Hopping transport in perylene diimide based organic solar cells: a DFT approach

Abstract

In this paper, a series of donor–π–acceptor (D–π–A) type oligomers were investigated for their structural and electronic properties through density functional theory (DFT) and time dependent-DFT (TD-DFT) calculations. The designed compounds consist of five different donor units linked at the ortho position of the perylene core of the perylene diimide (PDI) acceptor via five different π-bridging units. We have calculated the dihedral angle, average inter-ring bridge bond distance (l), inter-ring torsional angle (ϕ), distortion energy (ΔE_dis), bond length alteration (BLA) parameter (Δr), energy band gap (Δ_H–L) values, partial density of states (PDOS), ionization potential (IP), electron affinity (EA), molecular electrostatic potential surface (MEPS) analysis, transition dipole moment, reorganization energy (λ), electronic coupling matrix element (V), charge transfer rates (k_CT), hopping mobility (μ_hop), electron density difference (EDD), natural transition orbital (NTO) and spectral absorption properties of the studied compounds. The MEPS plots suggest that the studied compounds have significant charge transport properties. From the EDD map analysis, it is clear that the S_g → S₁ transition involves greater charge transfer from the donor to the acceptor unit via the π-bridging unit in all compounds. Further, the NTO analysis confirms the transfer of electron density from the donor to the acceptor unit after electronic excitation of the compounds. We have also studied the absorption and photovoltaic properties of the D/A blends by considering KM-10 as a standard donor and C₆₀-CN as a standard acceptor. The study suggests that complex KM-10/PDI-BITHN-4 with a theoretical PCE of 19.26% can be chosen as the potential candidate for application in organic solar cells (OSCs). Thus, our study provides recommendations for further development of efficient D/A complexes so as to improve the performance of OSCs.