Real-time, high-precision underwater monitoring and optical communication are critical for marine resource exploration yet remain hindered by the lack of photodetectors capable of stable, autonomous operation in harsh seawater environments. Herein, we report a robust, self-powered photoelectrochemical (PEC) deep-ultraviolet (DUV) photodetector based on an epitaxial La-doped SrSnO3 (LSSO)/SrRuO3 (SRO) architecture. The single-crystalline epitaxial LSSO film and the high-quality LSSO/SRO interface suppress defect-induced recombination and enable efficient separation and transport of photogenerated carriers. Through systematic optimization of La-doping concentration and operating bias, the device achieves a remarkable peak responsivity of 459.84 mA W-1 in natural seawater, alongside rapid response speeds (rise/fall times of 2.98/2.22 ms). The device shows excellent long-term stability in natural seawater during PEC operation, with the photoactive surface directly exposed to the electrolyte. Specifically, the photocurrent remains nearly unchanged after 100 days of ambient storage and 30 days of immersion in natural seawater, retaining over 90% of its initial value after 2000 s of continuous cycling. This work demonstrates that epitaxial oxide thin films can effectively bridge the gap between high-performance optoelectronics and harsh-environment adaptability, paving the way for next-generation deep-sea exploration technologies.
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