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

All-small molecule solar cells (ASM-OSCs) offer distinct advantages, such as well-defined material structures and straightforward purification processes, which help circumvent some of the limitations associated with polymer solar cells, including inconsistencies in device performance across batches. These qualities make ASM-OSCs a promising focus within the field of organic photovoltaics. However, challenges remain, particularly concerning the short conjugation length of small molecules and their rapid crystallization rates, which complicate efforts to control the morphology of the active layer. Consequently, the power conversion efficiency of ASM-OSCs still trails behind that of polymer-based counterparts. Currently, the most effective approach to enhancing ASM-OSCs' efficiency lies in developing novel, highly efficient small molecule donors and acceptors. In earlier studies, Wei Zhixiang’s team at the National Center for Nanoscience and Technology of the Chinese Academy of Sciences made significant strides in designing conjugated small molecule donors and optimizing their morphologies. Through enlarging the fused ring structure of the electron donor unit, they successfully synthesized small molecule donor ZR1, achieving multi-level morphologies upon blending with the non-fullerene acceptor Y6 (Nat. Commun., 2019, 10, 5393). Additionally, by altering the position of the side chain phenyl alkylsulfide chain, they designed a new structural donor, M-PhS. This modification enhanced the orderliness of stacking and compatibility, leading to the construction of an active layer morphology with phase-scale multi-order distribution, thus balancing charge separation and transfer. Devices based on M-PhS:BTP-eC9 achieved an impressive efficiency of 16.2% (Adv. Mater. 2022, 34, 2106316). Recently, building on these findings, 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 (Figure 1). By tuning intermolecular compatibility via surface tension differences, the blended film of ZR-SiO-EH:Y6 demonstrated a superior nanoscale dual continuous interpenetrating network morphology, featuring smaller phase domain sizes and ordered molecular accumulation. This ensured effective exciton dissociation and charge transport. Furthermore, the ordered molecular orientation and the reduced non-radiative energy loss of 0.2 eV, thanks to the energy level difference in the highest occupied molecular orbital (HOMO) between donors, resulted in a high open-circuit voltage of 0.87 V for ASM-OSCs. Consequently, the device based on ZR-SiO-EH:Y6 exhibited a high conversion efficiency of 16.4%. These findings underscore the importance of using siloxane chains to adjust intermolecular compatibility for achieving ordered phase separation morphologies, offering a valuable strategy for designing high-performance ASM-OSCs. The study 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 both intermediate electron donor units and terminal electron-withdrawing groups play critical roles in modulating solubility, energy levels, and molecular stacking modes. For instance, shortening the end-group alkyl chains of the small molecule donor from hexyl (MPhS-C6) to ethyl (MPhS-C2) reduces its sensitivity to thermal annealing while promoting tight π-π stacking (Fig. 2). The shorter alkyl chains of MPhS-C2 decrease thermal sensitivity, minimizing HOMO level increases and reducing crystallization scales. Combined with the compact packing properties of short alkyl chains, this ensures effective charge transport at smaller phase separation scales. When BTP-eC9 serves as the acceptor, non-radiative energy losses for MPhS-C2:BTP-eC9 devices drop from 0.247 eV to 0.192 eV, boosting efficiency from 16.2% to 17.11%, marking a breakthrough in ASM-OSCs efficiency. Improved active layer film density and reduced thermal aggregation sensitivity enhance the device's light and thermal stability. This work highlights the pivotal role of optimized active layer morphology in small molecule donor design, providing 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 been supported by projects like the National Natural Science Foundation of China and the Strategic Leading Science and Technology Special Project of the Chinese Academy of Sciences (Class B). [Figures omitted for brevity] The above work demonstrates the potential of innovative strategies in ASM-OSCs, setting a solid foundation for future advancements in organic photovoltaic technologies.

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