Optical and transport properties of an alkali-doped methanofullerene

We examine the effects of monoderivatization on the electronic properties of C60. For this we chose the phenyl-C61-octanoic acid cholesteryl ester, [6,6]PCOCr, whose nonlinear optical properties have been investigated in the past. While the optical absorption spectrum of this methano fullerene is similar to that of C60, substantial differences are observed upon doping with potassium. Similarly, the doping-dependent conductivity of the functionalized fullerene shows two maxima as opposed to the single maximum for C60. The experimental observations are consistent with the doping-induced degeneracy removal of the parent C60 LUMO (t1u) orbital, which in potassium-doped methanofullerene splits into two components separated by about 0.5 eV. We provide experimental evidence that the doping of [6,6]PCOCr proceeds, as in C60, with six consecutive reduction (electron transfer) steps, yielding K6[6,6]PCOCr stoichiometry at the end. The transport in partially doped [6,6]PCOCr thin films occurs by thermally activated hopping of the charge carriers with activation energy Ea~=0.25 eV and hopping probability proportional to the number of unpaired electrons in the reduced molecule.