When droplets containing non-volatile solutes dry on a substrate, the solution in the interior prefers migrating to the periphery caused by the capillary force and ultimately the solutes are carried to the edges of droplets forming “coffee rings” after drying, namely coffee ring effect (CRE). CRE is a general phenomenon in the printing process, however, seriously affects the uniformity and quality of films, especially in the dispersed deposition technology based on droplets such as spray coating.

In recent years, spray coating, a mature industrial technology, has been widely used in the fields of perovskite solar cells (PSCs) owing to its advantages of high-throughput, outstanding compatibility with diverse substrates and scalability. However, the inhomogeneity of sprayed films caused by CRE leads to poor reproducibility of devices and limits the improvement of devices performance. The power conversion efficiencies (PCEs) of sprayed perovskite devices are still far behind that of their spin-coated counterparts. Therefore, the elimination of the CRE is of great practical interest for the development of spraying technology in the field of perovskite.

In recent, Prof. Yong Peng and colleagues in Wuhan University of Technology reported a reaction-dependent regulating strategy. Unlike the inert conditions under which the solution cannot react with the substrate, here, the liquid droplets (FAI/Br) are reactive to the solid films (CsI/PbI₂) which will affect the distribution of the solutes. Considering this characteristic, authors regulated the reaction process via solvent selection and found that the solvent with proper drying rate, such as n-butyl alcohol (NBA), can effectively suppress the formation of “coffee rings”. On the one hand, an appropriate prolonged-drying-time enhances droplet spreading and permits the formation of an integrated wet film reducing the “edge” counts, on the other hand, the increased reaction between FAI/Br and CsI/PbI₂ film reduces the accumulation of solutes in the periphery, resulting in uniform deposition. Finally, dense and homogeneous Cs_0.19FA_0.81PbI_0.5Br_2.5 perovskite films were achieved and benefiting from the high-quality perovskite films, the PCEs of corresponding devices reached 19.17%, which is among the highest PCEs for spray-coated PSCs. In addition, the enlarged perovskite film exhibits excellent uniformity. This work provides not only an effective approach for CRE controlment in spraying technology to achieve high-performance devices, but a new idea for the development of other printing technologies.