National Nanocenter and others have obtained research on all small molecule organic solar cells

All-small molecule solar cells (ASM-OSCs) offer distinct advantages over polymer-based counterparts, particularly in terms of material clarity and straightforward purification techniques. These attributes help circumvent the challenges associated with significant batch-to-batch variations often seen in polymer solar cells, making ASM-OSCs a promising avenue within the field of organic photovoltaics. Despite these benefits, the short conjugated frameworks and rapid crystallization rates of small molecules pose difficulties in achieving optimal active layer morphology, which remains a key hurdle to surpassing the efficiencies attained by polymer solar cells. To date, the focus on developing novel, highly efficient small molecule donors and acceptors continues to be a pivotal strategy for enhancing ASM-OSC performance. In earlier studies, Dr. Wei Zhixiang’s team at the National Center for Nanoscience and Technology of the Chinese Academy of Sciences made notable strides in the design and morphological control of conjugated small molecule donors. Through the enlargement of the fused ring structure in the electron donor intermediate unit, they successfully synthesized the small molecule donor ZR1, which, when blended with the non-fullerene acceptor Y6, yielded a multi-order morphology (Nat. Commun., 2019, 10, 5393). Further innovation led to the creation of the new structural donor M-PhS via side-chain position isomerization of the phenyl alkylsulfide chain. This donor exhibited optimized orderly stacking and high compatibility, resulting in a phase-scale multi-order distribution in the active layer morphology, which balanced charge separation and transport. Devices based on M-PhS: BTP-eC9 achieved a remarkable efficiency of 16.2% (Adv. Mater. 2022, 34, 2106316). Recently, building upon previous work, the team introduced siloxane chains onto the ZR1 pendant thiophene unit to synthesize three small molecule donors with varying surface tensions—ZR1-C8, ZR-SiO, and ZR-SiO-EH. This modification allowed for the regulation of intermolecular compatibility by creating surface tension differences among the acceptors. Among these, the ZR-SiO-EH:Y6 blended film demonstrated superior nanoscale dual continuous interpenetrating network morphology, characterized by smaller phase regions and ordered molecular accumulation. This facilitated effective exciton dissociation and charge transport. Additionally, the ordered molecular orientation and the reduced non-radiative energy loss of 0.2 eV, attributed to the energy level difference of the highest occupied molecular orbital (HOMO) between the acceptors, resulted in a high open-circuit voltage of 0.87 V for ASM-OSCs. Consequently, devices based on ZR-SiO-EH:Y6 achieved an impressive power conversion efficiency of 16.4%. These findings underscore the effectiveness of introducing siloxane chains to achieve ordered phase separation morphology, offering a valuable approach for designing high-performance ASM-OSCs. This research was published in *Energy & Environmental Science* under the title “Regulating Phase Separation and Molecular Stacking by Introducing Siloxane to Small-Molecule Donors Enable High Efficiency All-Small-Molecule Organic Solar Cells.” In the realm of small molecule donor design, adjustments to the terminal electron-withdrawing units also play a crucial role in modulating solubility, energy levels, and molecular stacking modes. By shortening the end-group alkyl chains of the small molecule donor from hexyl (MPhS-C6) to ethyl (MPhS-C2), the sensitivity of its crystallinity to thermal annealing is diminished while maintaining tight π-π stacking. The longer alkyl chain in MPhS-C6 introduces flexibility through free rotation, making it more susceptible to thermal annealing, which enhances both its HOMO energy level and crystallization behavior. Conversely, the shorter alkyl chain in MPhS-C2 reduces thermal annealing sensitivity, lowering the HOMO level rise and crystallization scale. Combined with the compact packing properties of short alkyl chains, this ensures effective charge transport at a smaller phase separation scale. When paired with BTP-eC9 as the acceptor, the non-radiative energy loss of MPhS-C2:BTP-eC9 devices decreased from 0.247 eV to 0.192 eV, boosting efficiency from 16.2% to 17.11%, marking a breakthrough in ASM-OSC performance. This improvement, coupled with enhanced active layer film density and reduced thermal aggregation sensitivity, significantly bolstered the device's light and thermal stability. This study highlights the critical role of optimizing active layer morphology through small molecule donor design, providing invaluable guidance for synthesizing high-efficiency organic conjugated small molecules. The findings were published in *Advanced Materials* under the title “Donor End-Capped Alkyl Chain Length Dependent Non-Radiative Energy Loss in All-Small-Molecule Organic Solar Cells.” This research has received support from initiatives such as the National Natural Science Foundation of China and the Chinese Academy of Sciences Strategic Leading Science and Technology Special Project (Class B). [Figures omitted for brevity]

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