Phenyl-1,3,5-trithienyl-diketopyrrolopyrrole: a molecular backbone potentially affording high efficiency for...

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 C3 symmetry, planarity, and particularly high conjugation between the three arms through the core phenyl unit. When the three arms were functionalized with diketopyrrolopyrrole (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 °C 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 C3-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 C3 -symmetric structure can be a promising backbone, and enhancing the conjugation of the 3D-T-P backbone can effectively improve the device performance.