The Effect of Electrostatic Interaction on n-Type Doping Efficiency of Fullerene Derivatives

The molecular doping of organic semiconductors represents a key strategy for advancing organic electronic applications. However, the n-doping of organic materials is usually less efficient than p-doping and strategies toward the design of more efficient n-doping still remain less explored. In this contribution, the impact of electrostatic interaction is explored on the doping efficiency of fullerene derivatives. [6,6]-Phenyl-C-61-butyric acid methyl ester (PCBM) and a [60]fulleropyrrolidine with a more polarizable triethylene glycol type side chain (PTEG-1) are employed for a comparative study. It is found that the doping efficiency of lightly doped PCBM layers is limited to a few percent, while doped PTEG-1 films exhibit very high doping efficiency approaching 100%. The enhanced n-doping of PTEG-1 compared with that of PCBM is further substantiated by Raman and Fourier transform infrared spectroscopic studies. The activation energy for charge generation in doped PTEG-1 is much smaller than that of doped PCBM, which confirms a higher probability for dissociation of charge transfer complexes in the former compared to the latter. The enhanced molecular n-doping for PTEG-1 is attributed to the electrostatic interaction between the charge transfer complex and the polar environment offered by the triethylene glycol diether side chain.