Controlling the excited-state energy levels of 9,9′-bifluorenylidene derivatives by twisting their structure to attaining singlet fission character in organic photovoltaics

Abstract

9,9′-Bifluorenylidene (BFN) derivatives, in which the two fluorene moieties are connected via a CC double bond at the 9-position, are expected to show singlet fission (SF) character in organic photovoltaics (OPVs). Due to steric repulsion between the hydrogen atoms at the 1,8- and 1′,8′-positions, the two fluorene planes of BFN are twisted by 31° with respect to each other. DFT calculations suggest that this deviation from coplanarity lowers the T₁ energy level of BFN to approximately half that of S₁. We synthesized a molecular alkylbithiophene-substituted BFN and a copolymer of diketopyrrolopyrrole and BFN. The optoelectronic properties of these compounds were investigated, and OPV devices using these BFNs as a p-type material were fabricated. The device composed of the polymeric BFN derivative achieved a power conversion efficiency of 4.9%. The observed negative dependence of the photocurrent on the magnetic field suggested that the triplet excitons of the molecular BFN derivatives contribute to the photocurrent in these OPV devices.