Regulating Favorable Morphology Evolution by a Simple Liquid-Crystalline Small Molecule Enables Organic Solar Cells with over 17% Efficiency and a Remarkable Jsc of 26.56 mA/cm2

Liquid crystal small molecules (LCSMs) are manifested as the effective additives to regulate the morphology of active layers and elevate the performance of ternary organic solar cells (TOSCs) in fullerene systems. However, the current studies for TOSCs based on efficient LCSMs are most out of the LC phase transition temperature, which is not conducive to accurately disclosure the effect of LCSMs on the morphology evolution. Besides, the inner working mechanism of LCSMs has not been investigated systematically and in-depth. Herein, a structurally simple donor-acceptor-donor type LCSM DFBT-TT6 with a low liquid crystal phase transition temperature is utilized as the third component to construct TOSCs based on a highly efficient nonfullerene system PM6:Y6. To unveil the work mechanism of LCSMs on the TOSCs performance and eliminate other interferences simultaneously, a structurally similar non-LCSM DFBT-DT6 with a low glass-transition temperature is further synthesized for a more clear comparison. Interestingly, the addition of DFBT-TT6 can delicately control the crystallinity and phase separation of PM6:Y6, rendering the optimized morphology with only 3 wt % DFBT-TT6. In contrast, the non-LCSM DFBT-DT6 shows a negligible effect on morphology regulation, indicating the unique ability of LC molecules in morphology control. The underlying working mechanism is revealed by the combined study of miscibility and the wetting coefficient of the blends, elucidating that the LCSM DFBT-TT6 has good compatibility with PM6 and Y6. Therefore, DFBT-TT6 is more prone to being located at the interface of PM6 and Y6, and it is energetically favorable for charge transfer. The aforementioned favorable morphology evolution is associated with improved crystallinity, phase separation, charge transfer, exciton dissociation, and collection efficiency, ultimately boosting the power conversion efficiency of TOSCs from 15.76% to 17.05% with a remarkable short-circuit current density of 26.56 mA/cm(2). This work not only offers deep insight into the LCSM induced morphology evolution but also puts forward an affordable strategy to achieve high-performance TOSCs.

Automated summary

Liquid crystal small molecules (LCSMs) have been found to play a significant role in improving the performance of organic solar cells, particularly in regulating morphology and enhancing efficiency. The addition of LCSM DFBT-TT6 can delicately control the crystallinity and phase separation of the active layers, ultimately boosting the power conversion efficiency of the solar cells.