Phase Conversion from Hexagonal CuS<sub><i>y</i></sub>Se<sub>1–<i>y</i></sub> to Cubic Cu<sub>2–<i>x</i></sub>S<sub><i>y</i></sub>Se<sub>1–<i>y</i></sub>: Composition Variation, Morphology Evolution, Optical Tuning, and Solar Cell Applications

In this work, we report a simple and low-temperature approach for the controllable synthesis of ternary Cu–S–Se alloys featuring tunable crystal structures, compositions, morphologies, and optical properties. Hexagonal CuS<sub><i>y</i></sub>Se<sub>1–<i>y</i></sub> nanoplates and face centered cubic (fcc) Cu<sub>2–<i>x</i></sub>S<sub><i>y</i></sub>Se<sub>1–<i>y</i></sub> single-crystal-like stacked nanoplate assemblies are synthesized, and their phase conversion mechanism is well investigated. It is found that both copper content and chalcogen composition (S/Se atomic ratio) of the Cu–S–Se alloys are tunable during the phase conversion process. Formation of the unique single-crystal-like stacked nanoplate assemblies is resulted from oriented stacking coupled with the Ostwald ripening effect. Remarkably, optical tuning for continuous red shifts of both the band-gap absorption and the near-infrared localized surface plasmon resonance are achieved. Furthermore, the novel Cu–S–Se alloys are utilized for the first time as highly efficient counter electrodes (CEs) in quantum dot sensitized solar cells (QDSSCs), showing outstanding electrocatalytic activity for polysulfide electrolyte regeneration and yielding a 135% enhancement in power conversion efficiency (PCE) as compared to the noble metal Pt counter electrode.