The electrical and structural properties in thin films of an n-channel organic semiconductor, N,N′-dipentyl-3,4,9,10-perylene tetracarboxylic dimide (PTCDI-C 5), were analyzed. X-ray diffraction and atomic force microscopy were used to analyze the structure of polycrystalline thin films of PTCDI-C 5. Hydrophobic and hydrophillic substrates were used to synthesize organic thin film transistors (OTFT), which were based on PTCDI-C 5. It was observed that OTFTs exhibited significant contact resistance, which was characterized by the four-probe and resistance versus length method. In PTCDI-C 5 OTFTs, the TFT instability of threshold voltage shift (TVS) was found.