Benzoxadiazole and Benzoselenadiazole as π-Bridges in Nonfullerene Acceptors for Efficient Polymer Solar Cells

Fused-ring electron acceptors with acceptor-donor-acceptor (A-D-A) structure have become a hot research field in recent years. In this work, 2,1,3-benzoxadiazole (BO) and 2,1,3-benzoselenadiazole (BS) were introduced as pi-bridges between indaceno [1,2-b : 5,6-b'] dithiophene (IDT) cores and rhodanine termini for two nonfullerene acceptors IDTBOR and IDTBSR, to compare the structure-property relationship. Inserting BO as the pi-bridge afforded IDTBOR with deeper LUMO and HOMO levels, whereas BS-based IDTBSR possessed a smaller optical band gap. With PBDB-T as polymer donor, IDTBOR- and IDTBSR-based solar cells showed power conversion efficiencies of 7.74 and 6.95%, respectively. A higher open-circuit voltage of 0.95 V was found for IDTBSR, owing to its up-lying LUMO level. A higher short-circuit current density of 15.98 mA/cm(2) and fill factor of 54.67% were achieved for IDTBOR, mainly because of the higher electron mobility and better structure order in the blend film. The results suggest that BO and BS units are valuable pi-bridges to construct efficient nonfullerene acceptors.