Gradient Annealing of Halide Perovskite Films for Improved Performance of Solar Cells

A temperature gradient annealing process was developed to regulate grain growth in perovskite films, resulting in improved film quality and thereby enhanced solar cell performance. During a high-temperature stage of the anneal, crystal nucleation occurs continuously as the wet film becomes supersaturated due to solvent evaporation. The formation of pinholes is avoided during the later stage of the anneal by reducing the temperature, which suppresses the migration of perovskite grain boundaries. The resulting grain growth is preferentially along the [110] and [220] directions and produces dense and uniform films with high orientation and crystallinity. These films benefit from decreased nonradiative recombination and enhanced carrier transport due to a reduced defect density, and solar cells based on them exhibit a power conversion efficiency that is increased from 18.64 to 20.04% compared with devices prepared using a single temperature anneal.