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 D₁-A-bridge-D₂-bridge-A-D₁ 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 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 J_sc 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.