Nonadiabatic molecular dynamics simulations of charge transport in a covalent organic framework.

Abstract

Two-dimensional π-conjugated covalent organic frameworks (COFs) have recently been shown to exhibit exceptionally high charge-carrier mobilities, challenging conventional views of charge transport in organic materials. In this study, we investigate the microscopic origin of charge transport in single-layer phthalocyanine-based poly(benzimidazobenzophenanthroline) ladder-type COFs using large-scale nonadiabatic molecular dynamics simulations based on ab initio-parameterized Holstein-Peierls Hamiltonians. Density functional theory calculations reveal narrow yet dispersive valence bands with small reduced effective masses and weak electron-phonon coupling, reflecting the rigid, fully fused backbone of the framework. To go beyond static band-structure descriptions, here we employ mixed quantum-classical surface-hopping simulations that explicitly account for both local and nonlocal electron-phonon interactions in large two-dimensional lattices. Despite the limited electronic bandwidths, the simulations predict band-like hole transport with a power-law temperature dependence and room-temperature mobilities exceeding 103 cm2 V-1 s-1. This unusually high mobility is attributed to the exceptionally low dynamical energetic disorder and suppressed coupling fluctuations enabled by the structural rigidity and long-range order of the COF lattice. In contrast, the introduction of moderate static disorder leads to rapid localization and a crossover to hopping-dominated transport. These results provide a microscopic understanding of ultrahigh charge mobilities in ladder-type two-dimensional COFs and establish key design principles for achieving efficient charge transport in organic framework materials.