Charge transport in organic and polymer thin-film transistors: recent issues

Interest in organic and polymer thin film transistors (OTFTs and PTFTs; hereafter both are called PTFTs) has grown rapidly in the last decade. However, the theory of charge transport in PTFTs lags experiments, so it is difficult to provide the required feedback to device designers and technologists. Therefore, the authors present an analysis of charge transport in PTFTs. It is based on extensive published theoretical and experimental material. The discussion includes examples exploring the trend in experimental results. It is noted that because of the similarity in device structures and current-voltage characteristics between PTFTs and inorganic metal-oxide-semiconductor (MOS) field-effect transistors (FETs), the MOS theory and model have been used extensively in analysing the properties of PTFTs. The MOS model has also been significantly modified to obtain agreement with experiments, and these modifications are discussed. However, recent experimental data from state-of-the-art PTFTs reveal that even the adapted MOS model cannot reliably describe the charge transport in PTFTs, because important PTFT properties such as charge injection or charge build-up result in unrealistic values for several physical parameters (e.g. trap concentrations or time constants) in the adapted MOS model. Therefore, the injection-drift limited model (IDLM) is introduced in this work in an attempt to bring the theory of charge transport and the experimental data closer. The IDLM incorporates the important charge transport phenomena in PTFTs injection, drift and charge build-up. The paper concludes with a discussion of unsolved problems in charge transport in PTFTs.