Full-space built-in electric field inside gradient Sn-doped β-Ga₂O₃ photoanodes for enhanced photoelectrochemical solar-blind UV photodetection

β-Ga2O3 is a promising candidate for solar-blind ultraviolet photodetection owing to its suitable bandgap of approximately 4.9 eV, excellent photoresponse characteristics, and high stability. However, the lack of a sufficient driving force within the material leads to extensive bulk charge recombination, limiting its photocurrent and thus posing significant challenges in designing high-performance Ga2O3-based photodetection. In this study, we propose a gradient doping strategy to achieve a Sn-doping concentration gradient along the β-Ga2O3 film thickness. By combining sol–gel synthesis with rapid thermal annealing, a spatially graded band structure with a full-space built-in electric field is constructed, which increases the width of band bending over a large region and is crucial for significantly enhancing carrier separation and transport in the bulk. The resulting gradient Sn-doped β-Ga2O3 enables exceptional photoelectric performance without an external bias under 254 nm irradiation, including a superior responsivity of 66.88 mA W−1, a high detectivity of 8.12 × 1011 Jones, and a fast rise/decay time of 79/65 ms, outstanding most existing similar reported photoelectrochemical (PEC) type optoelectronic devices. Additionally, the device exhibits excellent long-term stability and enables high-resolution underwater ultraviolet imaging. This study demonstrates that the gradient doping strategy provides a feasible approach for enhancing the PEC performance of β-Ga2O3 photoelectrodes.