Polymer solar cells based on a furan-containing asymmetric nonfullerene acceptor

An asymmetric nonfullerene small molecule, TF-ORH, containing a nonfused electron-donating core of thiophene, furan and electron-withdrawing octylrhodanine (ORH) end groups was designed as an acceptor for polymer solar cells (PSCs). TF-ORH was synthesized via Knoevenagel condensation and the Suzuki coupling reaction. TF-ORH exhibited sufficient thermal stability and solubility for device fabrication. Compared to solution, TF-ORH film showed a red-shifted UV-Vis absorption with an absorption maximum at 596 nm (J-aggregation), which is complementary with that of the low bandgap polymer donor of poly[4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b:4,5-b']dithiophene-co-3-fluorothieno[3,4-b]thiophene-2-carboxylate] (PTB7-Th). The introduction of furan elevated the molecular orbital energy levels of TF-ORH compared to the thiophene analog, T2-ORH. Finally, a PSC fabricated using a PTB7-Th:TF-ORH blend film as the active layer exhibited a power conversion efficiency of 1.21% with an open-circuit voltage of 0.88 V and a small energy loss of 0.70 eV.