Asymmetric liquid crystalline donors with two different end groups enable efficient all-small-molecule organic solar cells

Abstract

Asymmetric substitution on donors has been shown to be an effective approach to optimize the morphology and photovoltaic performance of all-small-molecule organic solar cells (ASM-OSCs), but this strategy is rarely applied in liquid crystalline small-molecule donors (SMDs). Herein, one of the two rhodanine (R) end groups on the well-known liquid crystalline molecule BTR-Cl is replaced by 2-ethylhexyl cyanoacetate (CA), yielding three new asymmetric SMDs, namely, BT-CAR2, BT-CAR4, and BT-CAR6, whose alkyl chain lengths on the rhodanine groups are 2, 4, and 6 carbon atoms, respectively. The asymmetric structure enhances intermolecular interactions, and the three SMDs all exhibit highly ordered edge-on orientations in the solid states. Notably, the BT-CAR4:Y6 film achieves a finely-tuned morphology due to the optimal miscibility between BT-CAR4 and Y6. Consequently, all three ASM-OSCs exhibit efficiencies of around 15%, significantly surpassing the previously reported efficiency of the BTR-Cl based counterpart (13.6%). Specifically, the BT-CAR4:Y6 device achieves the highest efficiency of 15.52%. This work presents a promising avenue for designing efficient SMDs for ASM-OSCs.