Partial Substitution\nof Copper with Silver in Cu<sub>2</sub>ZnSnS<sub>4</sub>: An Efficient\nStrategy to Boost the Performance\nof Self-Powered Broadband Photodetectors in Superstrate Configuration

Self-powered broadband photodetectors (SPBPDs) hold great\npotential\nfor next-generation optoelectronic applications, but their performance\nis often limited by interface defects that impair charge transport\nand increase recombination losses. In this work, we report the enhancement\nof the photodetection efficiency of SPBPDs by partially substituting\ncopper (Cu) with silver (Ag) in kesterite Cu<sub>2</sub>ZnSnS<sub>4</sub> (ACZTS) thin films. Varying Ag concentrations (0%, 2%, 4%,\n6%) are incorporated into the CZTS layer, forming a TiO<sub>2</sub>/ACZTS heterojunction in superstrate configuration fabricated via\na low-cost sol–gel spin-coating technique with low-temperature\nopen air annealing avoiding conventional postdeposition sulfurization\nor selenization. Photodetection performance varied significantly with\nAg content in CZTS layer, where optimal performance is observed for\n4% Ag doping. Under the simulated solar spectrum (100 mW/cm<sup>2</sup>), the TiO<sub>2</sub>/4% ACZTS device demonstrates superior performance\nwith an ON/OFF ratio of 6.0 × 10<sup>2</sup>, a photoresponsivity\nof 0.42 mA/W, and a detectivity of about 2.5 × 10<sup>9</sup> Jones. In contrast, under 405 nm incident radiation, the device\nperformance improves significantly, achieving an ON/OFF ratio of approximately\n4.6 × 10<sup>3</sup>, a photoresponsivity of around 63.9 mA/W,\nand a detectivity of about 4.8 × 10<sup>11</sup> Jones which\nare the highest reported values observed for CZTS-based single-junction\nsuperstrate SPBPDs without a crystalline silicon wafer. Ag doping\neffectively reduces interface defects and enhances carrier dynamics,\nas evidenced by capacitance–voltage (<i>C</i>–<i>V</i>) and drive-level capacitance profiling (DLCP) measurements\nof the fabricated heterojunction. This approach offers a novel strategy\nfor enhancing SPBPD performance through partial cation substitution,\npaving the way for advanced photodetectors in diverse optoelectronic\napplications.