Crystallographic orientation propagation in metal halide perovskite thin films†
Metal halide perovskites have achieved remarkable efficiency in solar cells. However, the origin and impact of preferential crystallographic orientation of thin films are still not well understood. In this work, using in situ grazing-incidence X-ray scattering, we show that a preferential crystallographic orientation of a thin ‘seed’ layer (∼65 nm) on top of a thick (∼350 nm) randomly oriented film can propagate through the entire film. The thin cubic (100) oriented layer on top was formed by using a simple solution based treatment with methylammonium chloride (MACl) on randomly oriented methylammonium lead iodide (MAPbI3) films. Upon thermal annealing, the cubic (100) orientation propagates through the entire MAPbI3 film underneath which eventually cools down to tetragonal (110) orientation at room temperature. The treatment results in significantly improved solar cell power conversion efficiency, highlighting the importance of controlling the crystallographic orientation of grains.