Xinjue Zhong1, Xiaojuan Ni2, Siraj Sidhik3, Alan Kaplan4, Hong Li2, Aditya D. Mohite3, Yueh-Lin Loo4, Jean Luc Brédas2, and Antoine Kahn1 

1 Dept. of Electrical and Computer Engineering, Princeton Univ., Princeton, USA 

2 Dept. of Chemistry and Biochemistry, University of Arizona, Tucson, USA 

3 Dept. of Chemical and Biomolecular Engineering, Rice University, Houston, USA 4 Dept. of Chemical and Biological Engineering, Princeton Univ., Princeton, USA 

Two-dimensional (2D) halide perovskites exhibit remarkable tunability of optoelectronic properUes and  good environmental stability achieved through the selecUon of organic caUons. In parUcular, the  incorporaUon of bifuncUonal ligands featuring non-ammonium terminus and funcUonal groups capable of  forming extra bonding moUfs within the organic bilayer provides an effecUve strategy to engineer  perovskite structures and introduce addiUonal funcUonaliUes. 2D halide perovskites are therefore poised  to perform an important role, both acUve and passive, in the developing halide perovskite device field. 

This talk addresses the determinaUon of optoelectronic properUes, i.e. single parUcle gap (EG) and exciton  binding energy (EB), in several groups of Ruddlesden-Popper (RP) and Dion-Jacobson (DJ) 2D metal halide  perovskites via direct and inverse photoemission spectroscopy (UPS/IPES) aided by density funcUonal  theory (DFT). We first determine the electronic gap progression as a funcUon of inorganic layer thickness  in high purity films of BA2MAn-1PbnI3n+1 (n = 1-5).[1] We show that this series exhibits a type I, nested band  gap heterostructure arrangement. By subtracUng the opUcal from the electronic gap, we show that EB vs.  n ranges from 420 meV (n = 1) down to 100 meV (n = 5), well fided by the empirical scaling law developed  by Blancon et al.[2] We then turn to 2D DJ and RP perovskites incorporaUng organic symmetric vs.  asymmetric ligands (BDA vs. DMPD) and ligands with diverse funcUonal groups (-CN, -OH, -COOH, -Ph, and  -CH3), each exhibiUng disUnct bonding characterisUcs and dielectric properUes, and report on the impact  of these ligands on the electronic and excitonic properUes of these 2D perovskites.[3,4] These bifuncUonal  ligands featuring non-ammonium terminus and funcUonal groups form extra bonding moUfs within the  organic bilayer and provide an effecUve strategy to engineer perovskite structures and introduce addiUonal  funcUonaliUes. We observe a strong correlaUon between EG of the -CN, -COOH, -Ph, and -CH3-based  perovskites and the in-plane Pb-I-Pb bond angle, aligning with earlier findings regarding the relaUonship  between opUcal gaps and in-plane Pb-I-Pb bond angle.[5] EB in these 2D layers is found to range from 360  meV for (CH3–PA)2PbI4 to 70 meV for (OH–EA)2PbI4, a variaUon adributed to specific structural aspects,  such as in-plane Pb-I-Pb bond angle, interlayer spacing, and the dielectric constant of the bifuncUonal  ligands. Overall, these results provide deeper insight into the complex impact of organic ligands on the  electronic and excitonic properUes of 2D perovskites, in parUcular the substanUal role of interlayer  electronic coupling.  

[1] X. Zhong et al., Adv. Energy Mater. 12, 2202333 (2022) 

[2] J.C. Blancon et al., Nat. Commun. 9, 2254 (2018) 

[3] S. Silver et al., Adv. Energy Mater. 10, 1903900 (2020) 

[4] X. Zhong et al., Adv. Energy Mater., 2304345 (2024) 

[5] X. Zhao et al., Nat. Commun. 13, 3970 (2022)