Ultra-Efficient Epitaxial Liftoff Solar Cells Exploiting Optical Confinement in the Wave Limit: Final Technical Report, 19 July 1994 - 18 July 1998

This report describes work performed by the University of California during this subcontract. In this project, we pursued the epitaxial liftoff approach, which leaves a very clean substrate after use that can be readily reinserted into an epi-growth reactor. If, as many believe, the epi-growth step can be streamlined and reduced in cost, this would produce the highest possible performance cell,at a cost no higher than other thin-film technologies. We have found, as a number of other groups have, that the epitaxial liftoff process is vulnerable to microscopic cleavage cracks in the lifted-off films. The larger the area of the lifted-off epi-film, the greater the risk of microscopic cleavage cracks. Such cracks block the passage of electricity and are unacceptable in solar cells. Thishas restricted us to relatively small-area solar cells, which though they performed well, told us very little about scale-up. In the area of lifted-off films, a group in the Netherlands has recently published favorable results using a thin evaporated copper film as a mechanical support layer for the lifted-off GaAs. We have tested their approach during this past quarter, and we have not found itto be entirely satisfying. Instead, we suggest continuing to use organic polymer layers for mechanical support. In the past, the support layer has been a thick wax layer, or a thick photo-resist layer. We have now switched to very thin <1-mm-thick photo-resist layers for support. Such a thin layer has much less give to it, and it allows much less stretching of the lifted-off film.