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Recently, researchers from the National Institute of Nanoscience and Nanosystems of the Chinese Academy of Sciences and the Multi-Level Key Laboratory of Manufacturing have cooperated with Wei Zhixiang, Lu Wei, Dr. Deng Dan and Xi'an Jiaotong University Professor Ma Wei to design and synthesize soluble organic small-molecule photovoltaic materials. The layer topography is optimized, and the photoelectric conversion efficiency of 11.3% is obtained. This is the highest efficiency of the soluble organic small molecule solar cell reported in the literature, and it is also one of the highest efficiency of the organic solar cell. Relevant research results were published in Nature-Communications (2016, 7, 13740).
Because organic solar cells have the advantages of rich source of raw materials, low cost, light weight, and can be fabricated into large-area flexible devices through printing, they have become a solar energy utilization method with important application prospects and have attracted extensive attention in recent years. In the active layer material, the soluble organic small molecule has advantages of high purity, clear molecular structure and molecular weight compared to the polymer material. However, the efficiency of organic small molecule solar cells still needs to be further improved. In particular, the highest energy conversion efficiency of a more stable reverse device is less than 9%.
The two main ways to improve the photoelectric conversion efficiency are to adjust the energy level structure through molecular design, and to reduce the charge recombination and energy loss by improving the active layer morphology of the device. Wei Zhixiang's group achieved a synergistic optimization of these two aspects by changing the number of fluorine atoms in the terminal group of soluble small molecules. The fluorinated end groups help to reduce the HOMO energy level and the optical bandgap of the material; at the same time, the compatibility with the fullerene acceptor and the surface energy of the material can be reduced. Studies have shown that the fluorinated end groups induce a multi-level distribution of the material's sub-phase size in the horizontal direction, that is, the simultaneous existence of large-sized particles (about 100 nm) with high phase purity and favorable charge transport, as well as increased interface area to the acceptor and favorable charge Separated small size particles (approximately 15 nm). This multi-level distribution of secondary phases makes charge separation and transmission more balanced, reducing the recombination of charges and thus reducing energy loss. In the vertical direction, the fluorinated end groups increase the enrichment of the surface donor material, forming an electron blocking layer on the positive electrode surface, further reducing the energy loss, thereby achieving an increase in device efficiency. Based on this, the research team proposed an ideal morphology model of the active layer of the reverse device, forming a multi-scale nano-assembly structure on the horizontal, and forming a vertical phase distribution favorable for charge collection in the vertical direction. This work in-depth expounded the intrinsic relationship between molecular design, shape control and device performance of high-efficiency photovoltaic materials, and has important implications for the design of high-efficiency organic photovoltaic materials.
This achievement was supported by the key projects of the National Key Research Program “Nanotechnologyâ€, the National Natural Science Foundation of China, and the Nano Pilot Project of the Chinese Academy of Sciences.
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