Small Molecules Based on Alkyl/Alkylthio-thieno[3,2-b]thiophene-Substituted Benzo[1,2-b:4,5-b′]dithiophene for Solution-Processed Solar Cells with High Performance

Two acceptor donor acceptor small molecules based on thieno[3,2-b]thiophene-substituted benzo[1,2-b:4,5-b]dithiophene, DRBDT-TT with alkyl side chain and DRBDT-STT with alkylthio side chain, were designed and synthesized. Both molecules exhibit good thermal stability, suitable energy levels, and ordered molecular packing. Replacing the alkyl chain with alkylthio increases the dihedral angle between the thieno[3,2-b]thiophene (TT) and benzo[1,2-b:4,5-b']dithiophene (BDT) unit, and thus slightly decreases its intermolecular interactions leading to its blue-shift absorption in the solid state. The best devices based on DRBDT-TT and DRBDT-STT both exhibited power conversion efficiencies (PCEs) over 8% with high fill factors (FFs) over 0.70 under AM 1.5G irradiation (100 mW cm(-2)), which are attributed to their optimized morphologies with feature size of 20-30 nm and well-balanced charge transport properties. The devices based on DRBDT-STT exhibited relatively lower short-circuit current density (J(sc)) and thus slightly lower PCE as compared to the devices of DRBDT-TT, mainly due to its relatively poorer absorption. These results demonstrate that thieno[3,2-b]thiophene-substituted benzo[1,2-b:4,5-b']dithiophene derivatives could be promising donor materials for obtaining high efficiencies and fill factors.