Fullerene derivatives are used as electron acceptors in organic solar cells. CH₂ addends can effectively raise fullerene lowest unoccupied molecular orbital (LUMO) energy levels, which leads to high open-circuit voltage (V_oc). As the smallest addend, CH₂ does not affect fullerene packing in the solid state, thus keeping good electron mobility for the acceptors. 

Stepwise replacement of o-quinodimethane addends with CH₂ addends in 54π fullerenes improves the electron mobility of fullerene, enhances crystallinity of poly(3-hexylthiophene) (P3HT), increases donor-acceptor interfaces, and reduces bimolecular recombination, thus enhancing the power conversion efficiency of fullerene:P3HT solar cells remarkably from 2.26 % to 6.25 %. This is reported by the team of Liming Ding from the National Center for Nanoscience and Technology, Beijing, China.

This work clearly demonstrates the function of CH₂ addends in fullerenes, which is enlightening for designing high-performance fullerene acceptors for organic solar cells.

• Function of CH2 Addends on 54π Fullerene Acceptors
  Shan Chen, Zuo Xiao, Dan He, Lina Ma, Liming Ding
  Asian J. Org. Chem. 2014.
  DOI: 10.1002/ajoc.201402104