Organic Field-Effect Transistors (OFETs) are a key technology for flexible and low-cost electronics used e.g. for wearable electronics or flexible displays. However, OFETs are facing severe obstacles that delay their commer-cialization. Doping of organic semiconductors opens a new perspective on the OFET technology. This additional degree of freedom allows to realize new device concepts and to overcome current limitations of the OFET tech-nology. To take full advantage of the benefits of doping for OFETs, current doping ratios in the range of 0.1-1% have to be reduced significantly. The project addresses this challenge. Doping processes will be optimized to re-duce the doping level into the sub-100 ppm range. At these ultra-low doping concentrations, the influence of dop-ing on transistor behavior will be studied thoroughly: a) A quantitative model will be developed to describe charge carrier accumulation and depletion in doped organic transistors; b) Generation of minority charge carriers at ul-tralow doping concentrations will be studied; c) A new device concept - the organic tunnel field-effect transistor - will be realized, and its potential will be evaluated. To broaden the impact of the project, additional measures will be taken to increase the participation of minority students. Summer projects will be offered, which will provide undergraduate students from underrepresented groups with the opportunity to learn about experimental research and to inform them about potential choices for graduate school. Furthermore, research projects will be offered to local high-school students through the college credit plus program, and graduate students will be trained in a highly interdisciplinary field.
Doping organic semiconductors provides a new dimension in the design of OFETs and bears the potential of ena-bling new device concepts with increased performance. In light of these prospects, the research goal of this project is to study the influence of doping on OFETs and to provide an improved understanding of majority and minority charge carrier generation and recombination in doped OFETs. To reach this aim, the following objectives are pursued: a) to develop a consistent and experimentally validated model describing majority charge carrier accumulation and depletion in doped organic transistors; b) to study mi-nority charge carrier dynamics in doped organic transistors and clarify the mechanism of minority charge carrier generation and recombination; and c) to leverage on the potential of doping and realize vertical organic tunnel field-effect transistors (VOTFETs). Doping organic transistors necessitates the use of much lower doping concentrations as commonly used in organic devices. In this project, a rotating shutter system capable of controlling doping concentrations in the sub 100 ppm regime is introduced, which opens a new regime of doping. The influence of doping on the flatband, threshold, and pinch-off voltage at these ultra-low doping concentrations is studied by capacitance vs. voltage measurements, photoelectron spectroscopy, and transistor characterization. Minority charge carrier generation is studied in organ-ic metal-oxide-semiconductor structures and organic transistors, whereas the lifetime and diffusion length of mi-nority charge carriers are characterized in p-n-i-p structures. The operation mechanism of VOTFETs will be stud-ied by systematic device variations. In particular, the tunnel injection mechanism will be validated by an increase in the thickness of the intrinsic semiconductor layer. These experiments have the potential to advance the knowledge in the field: a) The mechanism of minority charge carrier generation will be clarified; b) An analytical model describing the influence of the flatband voltage on the threshold and pinch-off voltage will be tested; c) A detailed understanding of minority charge carrier diffusion will be developed and it will be studied how the lifetime of minority charge carriers depends on the doping con-centration and temperature; d) A new analytical solution describing current saturation in doped OFETs will be verified.
