Ultralow thermal conductivity of fullerene derivatives

Recently, Duda reported that the fullerene derivative [6,6]-phenyl-C ⁶¹-butyric acid methyl ester (PCBM) has the lowest thermal conductivity Λ ever observed in a fully dense solid, Λ ≈ 0.03 W m^-1 K^-1. We have investigated a variety of phases and microstructures of PCBM and the closely related compound [6,6]-phenyl-C ⁶¹-butyric acid n-butyl ester (PCBNB) and find that the thermal conductivities of PCBM and PCBNB films are mostly limited to the range 0.05 < Λ < 0.06 W m^-1 K^-1 with a few samples having slightly higher Λ. The conductivities we observe are ≈70% larger than reported by Duda but are still "ultralow" in the sense that the thermal conductivity is a factor of ≈3 below the conductivity predicted by the minimum thermal conductivity model using an estimate of the thermally excited modes per molecule.