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.