• Cosolvent-coordinated crystallization at ≤40 °C enables planar integration of micrometer-thick Sn–Pb single-crystal films with high structural and composition integrity.
• A tailored solvent matrix yields thickness-tunable single-crystal thin films with ultralow trap densities (~3.98 × 10^12 cm−3) and robust ambient stability.
• Integrated near-infrared photodetectors achieve 73.8% EQE, 0.51 A W−1 responsivity, 3.6 × 10^12 Jones specific detectivity, and stable performance over 25,000 cycles.
• This work establishes a scalable platform for redox-stable, low-temperature growth of Sn–Pb perovskite crystal films, expanding the processing–structure–function landscape for next-generation infrared optoelectronics.