Energy Band Alignment of ZnS_xSe_1-x Films on Si for Photovoltaic Carrier-Selective Contacts

ZnS_xSe_1-x films are promising materials for front carrier-selective contacts in silicon photovoltaics given their wide bandgaps and low resistivities compared to amorphous silicon. X-ray photoelectron spectra of ZnS_xSe_1-x films (x ranging from 0 to 1) grown on Si by molecular beam epitaxy were used to measure the conduction band and valence band offsets of ZnS_xSe_1-x with respect to Si for purposes of accurate optoelectronic simulations of photovoltaic devices incorporating ZnS_xSe_1-x carrier-selective contacts. Conduction band offsets ranged from 0.42 eV (x = 1) to 1.52 eV (x = 0) showing a significant departure from both Anderson model and density functional theory predictions. These offsets represent transmission probabilities through the ZnS_xSe_1-x depletion region of 94%–0% for an electron in the bulk silicon conduction band, suggesting Se-rich ZnS_xSe_1-x films will be necessary for effective ZnS_xSe_1-x electron-selective contacts on Si. The open-circuit voltage, fill factor, and conversion efficiency of ZnS_xSe_1-x/Si cells will be discussed.