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.