Selective CO2 reduction via photoelectrocatalysis is a creative approach to alleviating the energy crisis and environmental deterioration as well as to producing value-added chemicals. A considerable challenge is the development of electrocatalysts and photoelectrocatalytic systems to selectively and efficiently produce a target product. Here, we report an efficient and selective photocathode for converting CO2 to HCOOH in aqueous solution. A rational compositional screening strategy was first applied to pinpoint In0.4Bi0.6 from ternary In–Bi–Sn alloys as the most HCOOH-selective electrocatalyst composition among the electrocatalysts reported. A photocathode was then fabricated by coating the catalytic, protective, and conductive In0.4Bi0.6 alloy layer on a halide perovskite photovoltaic, effectively utilizing its low melting point eutectic molten state. The generated photovoltage assisted reduction of the overpotential by 680 mV while producing a stable current for nearly exclusive HCOOH production under simulated AM 1.5G irradiation. This work provides a promising approach to achieving efficient and selective solar–fuel conversion.