Band Edge Tuning of Two-Dimensional Ruddlesden–Popper Perovskites by A Cation Size Revealed through Nanoplates

Current understanding of the effects of various A-site cations on the photophysical properties of halide perovskites (APbI3) is limited by the compositional tunability. Here we report the synthesis and characterization of colloidal nanoplates of a series of 2D Ruddlesden–Popper (RP) perovskites (HA)2(A)Pb2I7 (HA = n-hexylammonium) with seven small and large A-site cations to reveal the size effects of such A cations. Absorbance and photoluminescence (PL) measurements show a clear parabolic trend of the optical band gap versus the A cation size, with methylammonium and formamidinium near the bottom. This band gap shifting is attributed to the changing chemical pressure inside the A-site cavity templating the Pb–I framework. PL quantum yield and time-resolved PL measurements show the effect of A cation size on the PL efficiencies and carrier lifetimes. This fundamental investigation can inform the choices of A-site cations that can be incorporated into halide perovskite materials for optoelectronic applications.