Synthesis and Characterization of a 2-(1,1-Dicyanomethylene) rhodanine-based Nonfullerene Acceptor for OPVs

We synthesized an A–π–D–π–A-type nonfullerene small molecule acceptor (Flu-CNRH) based on fluorene and 2-(1,1-dicyanomethylene)rhodanine (CNRH) by a Knoevenagel condensation of the diformyl compound (Flu-T-CHO) and CNRH for the use as acceptors in organic photovoltaic cells (OPVs). Introduction of CNRH end groups effectively lowered the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) energy levels of the acceptor compared to those of the corresponding rhodanine-based acceptor (Flu-RH). The HOMO and LUMO levels of Flu-CNRH are −5.71 and −3.60 eV, respectively. For comparison, Flu-RH has HOMO and LUMO energy levels of −5.58 and −3.53 eV, respectively. This lowered LUMO energy level enables Flu-CNRH to be combined with low bandgap polymer donors PTB7 and PTB7-Th by providing proper LUMO offsets between donor and acceptor. OPV cells were fabricated with the device configuration of ITO/PEDOT:PSS/active layer/LiF/Al. The best power conversion efficiency of 1.47% was obtained in the P3HT:Flu-CNRH devices at a D:A ratio of 1:1.5 and with annealing at 140°C. Moreover, Flu-CNRH showed compatibility with low bandgap polymer donor PTB7 (1.22%) with external quantum efficiency response at a longer wavelength region up to 750 nm, while Flu-RH had no photovoltaic behavior with PTB7 and PTB7-Th.