Film uniformity of solution-processed layers is the cornerstone of large-area perovskite light-emitting diodes, which is often determined by the ‘coffee-ring effect’. Here we demonstrate a second factor that cannot be ignored is the solid-liquid interface interaction between substrate and precursor and can be optimized to eliminate rings. A perovskite film with rings can be formed when cations dominate the solid-liquid interface interaction; whereas smooth and homogeneous perovskite emitting layers are generated when anions and anion groups dominate the interaction. This is due to the fact that the type of ions anchored to the substrate can determine how the subsequent film grows. This interfacial interaction is adjusted using carbonized polymer dots, who also orient the perovskite crystals and passivate their buried traps, enabling a 225 mm2large-area perovskite light-emitting diode with a high efficiency of 20.2%.