N-type organic thin film transistor (OTFT) with a top-contact structure was fabricated by thermal vapour deposition using N,N'-Dioctyl-3,4,9,10-perylenedicarboximide (PTCDI-C8) as an n-channel layer on Si/SiO2 substrate. The density of localised states (DOS) in the gap of PTCDI-C8 is estimated by studying the temperature dependence of the electrical characteristics of OTFT. The measurements were done immediately after the devices fabrication (non-degraded devices) and also after 2 h of exposure to air (degraded devices). The extracted field effect mobility decreased from 0.02 to 0.004 cm2 V-1 s-1 and threshold voltage increased from 25.3 to 40.5¿V for the degraded OTFT. The degradation of OTFTs was due to the trapping of majority charge carriers in the localised trap states created by adsorbed oxygen in the PTCDI-C8 layer. These localised trap states were found to be situated at around 0.15¿eV above the lowest unoccupied molecular orbit level. The study of DOS in OTFTs gives a complete understanding of trap-limited transport in organic thin film semiconductors.

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