Developing low boiling point solvent additives directly based on non-fullerene based active layer: Higher efficiency and better thickness tolerance

Abstract

Using solvent additive is an effective strategy to control the morphology of active layers. However, most additives with high boiling point (bp.) encounter an issue of long drying process of the active layer, which will increase the cost in commercial application. Herein, two different solvent additives 1,1-dimethoxyethylbenzene (DMB) and 6-mercapto-1-hexanol (MHA) with low bp. of ~200 and 225 °C, respectively, were employed to modify the PTB7-Th:EH-IDTBR non-fullerene system. The nonconjugated additive MHA has no solvation power on the PTB7-Th donor. Relative to the aromatic DMB, MHA can show sole affinity to the EH-IDTBR acceptor and the solidification of EH-IDTBR during film forming is more affected by the higher bp. MHA. It was found that phase separation, molecular aggregation, and balanced charge transport could be enhanced in an order of as-cast, DMB casted, and MHA casted blend films. At an active layer thickness of 100 nm, relative to 10.41% efficiency for as-cast device, solar cells casted with 0.5% DMB and MHA can show higher efficiencies of 10.81% and 11.36%, respectively. The morphological controls with DMB and MHA are also beneficial for thickness tolerance. DMB and MHA casted 300 nm thick active layers exhibit efficiencies of 9.07% and 8.11%, respectively, much higher than the 6.02% for the as-cast device. For the shelf-stability comparison up to 800 h in air, the DMB and MHA processed devices can remain 92.5% and 94% of the initial efficiency, respectively, better than 87.5% efficiency remaining for the as-cast device. Thereby, the DMB and MHA additives would be valuable toward highly efficient, more stable, and large scale NFA based solar cells.