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

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 pi-conjugated bridges between indacenodithiophene (IDT) as the electron-donating core and the electron-deficient difluorobenzothiadiazole unit and investigated the effects of the p-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 pi-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 CH2Cl2 solvent annealing exhibited superior device performance to those not subjected to CH2Cl2 solvent annealing. The PCE of BHJ-OSC devices based on BIT4FTT and PC71BM 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 CH2Cl2 vapor due to the obvious increase of both Jsc and FF. Interestingly, the devices based on BIT4FDT and PC71BM 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.