Effects of the length and steric hindrance of π-bridge on molecular configuration and optoelectronic properties of diindole[3,2-b:4,5-b′]pyrrole-based small molecules

Three new A-pi-D-pi-A type small molecules (DIP-T, DIP-HT, and DIP-HTT) employing diindole[3,2-b:4,5-b'] pyrrole (DIP) as the electron-rich donor (D) block, indenedione as the electron-deficient acceptor (A) group, and thiophene, 3-hexylthiophene, or 4-hexyl-2,2'-dithiophene as the conjugated pi-bridge, are designed, synthesized and characterized as donor materials for organic solar cells (OSCs). The effects of the length and steric hindrance of the thiophene-containing pi-bridges on the molecular configuration, optical, electrochemical and charge-transport properties were investigated by theory calculations, UV-vis spectroscopy, cyclic voltammetry, and the space-charge-limited current method, respectively. These DIP-based small molecules (SMs) show excellent solubility and good thermal stability. DIP-T with thiophene as pi-bridge shows wider and stronger absorption, higher-lying HOMO energy level, higher hole mobility and superior active layer morphology in comparison with that of DIP-HT and DIP-HTT with 3-hexylthiophene and 4-hexyl-2,2'-dithiophene as the conjugated pi-bridge, respectively. As a result, the optimized OSC based on DIP-T exhibits an obviously higher power conversion efficiency (PCE) of 4.65% with relative to the other two SMs. So far, this is the first report of developing the A-pi-D-pi-A type SMs with a DIP donor core for application in organic electronic devices. Our results indicate that the pentacyclic DIP can be a potential and attractive donor building block for developing efficient A-pi-D-pi-A type SMs donor materials.