Effect of dielectric roughness on performance of pentacene TFTs and restoration of performance with a polymeric smoothing layer

The morphology, structure, and transport properties of pentacene thin film transistors (TFTs) are reported showing the influence of the gate dielectric surface roughness. Upon roughening of the amorphous SiO2 gate dielectric prior to pentacene deposition, dramatic reductions in pentacene grain size and crystallinity were observed. The TFT performance of pentacene films deposited on roughened substrates showed reduced free carrier mobility, larger transport activation energies, and larger trap distribution widths. Spin coating roughened dielectrics with polystyrene produced surfaces with 2 A root-mean-square (rms) roughness. The pentacene films deposited on these coated surfaces had grain sizes, crystallinities, mobilities, and trap distributions that were comparable to the range of values observed for pentacene films deposited on thermally grown SiO2 (roughness also approximately 2 A rms).     

Cross‐linked Polymer‐Blend Gate Dielectrics through Thermal Click Chemistry

New cross‐linking reagents were synthesized and mixed with polystyrene (PS) in solution to form a blend. Thin‐films were spin‐coated from the blend and then cross‐linked by thermal activation at relatively low temperature (100 °C) to form cross‐linked gate dielectrics. This new method is compatible with plastic substrates in flexible electronics. The azide and alkyne cross‐linking reagents are kinetically stable at room temperature, so any premature cross‐linking is avoided during processing. This method also significantly improved the dielectric performances of PS thin films. Solution‐processed top‐gate organic field‐effect transistor devices with indacenodithiophene–benzothiadiazole copolymer as semiconductor layer and the cross‐linked PS blend as dielectric layer showed improved performances with lower gate leakages and higher operation stabilities than devices with neat PS film as dielectric layer.     

Conducting AFM and 2D GIXD studies on pentacene thin films

Among all organic semiconductors, pentacene has been shown to have the highest thin film mobility reported to date. The crystalline structure of the first few pentacene layers deposited on a dielectric substrate is strongly dependent on the dielectric surface properties, directly affecting the charge mobility of pentacene thin film OTFTs. Herein, we report that there is a direct correlation between the crystalline structure of the initial submonolayer of a pentacene film and the mobility of the corresponding 60-nm-thick films showing terrace-like structure, as confirmed by 2D grazing-incidence X-ray diffraction and atomic force microscopy. Specifically, multilayered pentacene films, grown from single crystal-like faceted islands on HMDS-treated surface, have shown much higher charge mobility (mu = 3.4 +/- 0.5 cm2/Vs) than those with polycrystalline dendritic islands (mu = 0.5 +/- 0.15 cm2/Vs) on OTS-treated ones.     

“Click” Dielectrics: Use of 1,3‐Dipolar Cycloadditions to Generate Diverse Core‐Shell Nanoparticle Structures with Applications to Flexible Electronics

We have synthesized a “universal ligand” incorporating a phosphonate surface anchor and a terminal alkyne moiety which binds to TiO₂ nanoparticles and exhibits excellent dispersity in organic solvents. The alkyne functionality permits attachment of azide terminated polymer shells using “click” chemistry. Thus TiO₂ core nanoparticles have been encapsulated with both polystyrene and poly(t‐butyl acrylate) shells. The TiO₂‐poly(t‐butyl acrylate) core shell nanoparticles are amenable to further chemical transformation into TiO₂‐poly(acrylic acid) nanoparticles through ester hydrolysis. These TiO₂‐polyacrylic acid nanoparticles are dispersible in aqueous solution. The resulting core‐shell nanoparticles have been incorporated as high K dielectric films in capacitor and organic thin film transistor devices and are promising new materials for flexible electronics applications.

     

Enhanced Performance of Solution‐Processed TESPE‐ADT Thin‐Film Transistors

A solution‐processed anthradithiophene derivative, 5,11‐bis(4‐triethylsilylphenylethynyl)anthradithiophene (TESPE‐ADT), is studied for use as the semiconducting material in thin‐film transistors (TFTs). To enhance the electrical performance of the devices, two different kinds of solution processing (spin‐coating and drop‐casting) on various gate dielectrics as well as additional post‐treatment are employed on thin films of TESPE‐ADT, and p‐channel OTFT transport with hole mobilities as high as ～0.12 cm² V^?1 s^?1 are achieved. The film morphologies and formed microstructures of the semiconductor films are characterized in terms of film processing conditions and are correlated with variations in device performance.     

Gate dielectric chemical structure-organic field-effect transistor performance correlations for electron, hole, and ambipolar organic semiconductors

This study describes a general approach for probing semiconductor-dielectric interfacial chemistry effects on organic field-effect transistor performance parameters using bilayer gate dielectrics. Organic semiconductors exhibiting p-/n-type or ambipolar majority charge transport are grown on six different bilayer dielectric structures consisting of various spin-coated polymers/HMDS on 300 nm SiO(2)/p(+)-Si, and are characterized by AFM, SEM, and WAXRD, followed by transistor electrical characterization. In the case of air-sensitive (generally high LUMO energy) n-type semiconductors, dielectric surface modifications induce large variations in the corresponding OTFT performance parameters although the film morphologies and microstructures remain similar. In marked contrast, the device performance of air-stable n-type and p-type semiconductors is not significantly affected by the same dielectric surface modifications. Among the bilayer dielectric structures examined, nonpolar polystyrene coatings on SiO(2) having minimal gate leakage and surface roughness significantly enhance the mobilities of overlying air-sensitive n-type semiconductors to as high as approximately 2 cm(2)/(V s) for alpha,omega-diperfluorohexylcarbonylquaterthiophene polystyrene/SiO(2). Electron trapping due to silanol and carbonyl functionalities at the semiconductor-dielectric interface is identified as the principal origin of the mobility sensitivity to the various surface chemistries in the case of n-type semiconductors having high LUMO energies. Thiophene-based n-type semiconductors exhibiting similar film morphologies and microstructures on various bilayer gate dielectrics therefore provide an incisive means to probe TFT performance parameters versus semiconductor-dielectric interface relationships.     

Fabrication of Carbon Nanotube Thin Films for Flexible Transistors by Using a Cross-Linked Amine Polymer

Owing to their remarkable properties, single-walled carbon nanotube thin-film transistors (SWCNT-TFTs) are expected to be used in various flexible electronics applications. To fabricate SWCNT channel layers for TFTs, solution-based film formation on a self-assembled monolayer (SAM) covered with amino groups is commonly used. However, this method uses highly oxidized surfaces, which is not suitable for flexible polymeric substrates. In this work, a solution-based SWCNT film fabrication using methoxycarbonyl polyallylamine (Moc-PAA) is reported. The NH₂-terminated surface of the cross-linked Moc-PAA layer enables the formation of highly dense and uniform SWCNT networks on both rigid and flexible substrates. TFTs that use the fabricated SWCNT thin film exhibited excellent performance with small variations. The presented simple method to access SWCNT thin film accelerates the realization of flexible nanoelectronics.     

Core–Shell Structured Polystyrene/BaTiO3 Hybrid Nanodielectrics Prepared by In Situ RAFT Polymerization: A Route to High Dielectric Constant and Low Loss Materials with Weak Frequency Dependence

A novel route to prepare core–shell structured nanocomposites with excellent dielectric performance is reported. This approach involves the grafting of polystyrene (PS) from the surface of BaTiO₃ by an in situ RAFT polymerization. The core–shell structured PS/BaTiO₃ nanocomposites not only show significantly increased dielectric constant and very low dielectric loss, but also have a weak frequency dependence of dielectric properties over a wide range of frequencies. In addition, the dielectric constant of the nanocomposites can also be easily tuned by varying the thickness of the PS shell. Our method is very promising for preparing high‐performance nanocomposites used in energy‐storage devices.     

A mesoporous Pt/TiO2 nanoarchitecture with catalytic and photocatalytic functions

A novel metal/semiconductor nanocomposite with catalytic and photocatalytic functions has been prepared. The new material consists of highly dispersed platinum (Pt) nanoparticles embedded in a cubic mesoporous nanocrystalline anatase (meso-nc-TiO2) thin film. The porous thin film possesses a narrow pore-size distribution and a large surface area. The diameter of the Pt cluster can be controlled to below 5 nm, and the high dispersion of these clusters gives rise to catalytic activity for the oxidation of carbon monoxide, an important reaction for automobile exhaust treatment. This novel ordered mesoporous Pt/TiO2 nanoarchitecture is also a promising photochemical material, as demonstrated by the photo-driven killing of Micrococcus lylae cells on the film.     

Dye-sensitized solid-state solar cells fabricated by screen-printed TiO_2 thin film with addition of polystyrene balls

The screen-printed nanoporous TiO2 thin film was employed to fabricate dye-sensitized solid-state solar cells using CuI as hole-transport materials. The solar cell based on nanoporous TiO2 thin film with large pores formed by the addition of polystyrene balls with diameter of 200 nm to the TiO2 paste exhibits photovoltaic performance enhancement, which is attributed to the good contact of CuI with surface of dye-sensitized thin film due to easy penetration of CuI in the film with large pores.     

Recent advances in characterization of CaCu3Ti4O12 thin films by spectroscopic ellipsometric metrology

CaCu3Ti4O12 (CCTO) thin films were successfully grown on LaAlO3(100) and Pt/TiO2/SiO2/Si(100) substrates by a novel MOCVD approach. Epitaxial CCTO(001) thin films have been obtained on LaAlO3(100) substrates, while polycrystalline CCTO films have been grown on Pt/TiO2/SiO2/Si(100) substrates. Surface morphology and grain size of the different nanostructured deposited films were examined by AFM, and spectroscopic ellipsometry has been used to investigate the electronic part of the dielectric constant (epsilon2). Looking at the epsilon2 curves, it can be seen that by increasing the film structural order, a greater dielectric response has been obtained. The measured dielectric properties accounted for the ratio between grain volumes and grain boundary areas, which is very different in the different structured films.     

Ag(core)-AgCl(shell) standard microelectrode-loaded TiO2

Micrometer-sized Ag(core)-AgCl(shell) composite crystals have been formed on TiO2 thin films by a two-st epelectrochemical method to provide information on the thermodynamic condition for efficient photoinduced interfacial electron transfer.     

Lamination method for the study of interfaces in polymeric thin film transistors

A method for the fabrication of polymeric thin-film transistors (TFTs) by lamination is described. Poly(dimethylsiloxane) stamps were used to delaminate thin films of semiconducting polymers from silicon wafers coated with a self-assembled monolayer (SAM) formed from octyltrichlorosilane. These supported films were laminated onto electrode structures to form coplanar TFTs. The fabrication process was used to make TFTs with poly(3-hexylthiophene), P3HT, and poly[5,5'-bis(3-dodecyl-2-thienyl)-2,2'-bithiophene], PQT-12. TFTs, where these polymers were laminated onto gate dielectrics coated with SAMs from octyltrichlorosilane, had effective field-effect mobilities of 0.03 and 0.005 cm2/(V s), respectively. TFTs where PQT-12 was laminated onto gate dielectrics that were not coated with a SAM also had mobility of 0.03 cm2/(V s). In contrast, TFTs fabricated by spin-coating PQT-12 onto the same structure had mobilities ranging from 10-3 to 10-4 cm2/(V s). These results suggest that the lower mobilities of polymer TFTs made with hydrophilic gate dielectrics are caused by molecular ordering in the semiconducting film rather than electronic effects of dipolar groups at the interface.     

TiO2光致双亲性与水性油墨转印的研究

研究了TiO2薄膜作为一种新型印刷材料的浸润性与光照时间和光照强度的关系以及光照时间对水性油墨在薄膜表面吸附的影响,实现了水性油墨由TiO2薄膜向纸张的多次转移.     

Structural characterization of a pentacene monolayer on an amorphous SiO2 substrate with grazing incidence x-ray diffraction

Grazing incidence X-ray diffraction reveals that a pentacene monolayer, grown on an amorphous SiO2 substrate that is commonly used as a dielectric layer in organic thin film transistors (OTFTs), is crystalline. A preliminary energy-minimized model of the monolayer, based on the GIXD data, reveals that the pentacene molecules adopt a herringbone arrangement with their long axes tilted slightly from the substrate normal. Although this arrangement resembles the general packing features of the (001) layer in single crystals of bulk pentacene, the monolayer lattice parameters and crystal structure differ from those of the bulk. Because carrier transport in pentacene OTFTs is presumed to occur in the semiconductor layers near the dielectric interface, the discovery of a crystalline monolayer structure on amorphous SiO2 has important implications for transport in OTFTs.     

TiO2/Au纳米棒阵列的制备及其光催化性能

采用电沉积和旋转涂膜相结合的方法成功制备了高度有序的TiO2/Au纳米棒阵列催化剂.扫描电镜和透射电镜结果表明,TiO2薄膜均匀地包覆在Au纳米棒的表面,形成核壳型的一维阵列结构.X射线衍射分析表明所获得TiO2为(101)晶面优先生长的锐钛矿晶相.紫外-可见漫反射光谱显示,由于Au和TiO2间肖特基结的存在造成吸收红...     

Use of laser drilling in the manufacture of organic inverter circuits

Inverter circuits have been made by connecting two high-quality pentacene field-effect transistors. A uniform and pinhole-free 900 nm thick polyimide gate-insulating layer was formed on a flexible polyimide film with gold gate electrodes and partially removed by using a CO₂ laser drilling machine to make via holes and contact holes. Subsequent evaporation of the gold layer results in good electrical connection with a gold gate layer underneath the gate-insulating layer. By optimization of the settings of the CO₂ laser drilling machine, contact resistance can be reduced to as low as 3 Ω for 180 μm square electrodes. No degradation of the transport properties of the organic transistors was observed after the laser-drilling process. This study demonstrates the feasibility of using the laser drilling process for implementation of organic transistors in integrated circuits on flexible polymer films.     

An Electrochemical Transducer Based on a Pentacene Double‐Gate Thin‐Film Transistor

We report an electrochemical transducer based on an organic double‐gate transistor. The bottom‐gate is given by a p‐doped silicon substrate, which is covered by 300 nm thermal oxide. A 20 nm pentacene film acts as the semiconducting layer, and a 50 nm tetratetracontane (TTC) alkane film is used as a top‐gate dielectric. An aqueous ionic solution acts as top‐gate. We record the transistor transfer characteristics by variation of the electrolyte potential via a Ag/AgCl electrode for various bottom‐gate settings. A change of the electrolyte potential results in a change of the transistor current and the characteristic behaviour of the device is in good agreement with the expected behaviour of a double‐gate transistor. The top‐gate capacitance of the alkane layer is as high as 2.6×10^?8 F cm^?2 determined by impedance measurements, indicating that TTC is a good choice as an organic top‐gate dielectric. The suitability of this transducer configuration for sensing in aqueous media is demonstrated by the detection of hexanoic acid and stearic acid molecules adsorbing to the alkane interface, respectively. We show that the transducer easily achieves a concentration sensitivity in the range of 100 nM.     

In-situ probe of gate dielectric-semiconductor interfacial order in organic transistors: origin and control of large performance sensitivities

Organic thin film transistor (OTFT) performance is highly materials interface-dependent, and dramatic performance enhancements can be achieved by properly modifying the semiconductor/gate dielectric interface. However, the origin of these effects is not well understood, as this is a classic "buried interface" problem that has traditionally been difficult to address. Here we address the question of how n-octadecylsilane (OTS)-derived self-assembled monolayers (SAMs) on Si/SiO(2) gate dielectrics affect the OTFT performance of the archetypical small-molecule p-type semiconductors P-BTDT (phenylbenzo[d,d]thieno[3,2-b;4,5-b]dithiophene) and pentacene using combined in situ sum frequency generation spectroscopy, atomic force microscopy, and grazing incidence and reflectance X-ray scattering. The molecular order and orientation of the OTFT components at the dielectric/semiconductor interface is probed as a function of SAM growth mode in order to understand how this impacts the overlying semiconductor growth mode, packing, crystallinity, and carrier mobility, and hence, transistor performance. This understanding, using a new, humidity-specific growth procedure, leads to a reproducible, scalable process for highly ordered OTS SAMs, which in turn nucleates highly ordered p-type semiconductor film growth, and optimizes OTFT performance. Surprisingly, the combined data reveal that while SAM molecular order dramatically impacts semiconductor crystalline domain size and carrier mobility, it does not significantly influence the local orientation of the overlying organic semiconductor molecules.     

Morphological and photoelectrochemical characterization of core-shell nanoparticle films for dye-sensitized solar cells: Zn-O type shell on SnO2 and TiO2 cores

Core-shell type nanoparticles with SnO2 and TiO2 cores and zinc oxide shells were prepared and characterized by surface sensitive techniques. The influence of the structure of the ZnO shell and the morphology ofnanoparticle films on the performance was evaluated. X-ray absorption near-edge structure and extended X-ray absorption fine structure studies show the presence of thin ZnO-like shells around the nanoparticles at low Zn levels. In the case of SnO2 cores, ZnO nanocrystals are formed at high Zn/Sn ratios (ca. 0.5). Scanning electron microscopy studies show that Zn modification of SnO2 nanoparticles changes the film morphology from a compact mesoporous structure to a less dense macroporous structure. In contrast, Zn modification of TiO2 nanoparticles has no apparent influence on film morphology. For SnO2 cores, adding ZnO improves the solar cell efficiency by increasing light scattering and dye uptake and decreasing recombination. In contrast, adding a ZnO shell to the TiO2 core decreases the cell efficiency, largely owing to a loss of photocurrent resulting from slow electron transport associated with the buildup of the ZnO surface layer.