Spacer cation electronic structure controls excited-state pathways in two-dimensional hybrid perovskites

Abstract

Conjugated organic cations have been explored in two-dimensional (2D) organic–inorganic hybrid perovskites (OIHPs) due to tunable optoelectronic functionality of such hybrid materials. However, relatively few conjugated organic cations have been successfully incorporated into 2D OIHPs, and many exhibit relatively large optical band gaps (2.5 eV or larger), largely due to constraints associated with cation size and solubility. Here, we investigate how the spacer-cation electronic structure biases excited-state pathways in 2D OIHPs using a series of compact, tetrazine-based organic cations with narrow optical gaps (∼2.1 eV) that are successfully incorporated into lead-based 2D perovskites via solution processing. Mixed-anion thin films (50% Br and 50% Cl) are used to shift the inorganic exciton into the visible spectral range, enabling spectroscopic investigation of excited-state processes. Steady-state photoluminescence reveals strong quenching of the inorganic emission, consistent with charge- and/or energy-transfer processes. Spacer-cation-dependent behavior is further examined using photoluminescence excitation (PLE) and transient grating (TG) spectroscopy, which indicate that [MeTzPMA]₂PbBr₂Cl₂ and [MeTzEA]₂PbBr₂Cl₂ favor short-range, Dexter-type energy transfer in competition with charge separation, while [MePTzEA]₂PbBr₂Cl₂ exhibits quenching dominated by charge transfer. Overall, these results reveal spacer-cation electronic structure as a key design parameter for regulating short-range interfacial coupling and excited-state pathways in 2D hybrid perovskites, providing useful principles for tailoring optoelectronic functionality.