Combining Organic Cations of Different Sizes Grants Improved Control over Perovskitoid Dimensionality and Bandgap
Isaiah W. Gilley, Hyoung Woo Kwon, Cheng Liu, Yi Yang, Chuying Huang, Haoyue Wan, Abdulaziz S. R. Bati, Evan H. Oriel, Mikaël Képénékian, Badri Vishal, Stefan Zeiske, Khasim Saheb Bayikadi, Taylor E. Wiggins, Eugenia S. Vasileiadou, Bin Chen, Richard D. Schaller, Jacky Even, Stefaan De Wolf, Edward H. Sargent, Mercouri G. Kanatzidis
Abstract
Because mixed-halide wide-bandgap (1.6–2.0 eV) perovskite solar cells suffer from operating instability related to light-induced halide segregation, it is of interest to study alternative means of bandgap widening. Perovskitoids combine wide bandgaps and structural stability resulting from face- or edge-sharing octahedral connections in their crystal structures. Unfortunately, there existed no prior reports of three-dimensional (3D) perovskitoids having direct bandgaps with optical absorption edges less than 2.2 eV. As the most significant predictor of perovskitoid bandgaps is the fraction of corner-sharing in their crystal structures, we hypothesized that increasing the amount of corner-sharing would access lower bandgaps than previously reported. We accomplished this by mixing a spacer cation within the size range for 3D perovskitoid formation with a smaller perovskite-forming cation. We explored three spacer cations of different sizes: ethylammonium (EA), cyclopropylammonium (c-C3A), and cyclobutylammonium (c-C4A), combining these with methylammonium (MA), and found that the middle cation, c-C3A, pairs with MA to form a 3D perovskitoid with the formula (c-C3A) 3 (MA) 3 Pb 5 I 16 and a direct bandgap with an optical absorption edge at 2.0 eV. Solution-processed films of this perovskitoid showed improved light stability over mixed-halide perovskites, and solar cells based on these films exhibit increased maximum power point operating stability compared to reference mixed-halide devices.