Providing sufficient driving force for charge separation and transfer (CST) is a critical issue in photoelectrochemical (PEC) energy conversion. Normally, the driving force is derived mainly from band bending at the photoelectrode/electrolyte interface but negligible in the bulk. To boost the bulky driving force, we report a rational strategy to create effective electric field via controllable lattice distortion in the bulk of a semiconductor film. This concept is verified by the lithiation of a classic TiO₂ (Li-TiO₂) photoelectrode, which leads to significant distortion of the TiO₆ unit cells in the bulk with well-aligned dipole moment. A remarkable internal built-in electric field of ~2.1 × 10² V m⁻¹ throughout the Li-TiO₂ film is created to provide strong driving force for bulky CST. The photoelectrode demonstrates an over 750% improvement of photocurrent density and 100 mV negative shift of onset potential upon the lithiation compared to that of pristine TiO₂ film.