Morphology Formation Pathways in Solution-Processed Perovskite Thin Films.

The active layer in a perovskite solar cell is usually composed of a polycrystalline thin film. Fabrication of this layer by solution processing is a promising candidate for up-scaling to the mass market. However, the evolution of an evaporating and simultaneously crystallizing thin film is not yet fully understood. To contribute to the understanding of the formation of thin films, a geometrical model is developed that deals with the effect of the interplay between solvent evaporation and crystal growth on the dry film morphology. The possible film formation mechanisms are investigated, depending on the processing conditions. Eleven formation pathways are found leading to four distinct morphologies. It is shown how these formation pathways can be utilized by adapting the process parameters to the material properties. Pinhole-free and flat films can be fabricated if the evaporation rate is high in comparison to the crystal growth rate. Alternatively, providing a high crystal number density on the substrate can lead to the desired film morphology at low drying rates. The generality of the model makes it applicable to any evaporating and simultaneously crystallizing thin film.