Driving Force for Exciton Dissociation in Organic Solar Cells: The Influence of Donor and Acceptor Relative Orientation

Abstract

The presence of a favorable driving force for electron transfer (ΔG) at the donor/acceptor (D/A) interface is fundamental to overcome the exciton binding energy and improve the efficiency of organic photovoltaic devices (OPVs). Using density functional theory (DFT), time-dependent DFT, and classical molecular dynamics, here we systematically investigate the dependence of ΔG on the molecular orientation between the donor and the acceptor in the dimer. This study was performed by considering the model configuration of D/A species of four donor A–D–A molecules named DRCN­(4–7)­T and C60 acceptor. We identified two configurations that gave the highest negative values of ΔG (which favors exciton dissociation): (i) face-on orientation when the fullerene is located at the central thiophene ring of the DRCN­(4–7)­T molecule. In this situation, the high degree of hybridization between the highest occupied molecular orbital of the acceptor and the lowest unoccupied molecular orbital of the donor stabilizes the energy of the lowest charge transfer state (CT1). (ii) Edge-on orientation when C60 is close to the positive partial charges of the D molecule. In this configuration, the fullerene further polarizes the D molecule which enhances the dimers’ dipole moment. The electrostatic potential produced by this dipole decreases then the energy of the CT1 state. Molecular dynamics calculations performed to simulate the deposition of DRCN5T/C60 blends showed that dimers with a very negative driving force (as identified by our DFT calculations) are indeed present in the film. We also found a tweezers-like effect produced by the lateral alkyl groups attached to the central thiophene ring of the DRCN5T molecule. Those lateral groups can capture the fullerenes and guide them toward the center of the molecule during the solvent evaporation. This process would favor the formation of face-on dimers that tend to have a negative ΔG. The comparison of our data with experimental results measured in DRCN­(4–7)­T suggests that there is a correlation between negative values of ΔG (averaged over some D/A configurations) and the performance of the corresponding solar cell. The fine-tuning of the D/A configuration can make the difference to decrease the germinated recombination and improve the efficiency of OPVs.