Recently, lead halide perovskites (LHPs) have been intensively investigated for use as photoelectrodes in photoelectrochemical (PEC) water splitting systems, and PEC water splitting systems with LHP electrodes have shown outstanding solar-to-hydrogen (STH) efficiencies. Despite the significant role of large-scale deployment for practical commercialization, the fabrication of large-area LHP photoelectrodes is rarely investigated in the field of PEC water splitting. Herein, we present a strategy involving immersing an as-coated LHP film in an antisolvent containing cyclohexylammonium iodide (CHAI) for the fabrication of high-quality large-size LHP films for use in unbiased PEC water splitting. CHAI molecules accelerate nucleation kinetics by participating in the formation of an intermediate phase, resulting in uniform high-crystallinity LHP films. Furthermore, CHAI molecules effectively passivate grain boundaries, thereby increasing the photocurrent density and onset potential of PEC devices. Consequently, parallelly illuminated coplanar LHP-based photoelectrodes with dimensions of 8 cm × 8 cm could be stably operated without any applied bias, and they exhibited a record-high STH efficiency of 9.89% and a T₈₀ (the time at which the photocurrent density drops to 80% of its initial value) of 24 h. The hydrogen production rate of our device was 145.56 μmol h⁻¹ cm⁻², which is highly comparable with previously reported values of state-of-the-art unassisted PEC devices.