Formamidine-assisted fast crystallization to fabricate formamidinium-based perovskite films for high-efficiency and stable solar cells

In the past decade, perovskite solar cells (PSCs) have experienced a rapid development with the goal of achieving both high efficiency and high stability. Avoiding the use of unstable methylammonium, formamidinium-based perovskites, with better thermal stability and lower band gap, have became promising candidate absorbers. However, the methylammonium-free PSCs usually show inferior PCEs, which is closely related to the crystalline quality of these perovskites. In this study, we propose a formamidine-assisted fast crystallization (FAAFC) method to regulate the nucleation and improve the crystallization by using an FA-containing chlorobenzene antisolvent. It was found that RbCsFAPbI(3) films prepared using the FAAFC method have a preferred orientation, larger crystal size and less grain boundary and traps. As a result, their inverted PSCs exhibit much better photovoltaic performance and stability compared to with the control method, achieving a PCE of 20.58% and <10% degradation after 500 h illumination.