STTR Phase I: P-Type CdSe for Thin-Film Top Cells Enabling High-Efficiency Monolithic Tandem Photovoltaics

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This Small Business Technology Transfer (STTR) Phase I project will develop a wide-bandgap, thin-film semiconductor to enable a high-efficiency light-absorber layer for the top cell in monolithic tandem or multi-junction thin-film photovoltaics. Because of relatively high device packaging costs, tandem junction thin-film photovoltaic devices can offer higher watt per square meter and lower cost per watt than their single junction counterparts. In order to overcome the current low thin-film device efficiencies for materials with bandgaps greater than 1.6 eV, it is proposed to use CdSe, with a bandgap of 1.72 eV, which is an optimum bandgap for a top cell of a tandem junction device when sharing the solar spectrum with an existing high-efficiency thin-film bottom cell. However CdSe likes to be an n-type semiconductor, which does not allow for its use as a top cell with the correct polarity on the high-efficiency p-type bottom cell for monolithically integrated (two-terminal) tandem devices. In this project an innovative approach to overcome this problem is proposed which involves using existing low-cost deposition techniques with added doping capability to fabricate p-type thin-films of CdSe. The retention of this conductivity type after high-temperature device processing will also be confirmed so as to enable high-efficiency devices as needed for application in the monolithic tandem device. Commercially, two of the most important near-term markets for high-efficiency flexible lightweight tandem-junction photovoltaics are space and high-altitude airships (HAA). In addition it is possible to leverage the monolithic tandem PV space/HAA product technology for the implementation of a low-cost monolithic tandem PV terrestrial product. The terrestrial product is anticipated to have a significant impact toward reducing the cost of alternative energy (solar electric).