Effect of Structure Modification of Benzothiadiazole Acceptor Unit on the D-A-D-A-D Typed Oligothiophene Based Donor Materials for Organic Small Molecules Solar Cells: A Theoretical Study

Four D-A-D-A-D structured organic small molecules (OSMs) DOBT-8T, BT-8T, FBT-8T and FFBT-8T have been designed for organic solar cells, which contain tetrathiophene as the core donor unit, bithiophene as the termial donor unit, combining different electron-withdrawing fragments DOBT, BT, FBT and FFBT as acceptor unit, respectively. The designed four OSMs were analyzed using density functional theory (DFT) and time dependent-DFT (TDDFT) calculations at B3LYP/6-31G(d) level. The effects of structure modification of benzothiadiazole acceptor unit on modulating the electron-donating ability of OSMs were fully investigated. Results showed that the geometrical structure, the band-gap, HOMO/LUMO energy levels, orbital spatial distribution, energetic driving force, open-circuit voltage and NPA atomic charge of these OSMs can be systematically altered by varying the electron-withdrawing properties with different benzothiadiazole acceptor units. Compared to other OSMs, FBT-8T displayed the more narrowed E, and relatively deeper HOMO level with FBT as acceptor. FFBT-8T exhibited the most deep-lying HOMO level of the four designed OSMs and suitable E-g value by using FFBT as acceptor. The power conversion efficiencies (PCEs) of ca. 4.7% and ca. 5.2% were achieved by the photovoltaic devices based on FBT-8T:PC61BM and FFBT-8T:PC61BM systems, respectively, predicting with Scharber models. On the basis of these results, FBT-8T and FFBT-8T as potential OSMs donor materials for high-efficiency organic bulk hetero-junction solar cell were proposed.