Theoretical Insights into the Effect of Conjugated Cores on Optical and Photovoltaic Properties of Small Molecule Acceptors for Organic Solar Cells

The research of organic photovoltaic materials based on small molecules is receiving high attention from researchers of organic solar cells (OSCs) to continuously improve their power conversion efficiency (PCE). To enhance the PCE, we have designed four small molecule acceptors P3-P6 based on reported IDT(TCV)(2) (P1) and IDTT(TCV)(2) (P2), with different central aromatic fused rings, and tricyanoethylene (TCV) as the terminal acceptor unit. The ground-state properties, excited-state properties and photovoltaic properties of donor/acceptor blends are calculated by density functional theory and time-dependent density functional theory. Also, the relationship between their photoelectric and structural properties has been discussed. The calculation results show that P5 and P6 can simultaneously increase open-circuit voltages (V-oc), fill-factors (FF), and short circuit current (J(sc)) due to their high lowest unoccupied molecular orbital, small energy gap and high charge collection efficiency. In terms of charge-transfer mechanism, PTB7-Th/P1-P6 can generate CT states through direct excitation and hot exciton, meanwhile there exist more opportunities of producing CT states via intermolecular electric field mechanism. Our results will provide theoretical guidance for further designing and synthesizing of acceptors to improve the performance of OSCs.