Tetrafluorene-9,9′-bifluorenylidene as a non-fullerene type electron acceptor for P3HT-based bulk-heterojunction polymer solar cells

2,2′,7,7′-Tetra(9,9-diethyl-9H-fluorene)-9,9′-bifluorenylidene (TFBF) was synthesized as a non-fullerene type electron acceptor for bulk-heterojunction polymer solar cells. The TFBF was synthesized from 2,7-di(9,9-diethyl-9H-fluorene)-9H-fluoren-9-one by a reductive dimerization using Lawessonʹs reagent. The TFBF was soluble in common organic solvents such as dichloromethane, chloroform, and 1,2-dichlorobenzene. The synthesis of TFBF was identified by 1H NMR, 13C NMR, FAB-Mass, and FT-IR, and the properties of the molecule were characterized by UV–visible absorption and fluorescence spectroscopy. The ionization potential of the TFBF solid thin film was determined by photo-electron spectroscopy, and morphology of the active layer was investigated by atomic force microscope. The measured optical band gap, the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) of TFBF were 2.32, −5.81, and −3.49 eV, respectively. Photoluminescence (PL) spectra of P3HT solution in chloroform were measured at different TFBF concentration. The PL intensity of P3HT solution was decreased by increasing the TFBF concentration. The electron mobility of P3HT:TFBF blend film was measured to be 6.82×10−5 cm2/Vs at the electric field of 500 (V/cm)1/2. Photovoltaic devices were fabricated with the structure of ITO/PEDOT:PSS/P3HT:TFBF/LiF/Al. One of the fabricated device showed the highest Voc of 0.70 V with Jsc=1.20 mA/cm2, FF=0.30, and PCE=0.26% at donor to acceptor composition ratio of 1:0.4.