Synthesis and characterization of fullerene based systems for photovoltaic applications: Evidence for percolation threshold

Acceptor polymers for photovoltaic applications were synthesized by grafting fullerene C60 onto polystyrene. The quality of the reaction was verified by various analytical techniques after each of the three steps of the reaction: nuclear magnetic resonance, infrared and UV–visible spectroscopies, and thermo-gravimetric analysis. In order to determine the optimal amount of C60, a series of polymers were prepared containing from 4 to 59 vol.% of fullerene. The optical (absorption, optical gap energy) and electrical (electron mobility, conductivity) properties have been measured. A percolation threshold at around 4 vol.% was identified for both conductivity and mobility measurements. This provides the lowest amount of C60 required for solar cells applications.