(Invited) New Insights on Charge Transport in Organic and Polymer Thin-Film Transistors

Abstract

There have been many reports of thin-film transistors based on semiconductors with mobilities in the range 1-20 cm2/Vs in the past few decades. The mobilities and mean free paths are simply too low for the application of conventional semiconductor transport theories based on solutions to the Boltzmann transport equation (BTE). We will present a very general solution based on the statistical nature of charge transport and introduction of a factor related to the probability of mean free path exceeding the minimum transport length. We are then able to apply the BTE with appropriate scattering mechanisms and obtain very good results that agree well with experiment. This approach is very well suited to thin-film transistors based on polymer and organic semiconductors with room temperature mobilities in the range 1-20 cm2/Vs. We compare theory with experimental results for DPP-based polymers and also oxides.Charge transport depends on carrier densities and electric fields in complex ways. We describe these dependencies in detail. Scaling down channel dimensions of organic and polymer semiconductor based thin-film transistors to submicron and nanoscale dimensions, required for higher speed operation, presents several challenges. Successful scaling will enable vastly improved device performance and hence the prospects are certainly very enticing. One of the biggest challenges is in making relatively low-resistance contacts. A more thorough understanding of velocity saturation mechanisms and charge transport at high electric fields is also necessary. We describe charge transport in oxide and polymer TFTs with an emphasis on scaling. We show that for some TFTs, scaling of the channel width facilitates scaling down of channel length and a nanostripe or nanogroove array channel geometry has advantages. Finally, we describe hybrid TFTs – with multiple materials that are better suited to scaling down.