Research on whole small molecule organic solar cells has made progress

Organic solar cells (OSCs), renowned for their lightweight design, solution-processability, and affordability, have garnered significant attention and research interest. Presently, high-efficiency OSCs typically feature an active layer composed of polymer donors paired with small-molecular non-fullerene acceptors. However, the inherent polydispersity of polymer molecular weights can lead to batch-to-batch variations, impacting the photoelectric conversion efficiency (PCE), which poses challenges for large-scale production. Conversely, full-small molecule organic solar cells, characterized by well-defined chemical structures, improved reproducibility, and reduced batch inconsistencies, have gained increasing recognition. Nevertheless, their development is hindered by suboptimal microphase separation morphologies, which limit further enhancements in PCE. Specifically, the introduction of ADA-type structures in acceptors presents a formidable challenge in regulating phase morphology, as high crystallinity and appropriate phase separation scales often counterbalance each other, resulting in relatively lower PCE values (typically below 16%). Dr. Ge Ziyi, leading the organic optoelectronic materials and devices team at the Ningbo Institute of Materials Technology and Engineering of the Chinese Academy of Sciences, has made substantial strides in this domain. Prior studies (Angew. Chem. Int. Ed., 2020, 59, 2808-2815) revealed that incorporating bifluorine atoms into small molecule donors significantly enhances molecular pi-pi stacking, exciton dissociation, and charge transport, achieving a PCE exceeding 13%. Further optimization of the post-treatment process elevated the PCE of full small molecule devices to 15.4%, elucidating the distinct operational mechanisms of thermal annealing and solvent annealing in modulating active layer morphology (Adv. Energy Mater., 2021, 2100800). Recently, the team achieved notable advancements in all-small molecule organic solar cells. For the first time, an asymmetric substitution strategy was applied to small molecule donor materials. By integrating various end groups with differing electron-withdrawing capabilities—cyanoacetate (CA), rhodamine (Reh), and indenone (ID)—a series of novel small molecule donor materials were synthesized: SM-CA, SM-CA-Reh, SM-Reh, SM-CA-ID, and SM-ID. When blended with the small molecule acceptor N3, the research demonstrated that the device based on SM-CA-Reh:N3 combines the high fill factor of SM-CA:N3 and the high current of SM-Reh:N3, resulting in a marked increase in efficiency from 15.41% (SM-CA:N3) to 16.34%. This efficiency represents the highest publicly reported value for binary all-small-molecule solar cells. Devices based on SM-CA-ID and SM-ID exhibited progressively lower efficiencies, at 8.20% and 2.76%, respectively. Microscopic molecular accumulation and overall phase distribution characterization revealed that molecular pi-pi stacking primarily determines phase separation morphology, rather than crystallinity or dipole. Furthermore, the introduction of rhodamine in SM-CA-Reh:N3 slightly increases phase size while preserving the superior phase separation network of SM-CA:N3, offering a compelling explanation for the integrated photovoltaic performance of SM-CA-Reh:N3. These findings were published in *Advanced Materials* under the title "Asymmetric Substitution of End-Groups Triggers 16.34% Efficiency for All-Small-Molecule Organic Solar Cells." The research received support from the National Nanoscience Center of the Chinese Academy of Sciences, South China University of Technology, and funding from the National Outstanding Youth Science Foundation and the National Natural Science Foundation, among others.  --- This breakthrough underscores the immense potential of asymmetric substitution in enhancing the efficiency of all-small-molecule organic solar cells. Moving forward, the team aims to refine these materials further, exploring additional modifications to optimize performance while maintaining reproducibility. Such innovations hold promise for advancing the commercial viability of organic solar cells, paving the way for sustainable energy solutions in the future.

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