Nonadiabatic Molecular Dynamics Modeling of the Intrachain Charge Transport in Conjugated Diketopyrrolo-pyrrole Polymers

Understanding the carrier transport processes and predicting the carrier mobility from first principle in organic electronic materials has been a longstanding challenge. We have applied the nonadiabatic Ehrenfest dynamics coupled with density functional tight binding (DFTB) to investigate the carrier motion in the donor–acceptor type polymer for photovoltaics. The equations of motion for the electrons are evolved under the fixed subspace spanned by the active molecular orbitals during each nuclear time step, and the feedback from charge to the nuclei motions, namely, the polaronic effect, is considered. We then use this methodology to investigate the charge transport dynamics for the ladder-type poly­(p-phenylenes) (LPPP) and poly­(diketopyrrolo-pyrrole (DPP)) series with ∼2 × 10³ atoms. The carrier mobilities are evaluated via the diffusion process. It was found that the diffusion abilities are determined by the magnitude of transfer integrals and localization length for frontier orbital, which is caused by the self-trapping effects (polaron) arising from the double bond stretching and twisting motions. This method can be useful in exploring the underlying charge transport behavior and improving the structure design of materials in organic electronics.