Conducting organic materials offer a unique combination of interesting properties that make them more attractive than inorganic materials currently used in microelectronics. It is advantageous to use organic thin film transistors (OTFT) when large display areas are required. This needs the enhancement of transistor performances by increasing the I-on/I-off ratio and the field effect mobility. In this report, the effects of annealing on the electrical properties of nickel phthalocyanine (NiPc) OTFTs were studied. Our work is based upon an analysis of different carrier-exchange mechanisms occurring at metal/organic material interface.