Efficient and Stable Nonfused Ring Small Molecule Acceptors Powered by an Electron Donating Unit for Organic Solar Cells

Designing electron acceptor materials with A-D-A '-D-A type structure is an effective way to achieve high-performance and low-cost organic solar cells (OSCs). Currently, almost all the A-D-A '-D-A type acceptors contain an sp3- hybridized bridge carbon in their donor part (D), which makes it possible to introduce bulky side chains to inhibit excessive intermolecular aggregation. However, the existence of an sp3-hybridized bridge carbon will deteriorate the photostabilities of the acceptors. Herein, an acceptor named TPD-BT bearing a 3,4-bis((2-ethylhexyl)oxy)-4 '-octyl-2,2 '-bithiophene (BT) donor unit was demonstrated to display a planar backbone and finely regulated aggregation. For comparison, the acceptor TPD-TT with a 3,6-bis((2-ethylhexyl)oxy)thieno[3,2-b]thiophene (TT) as donor unit was also synthesized and studied. Although both TPD-BT and TPD-TT exhibited good photostabilities, TPD-BT demonstrates more ordered packing and lower voltage loss. As a result, the power conversion efficiency (PCE) of the PBDB-T:TPD-BT-based device is 10.33%, which is higher than that of the PBDB-T:TPD-TT-based device (8.95%). All the results proved that introducing 3,4-bis((2-ethylhexyl)oxy)-4 '-octyl-2,2 '-bithiophene as a D unit is an effective approach to construct A-D-A '-D-A type acceptors, paving the way toward low-cost and photostable OSCs.