18.9% Efficiency Ternary Organic Solar Cells Enabled by Isomerization Engineering of Chlorine-Substitution on Small Molecule Donors

Ternary organic solar cells (OSCs) represent an efficient and facile strategy to further boost the device performance. However, the selection criteria and rational design of the third guest small molecule (SM) material still remain less understood. In this study, two new SM donor isomers, with a-chlorinated thiophene (aBTCl) and b-chlorinated thiophene (bBTCl) as side chains, are systematically designed, synthesized and incorporated as a third component in PM6:L8-BO binary blends. It is noticed that introducing the SM donors guest has extended the absorption of photo-active layer, induced desired component distribution vertically with enhanced crystallinity and reduced recombination process, leading to increased short-circuit current (J(SC)) and improved fill factor. Moreover, due to the synergetic suppressed nonradiative loss and preferable morphology, the ternary OSCs feature improves open-circuit voltage (V-OC). Consequently, an impressive champion power conversion efficiency of 18.96% and 18.55% is achieved by aBTCl-based and bBTCl-based ternary OSCs, respectively. Furthermore, a record efficiency of 17.46% is obtained with a 330 nm thickness of aBTCl-based ternary OSCs. This study demonstrates that molecular isomerization can be a promising design approach for SM donors to construct high-performance ternary OSCs with simultaneous enhancement of all photovoltaic parameters.

Automated summary

In this study, two new small molecule donor isomers were synthesized and incorporated into binary blends to improve the performance of ternary organic solar cells (OSCs). The introduction of the small molecule donors extended the absorption range, improved the component distribution and crystallinity, and reduced recombination, resulting in higher short-circuit current (JSC) and improved fill factor. Additionally, the molecular isomers also enhanced the open-circuit voltage (VOC), leading to record-breaking power conversion efficiency.