Surface Oxidation as a Cause of High Open-Circuit Voltage in CdSe ETA Solar Cells

TiO₂/CdSe/CuSCN extremely thin absorber (ETA) solar cells are found to give relatively high values of open-circuit voltage (>0.8 V) but low currents upon annealing the cadmium selenide (CdSe) in air (500 °C). Annealing in N₂ produces much lower photovoltages and slightly lower photocurrents. Band structure measurements show differences between the two annealing regimes that, however, appear to favor the N₂-annealed CdSe. On the other hand, chemically resolved electrical measurements (CREM) of the cells reveal marked differences in photo-induced charge trapping, in particular at absorber grain boundaries of the air versus N₂-annealed systems, correlated with the formation of Cd-O species at the CdSe surface. Using transient absorption and photovoltage decay, pronounced lifetime differences are also observed, in agreement with the strong suppression of charge recombination. The results point to a multiple role of grain surface-oxidation, which both impedes electron injection from the CdSe to the TiO_2, but, much more significantly, enhances hole injection to the CuSCN via passivation of hole traps that act as efficient recombination centers. Upon annealing in air, extremely thin absorber solar cells based on CdSe-sensitized titania show relatively high values of open-circuit voltage but low currents. This stems from the fact that while oxidation impedes electron injection from the CdSe to the TiO_2, the balance between hole extraction and recombination is improved in favor of the former.