Toward high performance indacenodithiophene-based small-molecule organic solar cells: investigation of the effect of fused aromatic bridges on the device performance

• Published in: Journal of materials chemistry. A, Materials for energy and sustainability Materials for energy and sustainability, 2016-01, Vol.4 (6), p.2252-2262
• Main Authors: Wang, Jin-Liang, Xiao, Fei, Yan, Jun, Liu, Kai-Kai, Chang, Zheng-Feng, Zhang, Ru-Bo, Wu, Hong-Bin, Cao, Yong
• Format: Article
• Language: English
• Subjects: Annealing; Blends; Devices; Energy levels; Molecular orbitals; Photovoltaic cells; Solar cells; Solvents
• ISSN: 2050-7488; 2050-7496
• DOI: 10.1039/c5ta09739h

Here we report the synthesis of a pair of D 1 -A-bridge-D 2 -bridge-A-D 1 type small molecules BIT4FDT and BIT4FTT which have different π-conjugated bridges between indacenodithiophene (IDT) as the electron-donating core and the electron-deficient difluorobenzothiadiazole unit and investigated the effects of the π-conjugated bridges on their photovoltaic properties. We found that the molecule BIT4FTT , containing thieno[3,2- b ]thiophene which has two fused thiophene rings as the π-conjugated bridges, exhibits different photophysical properties, HOMO/LUMO energy levels, charge carrier mobilities and morphologies of blend films, and photovoltaic properties compared with the analogous system BIT4FDT which has 2,2′-bithiophene rings as the conjugated bridges. Moreover, the devices based on the two molecules after CH 2 Cl 2 solvent annealing exhibited superior device performance to those not subjected to CH 2 Cl 2 solvent annealing. The PCE of BHJ-OSC devices based on BIT4FTT and PC 71 BM increased from 5.85% to 7.57% ( J sc = 11.33 mA cm −2 , V oc = 0.89 V, and FF = 0.75) after exposure to CH 2 Cl 2 vapor due to the obvious increase of both J sc and FF. Interestingly, the devices based on BIT4FDT and PC 71 BM showed a weaker response to solvent vapor annealing and much lower PCEs in comparison with those based on BIT4FTT . The results indicate that highly efficient small-molecule solar cells can be achieved using fused aromatic bridges and a suitable solvent vapor annealing process. Here we report the synthesis of a pair of D 1 -A-bridge-D 2 -bridge-A-D 1 type small molecules, which have different π-conjugated bridges and investigated the effects on their photovoltaic properties.