Electron-Accepting π-Conjugated Systems for Organic Photovoltaics: Influence of Structural Modification on Molecular Orientation at Donor–Acceptor Interfaces

In organic photovoltaics (OPVs) using nonfullerene acceptors, the fine-tuning of interfaces between donor and acceptor in the bulk-heterojunction (BHJ) structure has become an important factor to improve the performance. A series of electron-accepting π-conjugated compounds based on benzothiadiazole and arenedicarboximides were systematically synthesized to investigate the impact of structural modification on molecular orientation at donor–acceptor interfaces. X-ray diffraction and surface free energy measurements of these compounds in the film state revealed that the crystallinity correlates with the London dispersion (γ d ) and the polar components of their interfacial energies. BHJ solar cells prepared with our π-conjugated compounds as acceptors and poly(3-hexyl)thiophene as a donor exhibited that the structural modification exerts a significant influence on the photovoltaic characteristics, and afforded the highest power conversion efficiency of 2.05%. Absorption, photoluminescence, and carrier mobility measurements of the blend films showed that the OPV performance of our system are mainly governed by the efficiency of charge-separation into free carrier at the donor–acceptor interfaces. Furthermore, a strong correlation was found between the short-circuit current density of OPV and γ d of acceptors, indicating that this quantity promotes the formation of desirable charge-separated states. The findings provide novel information for the development of nonfullerene acceptors for OPVs.