Thin film growth by high vacuum evaporation of the n-type organic semiconductor 5, 5″′-diperfluorohexylcarbonyl-2,2′:5′,2″:5″,2″′-quaterthiophene (DFHCO-4T) on poly-(α-methylstyrene)-coated n++-Si/SiO2 substrates is investigated at various deposition fluxes and substrate temperatures. Film characterization by atomic force microscopy reveals typical Stransky-Krastanov growth. Transistors with Au source-drain top contacts and optimized DFHCO-4T deposition conditions attain an apparent saturation mobility of 4.6 cm2/Vs, whereas this parameter is 100× lower for similar transistors with LiF/Al top contacts. We explain this lower performance by the formation of a thin interfacial layer with poor injection properties resulting from a redox reaction between Al and DFHCO-4T.