(Invited) Isomer Effects of Nanocarbons in Organic Solar Cells
Authors: Imahori Hiroshi 
Published: February 2020

Fullerene derivatives are excellent electron-accepting materials in bulk heterojunction (BHJ) organic photovoltaic (OPV) devices. The spherical cage structure of fullerenes endows the small reorganization energy in electron transfer, thereby yielding superior electron-accepting and electron-transporting properties. A typical fullerene derivative is [6,6]-phenyl-C61-butyric acid methyl ester ([60]PCBM). Its [70]fullerene analogue ([70]PCBM) has been frequently employed in high-performance OPVs over [60]PCBM because of better light-harvesting ability in the visible region and solubility in common organic solvents arising from the rugby ball-shape with lower symmetry. High power conversion efficiencies (PCEs) of 8–11% have been achieved using [70]PCBM in combination with various low-bandgap conjugated polymers. However, the lower symmetric C70 cage possesses four distinct types of double bonds (α, β, γ, and δ), and mono-addition to C70 generally leads to a mixture of regioisomer products. In fact, [70]PCBM prepared by a conventional method using 4-benzoylbutyrate tosylhydrazone and C70 contains α-type (80-90%) and β-type (10-20%) isomers. Despite its prevalence use in OPV devices, [70]PCBM is usually used as the isomer mixture, which veils the device performances depending on the individual isomers of [70]PCBM.

In this talk, I will give an overview of the isomer effects of nanocarbons on BHJ OPV devices. In particular, I will focus on the comprehensive regioisomer and diastereomer separation effect of the most prevalent electron-acceptor ([70]PCBM) on the structures and photovoltaic properties of the blend films with a high-performance conjugated polymer (i.e., PffBT4T-2OD). In spite of the strong tendency of PffBT4T-2OD to form a bicontinuous structure with fullerene derivatives, one of the diastereomers of the β-isomer (β1-[70]PCBM) showed an extraordinary cohesion nature in the blend film, deteriorating the OPV device performance. OPV devices based on the other isomers (i.e., α-[70]PCBM (8.4%) and β2-[70]PCBM (8.4%)) rival or slightly surpass the device with the as-synthesized isomer mixture (mix-[70]PCBM, 8.2%) because of the existence of the poor performance β1-[70]PCBM. The results demonstrate the large impact of the substituent position and direction on a C70 surface on the photovoltaic properties.

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