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 pi-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 (gamma(d)) and the polar components of their interfacial energies. BHJ solar cells prepared with our pi-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 gamma(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.