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.

The project focusses on organic thin-film transistors. Compared to the operation of thin-film transistors relying on conventional semiconductors such as silicon, organic transistors require an additional crucial step to operate: The contacts must essentially provide mobile charge carriers for the organic semiconductor (injection) that are necessary to establish and to switch an electric current. The overarching aim of this project was to clarify whether this additional need to inject mobile charges and, subsequently, to distribute these charges within the organic semiconductor, could prolong or even entirely dominate the duration needed to switch-on time of such transistors. Device simulations were utilized to tackle this aim. An insufficient injection, i.e., when a less than ideal number of required mobile charges is provided, typically occurs when injection occurs across an injection barrier. A complex interplay of multiple factors determines the extent to which injection across a barrier affects the transistor current and operation. Most remarkably, the transistor architecture, i.e., how the charge-injecting facets of the contacts are oriented with respect to the main current path, is the factor that qualitatively determines, how the transistor operation responds to injection. While insufficient injection essentially lowers the perceived charge mobility in staggered architectures, it shifts the turn-on voltage in coplanar transistor architectures (i.e., specifically in bottom-contact bottom-gate transistors). Particularly for the latter architecture, it was proposed (i) how to unambiguously link current losses to insufficient injection and (ii) how to compensate for these losses. The consideration of suggested compensation measures, i.e., the reduction of the insulator film thickness and the injection barrier, enabled for the first time the fabrication of coplanar transistors that surpass the switching speed of staggered transistors with identical materials and dimensions. This demonstration falsifies the commonly accepted notion that switching speeds of coplanar transistors are inherently inferior to the ones of staggered transistors. The analysis of switching times is only relevant for small to mediocre injection-related current losses. In such a case, the process of injection indeed prolongs the switch-on time compared to the ideal injection case. However, the largest portion of switch-on time is required for charging all those semiconductor regions besides the main current pathway that are sandwiched between two electrodes. In conditions, however, in which current flow is essentially suppressed due to a highly insufficient injection process, the time associated to the injection process (trivially) dominates the switch-on times.