Oligomeric semiconductors enable high efficiency open air processed organic solar cells by modulating pre-aggregation and crystallization kinetics

Abstract

Although massive progress in power conversion efficiencies (PCEs) has been made on organic solar cells (OSCs), the challenge remains to scale up high-performance OSCs that are compatible with industrial production conditions, such as post-treatment-insensitive, non-halogenated solvent, large-area printing, processing under atmospheric environment. In this study, two A–D–A–D–A type oligomers, named 5BDDBDT-F and 5BDDBDT-Cl, were designed and synthesized to be incorporated into binary OSCs. Ex situ and in situ characterizations were performed to reveal the underlying working mechanisms. The two-phase transitions: liquid-to-liquid and liquid-to-solid can be synergistically tailored to promote the molecular arrangement of the binary blend. Benefiting from the enhanced crystallinity, the oligomer can act as a bridging linker to rehabilitate the interfacial defects to reduce the trap-assisted recombination losses and thus the non-radiative recombination losses. As a result, using non-halogenated solvent (o-xylene), the best-performing OSCs with PCEs of 18.32% with a VOC of 0.850 V, an FF of 79.15% were achieved based on the PM6:5BDDBDT-F:BTP-eC9 ternary OSCs, and 18.43% with a VOC of 0.854 V, an FF of 79.29% were obtained based on PM6:5BDDBDT-Cl:BTP-eC9 ternary OSCs without any post-processing. More importantly, it is noteworthy that the large-area (1 cm2) blade coated devices based on PM6:5BDDBDT-F:BTP-eC9 and PM6:5BDDBDT-Cl:BTP-eC9, processed using non-halogenated solvent in an open-air environment without thermal-annealing treatment, obtained high PCE values of 17.11% and 17.06%, respectively, which are among the highest PCE values of blade-coated OSCs in open air. This study demonstrated an efficient strategy of controlling crystallization kinetics to improve the material crystallinity, as an effective guideline for achieving high photovoltaic performance in printing ternary OSCs, pushing the OSCs towards future manufacturing. Broader context: Along with the rapidly improved performance of organic solar cells (OSCs), it is imminent to develop efficient green solvent-processed OSCs towards industrial production requirements. Herein, in the halogen-free solvent processed OSCs, we report an efficient oligomer ternary strategy where the oligomer can act as a bridging linker to rehabilitate the interfacial defects to reduce the trap-assisted recombination losses and thus the non-radiative recombination losses by modulating pre-aggregation and crystallization kinetics. As a result, the best performing OSCs with PCEs of 18.32% and 18.43% were achieved based on the PM6:5BDDBDT-F:BTP-eC9 and PM6:5BDDBDT-Cl:BTP-eC9 ternary OSCs, respectively, without any post-processing. More importantly, the large-area (1 cm2) halogen-free solvent blade coated devices based on PM6:5BDDBDT-F:BTP-eC9 and PM6:5BDDBDT-Cl:BTP-eC9 processed in air without thermal-annealing obtained high PCE of 17.11% and 17.06%, respectively.