Tricyclic or Pentacyclic D Units: Design of D-π-A-Type Copolymers for High VOC Organic Photovoltaic Cells.

Although there are several electron-donating (D) units, only the classic benzo[1,2-b:4,5-b']dithiophenes (BDT) unit was utilized to develop D-π-A-type copolymers for high-voltage organic photovoltaic (OPV) cells. Hence, in this work, we chose two tricyclic D units, BDT and benzo[1,2-b:4,5-b']difurans (BDF), together with one pentacyclic ring, dithieno[2,3-d;2',3'-d']benzo[1,2-b;4,5-b']dithiophenes (DTBDT), to comprehensively study the effect of different D units on the optoelectronic properties and photovoltaic performance. By copolymerized with the benzo[1,2,3]triazole (BTA) electron-accepting unit, the final copolymers J52-Cl, F11, and PE52 were combined with a nonfullerene acceptor (NFA) F-BTA3 according to the "Same-A-Strategy." As we preconceived, all the three single-junction OPV cells can obtain high open-circuit voltage (VOC) over 1.10 V. Although the tricyclic D unit of BDF exhibits a slightly lower VOC of 1.12 V because of its mildly larger energy loss of 0.698 eV, its higher carrier mobilities and exciton dissociation efficiency strikingly boost the short-circuit current (JSC) and fill factor, which contribute to a comparable PCE of 10.04% with J52-Cl (10.10%). However, the DTBDT-based polymer PE52 shows the worst performance with a PCE of 6.78% and a VOC of 1.14 V, owing to the higher bimolecular recombination and disordered molecular stacking. Our results indicate that tricyclic D units should be a better choice for constructing D-π-A-type polymers for high-voltage photovoltaic materials than the pentacyclic analogues.