Effect of aromatic π-bridges on molecular structures and optoelectronic properties of A-π-D-π-A small molecular acceptors based on indacenodithiophene

Investigation on the relationship between molecular structure and device performance is of great important to develop highly efficient A-π-D-π-A small molecular acceptors (SMAs). However, there is still lack of a complete and in-depth study on effects of π-bridge on molecular structure, optoelectronic properties and photovoltaic performances. Herein, we reported the design, synthesis and photovoltaic application of four A-π-D-π-A type SMAs, denoted as IDT-Py-IC, IDT-Fu-IC, IDT-Th-IC, and IDT-Ph-IC, which possess an identical central D unit of indacenodithiophene and the terminal A group of 3-(dicyanomethylidene)indol-1-one, linked by various aromatic π-bridges of pyrrole, furan, thiophene, and benzene, respectively. The impact of the different aromatic π-bridge on molecular structures, optoelectronic and photovoltaic properties as well as active layer morphologies was comprehensively explored. Results show that both molecular co-planarity and electron-donating ability of aromatic π-bridges distinctly affect optical bandgaps (Egopt) and HOMO/LUMO levels of these SMAs. The poor backbone planarity of pyrrole-bridged IDT-Py-IC observed by theory calculation leads to a blue-shifted absorption and up-shifted HOMO/LUMO levels. The Egopt of these SMAs is gradually increased and HOMO levels are gradually down-shifted with the decrease of the electron-donating ability of aromatic π-bridges. Polymer solar cells (PSCs) based on these SMAs exhibit a high Voc over 0.93 V, especially for PBDB-T:IDT-Py-IC-based PSCs, producing a rather high Voc up to 1.06 V due to the high-lying LUMO level. After optimizations, the PBDB-T:IDT-Th-IC-based PSC outperforms the other three SMAs with a high PCE up to 8.72% mainly due to the large Jsc and FF, which could be ascribed to better absorption characteristics, higher and more proportional carrier mobility, efficient exciton dissociation and charge collection, reduced bimolecular recombination and superior active layer morphology. This finding demonstrates that the π-bridge plays a crucial role in tailoring molecular structures, optoelectronic properties and device performance of A-π-D-π-A type SMAs.