Contact effects in compact models of organic thin film transistors: Application to zinc phthalocyanine-based transistors

A new compact model is developed for organic thin film transistors (OTFTs) by incorporating the effects of the contacts on the transistor's output characteristics. The model is based on physically realistic expressions that describe charge flow in the vicinity of the contacts and a previously developed drift model. The resulting new model maintains the compactness of our original drift-based model and is very suitable for circuit simulations. For proper modeling, accurate model parameters are important. Therefore, we also propose a modified method to extract the transistor's parameters, including ones related to the contact region. The validity of the model is examined by applying it to experimental data obtained for OTFTs which used solution processed films of substituted zinc phthalocyanine derivatives as an active layer between the gold contacts as the source and the drain terminals. Both linear and non-linear responses in the low drain voltage regime of the output characteristics of the transistors are analyzed. The new parameter extraction scheme provides a way to study the evolution with the gate voltage of the ratio of free to total charge density in the low conductivity region close to the contact.