Positional Effect of the 2-Ethylhexyl Carboxylate Side Chain on the Thiophene pi-Bridge of Nonfullerene Acceptors for Efficient Organic Solar Cells

Abstract

Three nonfullerene acceptors (NFAs) with an acceptor-pi-donor-pi-acceptor (A-pi-D-pi-A) structure, i.e., IDT-3EsT, IDT-3EsT-2IC2F, and IDT-4EsT-2IC2F, were designed and synthesized by inserting a 2-ethylhexyl carboxylate-substituted thiophene pi-bridge between the indacenodithiophene (IDT) core and acceptor end group [2-(3-oxo-2,3-dihydro-1H-inden-1ylidene)malononitrile (IC) or 2-(5,6-difluoro-3-oxo-2,3-dihydro-1H-inden-1-ylidene)malononitrile (IC2F)]. These acceptors were systematically modified through variation of the end group and the position of 2-ethylhexyl carboxylate on the thiophene pi-bridge. The effect of the 2-ethylhexyl carboxylate group on absorption and energy levels, as well as photovoltaic performance, was investigated. The wide-bandgap PBDB-T polymer was selected as the donor material to fabricate the conventional type of organic solar cells (OSCs) based on the PBDB-T:NFAs blend. Among three NFA-based OSCs, those fabricated using the PBDB-T:IDT-3EsT-2IC2F blend film exhibited an optimized power conversion efficiency (PCE) of 7.54% with a short-circuit current density (J(SC)) of 14.68 mA cm(-2), an open-circuit voltage (V-OC) of 0.88 V, and a fill factor (FF) of 58%. OSCs fabricated using PBDB-T:IDT-3EsT showed a promising PCE of 7.43% with a significantly improved V-OC of 0.98 V. PBDB-T:IDT-4EsT-2IC2F-based OSCs showed the lowest device performance with a PCE of 5.45% due to the formation of larger domains in the PBDB-T:IDT-4EsT-2IC2F blend film, which had adverse effects on exciton diffusion, dissociation, and charge transport properties. These results demonstrated that modifying the position of substituents on the thiophene pi-bridge of NFAs is critical in determining the photovoltaic performance of OSCs.