Self-Powered Broadband High-Responsivity Photodetectors: p-(CuS)_x(ZnSe)_1-x/p-Si Isotype Heterojunction

The increasing demand for cost-effective, high-performance, and broadband photodetectors motivates the development of novel material systems and device architectures. In this study, compositionally tunable p-(CuS)_x(ZnSe)_1-x/p-Si isotype heterojunction photodetectors were fabricated via thermal evaporation. The CuS/ZnSe ratio was systematically varied to investigate its impact on the morphological, optical, and electrical properties of the heterojunction. At the optimized x = 0.5 composition, densely and homogeneously distributed nanopillars enhanced light trapping and photon harvesting. Consequently, the device achieved a responsivity of 1.1 A/W, a detectivity of 1.16 × 10¹¹ Jones, and an external quantum efficiency (EQE) of 161% at 850 nm under zero bias. The remarkably high EQE originates from the combined effect of enhanced light harvesting and a trap-assisted internal photoconductive gain mechanism. The valence-band offset at the heterojunction interface acts as a hole-blocking barrier, while copper vacancies identified by XPS serve as hole-trapping centers that prolong carrier lifetime and trigger electron recirculation. This mechanism was further supported by the asymmetric transient response with rise and fall times of 21 and 54 µs, respectively. Ultimately, this convergence of maximized photon absorption and prolonged carrier lifetimes provides a viable strategy for designing high-gain and chalcogenide-based photodetectors for high sensing of light with very low intensity.