Structural and excitonic properties of the polycrystalline FAPbI3thin films, and their photovoltaic responses.

Faormamadinium based perovskites have been proposed to replace the methylammonium lead tri-iodide (MAPbI₃) perovskite as the light absorbing layer of photovoltaic cells owing to their photo-active and chemically stable properties. However, the crystal phase transition from the photo-activeα-FAPbI₃to the non-perovksiteδ-FAPbI₃still occurs in un-doped FAPbI₃films owing to the existence of crack defects, which degrads the photovoltaic responses. To investigate the crack ratio (CR)-dependent structure and excitonic characteristics of the polycrystalline FAPbI₃thin films deposited on the carboxylic acid functionalized ITO/glass substrates, various spectra and images were measured and analyzed, which can be utilized to make sense of the different devices responses of the resultant perovskite based photovoltaic cells. Our experimental results show that the there is a trade-off between the formations of surface defects and trapped iodide-mediated defects, thereby resulting in an optimal crack density or CR of the un-dopedα-FAPbI₃active layer in the range from 4.86% to 9.27%. The decrease in the CR (tensile stress) results in the compressive lattice and thereby trapping the iodides near the PbI₆octahedra in the bottom region of the FAPbI₃perovskite films. When the CR of the FAPbI₃film is 8.47%, the open-circuit voltage (short-circuit current density) of the resultant photovoltaic cells significantly increased from 0.773 V (16.62 mA cm^-2) to 0.945 V (18.20 mA cm^-2) after 3 d. Our findings help understanding the photovoltaic responses of the FAPbI₃perovskite based photovoltaic cells on the different days.