n-channel organic thin film transistors were fabricated on polyethylene naphthalate substrates. The first part of the paper is devoted to a critical analysis of eight methods to extract the threshold voltage from the transfer characteristic in the linear regime. Next, to improve electron injection and reduce contact resistance, self-assembled monolayers (SAMs) were deposited on the gold source and drain electrodes. The subsequent modification on the current-voltage characteristics of the transistors is analyzed by the transfer line method, using a threshold-voltage-corrected gate voltage. The improved performance of the device obtained with some of the SAM treatments is attributed to both a better morphology of the semiconductor film, resulting in an increased channel mobility, and to easier electron injection, which manifests itself through a lowering of the contact resistance. Interestingly, the modulation of the contact resistance exactly follows an opposite behavior to what reported in the case of p-channel devices, which brings further evidence for that charge injection is tuned by the direction and magnitude of the dipole moment of the SAM.