Alkali metal cation modulation toward high-electronic-quality perovskite films requires strict control over trap densities in the devices. By introducing tailor-made potassium cation (K+)-functionalized carbon nanodots (CNDs@K) into the perovskite precursor solution, we succeeded in defect passivation and crystallization control of the perovskite film. X-ray diffraction indicated that the binding effect of carbon dots confined the K+ ions in the grain boundary and prevented excessive cations from occupying interstitial sites, thereby reducing the microstrain of the polycrystalline film. Consequently, the synergistic effect of the tailored crystal size and suppressed grain boundary defects could reduce the charge trap density, facilitate charge generation, and lengthen the carrier lifetime, leading to a boosted efficiency of 21.01% with a high fill factor of 84%. This performance is among the best reported for carbon dot-doped PSCs.