Developing an efficient thiophene additive for 18.53% efficiency of binary polymer solar cells by adopting asymmetric strategy

Solvent additives have been proved as a simple and effective method to optimize molecular stacking and control the active layer morphology of polymer solar cells (PSCs). However, most of the reported additives are difficult to weed out, which result in morphological degradation and worse stability of PSCs. To solve this problem, an efficient solvent additive 2-bromo-5-iodothiophene (BIT) has been developed and functions as a morphology-controlling agent for PM6:Y-series binary PSCs. Studies indicate that the BIT can be weed out easily and asymmetric effect of BIT enables it mixed well and formed dimers with nonfullerene accepter Y6 with the smallest steric hindrance in active layer, which is of great benefit to achieve a tighter intermolecular packing after annealing treatment when compared with the PSCs processed with 2,5-dibromothiophene (DBT) and 2,5-diiodothiophene (DIT). Therefore, the BIT treated active layer achieves improved aggregation, enhanced crystallinity and balanced charge carrier mobility and then facilitates exciton dissociation and charge transport of devices, which contribute to the improved fill factor (FF) and short circuit current density (Jsc) of binary PSCs. Finally, the PSCs based on PM6:Y6 after BIT treatment showed a high PCE of 17.43 % with FF of 77.90 %, which are higher than that of the PSCs (PCE = 15.15 %, FF = 71.95 %) without treatment. It is worth noting that BIT has also been successfully applied in various photovoltaic systems and a high PCE of 18.53 % with FF of 78.88 % were obtained in PM6:L8-BO based PSCs.