A photoelectrochemical (PEC) device induces electrochemical reactions on the surfaces of light-absorbing semiconductors to harness sunlight for producing valuable chemicals. The most critical issue in PEC is the poor stability of semiconductors in electrochemical environments. The stability can be enhanced by applying a transparent and conductive protection layer, which is usually prepared by an oxide thin film with tens of nanometer, on the semiconductor. Nevertheless, ensuring the complete impermeability to an electrolyte remains a significant challenge as even a single pinhole in the thin film can lead to the dissolution of the entire underlying semiconductor layer. In this study, we present a facile and reliable protection method applicable to various semiconductors using a thick (200-500 μm) single crystal TiO2. The impermeability is ensured by the exceptionally thick thickness without compromising the device performance. We applied the protection layer on a halide perovskite semiconductor well-known for moisture instability, and inductively coupled plasma mass spectrometry rigorously confirmed that there was no dissolution of elements from the halide perovskite film. The robust protection layer also enabled the safe integration of the halide perovskite PEC device and a biocatalyst without concerns about Pb or halide toxicity. An old yellow enzyme from Thermus scotoductus (TsOYE) coupled with the PEC carried out trans-hydrogenation of C=C bonds, demonstrating the expanded applicability and economic potential of PEC systems for producing fine chemicals and pharmaceutical intermediates.