Organic electronics based on tetrathiafulvalene-derivatives: organic field-effect transistors and sensors
- Autores: Raphael Pfattner
- Directores de la Tesis: Concepción Rovira Angulo (dir. tes.), Marta Mas Torrent (codir. tes.)
- Lectura: En la Universitat Autònoma de Barcelona ( España ) en 2011
- Idioma: inglés
- Tribunal Calificador de la Tesis: Ramon Alcubilla González (presid.), Germà García Belmonte (secret.), Fabio Biscarini (voc.)
- Materias:
- Enlaces
- Tesis en acceso abierto en: TESEO
- Resumen
Nowadays, organic electronics has become a popular topic which is discussed all around with an increasing appearance of potential real applications on the market. This has been possible due to the pioneering work performed by scientists to study and understand the working mechanisms of those materials to be able to transfer this technology to easy, low-cost but nevertheless high performing device fabrication techniques.
This thesis starts with a general introduction to the field of organic electronics keeping their inorganic counterparts in mind. Specially low-cost, light weight and flexibility are key features, that motivate the transfer of the outstanding properties of organic active materials towards device fabrication and technology on large areas (OLAE - Organic Large Area Electronics). Organic Field-Effect Transistors (OFETs) are one important potential application where organic active materials can be found.
The main working mechanisms, including simple charge transport models, as well as the main device parameters of such devices are introduced and discussed. Tetrathiafulvalenes (TTFs), which exhibit two reversible oxidation processes, are an important family of organic donor molecules, that have a key impact on the world of organic electronics. The main characteristics of these molecules, which can form conducting charge transfer salts and can also work as organic semiconductors in their neutral state, are described. Further the potential of employing TTFs in devices, is highlighted.
Two polymorphic forms of dithiophene-tetrathiafulvalene (DTTTF), i.e. alpha- and beta-DT-TTF were studied by means of confocal Raman spectroscopy and X-ray diffraction and employed as active material in different Organic Field-Effect Transistor (OFET) architectures. Both polymorphs exhibited excellent device performance, although the -phase revealed mobilities between two and ten times higher in accordance to the closer stacking of the molecules. Additionally, the OFET properties of three TTF families bearing electron-withdrawing groups were investigated, since it is known that the incorporation of such substituents to the organic semiconductor core can improve the device stability and give rise to electron transport. High p-channel OFET mobilities of up to 0.44 cm2/Vs were achieved for some of these materials and even some hints of electron transport were observed in one of them. A new technique, Electrical Time of Flight (EToF) measurements in devices with OFET structures, revealed ambipolar behavior in some TTF-derivatives. Moreover an easy, solution-based technology, zone casting, was used for the preparation of highly crystalline layers of different TTF-derivatives on flexible substrates and extended over large areas.
An important issue to improve device performance in Organic Field-Effect Transistors (OFETs) is to minimize the contact resistance (RC) (i.e. enhance charge injection) which is mainly determined by the material used as source-drain electrodes as well as by the device configuration. In case of p-channel OFETs, for instance, the Fermi level of these electrodes should be close to the Highest Occupied Molecular Orbital (HOMO) level of the active material. Here we report a novel approach to grow organic source and drain contacts in organic single crystals of alpha-dithiophene-tetrathiafulvalene (DT-TTF) which leads to very high performing field-effect transistors (maximum muFE = 6.15 cm2/Vs). The investigation of the organic semiconductor/organic metal interface by means of Atomic Force Microscopy (AFM), Focused Ion Beam - Scanning Electron Microscopy (FIBSEM), Raman analysis and gated four probe measurements shows that the crystallization of DT-TTF on the electrodes does not damage the materials but causes a blending of them improving the contact. This work elucidates the larger versatility of organic metals in contrast to the inorganic ones, and highlights the importance of the contacts in OFETs not only in terms of the energy levels alignment but also with respect to the interface morphology. In a further study the temperature dependence of the best performing single crystal OFET was measured in both the linear and the saturation regime which allowed the estimation of the Density Of States (DOS) in the band gap.
Ultra strain sensitive all-organic flexible thin-Films composed by a thin topmost conducting layer of Ion-Radical Salts (IRSs) based on tetrathiafulvalene (TTF)-derivatives embedded in a polymeric matrix, called also Bilayer (BL)-Films. Four BL-Films based on different IRS were studied in more detail, namely alpha-(BEDT-TTF)2I3, beta-(BEDT-TTF)2I3, (BET-TTF)2IxBr3-x and bet-(BET-TTF)2I3, by means of Energy-Dispersive X-ray spectroscopy (EDX), Scanning Electron Microscopy (SEM), X-ray and Electronic Paramagnetic Resonance (EPR). All samples exhibited a sheet resistance (RSHEET) of the order of 10 kOmega. To study the influence of strain on the change of the electrical resistance, an electromechanical characterization system was developed. One of the most important parameters in this regard is the gauge factor (k) which, gives information about the sensitivity of BL-Films with respect to tensile stress. It was found that BL-Films exhibit k values of one order of magnitude higher compared to conventionally used strain gauges. In addition to some proof-of-concept experiments showing the potential of these Films as sensors, the incorporation of such BL-Films in textiles was studied for possible applications in smart, wearable organic electronics.
The work described in this thesis has contributed to the development of organic electronics for potential applications, where TTF-derivatives will have a strong impact due to their high performance and facile processability.
