Effect of Indium Doping on Surface Optoelectrical Properties of Cu₂ZnSnS₄ Photoabsorber and Interfacial/Photovoltaic Performance of Cadmium Free In₂S₃/Cu₂ZnSnS₄ Heterojunction Thin Film Solar Cell

Maximum conversion efficiency of 6.9% was obtained over an electrodeposited Cu₂ZnSnS₄-based thin film solar cell with a Cd-free In₂S₃ buffer layer by applying a rapid post-heat treatment to the In₂S₃/Cu₂ZnSnS₄ stacked layer. It was found that post-heating of the In₂S₃/Cu₂ZnSnS₄ stack promoted an increment of the acceptor density of the Cu₂ZnSnS₄ layer close to the In₂S₃–Cu₂ZnSnS₄ heterointerface of the In₂S₃/Cu₂ZnSnS₄ stack. Moreover, the diffusion of In also resulted in a red-shift of the band gap energy of Cu₂ZnSnS₄ from 1.47 to 1.40 eV. Due to extension of external quantum efficiency response of the solar cell to the long wavelength region, the solar cell based on the post-heated In₂S₃/Cu₂ZnSnS₄ stack reached appreciably large short circuit current density of more than 20 mA cm^–2. The energy difference between the conduction band minimum of In₂S₃ and that of Cu₂ZnSnS₄ at the In₂S₃/Cu₂ZnSnS₄ heterointerface was determined to be a slightly positive value of 0.11 eV, indicating formation of a “notch-type” conduction band offset for efficient suppression of the interface recombination.