Organic electronics on banknotes

Organic transistors and circuits are fabricated directly on the surface of banknotes. The transistors operate with voltages of 3 V and have a field-effect mobility of about 0.2 cm2 V?1s?1. For an array of 100 transistors a yield of 92% is obtained.     

Ambipolar microcrystalline silicon thin-film transistors

Hydrogenated microcrystalline silicon (µc-Si:H) has recently received significant attention as a promising material for thin-film transistors (TFTs) in large area electronics due to its high electron and hole charge carrier mobilities. We report on ambipolar TFTs based on microcrystalline silicon prepared by plasma-enhanced chemical vapor deposition at temperature of 160 °C with high electron and hole charge carrier mobilities of 40 cm²/Vs and 10 cm²/Vs, respectively. The ambipolar microcrystalline silicon TFTs provide a simple route in realizing large area integrated circuits at low cost. The electrical characteristics of the ambipolar microcrystalline silicon TFTs will be described and the first results on ambipolar inverters will be presented. The influence of the ambipolar TFT characteristics on the performance of the inverter will be also discussed.     

Inside Front Cover: Buckled and Wavy Ribbons of GaAs for High‐Performance Electronics on Elastomeric Substrates (Adv. Mater. 21/2006)

Stretchable single‐crystalline GaAs nanoribbons and stretchable electronic devices fabricated with these ribbons are reported on p. 2857 by Sun, Rogers, and co‐workers. The inside cover shows an array of ‘wavy' GaAs nanoribbons (background) sitting on an elastomeric poly(dimethylsiloxane) (PDMS) support. Wavy and buckled ribbons integrated with metal electrodes (foreground inset) enable high‐performance, fully stretchable electronics, i.e., metal–semiconductor field‐effect transistors.     

Organic Thin Film Transistors for Large Area Electronics

Organic thin‐film transistors (OTFTs) have lived to see great improvements in recent years. This review presents new insight into conduction mechanisms and performance characteristics, as well as opportunities for modeling properties of OTFTs. The shifted focus in research from novel chemical structures to fabrication technologies that optimize morphology and structural order is underscored by chapters on vacuum‐deposited and solution‐processed organic semiconducting films. Finally, progress in the growing field of the n‐type OTFTs is discussed in ample detail. The Figure, showing a pentacene film edge on SiO₂, illustrates the morphology issue.     

Organic thin-film transistors using thin ormosil-based hybrid dielectric

We synthesized a novel thermally-crosslinkable ormosil-based hybrid material as a solution-processable dielectric layer for organic thin-film transistors (OTFTs). Dielectrics with a thickness of 50-260 nm were fabricated via spin-coating in order to evaluate their applicability as an ultra-thin gate dielectric. It was observed that the capacitance of the hybrid dielectric increases with decreasing film thickness. Hybrid dielectrics with a thickness of 260 nm and 160 nm, respectively, exhibited adequate leakage current behavior. Coplanar-type OTFTs were fabricated using each of the hybrid dielectrics (i.e., thickness of 260 nm and 160 nm). The off-current, threshold voltage, and field-effect mobility of both transistors were analyzed to investigate the effects of capacitance and film thickness on the electrical performance of the transistors.     

Electrochemical investigation and characterization of thin-film porosity

In thin-film applications it is necessary to control film properties such as homogeneity and porosity to obtain high-quality coatings. Electrochemistry can be a very helpful tool since it can provide information about processes taking place at the interface between substrate and coating. Different thin carbon-based coatings were deposited via physical vapour deposition methods and vapour phase polymerization on pure iron substrates: fullerene films, which were modified by an ion bombardment and thin films of poly(p-xylylene), which is a very good insulating polymer. The film porosity and stability of the film/substrate system against aqueous corrosion were investigated and compared using cyclic voltammetry. The dependence of porosity and film stability on various deposition process parameters such as film thickness and plasma conditions was measured via the dissolution current density and the open circuit potential shift of the substrate material. It could be shown that the two measurements, current density I_crit. and open circuit potential E_ocp. can provide useful complementary information about film porosity that can lead to a better understanding of the coatings properties and the deposition process as well.     

Hot-wire thin-film transistors on PET at 100 °C

Bottom-gate amorphous silicon thin-film transistors were fabricated using active layers deposited by r.f. and hot-wire (HW) chemical vapor deposition on polyethylene terephthalate (PET) and polyimide (PI) substrates. The maximum processing temperature was 100 °C for PET and 250 °C for PI. For transistors deposited at 100 °C by r.f. on PET and at 175 °C by HW on PI the transistor characteristics are comparable, although still inferior, to those of standard amorphous silicon transistors fabricated on glass substrates at 250 °C. HW transistors fabricated at 100 °C showed poor device characteristics. For devices fabricated at 100 °C, an extended anneal at this temperature was required to improve the transistor characteristics, independently of the film deposition technique used.     

Aluminum scandium nitride thin-film bulk acoustic resonators for wide band applications

Piezoelectric c-textured Al_(1−x)Sc_xN thin films, where the Sc relative concentration, x, varies in the range 0-0.15 have been studied in view of radio frequency (RF) electro-acoustic applications. Thin film bulk acoustic wave resonators (FBARs) employing these films were fabricated and characterized as a function of the Sc concentration for the first time. The measured electromechanical coupling is found to increase by as much as 100% in the above concentration range. The results from this work underline the potential of the c-textured Al_(1−x)Sc_xN based FBARs for wide band RF applications.     

Nonvolatile memory characteristics of solution-processed oxide thin-film transistors using Ag nanoparticles

We developed a nonvolatile memory device based on a solution-processed oxide thin-film transistor (TFT) with Ag nanoparticles (NPs) as the charge trapping layer. We fabricated the device using a soluble MgInZnO active channel on a SiO₂ gate dielectric, Ag NPs as a charge trapping site at the gate insulator-channel interface, and Al for source and drain electrodes.The transfer characteristics of the device showed a high level of clockwise hysteresis that can be used to demonstrate its memory function, due to electron trapping in the Ag NPs charge trapping layer. A large memory window (?V_th) was observed with a forward and backward gate voltage sweep, and this memory window was increased in size by increasing the gate voltage sweep. These results show the potential application of memory on displays and disposable electronics.     

Inorganic Semiconductors for Flexible Electronics

This article reviews several classes of inorganic semiconductor materials that can be used to form high‐performance thin‐film transistors (TFTs) for large area, flexible electronics. Examples ranging from thin films of various forms of silicon to nanoparticles and nanowires of compound semiconductors are presented, with an emphasis on methods of depositing and integrating thin films of these materials into devices. Performance characteristics, including both electrical and mechanical behavior, for isolated transistors as well as circuits with various levels of complexity are reviewed. Collectively, the results suggest that flexible or printable inorganic materials may be attractive for a range of applications not only in flexible but also in large‐area electronics, from existing devices such as flat‐panel displays to more challenging (in terms of both cost and performance requirements) systems such as large area radiofrequency communication devices, structural health monitors, and conformal X‐ray imagers.     

Crystalline Silicon Thin‐Film Solar Cells on Silicon Nitride Ceramic Substrates

Photovoltaics—the generation of electricity from sunlight—is a technically challenging but environmentally benign technology to generate electricity with a large economical potential. However, the major hurdle for its widespread usage is its present high cost. Various thin‐film solar cell technologies are investigated to bring down the total cost to an economic value. One of them, the crystalline silicon thin‐film (CSiTF) solar cell combines the advantages of conventional wafer‐based silicon solar cells such as high efficiency and non‐toxicity with the benefits of thin‐film technologies such as serial interconnection and large area deposition. This paper reports for the first time the preparation of CSiTF solar cells on specially developed Si₃N₄ ceramic substrates. Three different types of Si₃N₄ ceramic wafers were single‐sided coated with 10μm of microcrystalline silicon, which was recrystallized by a zone melting step and subsequently thickened to approx. 30 μm. Optical analysis of the layer surface and cross sections was done to determine the crystallographic properties of the silicon layers, as well as mass spectroscopy to measure the concentration of transition metal impurities. A one‐side contacted solar cell process was applied on non‐conducting Si₃N₄ substrates. The best 1 cm² cells achieved an efficiency of 8.0 % with an excellent fill factor of 74 % and an open circuit voltage of 554 mV. The solar cell characterization was complemented by measurements of dark current–voltage characteristics, spectrally resolved light beam induced current mapping, and external quantum efficiency.     

Flexible organic thin-film transistors using single-walled carbon nanotubes as an activated channel

We report on the fabrication of organic thin-film transistors (OTFTs) with a spun cross linked poly-4-vinylphenol (PVP) dielectric on a polyethersulphone (PES) flexible substrate. To improve the electrical performance of OTFTs, we employed a random single-walled carbon nanotubes (SWNTs) network as a carrier transfer underlay without sacrificing the flexibility of the TFTs. The random SWNTs showed that they can act as a semiconducting channel and conduction path to shorten the channel length in our TFTs. The flexible thin-film transistors (TFTs) with a random SWNTs/pentacene bilayer as an active channel exhibited an improved saturation field effect mobility (µ_sat) of 2.6 × 10^− 1 cm²/Vs compared to that of TFTs without the SWNTs underlay, while creating only a minor reduction of the current on/off ratio.