature with better IR transparency, the option of backside light collection with a suitable visibly transparent substrate, and is a precursor for use as a top cell in a monolithic tandem device configuration. CIAS devices with transparent back contacts show no loss in performance compared to standard Mo back contacts.