Asymmetric 9,9′-bifluorenylidene-based small molecules as the non-fullerene acceptors for organic photovoltaic cells

Abstract

Three new asymmetric 9,9′-bifluorenylidene-based derivatives, 2,7-dibutoxyl-3′,6′-bis(5-methylenemalononitrile-3-octylthiophen-2-yl)-9,9′-bifluorenylidene (BF-TDCN2), 2,7-dibutoxyl-3′,6′-bis(5-(methylene-indene-1,3-dione)-3-octylthiophen-2-yl)-9,9′-bifluorenylidene (BF-TID2) and 2,7-dibutoxyl-3′,6′-bis(5-(2-methylene-3-oxo-2,3-dihydro-1H-inden-1-ylidene)malononitrile)-3-octylthiophen-2-yl)-9,9′-bifluorenylidene (BF-TDCI2), were successfully synthesized by grafting different electron-withdrawing groups (malononitrile (DCN), 1H-indene-1,3(2H)-dione (ID) and 2-(3-oxo-2,3-dihydro-1H-inden-1-ylidene)malononitrile (DCI)), which were used as the electron acceptors for organic photovoltaic cells. By changing the electron-withdrawing ability of the terminal group, the molecular energy level and band gap can be easily adjusted. The optical bandgaps of the three compounds in the thin films decreased with increasing the electron-withdrawing ability of the terminal group. Besides, the lateral chains of alkoxy groups located at the asymmetric end also play a certain influence on the solubility, molecular aggregation and the miscibility with polymer donor. Among these electron acceptors, the photovoltaic cell fabricated PBDB-T:BF-TDCI2 exhibited a maximum power conversion efficiency of 4.85% with an open-circuit voltage of 0.88 V and a low energy loss of 0.62 eV. By investigating different processing processes, the results showed that the power conversion efficiency can be improved by 20% with simple solvent annealing treatment. Through further study on the morphology and photophysical properties of the active layers, it was found that the processed device had better phase separation size and morphology, which was favorable to enhancing the intermolecular interaction, thus improving exciton separation and charge transfer in the active layer.