Fabrication and characteristics of high-V_OC single-crystalline Cu₂ZnSnSe₄ solar cells

Cu₂ZnSnSe₄ single-crystal solar cells with open-circuit voltages reaching 450 mV are demonstrated. The key differences in performance between high- and low-voltage cells are analyzed and compared with state-of-the-art thin film devices. Copper-poor absorbers of two different compositions were evaluated as a function of surface treatment. Temperature-dependent JV measurements were used to assess the efficacy of interface passivation. Crystals with lower copper content are shown to require more aggressive chemical treatments to achieve the maximal benefits than does their higher-Cu counterparts. Hole concentration is confirmed via Hall characterization, with smaller densities corresponding to lower Cu concentration. In conjunction with poor lifetimes, these carrier densities are shown to limit collection at long wavelength, which reduces current in single-crystal devices. The decrease in Cu concentration is also shown to increase the bandgap from 0.98 to 1.03 eV while maintaining the same level of subbandgap absorption. Postfabrication device annealing was shown to benefit devices with higher Cu/Zn + Sn ratios but hinder devices further deviating from stoichiometry, which is attributed to pn-junction degradation.