Influence of Formamidine Formate Doping on Performance and Stability of FAPbI₃-Based Perovskite Solar Cells

Formamidine lead iodide (FAPbI₃) perovskite material is very suitable for solar photovoltaic devices because of its ideal low band gap, theoretically high efficiency, and wide range of solar spectral absorption, coupled with its good thermal stability. A two-step spin coating method could control the crystallization process of formamidine lead iodide perovskite films better, resulting in more easily repeatable high-quality films. However, it is still difficult to avoid the formation of halide I-vacancy during the preparation of films, which will affect device performance and stability. In this paper, we added small molecular formamidine formate (FAHCOO) into the PbI₂ precursor solution. Due to the high binding energy between HCOO⁻ and I-vacancy, film defects caused by I-vacancies could be passivated. A molecular exchange process could be introduced in the two-step method with the addition of FAHCOO. The exchange process could delay the crystallization process in perovskite films and make them transform more fully; thus, ultimately improving the crystallization quality of the films. In addition, by adding FAHCOO to the PbI₂ precursor solution, a small number of FAPbI₃ can be pre-generated as templates. These templates could induce the growth of specific crystal planes of FAPbI₃ in the second step reaction; thereby, improving the crystallinity of FAPbI₃ films. The FAPbI₃ of devices with optimized FAHCOO show a champion power conversion efficiency (PCE) of 19.04%, apparently higher than that of the controlled devices without FAHCOO (16.69%). For working stability tests under AM 1.5G illumination in an air environment, PSCs with FAHCOO showed nearly 100% of their initial efficiency after a 4100 s tracking test, while the original control device dropped to about 94%.