(1. School of Chemical and Biological Engineering, Lanzhou Jiaotong University, Lanzhou 730070, China) （2. Key Laboratory of OptoElectronic Technology and Intelligent Control (Ministry of Education), Lanzhou Jiaotong University, Lanzhou 730070, China）（3. School of Mechatronic Engineering, Lanzhou Jiaotong University, Lanzhou 730070, China)
As a new type of organic photovoltaic materials, 9,9′-bifluorenylidene (9,9′-BF) derivatives have attracted much scientific attention due to the aromaticity and twisted structure of the 9,9′-bifluorenylidene core. 9,9′-Bifluorenylidene derivatives contain two rigid coplane biphenyl units which can lead to good thermal and chemical stability of compounds, moreover, they have excellent photoelectric properties. These compounds could easily accept one electron to form 14-π-electron system, and thus they have aromaticity and improve the LUMO energy level of materials for increasing the open circuit voltage in organic photovoltaic (OPV) cells. In addition, 9,9′-bifluorenylidene has 12 different substitution positions, they can be easily replaced by suitable groups to systhesize the needed molecules, and its structure is more flexible than fullerene derivatives. However, the bulk heterojunction organic photovoltaic cells based on the reported 9,9′-bifluorenylidene derivatives have lower power conversion efficiency (PCE) because of their lower electron mobility. The key issues for improving their electron mobility and spectral absorption in the UV-vis region have been attractive. In this paper, we review the recent advances on 9,9′-bifluorenylidene derivatives as organic optoelectronic functional materials in organic photovoltaic cells, and the performance of these materials is analyzed from the view of molecular structures. Finally, the prospect of the development of bifluorene-based materials is addressed.