Exceptionally Low Thermal Conductivities of Films of the Fullerene Derivative PCBM

We report on the thermal conductivities of microcrystalline [6,6]-phenyl C₆₁-butyric acid methyl ester (PCBM) thin films from 135 to 387 K as measured by time domain thermoreflectance. Thermal conductivities are independent of temperature above 180 K and less than 0.030±0.003 W m^-1 K^-1 at room temperature. The longitudinal sound speed is determined via picosecond acoustics and is found to be 30% lower than that in C₆₀/C₇₀ fullerite compacts. Using Einstein's model of thermal conductivity, we find the Einstein characteristic frequency of microcrystalline PCBM is 2.88×10¹² rad s^-1. By comparing our data to previous reports on C₆₀/C₇₀ fullerite compacts, we argue that the molecular tails on the fullerene moieties in our PCBM films are responsible for lowering both the apparent sound speeds and characteristic vibrational frequencies below those of fullerene films, thus yielding the exceptionally low observed thermal conductivities.