Unveiling Electronic and Excitonic Properties of 2D Ruddlesden-Popper Perovskites Based on Bifunctional Ligands

Two-dimensional (2D) Ruddlesden-Popper halide perovskites exhibit remarkable tunability of optoelectronic properties and good environmental stability achieved through the selection of organic cations. The incorporation of bifunctional ligands featuring non-ammonium terminus and functional groups capable of forming extra bonding motifs within the organic bilayer provides an effective strategy to engineer perovskite structures and introduce additional functionalities.<br/>Here, we explore a series of <i>n </i>= 1 2D perovskites incorporating organic ligands with diverse functional groups (-CN, -OH, -COOH, -Ph, and -CH₃), each exhibiting distinct bonding characteristics and dielectric properties, and report on the impact of these bifunctional ligands on the electronic and excitonic properties of these 2D perovskites.^[1]<br/>We perform ultraviolet and inverse photoemission spectroscopies (UPS/IPES) to determine the energy positions of the valence band maximum and conduction band minimum, and the electronic, or single-particle, gap of these 2D perovskites. We observe a strong correlation between the electronic gaps of the -CN, -COOH, -Ph, and -CH₃-based perovskites and the in-plane Pb-I-Pb bond angle, aligning with earlier findings regarding the relationship between optical gaps and the in-plane Pb-I-Pb bond angle.^[2]Interestingly, the gap of the -OH-based perovskite deviates significantly from this trend. We conduct density functional theory calculations and tight-binding model analysis to further elucidate this structure-property relationship. We attribute the “-OH” anomaly to band dispersion along the out-of-plane direction, caused primarily by interlayer electronic coupling present in (OH-EA)₂PbI₄. By subtracting optical gap from single-particle gap, we estimate the exciton binding energy (E_b) in these 2D layers, which ranges from 420 meV for (CH₃–PA)₂PbI₄to 130 meV for (OH–EA)₂PbI₄. This large variation is attributed to specific structural aspects, such as in-plane Pb-I-Pb bond angle, interlayer spacing, and the dielectric constant of the bifunctional ligands.<br/>Our results provide deeper insights into the complex impact of organic ligands on the electronic and excitonic properties of 2D perovskites, in particular the substantial role of strong interlayer electronic coupling. These findings contribute to a better understanding of the structure-property relationship in such materials, facilitating the design and optimization of perovskite-based devices for various applications.<br/><br/>[1] X. Zhong et al., Energy & Environ. Sci., (under review)<br/>[2] X. Zhao et al., <i>Nat. Commun.</i> 2022, <i>13</i>, 3970.