Unveiling the important role of non-fullerene acceptors crystallinity on optimizing nanomorphology and charge transfer in ternary organic solar cells

The crystallinity of non-fullerene acceptors as the third component has an important role on nanomorphology optimization and charge transfer dynamics of ternary organic solar cells (OSCs). Herein, efficient ternary OSCs were fabricated by incorporating two typical non-fullerene acceptors with different crystallinity, (Z)-5-{[5-(15-{5-[(Z)-(3-Ethyl-4-oxo-2-thioxo-1,3-thiazolidin-5-ylidene)methyl]-8-thia-7.9-diazabicyclo [4.3.0] nona-1 (9),2,4,6-tetraen-2-yl}-9,9,18,18-tetrakis (2-ethylhexyl)-5.14-dithiapentacyclo [10.6.0.0(3,10).0(4,8).01(3,17)]octadeca-1(12),2,4(8),6,10,13 (17),15-heptaen-6-yl)-8-thia-7.9-diazabicyclo [4.3.0]nona-1 (9),2,4,6-tetraen-2-yl]methylidene}-3-ethyl-2-thioxo-1,3-thiazolidin-4-one (EH-IDTBR) or (5Z, 5'Z)-5,5'-((7,7'-(4,4,9,9-tetraoctyl-4,9-dihydro-s-indaceno [1,2-b:5,6-b']dithiophene-2,7-diyl)bis (benzo [c][1,2,5]thiadiazole7,4diyl)) bis(methanylylidene))bis (3-ethyl-2-thioxothiazolidin-4-one) (O-IDTBR), into the host donor/acceptor active layers comprising of poly (3-hexythiophene-2,5-diyl) (P3HT) and [6,6]-phenyl-C71-butyric acid methylester (PC71BM). As a result, the 21.5% and 22.7% increase of the power conversion efficiency (PCE) for the two ternary systems were achieved, respectively, which was attributed to the enhanced light harvesting capability, optimized bulk-heterojunction morphology and the formation of cascade energy level alignments that could introduce an additional pathway for efficient charge transfer. Although both the short-circuit current density (Jsc) and fill factor (FF) values were increased significantly by regulating the weight ratios of non-fullerene acceptors of two ternary systems, the O-IDTBR-based ternary OSCs showed the higher Jsc while P3HT:EH-IDTBR:PC71BM system exhibited the higher FF values. The main difference of improved photovoltaic performance in the two ternary systems could be associated with the different blend morphology and charge carrier mobilities. In addition, nanomorphology studies suggested that lamellar stacking coherence lengths of P3HT in face-on orientation for EH-IDTBR- and O-IDTBR-based systems can be increased from 18.48 nm to 20.94 nm and 21.67 nm respectively, resulting from the stronger crystallinity of O-IDTBR than that of EH-IDTBR, which was beneficial for charge transport in the vertical direction. These results indicate that selecting the appropriate crystalline non-fullerene acceptors may be an effective strategy to optimize nanomorphology to further achieve high efficiency ternary OSCs.