High performance solution-processed n-channel organic field-effect transistors based on [6,6]-phenyl C61 butyric acid methyl ester with low operating voltages (3 V) are demonstrated using a high- k hafnium dioxide gate dielectric grown by atomic layer deposition. Devices exhibit excellent n-channel performance with electron mobility values up to 0.14 cm2 /V s, threshold voltages of ∼0.3 V, current on/off ratios > 105, and very low values of subthreshold slope (∼140 mV/decade). © 2009 American Institute of Physics.