In this study we investigate charge injection into a methanofullerene. The temperature and electric field dependent source-drain currents from contact limited [6,6]-phenyl C61-butyric acid methyl ester (PCBM) thin film transistors (TFTs) were analyzed. A form for the temperature and field dependent behavior of the parasitic contact resistances between metal and semiconductor was proposed based on a diffusion limited thermionic emission (DLTE) injection current and accounting for the disorder in the system. The temperature dependent current-voltage characteristics were initially modeled with a model for field effect behavior in amorphous organic semiconductors from the literature to determine material parameters. The initial fit resulted in discrepancies between the experimental data and the data predicted by the model. Modifying the model to account for the effects of contact resistances led to much better agreement between the experimental and predicted data. Based on these results, the DLTE injection current describes the injection process in PCBM TFTs well.