Perovskite solar cells have attracted much attention as a promising printable photovoltaic with high power conversion efficiency. The performance of perovskite solar cells highly depends on the quality of the perovskite layer, and thus the development of the fabrication technology to obtain a dense and flat perovskite layer is one of crucial issues in this field. We have recently optimized our solution method with solvent engineering using a mix solvent of DMF and DMSO, which leads to a high power conversion efficiency ~20% even in the cells composed of a pure CH₃NH₃PbI₃ perovskite as a photo-absorber. In this method, the use of DMFwas found to be effective to obtain a thicker perovskite layer, which leads to high J_SC, due to high solubility of perovskite components (PbI₂ and CH₃NH₃I) in DMF, whereas the higher volatility of DMF relative to DMSO needs a rather narrow process window in its spin-coating. For the application to the larger scale fabrication of the devices, the development of fabrication method for perovskite layer with a wider process window, which leads to higher reproducibility, is strongly required.  

During the course of our investigation of solvent effects on the perovskite film, we found that the color of CH₃NH₃PbI₃ perovskite film immediately changes from black to colorless upon exposure to DMF vapor. The SEM images exhibited that the morphology of the surface drastically changes from cubic to needle-like shape. To confirm the colorless crystalline component, we isolated colorless crystals from a mixture of PbI₂ and CH₃NH₃I in DMF. The single crystal X-ray analysis confirmed that these colorless crystals correspond to a complex of CH₃NH₃PbI₃·DMF, in which a solvent molecule of DMF is intercalated between the perovskite components. This complex shows higher solubility in DMSO(2.0 M at room temperature) compared to that of a mixture of PbI₂ and CH₃NH₃I(1.2 M), suggesting its utility as a highly purified precursor for perovskite layer even in the solution method using a DMSO solution, which enables the fabrication of perovskite layer with a wider process window due to a low volatility of DMSO. After optimization of the fabrication protocol, we achieved a J_SC value of 23.5mA/cm², a V_OC of 1.14V, and a FF of 0.73, leading to high PCE of 19.6% with excellent reproducibility. In this presentation, we will report the details of our fabrication method using this complex as a precursor as well as its properties.