Phase-engineered CdS for ultrafast self-powered photoelectrochemical photodetector in underwater optical communication

Photoelectrochemical (PEC) photodetectors are attractive for underwater optical communication (UOC) owing to their facile fabrication, high sensitivity, and self-powered capability. However, their practical implementation remains constrained by severe wavelength-dependent attenuation in deep-sea environments and the need for high-speed signal transmission. Specifically, wireless UOC is restricted to a narrow band around 450 nm, making CdS, whose intrinsic absorption band lies in this region, a promising material platform. Here, an additive-mediated hydrothermal strategy is developed to achieve phase-selective deposition of high-quality CdS films with either a cubic or a hexagonal crystal structure. While both phases exhibit good self-powered photoresponse, cubic-phase CdS demonstrates record PEC performance at 450 nm, achieving high responsivity of 180.3 mA W⁻¹ (1.61 μW cm⁻²), ultrafast response time of 4/36 μs (rise/fall), and a cutoff frequency of 32.2 kHz, supporting potential data rates over 60 kbps. Studies combining experimental characterization and theoretical calculations reveal that cubic-phase CdS possesses a smaller hole effective mass, a lower valence band edge, and stronger surface hydroxyl adsorption, which collectively accelerate carrier transport and enhance interfacial charge transfer. Overall, this work highlights the critical role of designing spectrally-matched photodetectors for UOC and establishes phase engineering as a promising strategy for pushing the boundaries of high-speed, high-performance optoelectronic devices.