Understanding what limits the voltage of polycrystalline CdSeTe solar cells

The origin of voltage deficits in polycrystalline cadmium selenide telluride (CdSeTe) solar cells is unclear. Here, we present a comprehensive voltage loss analysis performed on state-of-the-art CdSeTe devices—fabricated at Colorado State University and First Solar—using photoluminescence techniques, including external radiative efficiency (ERE) measurements. More specifically, we report the thermodynamic voltage limit V oc,ideal , internal voltage i V oc and external voltage V oc of partially and fully finished cells fabricated with different dopant species, dopant concentrations and back contacts. Arsenic-doped aluminium-oxide-passivated cells made at Colorado State University present remarkably high ERE (>1%)—translating into i V oc above 970 mV—but suffer from poor back-contact selectivity. On the other hand, arsenic-doped devices from First Solar present almost perfect carrier selectivity ( V oc = i V oc ), leading to V oc above 840 mV, and are limited by recombination in various parts of the device. Thus, development of contact structures that are both passivating and selective in combination with highly luminescent absorbers is key to reducing voltage losses. The open-circuit-voltage deficit of cadmium selenide telluride solar cells is typically higher than that of other photovoltaic technologies yet the reasons are unclear. Now, Onno et al. use photoluminescence techniques to break down the contributions of dopants and back contacts to voltage losses.