Methanofullerenes used as electron acceptors in polymer photovoltaic devices

A series of [6,6]-phenyl C₆₁-butyric acid esters, including methanofullerene [6,6]-phenyl C₆₁-butyric acid methyl ester (PCBM) with different alkyl chain lengths (C₁-C₁₆) was synthesized from the reaction of C₆₀ and alkyl 4-benzoylbutyrate p-tosylhydrazone in the presence of sodium methylate. The solubility of C ₆₀ derivatives in organic solvents increased with the increase in the length of alkyl substitutions. Photovoltaic cells with these derivatives were fabricated with the structure of ITO/PEDT/MEH-PPV + C₆₀ derivatives/ Ba/Al. Device performances with such PCBM analogues were investigated and discussed in terms of Donor/ Acceptor (D/A) phase separation and mobility of acceptor phase. The results clearly indicate that both interfacial properties of the two phases (donor and acceptor) and mobility of electrons and holes within corresponding phases play an important role in the efficiencies of PV cells. This study revealed that methanofullerenes [6,6]-phenyl C₆₁-butyric acid butyl ester, PCBB, possesses better photosensitivity than the PCBM, a widely investigated and well-recognized C₆₀ derivative for polymer PV cells The energy conversion efficiency reaches 2.84% for PCBB under AM1.5 illumination (78.2 mW/cm²), but 2.0% for PCBM fabricated in the same conditions.