This Small Business Innovation Research (SBIR) Phase I project provides a new approach to low-temperature processing of a compound semiconductor material, copper indium gallium diselenide (CIGS). CIGS is presently being used as the solar absorber layer in some thin film polycrystalline solar cells with world-record efficiencies of 19 percent. While there is a move in the display and photovoltaic (PV) communities towards continuous roll-to-roll manufacturing owing to cost benefits, roll-to-roll CIGS solar cell processes currently give 6 to 8 percent efficient modules. This is thought to be due to microstructural limitations in the CIGS absorber layer as a consequence of lower temperature sintering required when using a polyimide substrate. The research objective of this project is to demonstrate improved conversion efficiencies for CIGS solar cells using a low-temperature processing step. To do this, small grain CIGS films will be subjected to conditions that favor grain growth yielding a large-grained polycrystalline semiconductor. Temperature will be carefully controlled and optimized to determine if this process might enable economical substrates such as polyethylene terephthalate. If successful, the approach would be generally applicable to polycrystalline metal chalcogenide electronic materials where performance improvements might be anticipated with a reduction in the number of grain boundaries.
The commercial application of this project is in the manufacture of high efficiency, flexible, solar cell semiconductor material. This project allows a feasibility demonstration for a semiconductor growth methodology using CIGS solar cells as the first example. Assuming the low-temperature treatment results in the formation of large-grained materials and gives increased solar conversion efficiencies, the process could be utilized as a plug-in at an existing roll-to-roll CIGS manufacturing facility. The development of 15 percent efficient CIGS solar cells on flexible and lightweight substrates would address the needs of higher-end solar cell products used in portable electronics such as cell phones and laptops where a 10-year market estimate of $5 billion is not unreasonable. While existing PV technologies may meet the cost target for portable PV (i.e. $10/W) these are not applicable given the low-specific power density and inflexibility of the modules thus providing a significant opportunity for an emerging solar cell technology. For this consumer application, the value added through the use of portable PV is the convenience of never "plugging in" to recharge a power system. In addition to CIGS solar cells, this low-temperature growth approach could impact the emerging fields of flexible electronics and electronic textiles through new routes to transistors and/or thermo-electrics.
