To date, there have been a plethora of reports on different means to fabricate organic–inorganic metal halide perovskite thin films; however, the inorganic starting materials have been limited to halide-based anions. Here we study the role of the anions in the perovskite solution and their influence upon perovskite crystal growth, film formation and device performance. We find that by using a non-halide lead source (lead acetate) instead of lead chloride or iodide, the perovskite crystal growth is much faster, which allows us to obtain ultrasmooth and almost pinhole-free perovskite films by a simple one-step solution coating with only a few minutes annealing. This synthesis leads to improved device performance in planar heterojunction architectures and answers a critical question as to the role of the anion and excess organic component during crystallization. Our work paves the way to tune the crystal growth kinetics by simple chemistry.

Crystal Growing Kit: For single crystals to remain intact, there is a limit to the size and number of defects that can be included before the underlying lattice is destroyed. Biological crystals, however, are known to include large macromolecules. H. Li et al. (p. 1244 ; see the Perspective by Hollingsworth ) used electron tomography to study the crystallization of calcium carbonate inside an agarose gel, observing that the crystals physically entrapped the agarose macromolecules. To accommodate the curvature induced by the polymer chains, both high- and low-energy facets formed at the fiber-crystal interfaces. Thus, physical interactions alone may be sufficient for the incorporation of macromolecules in biological crystals and it may be possible to grow unusually shaped single crystals.

Structure control in solution-processed hybrid perovskites is crucial to design and fabricate highly efficient solar cells. Here, we utilize in situ grazing incidence wide-angle X-ray scattering and scanning electron microscopy to investigate the structural evolution and film morphologies of methylammonium lead tri-iodide/chloride (CH₃NH₃PbI_3–xCl_x) in mesoporous block copolymer derived alumina superstructures during thermal annealing. We show the CH₃NH₃PbI_3–xCl_x material evolution to be characterized by three distinct structures: a crystalline precursor structure not described previously, a 3D perovskite structure, and a mixture of compounds resulting from degradation. Finally, we demonstrate how understanding the processing parameters provides the foundation needed for optimal perovskite film morphology and coverage, leading to enhanced block copolymer-directed perovskite solar cell performance.