Photo-induced Charge Transfer of Fullerene and Non-Fullerene Conjugated Polymer Blends via Density Functional Theory*

Donor-acceptor (D-A) type semiconducting conjugated polymers (CPs) are promising candidates for organic photovoltaic (OPV) devices due to their unique tunable mechanical compliance and optoelectronic performance. One of the most important parameters in the PV devices is photo-induced charge transfer (CT) at the interface of the CP and acceptor unit.  It would be largely beneficial to computationally examine the ability of a different molecular configuration serving as an efficient charge transfer device before any synthesis process to narrow down search list of possible high-performance CPs. In this study, we employ density functional theory (DFT) to explore photo-induced charge transfer of diketopyrrolopyrrole (DPP) based polymer as a blend with non-fullerene (ITIC) and fullerene (PCBM) acceptor units.  To evaluate the efficiency of charge transfer, we study the non-radiative relaxation of photoexcited electrons and holes using the reduced density matrix on the basis of the Redfield theory. Non-adiabatic couplings between electronic orbitals are computed based on nuclear trajectories obtained from ab initio calculations. We track the relaxation rates of charge carriers over time, where the derivative of difference between the rate of electron and hole can qualitatively represent the current density at zero voltage. This can be utilized to characterize the charge transfer performance of CPs blended with different acceptor units. Relaxation rate results indicate that CPs blend with ITIC offers a better PV effect, illustrating the potential of the current approach to explore CPs blend electronic performance for OPV devices and narrowing down the list of potential candidate CPs.