Reconfiguring Band-Edge States in 2D Metal Halide Perovskites via Electron-Donating and -Withdrawing Organic Cations.

Hybrid metal halide perovskites consist of the organic cation (A-site) and inorganic skeleton (B-site metal cation and X-site halide anion). Traditionally, the electronic structures near the band edges are mainly controlled by the inorganic component, with the A-site having negligible influence as its electronic states lie far from the band edges. In this work, we show that by adjusting the configuration of organic cations and introducing substituents, the A-site can directly contribute to the band-edge electronic states. Using density functional theory calculations, ab initio molecular dynamics simulations, confocal spectroscopy, and synchrotron high-pressure diffraction, we uncover the origins of band-edge electronic state reconfiguration in 2D hybrid lead halide perovskites. Our results demonstrate that the conjugation of organic cations, combined with the electron-donating and withdrawing substituents, enables the reconfiguration of band-edge electronic states. Specifically, electron-donating groups raise the energy levels of the organic cation, allowing its contribution to the valence band maximum. Conversely, electron-withdrawing groups lower these levels, enabling participation at the conduction band minimum. This mechanism is confirmed by our use of tailored cations together with temperature- and pressure-dependent studies. This work offers a new approach for tailoring the electronic states of hybrid metal halide perovskites with desired optoelectronic performance.