Synthesis, optical and electrochemical properties of small molecules DMM-TPA[DTS(FBTTh3)3] and TPA[DTS(FBTTh3)3], and their application as donors for bulk heterojunction solar cells

Abstract

Two symmetrical planar star-shaped organic small molecules, i.e., DMM-TPA[DTS(FBTTh₃)₃] (1) and TPA[DTS(FBTTh₃)₃] (2), with fused TPA and TPA core donors, respectively, and three branched DTS(FBTTh₃) units were synthesized and characterized. These small molecules were used donor materials along with a PC₇₁BM acceptor for solution-processed bulk heterojunction solar cells. The power conversion efficiency (PCE) of the solar cells based on 1 and 2 is about 2.87% and 2.51%, respectively, when the active layers were processed from a CB solvent. The higher PCE of the solar cell based on 1 may be attributed to its low bandgap and broad absorption profile as compared to 2. The PCE of the solution-processed SMBHJ solar cells was improved up to 5.16% and 4.70% for 1 and 2, respectively, when the active layers were processed with 4 v% DIO as an additive in the CB solvent. The enhancement in the PCE was mainly due to the increase in J_sc and FF, which is attributed to the balanced charge transport between the electron and hole transport and reduction in the bimolecular recombination, leading to an increase in the PCE.