Isomer Separations of [70]PCBM for Organic Photovoltaic Applications
Tomokazu Umeyama a, Hiroshi Imahori a
a Kyoto University, Goryo-Ohara, Nishikyo-ku, Kyoto 615-8245, Japan
Asia-Pacific International Conference on Perovskite, Organic Photovoltaics and Optoelectronics
Proceedings of International Conference on Perovskite and Organic Photovoltaics and Optoelectronics (IPEROP19)
Kyōto-shi, Japan, 2019 January 27th - 29th
Organizers: Hideo Ohkita, Atsushi Wakamiya and Mohammad Nazeeruddin
Oral, Tomokazu Umeyama, presentation 025
Publication date: 23rd October 2018

[70]Fullerene derivatives are preferentially employed in organic photovoltaic (OPV) devices compared with [60]fullerene derivatives. A C70 derivative of [6,6]-phenyl-C61 butyric acid methyl ester ([60]PCBM), i.e. [70]PCBM, and its analogues are widely used in high-performance OPV devices in combination with low-bandgap conjugated polymers. However, an important aspect of C70 derivatives, which is usually overlooked in photovoltaic applications, is that as-prepared C70 mono-adducts consist of regioisomers due to the lower symmetry of rugby-ball shaped C70 over spherical C60. As contrasted with C60, C70 has four distinct types of [6,6]-bonds (a-, b-, e-, and k-type bonds), and thereby [70]fullerene derivatives including [70]PCBM have been used as a regioisomer mixture in OPV devices. Considering that the miscibility with conjugated polymer donors and the molecular arrangement of [70]fullerene derivatives in the active layer have a large impact on charge separation and charge-transporting properties, pure isomers of [70]fullerene derivatives rather than the isomer mixtures would yield a more desirable donor-acceptor network toward highly efficient OPVs.

In this study, by using a high-performance liquid chromatography (HPLC) suitable for the separation of fullerenes, we separated as-purchased [70]PCBM (denoted as mix-[70]PCBM) into isomerically pure ones and investigated the isomer separation effects on the photovoltaic properties [1-4]. For instance, in combination with a high-performance conjugated polymer donor (PffBT4T-2OD), one of the diastereomers of the b-isomer (b1-[70]PCBM) showed an extraordinary cohesion nature in the blend film, deteriorating the OPV device performance (0.4%). OPV devices based on the other isomers, i.e. a-[70]PCBM (8.4%) and b2-[70]PCBM (8.4%), rival or slightly surpass the device with the as-synthesized isomer mixture (mix-[70]PCBM, 8.2%).

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