Although lead halide perovskite (LHP) solar cells can be externally applied to water electrolysis for solar-to-hydrogen conversion, an LHP device with a monolithic configuration for photoelectrochemical (PEC) cells is more economically ideal for viable hydrogen production. To this end, the hydrogen evolution reaction (HER)/oxygen evolution reaction (OER) catalyst attachment on the LHP solar cell terminals is a governing factor for high efficiency and stable operation. Herein, we report monolithic PEC cells with a double-cation perovskite solar cell connected to a robust 3D OER catalyst using conductive carbon powders for minimal ohmic contact loss. The monolithic PEC cell exhibits 0.56 V onset potential and a maximum photocurrent density of 24.26 mA/cm(2) at 1.23 V-RHE (reversible hydrogen electrode (RHE)) with a 9.16% applied bias photon-to-current efficiency. This result will stimulate commercial-level application of LHP-based PEC cells for solar water splitting as well as solar-to-fuel conversion with high efficiency.