Lead halide perovskite solar cells (PSCs) have advanced rapidly in performance over the past decade. Single-crystal PSCs based on microns-thick grain-boundary-free films with long charge carrier diffusion lengths and enhanced light absorption (relative to polycrystalline films) have recently emerged as candidates for advancing PSCs further toward their theoretical limit. Thus far, the preferred method to grow MAPbI3 single-crystal films for PSCs involves solution-processing at temperatures ⪆120

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... °C, which adversely affects the films' crystalline quality, especially at the surface, primarily due to methylammonium iodide loss at such high temperatures. Here we devise a solvent-engineering approach to reduce the crystallization temperature of MAPbI3 single-crystal films (<90 °C ), yielding better quality films with longer carrier lifetimes. Single-crystal inverted PSCs fabricated with this strategy show markedly enhanced open-circuit voltages (1.15 V vs. 1.08 V for controls), leading to power conversion efficiencies of up to 21.9%, which are among the highest reported for MAPbI3-based devices.