Department of Chemistry, University of North Carolina at Chapel Hill , Chapel Hill, North Carolina 27599-3290, United States.
Department of Applied Physical Sciences, CB#3216, University of North Carolina , Chapel Hill, North Carolina 27599-3216, United States.
Acc Chem Res. 2017 Sep 19;50(9):2401-2409. doi: 10.1021/acs.accounts.7b00326. Epub 2017 Sep 5.
Organic solar cells (OSCs) have been a rising star in the field of renewable energy since the introduction of the bulk heterojunction (BHJ) in 1992. Recent advances have pushed the efficiencies of OSCs to over 13%, an impressive accomplishment via collaborative efforts in rational materials design and synthesis, careful device engineering, and fundamental understanding of device physics. Throughout these endeavors, several design principles for the conjugated donor polymers used in such solar cells have emerged, including optimizing the conjugated backbone with judicious selection of building blocks, side-chain engineering, and substituents. Among all of the substituents, fluorine is probably the most popular one; improved device characteristics with fluorination have frequently been reported for a wide range of conjugated polymers, in particular, donor-acceptor (D-A)-type polymers. Herein we examine the effect of fluorination on the device performance of solar cells as a function of the position of fluorination (on the acceptor unit or on the donor unit), aiming to outline a clear understanding of the benefits of this curious substituent. As fluorination of the acceptor unit is the most adopted strategy for D-A polymers, we first discuss the effect of fluorination of the acceptor units, highlighting the five most widely utilized acceptor units. While improved device efficiency has been widely observed with fluorinated acceptor units, the underlying reasons vary from case to case and highly depend on the chemical structure of the polymer. Second, the effect of fluorination of the donor unit is addressed. Here we focus on four donor units that have been most studied with fluorination. While device-performance-enhancing effects by fluorination of the donor units have also been observed, it is less clear that fluorine will always benefit the efficiency of the OSC, as there are several cases where the efficiency drops, in particular with "over-fluorination", i.e., when too many fluorine substituents are incorporated. Finally, while this Account focuses on studies in which the polymer is paired with fullerene derivatives as the electron accepting materials, non-fullerene acceptors (NFAs) are quickly becoming key players in the field of OSCs. The effect of fluorination of the polymers on the device performance may be different when NFAs are used as the electron-accepting materials, which remains to be investigated. However, the design of fluorinated polymers may provide guidelines for the design of more efficient NFAs. Indeed, the current highest-performing OSC (∼13%) features fluorination on both the donor polymer and the non-fullerene acceptor.
有机太阳能电池（OSC）自 1992 年提出体异质结（BHJ）以来，一直是可再生能源领域的一颗冉冉升起的新星。近年来的进展将 OSC 的效率推高到了 13%以上，这是通过合理的材料设计和合成、精细的器件工程以及对器件物理的基本理解的协同努力实现的。在这些努力中，已经出现了用于此类太阳能电池的共轭给体聚合物的几个设计原则，包括通过明智地选择构建块、侧链工程和取代基来优化共轭主链。在所有的取代基中，氟可能是最受欢迎的；对于广泛的共轭聚合物，特别是给体-受体（D-A）型聚合物，氟化通常会带来改善的器件特性，这一现象已被频繁报道。在此，我们研究了氟化对太阳能电池器件性能的影响，作为氟化位置（在受体单元或供体单元上）的函数，旨在概述对这种奇特取代基的好处的清晰理解。由于氟化受体单元是 D-A 聚合物最常用的策略，我们首先讨论了氟化受体单元的影响，重点介绍了五个最广泛使用的受体单元。虽然用氟化受体单元已经观察到了器件效率的提高，但背后的原因因情况而异，并且高度依赖于聚合物的化学结构。其次，我们研究了氟化供体单元的影响。在这里，我们关注的是已经用氟化研究最多的四个供体单元。虽然已经观察到供体单元氟化对器件性能的增强效果，但氟是否总能提高 OSC 的效率并不清楚，因为有几个案例中效率下降，特别是在“过度氟化”时，即当掺入过多的氟取代基时。最后，虽然本账户重点研究了聚合物与富勒烯衍生物作为电子受体材料配对的情况，但非富勒烯受体（NFAs）在 OSC 领域迅速成为关键参与者。当 NFAs 用作电子受体材料时，聚合物的氟化对器件性能的影响可能会有所不同，这仍有待研究。然而，氟化聚合物的设计可为更高效的 NFAs 的设计提供指导。事实上，目前性能最高的 OSC（约 13%）在供体聚合物和非富勒烯受体上都进行了氟化。
