The Impact of Diammonium Cation Dipole Moment on Charge Transport in 2D/3D Perovskite

2D/3D heterojunction perovskite solar cells have emerged as a highly promising photovoltaic architecture, combining high efficiency and exceptional long-term stability. Understanding the energy band alignment at the 2D/3D interface is crucial for optimizing device performance. In this study, we utilize a multiscale computational framework─incorporating density functional theory, ab initio molecular dynamics, and nonadiabatic molecular dynamics simulations─to investigate how dipole engineering of diammonium cations in Dion-Jacobson (DJ) perovskites influences band structure, charge carrier dynamics, and nonradiative recombination mechanisms. Our results demonstrate that increasing the alkyl chain length of diammonium cations significantly enhances the electrostatic potential polarization. This modification not only increases the dipole moments but also strengthens the hydrogen bonding interactions with adjacent iodide anions. Notably, the increased dipole moment shifts the heterojunction band alignment from type-I to type-II, facilitating the spatial delocalization of electron-hole, reducing pure-dephasing and nonadiabatic coupling, thereby suppressing nonradiative recombination. Moreover, the dipole-induced built-in electric field promotes upward band bending and enhances work function, which together improve spatial charge localization, extend carrier recombination distances, and shorten carrier transport paths─optimizing carrier dynamics across the 2D and 3D phases. Additionally, the hydrogen bonding between diammonium cations and the [PbI₆]^4- framework suppresses FA rotation and strengthens cation-inorganic dynamic coupling, leading to reduced atomic vibrations. Rigid diammonium cations enhance low-frequency phonon vibration modes, which stabilize the type-II heterojunction and weaken nonadiabatic coupling. Conversely, π-conjugated diammonium cations introduce higher-frequency and molecular phonon vibration modes, accelerating the charge transport. This study establishes a mechanistic link between diammonium cations, dipole engineering, band structure modulation, and carrier dynamics in DJ-phase 2D/3D perovskites, providing essential design principles for developing high-efficiency and stable perovskite photovoltaics.