Non-fullerene small molecular acceptors based on dithienocyclopentafluorene and dithienocyclopentacarbazole cores for polymer solar cells

Recently, acceptor-donor-acceptor (A-D-A) type of small molecules based on indacenodithieno[3,2-b]thiophene (IDTT) or indacenodithiophene (IDT) as the electron-donating cores were very promising non-fullerene (NF) acceptors to overcome the energy levels and absorption drawbacks of the fullerene-based acceptors in polymer solar cells (PSCs). However, few attempts have been done to develop novel electron-donating cores to construct highly efficient A-D-A acceptors in PSCs. We design and prepare two novel non-fullerene small molecular acceptors (DTF-IC and DTC-IC) based on dithienocyclopentafluorene and dithienocyclopentacarbazole as the electron-donating cores sealed with 2-(3-oxo-indane-1-ylidene)-malononitrile as the electron-withdrawing groups. The two molecules exhibit relatively high lowest unoccupied molecular orbital (LUMO) energy levels and strong absorption in the region from 550 to 740 nm. Relative to the molecule DTF-IC possessing dithienocyclopentafluorene core, the dithienocyclopentacarbazole based acceptor DTC-IC shows slightly red-shifted absorption spectrum, enhanced extinction coefficients, better crystallinity and improved electron mobility. The polymer solar cells based on these two NF acceptors exhibit power conversion efficiencies of 7% when blended with the narrow bandgap polymer poly({4,8-bis[(5-(2-ethylhexyl)-2-thienyl]benzo[1,2-b:4,5-b′]dithiophene-2,6-diyl}{3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4-b]thiophenediyl}) (PTB7-Th) in inverted solar cells.