Barrier Molecules Based on Formamidinium for Low-Dimensional Perovskite Solar Cells

Abstract

Low-dimensional perovskites are attractive materials for solar cells; this mainly includes barrier molecules based on ammonium or diammonium groups. In this work, we used barrier molecules based on a formamidinium (FA) functional group instead of the traditional ammonium functional group. Two FA-based barrier molecules were studied and compared: phenyl acetimidamide chloride (PhACCl) and benzamidine chloride (BzmCl). The main difference between the barrier molecules is the distance of the FA group from the aromatic ring, which results in the formation of two-dimensional (2D) perovskite in the case of PhACCl, whereas BzmCl formed a quasi-1D (one-dimensional) structure based on single-crystal X-ray diffraction (XRD). Absorbance, photoluminescence (PL), and XRD measurements show the existence of a 2D structure for PhACCl. The existence of a 1D phase in perovskite based on BzmCl as the barrier molecule inhibits charge transfer and forms pinholes in the film. Charge extraction and PL decay measurements show a high recombination rate with BzmCl, whereas with PhACCl there is better alignment of the energy levels to the selective contacts; this minimizes the hysteresis and enhances the cycle stability. The photovoltaic performance of low-dimensional n = 7 cells, based on PhACCl, shows 11.5% efficiency, with an open-circuit voltage of 1.08 V. This study investigates the use of FA-based barrier molecules and shows the importance of steric hindrance in the formation of a 2D perovskite structure.