We report the characterization of interface formation between Au(Au) and pentacene, an organic material used as an active material in Organic Thin-Film Transistors, using x- ray and ultraviolet photoelectron spectroscopy (XPS and UPS). XPS results indicate that there is no chemical reaction between Au and pentacene regardless of deposition order. UPS results indicate the presence of interface dipoles at both the pentacene/Au and the Au/pentacene interfaces. In addition there are indications of band- bending at the Au/pentacene interface.
The structural and transport properties of pentacene thin film transistors are reported, showing the influence of the deposition temperature, the deposition rate and the substrate on the structural and transport properties of oTFTs. The structure and morphology of pentacene films on thermal oxide and plasma CVD silicon nitride were compared by x-ray diffraction measurements and atomic force microscopy. There is a clear correlation between the morphology and the structural properties of the highly polycrystalline films on the two dielectrics. In the case of silicon nitride the roughness of the film has a distinct influence on the morphology and the structural properties, whereas the films on thermal oxide are in general highly ordered independent of the deposition conditions. The ordered films exhibit a thin film and a crystalline bulk phase, and the crystalline bulk phase fraction increases with the deposition temperature and the film thickness. We find that careful control of the deposition conditions give virtually identical films on the oxide and nitride dielectrics. To study the electronic properties we have realized inverted staggered transistors. The mobility of the TFTs is correlated with the morphology and the structural properties of the films, and increases with the size of the crystals. The TFTs exhibit very similar mobilities of ~0.4 cm2/Vs and on/off ratios >108 on thermal oxide and flat silicon nitride. The impact of the dielectric on the device parameters of mobility, threshold voltage and sub threshold voltage slope are discussed. Bias stress experiments are performed to investigate the stability of the TFTs, and to gain understanding of the transport mechanisms of thermally evaporated pentacene TFTs.
The early demonstrations of field-effect transistors based on organic semiconductors with high dynamic range utilized single devices incorporating p-channel materials such as thiophene oligomers and pentacene. Through end-group substitution and use of electron-deficient cores, we have built a library of semiconductors with a variety of attributes, including low off-current, n-channel function and chemical sensitivity. Devices containing these materials can be assembled to form complementary circuits, pixel drivers, and chemoselective sensors. Specific properties of these materials have led to a new approach to a polymer-based memory element.
Recent research on organic and polymeric semiconductors is directed towards highly ordered molecular structures in solid states. Through molecular design and engineering, it has been shown possible to control the molecular orientation and processing conditions of these materials as well as fine tuning their energy levels and color emissions. Thin film field-effect transistors (FETs) have been used as testing structures for evaluating the semiconducting properties of new organic semiconducting materials. Performance similar to amorphous-Si can now be realized with some organic materials. Large-scale integration of organic transistors has been demonstrated. In addition, several low cost novel non-lithographic patterning methods have been developed, which resulted in the first flexible electronic paper. The field-effect transistor device structure can also be utilized as a means to induce a great amount of charge carriers in organic thin films through the gate field. Using this type of structure, superconductivity was observed in a highly ordered conjugated regioregular poly(3-hexylthiophene).
Organic field effect transistors FET have been fabricated with active semiconducting organic thin films that are only a few monolayers thick. The motivation of this study has been to establish a direct correlation between transistor performance and the polymer microstructure in the ultrathin accumulation layer of the transistor. Monolayer thick films of a block copolymer and several model oligomers consisting of a rigid conjugated sexithiophene (6T) block and a flexible polyethyleneoxide (PEO) block have been deposited onto the surface of e.g. SiO2 gate dielectrics functionalized with a self-assembled monolayer. Block copolymer phase behavior and surface morphology has been studied as a function of chain length, solvent and film thickness. Operational transistors have been demonstrated with film thicknesses of only one or two monolayers. Typical device characteristics show a carrier mobility on the order of 10-2 - 10-3 cm2/Vs and ON-OFF current ratio of 103 - 105. Film microstructure, orientation of micro-crystallites and film thickness have been studied by atomic force microscopy (AFM), UV-Vis absorption spectroscopy and X-ray diffraction.
Recently it has been discovered that some types of liquid crystals exhibit fast electronic conduction, whose carrier transport properties are characterized by high mobility over 10-2cm2Vs independent of electric field and temperature. Theses materials feature crystal-like self- organizing molecular alignment and liquid-like fluidity, providing us with a good basis of the excellent photo- electrical properties and large-are uniformity for practical application. The general aspects of carrier transport properties in the present materials, which we call Self- organizing molecular semiconductors, are reviewed on the basis of our experimental results on the smectic liquid crystals and discus their high potential as a new type of organic semiconductors.
We report on the use of silicon dioxide gate dielectric chemically-modified with vapor-deposited octadecyltrichlorosilane (OTS) monolayers for improved organic thin film transistor (OTFT) performance. To date, silicon dioxide gate dielectric chemically-modified with OTS monolayers deposited from solvent solution have demonstrated the highest reported OTFT performance using the small-molecule organic semiconductor pentacene as the active layer. Vapor treatment is an attractive alternative, especially for polymeric substrates that may be degraded by solvent exposure. Using our OTS vapor treatment we have fabricated photolithographically defined pentacene OTFTs on flexible polymeric substrates with field-effect mobility greater than 1.5 cm2/V-s. We find the performance of pentacene as well as several other small-molecule organic active layer materials can be significantly improved using silicon dioxide gate dielectric chemically-modified with vacuum vapor prime OTS. Pentacene, naphthacene, Cu-phthalocyanine, and alpha-sexithienyl OTFTs fabricated on thermally oxidized silicon substrates with photolithographically defined bottom contacts typically show a factor of 2 to 5 improvement in field-effect mobility and reduced subthreshold slope when using silicon dioxide gate dielectric vacuum vapor treated with OTS compared to OTFTs on untreated gate dielectric.
The performance of novel organic devices such as organic light-emitting diodes or organic field-effect transistors is intimately connected to the nature and dynamics of the charge carriers in the device components. Carrying out intercalation studies of solid model oligomers, it is experimentally demonstrated that the low lying electronic excitations in p-type doped systems are significantly confined on the individual molecules due to polaronic effects and thus deserve the name polaron excitations. These results allow for a quantitative experimental estimate of the charge carrier (polaron) extension which is of the order of 20 Angstrom . In addition, it is shown that electron correlation effects play an important role in the determination of the band gap of molecular organic semiconductors. The implications of these results for organic devices are discussed.
Lower cost and high throughput printing techniques, such as screen printing, have a great promise in the fabrication of organic light-emitting devices. We investigate the effects of solution viscosity and screen mesh count on the printed layer thickness, and device performance. The results also demonstrate, for the first time to our knowledge, the use screen-printing to deposit an ultra-thin layers of less than 15nm with RMS surface roughness of less than 1.5 nm.
All-polymer thin film transistors and circuits have been fabricated by inkjet printing. Source, drain and gate electrodes were printed with a solution of conducting conjugated polymer, poly-ethylenedioxythiophene (PEDOT), and semiconductor and gate dielectric were spin-coated from solutions of conjugated polymer and insulator polymer, respectively. The transistors printed in air show comparable performances to the reference samples with gold electrodes. In order to overcome the resolution limit of inkjet printing, water-based PEDOT solution has been deposited onto a pre-patterned substrate which defines a channel by wettability contrast between hydrophilic and hydrophobic surface regions. Polymer transistors with a channel length of 5 microns have been achieved by this approach. In order to improve carrier mobility, main chains of the polymer semiconductor were self-aligned along the channel direction, and a mobility of 0.02 cm2/V+s has been achieved in the printed transistor. We demonstrate simple printed circuits (inverters) with via-holes and load resistors formed by inkjet technology.
OTFTs have recently received much attention because organic materials are more compatible with low temperature and low cost on flexible substrates than a-Si TFTs1. In addition, printing processes are more compatible with OTFT fabrication than a-Si TFTs2. This paper reports a materials compatibility study of various organic gate dielectrics with screen-printed electrodes for pentacene based TFTs. Standards were prepared using the polymer dielectrics and gold drain/source electrodes on a glass substrate. Poly(vinylphenol) (PVP) and benzocyclobutene (BCB) were spin coated and parylene was vapor deposited as dielectric materials. Pentacene was vapor deposited onto the substrates. Test devices were fabricated by painting drain/source electrodes onto the 3 dielectrics. The pentacene/PVP/gold devices had the best performance with mobilities near 0.3 cm2/V-sec, on/off current ratios between 103-104. When screen printable electrodes were introduced to the PVP system, device performance was significantly degraded. The BCB device performance was nearly the same for each device material; the parylene device performance was superior with the screen printable conductive inks relative to the gold devices. Using these data, a potentially low cost device was fabricated on a Mylar substrate with an ITO gate electrode, parylene dielectric, and screen-printed silver ink for drain/source electrodes. On/off current ratios were between 103 and 104. Mobilities ranged from 0.02 - 0.08 cm2/V-sec.
Organic Field-effect Transistors (OFETs) with poly(3-alkylthiophene) (P3AT) as semiconductor on flexible polymeric substrate and organic insulator in top gate construction were prepared. These transistors show a high field-effect mobility, good saturation behavior for low biases and rather high on-off ratio. The transistors were characterized through dynamic and lifetime measurements. A switching speed of 38 kHz at a 3 dB modulation depth was reached in a single transistor. The shelf-lives of the transistors exceeded six months without any special encapsulation. In addition different integrated plastic circuits (IPC) were constructed consisting of several organic transistors including NAND/NOR gates, bi-stable flip-flops and ring-oscillators with a oscillating frequency of 68 Hz.