Solution processed n -channel organic field-effect transistors based on [6,6]-phenyl C61 butyric acid methyl ester with high mobility and low contact resistance are reported. Ca, Au, or Ca capped with Au (Ca/Au) was used as the top source/drain electrodes. The devices with Ca electrodes exhibit excellent n -channel behavior with electron mobility values of 0.12 cm2 /V s, low threshold voltages (∼2.2 V), high current on/off ratios (105- 106) and subthreshold slopes of 0.7 V/decade. By varying the channel lengths (25-200 μm) in devices with different metal/semiconductor interfaces, the effect of channel length scaling on mobility is studied and the contact resistance is extracted. The width-normalized contact resistance (RC W) for Au (12 k cm) is high in comparison to Ca (7.2 kcm) or Ca/Au (7.5 kcm) electrodes at low gate voltage (VGS =10 V). However, in the strong accumulation regime at high gate voltage (VGS =30 V), its value is nearly independent of the choice of metal electrodes and in a range of 2.2-2.6 kcm. These devices show stable electrical behavior under multiple scans and low threshold voltage instability under electrical bias stress (VDS = VGS =30 V, 1 h) in N2 atmosphere. © 2009 American Institute of Physics.