Organic Thin Film Transistors as Chemical Sensors

The goal of the project is to develop novel types of organic thin-film transistors (OTFT) containing a functionalized interfacial layers between the gate dielectric and the active layer to control the device-characteristics. In particular, we will focus on chemically reactive layers, which are suitable for chemical sensing, translating the presence of an analyte into a strong modification of threshold voltage and source-drain current of the transistor. This will allow the demonstration of new concepts for chemical or photochemical probes (to measure the total exposure of the device utilizing irreversible reactions) and sensors (to determine the current level of the analyte using reversible reactions). In preliminary tests, we have already realized devices with a covalently bound silane-based interfacial layer, in which exposure to NH3 shifts the threshold-voltage by up to 70V ! The main task of the project will be the realization of OTFT based sensors, and, most importantly, to understand the physical and chemical details of the involved processes. To achieve the latter, we will apply a multitude of analytical techniques: In addition to an in depth electrical characterisation of the devices, these include numerous surface-sensitive techniques to investigate layer thickness, structure, and morphology as well as the chemical composition of the layers. To complement the experiments, we will also perform standard quantum-mechanical calculations on suitable model systems. As interfacial layers, we will apply (i) covalently bound functional molecules having suitable docking groups (trichlorosilane or trialkoxysilane) groups to link to the substrate (i.e., the SiOx dielectric) and bearing also chemically reactive or photosensitive end groups; for these layers we have gathered significant experience during the past months, especially regarding their application in OTFTs. (ii) Spin-cast polymers (as an additional insulating layer of the dielectric on top of SiOx ) bearing the same functional sensing units as the molecules mentioned above. Here, the sensing functionality will be either included directly during synthesis (by our partners), or added a posteriori through surface reactions on the spin-cast films. (iii) Langmuir-Blodgett (LB) type mono- and multilayers of analogous materials; the main potential of these materials is that they allow for well controlled self-assembly processes and a full control over the layer thickness (enabling, e.g., the fabrication of well defined multilayer structures). As reactive functional groups, we will apply analyte-docking groups like (e.g., sulfochloride, crown-ethers and non-protic bases) or photo-isomerisable/cleavable units. The proposed project aims at forming a bridge between the two big national research clusters on organic materials currently under way in Austria: The ISOTEC project of the Austrian Nanotechnology Initiative and the NFN "Interface controlled and functionalized organic films". With a thematic position between the two clusters (interface controlled organic sensors), this project will help linking those activities to generate additional synergetic effects and simultaneously strongly benefit from numerous collaborations, which are detailed in this proposal.

Organic electronic devices have a huge potential for applications in displays, solar cells, cheap electronic circuits and sensors. In essentially all of these devices transistors are applied. In the course of the present project we, thus, intensively studied how the characteristics of such organic transistors can be controlled by chemical and photochemical means. This was achieved by inserting specifically designed interfacial layers between the active semiconductor material and the gate dielectric. Using, for example, covalently bonded and also spin-cast acidic layers we succeeded in developing a strategy to chemically shifting the voltage at which organic transistors turns on by several ten volts. This allows switching the transistor from "depletion" to "enhancement" type operation. The underlying process could be characterized as proton-transfer doping of the organic semiconductor and can be compensated by exposing the device to a base like ammonia. Carefully controlling the ammonia dose or the layer thickness, in fact, allows a controlled tuning of the turn-on voltage of the transistors by many ten volts. The insights gained in this study then enabled us to realize a novel sensor element that allows for a discriminative sensing of ammonia in humid environment down to the ppm region and to develop a chemically-triggered switch that switches between logic "0" and logic "1" depending on whether it has been exposed to a basic or an acidic environment. Moreover, replacing the acidic layer by a photoacid, we were able to photochemically control the growth of the organic semiconductor film and, thus, the resulting charge-carrier mobility. More importantly, it also allowed us to tune the acid-doping process by the illumination dose, which enables an exceedingly simple procedure for realizing inverters and in the future also more complex electronic circuits. Far beyond these device-focused aspects, a central element of the project was to obtain a profound understanding of the microscopic details of the processes resulting in the above-mentioned effects. This was achieved by well designed control-experiments, the application of various spectroscopic and X-ray techniques, as well as by comparing the experimental results to the outcome of device simulations and quantum-chemical modeling. This was possible only through intensive collaborations with various partners from chemistry, theoretical physics, and solid-state physics providing the project with a strong multi-disciplinary character.