Umeyama Tomokazu, Imahori Hiroshi
Department of Molecular Engineering, Graduate School of Engineering , Kyoto University , Nishikyo-ku, Kyoto 615-8510 , Japan.
Institute for Integrated Cell-Material Sciences (WPI-iCeMS) , Kyoto University , Sakyo-ku, Kyoto 606-8501 , Japan.
Acc Chem Res. 2019 Aug 20;52(8):2046-2055. doi: 10.1021/acs.accounts.9b00159. Epub 2019 Jul 18.
Solar energy conversion is one of the most important issues for creating and maintaining a future sustainable society. In this regard, photovoltaic technologies have attracted much attention because of their potential to solve energy and environmental issues. In particular, thin-film solar cells, such as organic photovoltaics (OPVs) and perovskite solar cells (PSCs), are highly promising owing to their flexibility, light weight, and low-cost production. One of the most important factors used to evaluate solar-cell performance is the power conversion efficiency (PCE), which is the ratio of the output electric power divided by the input light power. The PCEs of PSCs have become comparable to those of multicrystalline silicon solar cells in a laboratory level, but the PCEs of OPVs have yet to catch up with them and still need to be improved. The insufficient durability of PSCs and OPVs is also a challenge that needs to be addressed. Fullerene derivatives have been utilized as electron acceptors and electron-transport materials in OPVs and PSCs. However, the use of fullerene derivatives requires attention to their isomers if they are multiadducts or even monoadducts produced from fullerenes with low symmetry. Their nonuniform structures and electronic properties may exert a negative effect on photovoltaic properties. However, most researchers in the field of OPVs and PSCs have been unaware of the importance of the isomerism. Even the most prevalent, high-performance fullerene acceptor, [6,6]-phenyl-C-butyric acid methyl ester ([70]PCBM), has been used as an isomer mixture. In this Account, we summarize recent studies on the effects of isomer separation of fullerene derivatives on the device performances of OPVs and PSCs. Largely, fullerene derivatives containing various isomers are categorized into [60]fullerene bisadducts, [70]fullerene bisadducts, and [70]fullerene monoadducts. In all cases, the difference in isomerism was found to have a large impact on PCEs. The miscibility with polymer donors and film-forming property of fullerene derivatives were affected by the isomer separations, which exert the most potent influence on device performances. Although the disorders in energy levels among isomers are not definitely influencing on photovoltaic properties of isomer mixtures, the molecular packing structures of fullerene derivatives make a significant effect on their photovoltaic properties. Notably, isomerically pure fullerene derivatives often-but not always-exhibit higher PCEs than the isomer mixture. The search for the best isomers of fullerene derivatives and their optimal compositional ratios, which extensively depend on their roles and the combined materials, will be an indispensable step to achieving consistently higher device performances for OPVs and PSCs.
太阳能转换是创建和维持未来可持续社会的最重要问题之一。在这方面，光伏技术因其解决能源和环境问题的潜力而备受关注。特别是，薄膜太阳能电池，如有机光伏电池（OPV）和钙钛矿太阳能电池（PSC），因其灵活性、重量轻和低成本生产而极具前景。用于评估太阳能电池性能的最重要因素之一是功率转换效率（PCE），它是输出电功率与输入光功率的比值。在实验室水平上，PSC的PCE已与多晶硅太阳能电池相当，但OPV的PCE尚未赶上它们，仍需提高。PSC和OPV耐久性不足也是一个需要解决的挑战。富勒烯衍生物已被用作OPV和PSC中的电子受体和电子传输材料。然而，如果富勒烯衍生物是由低对称性富勒烯产生的多加成物甚至单加成物，则使用它们时需要注意其异构体。它们不均匀的结构和电子性质可能会对光伏性质产生负面影响。然而，OPV和PSC领域的大多数研究人员尚未意识到异构体的重要性。即使是最普遍、高性能的富勒烯受体，[6,6]-苯基-C-丁酸甲酯（[70]PCBM），也一直作为异构体混合物使用。在本综述中，我们总结了关于富勒烯衍生物异构体分离对OPV和PSC器件性能影响的最新研究。大体上，含有各种异构体的富勒烯衍生物可分为[60]富勒烯双加成物、[70]富勒烯双加成物和[70]富勒烯单加成物。在所有情况下，都发现异构体的差异对PCE有很大影响。富勒烯衍生物与聚合物供体的混溶性和成膜性能受异构体分离的影响，这对器件性能影响最大。虽然异构体之间能级的无序对异构体混合物的光伏性质没有明确影响，但富勒烯衍生物的分子堆积结构对其光伏性质有显著影响。值得注意的是，异构体纯的富勒烯衍生物通常（但并非总是）比异构体混合物表现出更高的PCE。寻找富勒烯衍生物的最佳异构体及其最佳组成比例，这在很大程度上取决于它们的作用和组合材料，将是实现OPV和PSC器件性能持续提高不可或缺且必不可少的一步。
