The case for contacts: enabling high efficiency thin film solar cells

Cadmium telluride (CdTe) solar cell devices have reached efficiencies up to 22.1% and are the most dominant thin film photovoltaic (PV) technology on the market today. As CdTe gains prominence, optimizing back contacts and interfaces to transfer charge without losses in efficiency in essential, and transparent back contacts have not been developed for bifacial solar technology or multijunction solar cells. To address these challenges, this study investigates sputtered alloy Cu-Zn-S as a new p-type semi-transparent back contact material to thin film polycrystalline CdTe. We report that without device optimization, CdTe solar cells with these new back contacts perform as well as control cells with standard back contacts (fill factors of approximately 75%), with low contact barrier heights of approximately 0.3 eV and carrier lifetimes on par with those of control cells. This investigation explains some of the physical mechanisms governing the device stack, inspires future work to understand interfacial chemistry and charge transfer, and elicits optimization to achieve higher efficiency cells and accelerate society’s renewable energy transformation.