Electronic State Modulation by Large A‑Site Cations in Quasi-Two-Dimensional Organic–Inorganic Lead Halide Perovskites

Organic–inorganic lead halide perovskites (OLHPs) with the general chemical formula of APbX3 have attracted wide interest for their application in the optoelectronics field within the past decade. Despite the performance breakthroughs enabled by modifying A-site cations, a fundamental understanding of the mechanistic origins is still largely neglected. Especially, the critical role of A-site cations in modulating the physicochemical properties of perovskites is challenging to reveal because the choice of A-site cations is limited by the geometrical Goldschmidt tolerance factor. Herein, we for the first time successfully incorporated two large A-site cations in the lead halide cage by relaxing the tolerance factor in a 2D Ruddlesden–Popper system, and we obtained two novel single crystals, namely, (PA)2(Gua)Pb2I7 and (PA)2(Ace)Pb2I7, where PA = n-pentylamine, Gua = guanidine, and Ace = acetamidine. The sizes of Gua (278 pm) and Ace (277 pm) are almost identical and thus enable us to individually study the influence of other properties of the A-site cation on the optoelectronics properties including the polarity, electron donating abilities, etc. By combining with first-principles theoretical perspectives, we deliver insights into the electronic and structural features of perovskites associated with A-site electron donating properties. This adds more versatility to the roles of the A site cations in OLHPs.