Phenyl-1,3,5-Trithienyl-Diketopyrrolopyrrole: A Molecular Backbone Potentially Affording High Efficiency for Solution-Processed Small-Molecule Organic Solar Cells through Judicious Molecular Design

摘要

Finding new molecular backbones is necessary for further advances in solution-processed small-molecule organic solar cells (SM-OSCs). Increasing molecular π conjugation generally enhances the light-harvesting ability, and the resulting strong π-π-stacking interactions improve the charge-carrier transport ability; both increase the efficiency. In this study, we focus on the phenyl-1,3,5-trithienyl (3T-P) backbone because of its C_3 symmetry, planarity, and particularly high conjugation between the three arms through the core phenyl unit. When the three arms were functionalized with diketopyrrolopyr-role (DPP) units to afford 3D-T-P, only modest efficiency was achieved (1.16%). Introduction of 4,8-bis(2-(2-ethylhexylthienyl)) benzodithiophene (BDT) between the 3T-P and DPP units to give 3D-B-T-P enhanced the light-harvesting ability, and particularly improved the hole mobility by 1.5 orders of magnitude (5.91 × 10~(-2) versus 1.05 × 10~(-3) cm~2 V~(-1) s~(-1)). When using PC71BM as the acceptor material, 3D-B-T-P gave the best power conversion efficiency (PCE) of 2.27%, which is about 1.9 times higher than the best efficiency from 3D-T-P (≈1.16%). The efficiency can be improved up to 3.60% with 3% (v/v) of 1,8-diiodooctane (DIO) as the cosolvent and thermal annealing at 100℃ for 10 min. This PCE is, to the best of our knowledge, the highest efficiency reported to date among the phenyl-1,3,5-based C_3-symmetric molecules. Removing one DPP unit from 3D-T-P to form 2D-T-P, or from 3D-B-T-P to form 2D-B-T-P both decreased the light-harvesting ability and the hole mobility, thereby affording lower efficiency. Taken together, our results demonstrate that the planar phenyl-1,3,5-trithienyl-based C_3-symmetric structure can be a promising backbone, and enhancing the conjugation of the 3D-T-P backbone can effectively improve the device performance.