Solution processed CuSbS2 films for solar cell applications

CuSbS2 is a semiconductor with a band gap of 1.5 eV and earth-abundant constituent elements, indicating potential promise as a photovoltaic absorber material. However, strategies to fabricate CuSbS2 films, especially using solution processing, have not been thoroughly developed. We report on two solution-based approaches to deposit CuSbS2 films: chemical bath deposition (CBD) and deposition of colloidal nanoplates. Conditions to directly deposit ternary CuSbS2 (chalcostibite) films were not found, but CuSbS2 films could be formed by annealing CBD-grown bilayers of CuS and Sb2S3. Simultaneous control over phase purity and film morphology proved elusive. To address this challenge, we synthesized colloidal nanoplates of phase-pure chalcostibite CuSbS2 capped with oleylamine ligands following a literature procedure. When colloids are condensed into thin films, these synthesis ligands are insulating and inhibit the inter-crystal charge transfer that is necessary for long-range charge transport. To solve this problem, two approaches were pursued: convective assembly followed by solid-state ligand exchange and a novel process involving solution-phase ligand exchange followed by electrophoretic deposition (EPD). Replacement of oleylamine with S2− increased the film conductivity by two orders of magnitude. S2− capping groups also increased the electrophoretic mobility and enabled EPD at bias voltages as low as 5 V. Time-resolved terahertz spectroscopy indicated transient photoconductivity persisting beyond 1 ns and carrier mobilities of ~ 1 cm2 V−1 s−1. While many challenges remain, this work indicates the potential promise of solution-processed CuSbS2 nanoplates as building blocks for photovoltaic devices.