Controllable blading interdiffusion of formamidinium iodide on thermal evaporated scalable and conformal lead iodide for efficient perovskite solar cells

Abstract

Organic-inorganic perovskite solar cells with a rapid improving performance have attracted a broad interest, which are mainly fabricated by either one step antisolvent or sequential two-step spin coating process. The sequential two-step coating process has a fine controllability by separating the deposition of PbI2 and organic salt like formamidinium iodide. However, the spin coating has issues such as the use of toxic solvents, a high solution waste ratio and is not suitable for large-area and textured surface. The developed sequential two-step process combining the thermal evaporation of scalable and conformal PbI2 and a blading process of organic salt is expected for the formation of high-quality perovskite with a high uniformity and conformability. However, the device performance is inferior to the two-step solution coating process. Here we systematically investigated the structure and morphology difference of the PbI2 that deposited by solution coating process and thermal evaporation process, and its effect on perovskite layer quality and device performance. The spin coated solvent coordinated PbI2 formed a porous structure, which is beneficial for organic salt penetration. In contrast, evaporated PbI2 is compact with a smooth surface, and may lead to unwanted 2D structures or un-completed conversion when reacted with organic ammonium salt. A temperature-assisted crystal growth is developed to modulate the crystal growth and a superior perovskite film morphology was obtained from thermal evaporated PbI2. Consequently, a champion efficiency of 19.8 % was realized, which is among the highest values for perovskite solar cells prepared by sequential two-step process with thermal evaporation of PbI2 and blading of organic salts.