The critical role of composition-dependent intragrain planar defects in the performance of MA_1–xFA_xPbI₃ perovskite solar cells

Perovskite solar cells show excellent power conversion efficiencies, long carrier diffusion lengths and low recombination rates. This encourages a view that intragrain defects are electronically benign with little impact on device performance. In this study we varied the methylammonium (MA)/formamidinium (FA) composition in MA_1–xFA_xPbI₃ (x = 0–1), and compared the structure and density of the intragrain planar defects with device performance, otherwise keeping the device nominally the same. We found that charge carrier lifetime, open-circuit voltage deficit and current density–voltage hysteresis correlate empirically with the density and structure of {111}_c planar defects (x = 0.5–1) and {112}_t twin boundaries (x = 0–0.1). The best performance parameters were found when essentially no intragrain planar defects were evident (x = 0.2). Similarly, reducing the density of {111}_c planar defects through MASCN vapour treatment of FAPbI₃ (x ≈ 1) also improved performance. These observations suggest that intragrain defect control can provide an important route for improving perovskite solar cell performance, in addition to well-established parameters such as grain boundaries and interfaces.