Slot-die coating is highly promising for scaled deposition of metal halide perovskite thin films. However, the power conversion efficiencies (PCEs) of slot-die-prepared perovskite solar cells (PSCs) still lag behind their spin-casted counterparts. To resolve this issue, the crystal size and quality of slot-die-coated methylammonium lead triiodide (MAPbI3) perovskite films are dramatically improved via additive engineering using potassium thiocyanate (KSCN). The modified micrometer-thick films have an average grain size of ≈11 μm and charge-carrier parameters that are comparable with single-crystal perovskites, such as a 1.89 μs lifetime, 136.65 ± 31.52 cm2 V−1 s−1 mobility, and 25.15 ± 3.55 μm diffusion length. Exploiting these enhanced properties, planar inverted PSCs with negligible hysteresis are fabricated and an average and a maximum PCE of 20.14% and 21.38%, respectively, are achieved which are among the highest reported values for slot-die-coated PSCs. Notably, our devices have a narrow PCE distribution along the slot-die coating axis, highlighting slot-die coating's promise to fabricate large-scale, high-performance PSCs.