(Invited) Novel Fullerenes and Their Photovoltaic Properties
Authors: Xie Su-Yuan , Xiao Jun 
Published: February 2020

Thousands fullerenes with even-numbered Cn (n > 20 but with exception of 22) were produced in the original gas-phase laser vaporization experiment conducted by Kroto, Curl and Smalley.[1] However, only a few fullerenes survive the ambient conditions and were isolated in the form of bare hollow fullerene. In recent years, tens labile fullerenes with novel cage configuration have been stabilized by exohedral chlorination or hydrogenation using carbon arc or combustion method in our lab, such as #271C50, #540C54, #864C56, #913C56, #916C56, #1,809C60, #1,804C
60, #4,169C66, #11,188C
72 and #23,863C
78.[2
-6
] The availability of these unprecedented fullerenes provides experimental opportunities for scientists from a wide of variety of disciplines to expand their insight into the new world of these fullerenes.

We have paid special attention to the possibility of using these novel fullerenes as electron acceptors in polymer solar cells. Cyclic voltammetric analysis revealed that C50H10 exhibited reversible cathodic behavior and useful reduced potentials for possible application in photovoltaic devices. The first reduction potential of C50H10 (-1.41 V vs Fc/Fc+ in acetonitrile/o-DCB) is about 0.34 V more negative than the corresponding value of C60 as well as its hydride C60H2. The LUMO of C50H10 (-3.37 eV) is about 0.34 eV higher than that of C60, with implication to utilize C50H10 as promising electron acceptor for making polymer solar cells with higher open circuit voltage.[
7
]

In addition, two classes of novel C60 derivatives with higher LUMO energy levels were synthesized, including alkoxy substituted dihydronaphthyl-C60 derivatives and PCBM-C60 derivatives (PCBM = [6,6]- phenyl- C61- butyric acid methyl ester). Photovoltaic devices using each of these fullerene derivatives as acceptor in combination with poly(3-hexylthiophene) as donor were fabricated. Power conversion efficiencies of the device based on 2-methoxy-(5,8)-dihydronaphthyl-(6,7)-[6,6]-C60 is 4.58%, better than PCBM-based polymer solar cell.[
8
] The higher power conversion efficiency implies that photovoltaic performance of fullerene can be improved by chemically modified with electron-donating group(s), and lends credit to a bright future of more efficient polymer (or even perovskite) solar cells based on novel fullerenes.[9-11]

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