The N-type semiconducting polymer, P(NDI2OD-T2), with different molecular weights (MW = 23, 72, and 250 kg/mol) was used for the fabrication of field-effect transistors (FETs) with different Semiconductor layer thicknesses. FETs with semiconductor layer thicknesses from similar to 15 to 50 nm exhibit similar electron mobilities (mu's) of 0.2-0.45 cm(2) V-1 s(-1). Reduction of the active film thickness led to decreased mu values; however, FETs with similar to 2 and similar to 5 nm thick P(NDI2OD-T2) films still exhibit substantial mu's of 0.01-0.02 and similar to 10(-4) cm(2) V-1 s(-1), respectively. Interestingly, the lowest molecular weight sample (P-23, MW approximate to 23 kg/mol, polydispersity index (PDI) = 1.9) exhibited higher mu than the highest molecular weight sample (P-250, MW approximate to 250 kg/mol, PDI = 2.3) measured for thicker devices (15-50 nm). This is rather unusual behavior because typically charge carrier mobility increases with MW where improved grain-to-grain connectivity usually enhances transport events. We attribute this result to the high crystallinity of the lowest MW sample, as confirmed by differential scanning Calorimetry and X-ray diffraction studies, which may (over) compensate for other effects.