FWF - TRANSIENT - Understanding the transient characteristic of organic transistors

Flexible, transparent circuits based on organic thin-film transistors (OTFTs) presently reach operation frequencies of up to ca. 30MHz. Due to tremendous improvements of the charge carrier mobility of the organic semiconductors and of scaling the device dimensions, operation frequencies were boosted by three orders of magnitude within the last 5-7 years. However, thin-film transistors (TFTs) based on nanocrystalline or amorphous silicon operate in the GHz range, even though the carrier mobilities are almost comparable to those in organic semiconductors. While present frequencies allow one to realize thin, high volume applications such as sensors, pixel drivers for organic matrix displays, or RFID tags, there are numerous efforts to achieve at least a ten-fold increase in the operation frequency. The typically applied optimization strategies were, however, originally developed for inorganic TFTs. Thus, they do not account for the most important difference between OTFTs and TFTs: Owing to their large bandgap, organic semiconductors possess only a negligible amount of intrinsic mobile carriers. The functionality of OTFTs relies, thus, entirely on mobile carriers injected from the contacts. Hence, injection processes play a pivotal role for the dynamic response of organic thin-film transistors. The understanding of injection processes in OTFTs, however, is still in its infancy. The goal of this proposal is to gain fundamental insights into the impact of injection on the dynamic behavior of OTFTs. To that aim, the dynamic behavior will be, for the first time, systematically studied with respect to the impact of charge carrier mobilities, injection barriers, local electric field distributions, trap-, and static charge distributions (incl. doping) within the device. This will be achieved by combining alternate current and time-resolved, drift-diffusion-based simulations for organic devices. The studies will be conducted utilizing our own simulation tool being tailor-made for the description of organic devices. This will allow to self-consistently combine the relevant injection- and back-flowing currents at the contacts with the charge transport in the device on a highly sophisticated level. Based on these fundamental studies, the impact of injection for OTFTs will be unambiguously established and, accordingly, the role of charge carrier mobilities and device dimensions for the dynamic response reassessed. The fundamental insights gained in the course of this project will pave the way for the dsign of novel device structures. The modeling results will be evaluated in a tight-feedback loop with experimental groups, which will ensure access to high-level data.