CdSe Top Cells Enabling CdSe/CIGS Tandem Junction Photovoltaics

Ascent Solar Technologies (ITN) proposes to take the next step in spacecraft solar array development, building upon previous development to make the definitive thin-film photovoltaic (TFPV) device for high-efficiency (> 20%) and high-specific power (greater than 2000 W/kg) when combined with lightweight and flexible substrates. ITN will develop wide-bandgap cadmium selenide (CdSe) as a high-efficiency top cell for monolithic (two-terminal) tandem-junction photovoltaics. CdSe has the optimum bandgap (1.72 eV) for a top cell when sharing the solar spectrum with a high-efficiency, but low-bandgap CuInGaSe2 TFPV bottom cell. Tandem-junctions offer the biggest potential for increasing TFPV efficiencies in addition to lower module related losses, higher voltages and better temperature coefficients than low-bandgap single-junction devices. ITN’s innovative approach to achieving high-efficiency top cells is to fabricate CdSe devices leveraging a newly developed proprietary process to convert normally n-type CdSe, to p-type, while also improving material quality. This process should enable CdSe to take advantage of high-efficiency enabling structures and processes demonstrated in lower bandgap CdTe TFPV, but are not yet tested with CdSe solar absorbers. The process also enables the CdSe to have the correct polarity for monolithic connection to the p-type CuInGaSe2 bottom cell tandem devices.