Interrelation between processing condition, morphology, crystal orientation and efficiency of CH3NH3PbI3-based perovskite solar cells

Abstract

The power conversion efficiency of organic-inorganic perovskite has dramatically been increased for the past few years. Until now, the most widely used method to fabricate perovskite films for photovoltaic applications is one-step deposition owing to its cost-effective, simple and easy fabrication. However, uncontrolled solidification of perovskite crystals during the one-step deposition very often results in poor-quality films with low surface coverage. Herein, we report a simple method to fabricate high-quality CH3NH3PbI3 film for high efficiency perovskite solar cells. The tree-like morphology with many pinholes is formed when the film is fabricated by the conventional one-step deposition as described in previous report, where the solidification takes place during spin-coating due to solvent evaporation accompanied by crystallization. However, when the spin-coating time is shorten so as to avoid the solidification during spin-coating in this study, CH3NH3PbI3 is crystallized during post heat-treatment, resulting in flower-like morphology without pinhole. In-situ OM and XRD experiments reveal that the solidification during spin-coating arises from the formation of CH3NH3I–PbI2–solvent complex, which induces the nucleation of CH3NH3PbI3 crystals followed by crystal growth in one-dimensional direction. The crystal growth in one-dimensional direction yields tree-like morphology with many pinholes in CH3NH3PbI3 perovskite film, resulting in very low PCEs of solar cell devices. However, when the solidification during spin-coating is suppressed by shortening the spin-coating time and thereafter CH3NH3PbI3 is crystallized directly from the wet film by post heat-treatment, the flower-like morphology is developed in the film with high surface coverage. Especially, the film from DMSO solution showing the flower-like morphology with good coverage has large-sized crystals on the length scale of tens of micrometers and the crystals in the film are highly orientated along (112) or (200) directions, resulting in a high PCE of 13.85%.