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N-type organic field-effect transistors (OFETs), based on two perylene diimide semiconductors (PDI-8 and PDI-8CN2) exhibiting very different air sensitivities, have been fabricated on Si/SiO2 substrates. These OFETs have been electrically characterized in vacuum both in the dark and under white-light illumination by dc transfer and output curves, bias stress experiments and variable temperature measurements. In particular, the combination of variable temperature and light illumination experiments is shown to be a powerful tool to clarify the influence of charge trapping on the device operation. Even if, in vacuum, the air-sensitive PDI-8 devices display slightly better performances in terms of field-effect mobility and maximum current values, according to our results, charge transport in PDI-8 films is much more affected by charge trap states compared to PDI8-CN2 devices. These trapping centers are mainly active above 180 K, and their physical nature can be basically ascribed to the interaction between silanol groups and water molecules absorbed on SiO2 surface that is more active above the H2O supercooled transition temperature.