Organic semiconductors have been studied as potential materials for flexible electronics due to their mechanical flexibility and solution processability. Moreover, organic semiconductors offer great potential as thermoelectrics because of their low thermal conductivity (~ 0.1 W/m^.K). Prior research has focused on p-type conduction, but for practical thermoelectrics the n-type counterpart is essential to engineer efficient devices. In this project we studied the effect of extrinsic n-type doping with a 1,3-bis(diisopropylphenyl)imidazol-2-ylene-C₆₀ Lewis acid base adduct by fabricating field-effect transistors from both doped and undoped Phenyl-C₆₁-butyric acid methyl ester (PCBM). The transfer and output characteristics of the devices were studied in order to estimate the electron mobility and the threshold voltage, which is related to the carrier concentration. The results show it is possible to control the threshold voltage in PCBM transistors by adding a structurally similar dopant. Futhermore, we found that the threshold voltage can be tuned via device processing – either by annealing the films or by heating the PCBM solution before spin coating. Controlling the carrier concentration of n-type organic semiconductors is an important first step towards practical organics and thermoelectrics.