Low-dimensional perovskite films afford solar cells with better stability against environmental stresses than three-dimensional counterparts. Yet, the perovskite crystals need to be vertically aligned relative to the substrate to ensure efficient charge transport and hence high-power conversion efficiency. Alkylammonium chloride additives are used to induce such vertical alignment but the underlying mechanism is not fully understood, especially for perovskite processed from widely used solvents such as N,N-dimethylformamide. Now, Martin Kaltenbrunner and colleagues in Austria, Germany and the United Kingdom show that methylammonium chloride initiates the growth of the perovskite at the liquid–air interface where surface tension induces vertical orientation of the crystals.
The researchers show that the additive initially forms a complex with the perovskite precursors and then starts to evaporate upon annealing, leaving behind unstable precursor complexes. As evaporation occurs at the liquid–air interface, these complexes act as nucleation centres and start to crystallise at the liquid surface. The researchers suggest that the same mechanism could also explain the preferential alignment for approaches that do not rely on additives. For instance, for films processed on a pre-heated substrate the rapid evaporation would induce the nucleation of perovskite at the liquid–air interface. They conclude that if the formation of the perovskite crystals starts at the top liquid–air interface, the roughness of the underlying substrate plays a minor role in the formation of the perovskite layer, easing restrictions on the fabrication of solar cells.
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