Cover Picture: High Definition Digital Fabrication of Active Organic Devices by Molecular Jet Printing (Adv. Funct. Mater. 15/2007)

A new method for direct patterning of organic optoelectronic/electronic devices using a reconfigurable and scalable printing method is reported by Vladimir Bulovic and co‐workers on p. 2722. The printing technique is applied to the fabrication of high‐resolution printed organic light emitting devices (OLEDs) and organic field effect transistors (OFETs). Remarkably, the final print‐deposited films are evaporated onto the substrate (rather than solvent printed), giving high‐quality, solvent‐free, molecularly flat structures that match the performance of comparable high‐performance unpatterned films. We introduce a high resolution molecular jet (MoJet) printing technique for vacuum deposition of evaporated thin films and apply it to fabrication of 30 μm pixelated (800 ppi) molecular organic light emitting devices (OLEDs) based on aluminum tris(8‐hydroxyquinoline) (Alq₃) and fabrication of narrow channel (15 μm) organic field effect transistors (OFETs) with pentacene channel and silver contacts. Patterned printing of both organic and metal films is demonstrated, with the operating properties of MoJet‐printed OLEDs and OFETs shown to be comparable to the performance of devices fabricated by conventional evaporative deposition through a metal stencil. We show that the MoJet printing technique is reconfigurable for digital fabrication of arbitrary patterns with multiple material sets and high print accuracy (of better than 5 μm), and scalable to fabrication on large area substrates. Analogous to the concept of “drop‐on‐demand” in Inkjet printing technology, MoJet printing is a “flux‐on‐demand” process and we show it capable of fabricating multi‐layer stacked film structures, as needed for engineered organic devices.     

Cover Picture: Self‐Organization of Ink‐jet‐Printed Triisopropylsilylethynyl Pentacene via Evaporation‐Induced Flows in a Drying Droplet (Adv. Funct. Mater. 2/2008)

Charge carrier transport in organic electronic devices is influenced by the crystalline microstructure and morphology of the organic semiconductor film. Evaporation behavior during drying plays a vital role in controlling the film morphology and the distribution of solute in inkjet‐printed films. On p. 229, Kilwon Cho and co‐workers demonstrate the influence of the evaporation‐induced flow in a single droplet on the crystalline microstructure and film morphology of inkjet‐printed 6,13‐bis((triisopropylsilylethynyl) pentacene. The results provide an excellent method for direct‐write fabrication of high‐performance organic electronics. We have demonstrated the influence of evaporation‐induced flow in a single droplet on the crystalline microstructure and film morphology of an ink‐jet‐printed organic semiconductor, 6,13‐bis((triisopropylsilylethynyl) pentacene (TIPS_PEN), by varying the composition of the solvent mixture. The ringlike deposits induced by outward convective flow in the droplets have a randomly oriented crystalline structure. The addition of dichlorobenzene as an evaporation control agent results in a homogeneous film morphology due to slow evaporation, but the molecular orientation of the film is undesirable in that it is similar to that of the ring‐deposited films. However, self‐aligned TIPS_PEN crystals with highly ordered crystalline structures were successfully produced when dodecane was added. Dodecane has a high boiling point and a low surface tension, and its addition to the solvent results in a recirculation flow in the droplets that is induced by a Marangoni flow (surface‐tension‐driven flow), which arises during the drying processes in the direction opposite to the convective flow. The field‐effect transistors fabricated with these self‐aligned crystals via ink‐jet printing exhibit significantly improved performance with an average effective field‐effect mobility of 0.12 cm² V^–1 s^–1. These results demonstrate that with the choice of appropriate solvent ink‐jet printing is an excellent method for the production of organic semiconductor films with uniform morphology and desired molecular orientation for the direct‐write fabrication of high‐performance organic electronics.     

Guard flow-enhanced organic vapor jet printing of photovoltaic donor materials in air

We demonstrate the ability to rapidly deposit (local deposition rates well exceeding 1000 Å/s) in air a small molecular electron donor material boron subphthalocyanine chloride (SubPc) for use in photovoltaic devices. The method employs a highly collimated jet of hot nitrogen gas carrying organic vapor toward a substrate, on which the organic molecules condense. A secondary jet of nitrogen coaxially surrounds the primary jet, shielding the organic vapor from the atmosphere. We study how the guard flow rate affects the printed film morphology and the resulting device properties. Notably, the crystallinity of the donor film can be increased while lowering the thin-film roughness, enhancing short circuit photocurrent nearly twofold over films printed without guard flow, allowing the photovoltaic power conversion efficiency of planar heterojunction SubPc/C₆₀ cells to exceed 2% with SubPc deposited at ambient conditions.     

High Definition Digital Fabrication of Active Organic Devices by Molecular Jet Printing

We introduce a high resolution molecular jet (MoJet) printing technique for vacuum deposition of evaporated thin films and apply it to fabrication of 30 μm pixelated (800 ppi) molecular organic light emitting devices (OLEDs) based on aluminum tris(8‐hydroxyquinoline) (Alq₃) and fabrication of narrow channel (15 μm) organic field effect transistors (OFETs) with pentacene channel and silver contacts. Patterned printing of both organic and metal films is demonstrated, with the operating properties of MoJet‐printed OLEDs and OFETs shown to be comparable to the performance of devices fabricated by conventional evaporative deposition through a metal stencil. We show that the MoJet printing technique is reconfigurable for digital fabrication of arbitrary patterns with multiple material sets and high print accuracy (of better than 5 μm), and scalable to fabrication on large area substrates. Analogous to the concept of “drop‐on‐demand” in Inkjet printing technology, MoJet printing is a “flux‐on‐demand” process and we show it capable of fabricating multi‐layer stacked film structures, as needed for engineered organic devices.     

Self‐Organization of Ink‐jet‐Printed Triisopropylsilylethynyl Pentacene via Evaporation‐Induced Flows in a Drying Droplet

We have demonstrated the influence of evaporation‐induced flow in a single droplet on the crystalline microstructure and film morphology of an ink‐jet‐printed organic semiconductor, 6,13‐bis((triisopropylsilylethynyl) pentacene (TIPS_PEN), by varying the composition of the solvent mixture. The ringlike deposits induced by outward convective flow in the droplets have a randomly oriented crystalline structure. The addition of dichlorobenzene as an evaporation control agent results in a homogeneous film morphology due to slow evaporation, but the molecular orientation of the film is undesirable in that it is similar to that of the ring‐deposited films. However, self‐aligned TIPS_PEN crystals with highly ordered crystalline structures were successfully produced when dodecane was added. Dodecane has a high boiling point and a low surface tension, and its addition to the solvent results in a recirculation flow in the droplets that is induced by a Marangoni flow (surface‐tension‐driven flow), which arises during the drying processes in the direction opposite to the convective flow. The field‐effect transistors fabricated with these self‐aligned crystals via ink‐jet printing exhibit significantly improved performance with an average effective field‐effect mobility of 0.12 cm² V^–1 s^–1. These results demonstrate that with the choice of appropriate solvent ink‐jet printing is an excellent method for the production of organic semiconductor films with uniform morphology and desired molecular orientation for the direct‐write fabrication of high‐performance organic electronics.     

Room‐Temperature Printing of Organic Thin‐Film Transistors with π‐Junction Gold Nanoparticles

Printing semiconductor devices under ambient atmospheric conditions is a promising method for the large‐area, low‐cost fabrication of flexible electronic products. However, processes conducted at temperatures greater than 150 °C are typically used for printed electronics, which prevents the use of common flexible substrates because of the distortion caused by heat. The present report describes a method for the room‐temperature printing of electronics, which allows thin‐film electronic devices to be printed at room temperature without the application of heat. The development of π‐junction gold nanoparticles as the electrode material permits the room‐temperature deposition of a conductive metal layer. Room‐temperature patterning methods are also developed for the Au ink electrodes and an active organic semiconductor layer, which enables the fabrication of organic thin‐film transistors through room‐temperature printing. The transistor devices printed at room temperature exhibit average field‐effect mobilities of 7.9 and 2.5 cm² V^?1 s^?1 on plastic and paper substrates, respectively. These results suggest that this fabrication method is very promising as a core technology for low‐cost and high‐performance printed electronics.     

有机电致发光器件中有机薄膜的制备方法

有机电致发光器件中有机薄膜的制备方法非常重要,不同方法制备的薄膜质量不同,这直接影响着器件的效率;制备方法直接影响到产业化中的器件制备成本。根据材料的不同,有机小分子常用真空蒸镀的方法,而高分子材料常用旋涂的方法制备薄膜。随着有机电致发光器件制备工艺的发展,相继出现了其他的制备工艺,如:有机蒸汽喷印(organic vapor jet printing)、有机气相沉积(organic vapor phase deposition)、丝网印刷(screen printing)和喷墨打印(ink jet printing)技术等,这对有机电致发光显示器产业化发展具有巨大的推动作用。文章综述了这些制备方法,比较了它们的优缺点,以及这些工艺对产业化的影响。     

Polypyrrole top-contact electrodes patterned by inkjet printing assisted vapor deposition polymerization in flexible organic thin-film transistors

Highly conductive polymer, polypyrrole (PPy) was successfully patterned as source and drain (S/D) electrodes for flexible pentacene thin film transistors in top-contact structure by combining inkjet printing and vapor deposition polymerization. Facile inkjet printing of initiator and subsequent exposure of pyrrole monomers resulted in selective absorption and polymerization of pyrrole monomers on the patterned initiator region. Pentacene transistors based on printed PPy electrodes exhibited higher electrical characteristics than that of the devices with thermally evaporated Au electrodes. Improved performance of the devices based on PPy electrodes could be attributed to the reduction of contact resistance at the interface between polymer and organic semiconductor. For the replacement of metal electrodes, vapor deposition polymerization assisted inkjet printing technique can provide a versatile method to utilize highly conductive polymer as a functional electrode of flexible organic electronic devices.     

高性能有机薄膜晶体管(OTFT)制备及其导电机制

详细介绍了在SiO2和高kHfO2介质层上制备并五苯薄膜晶体管方面的研究,特别是利用原子力显微技术(AFM)和静电力显微技术(EFM)研究了并五苯分子初始生长模式,揭示了衬底形貌、表面化学性能(包括化学清洗和聚合物层修饰)对有机半导体成膜结构和薄膜场效应晶体管性能之间的关联,包括晶体管迁移率、开关比和阈值电压等;针对并五苯初始生长成核模式的差异,分析了不同岛(畴)间畴边界对载流子在有机薄膜内输运的影响,有助于理解有机半导体薄膜导电机理。通过优化和控制介电层和有机半导体薄膜层的界面化学性质,在SiO2介质层上成功制备出迁移率为1.0cm2/V.s、开关电流比达到106的OTFT器件;在高kHfO2介质层上获得的OTFT器件的工作电压在-5V以下,开关电流比达到105,载流子迁移率为0.6cm2/V.s;器件性能指标已经达到目前国际上文献报道的最好水平。     

并五苯薄膜晶体管及其应用

有机薄膜晶体管(TFT)在数据存储、集成电路、传感器诸方面的广泛应用引起了人们极大的兴趣.在有机TFT的研究中,并五苯TFT占有很重要的地位.介绍了并五苯薄膜晶体管的结构、工作原理及其应用,评述了该领域的研究进展,并对其前景进行了展望.     

Influence of Electric Field on Microstructures of Pentacene Thin‐Films in Field‐Effect Transistors

We report on electric‐field‐induced irreversible structural modifications in pentacene thin films after long‐term operation of organic field‐effect transistor (OFET) devices. Micro‐Raman spectroscopy allows for the analysis of the microstructural modifications of pentacene in the small active channel of OFET during device operation. The results suggest that the herringbone packing of pentacene molecules in a solid film is affected by an external electric field, particularly the source‐to‐drain field that parallels the a–b lattice plane. The analysis of vibrational frequency and Davydov splitting in the Raman spectra reveals a singular behavior suggesting a reduced separation distance between pentacene molecules after long‐term operations and, thus, large intermolecular interactions. These results provide evidence for improved OFET performance after long‐term operation, related to the microstructures of organic semiconductors. It is known that the application of large electric fields alters the semiconductor properties of the material owing to the generation of defects and the trapping of charges. However, we first suggest that large electric fields may alter the molecular geometry and further induce structural phase transitions in the pentacene films. These results provide a basis for understanding the improved electronic properties in test devices after long‐term operations, including enhanced field‐effect mobility, improved on/off current ratio, sharp sub‐threshold swing, and a slower decay rate in the output drain current. In addition, the effects of source‐to‐drain electric field, gate electric field, current and charge carriers, and thermal annealing on the pentacene films during OFET operations are discussed.     

Printable Ultra-Flexible Fluorinated Organic–Inorganic Nanohybrid Sol–Gel Derived Gate Dielectrics for Highly Stable Organic Thin-Film Transistors and Other Practical Applications

A novel fluorinated organic–inorganic (O–I) hybrid sol—gel based material, named FAGPTi, is successfully synthesized and applied as a gate dielectric in flexible organic thin-film transistors (OTFTs). The previously reported three-arm-shaped alkoxysilane-functionalized amphiphilic polymer yields a stable O–I hybrid material consisting of uniformly dispersed nanoparticles in the sol-state. Here, a fluorinated precursor is introduced into the system, making it possible to realize more stable spherical composites. This results in long-term colloidal stability (≈1.5 years) because composite growth is strongly inhibited by the presence of fluorine groups with intrinsically strong repulsive forces. Additionally, the FAGPTi film is easily deposited via thermally annealed sol–gel reactions; the films can be successfully fabricated through the printing method, and exhibit excellent flexibility and enhanced insulating properties compared to existing materials. OTFTs with FAGPTi layers show highly stable driving characteristics under severe bending conditions (1.9% strain). Integrated logic devices are also successfully operated with these OTFTs. Additionally, it can facilely be applied to amorphous indium-gallium-zinc-oxide (a-IGZO) TFT devices other than OTFT. Therefore, this synthetic strategy can provide useful insights into the production of functional O–I hybrid materials, enabling the efficient fabrication of electronic materials and devices exhibiting these properties.     

Electric Field Tuning Molecular Packing and Electrical Properties of Solution‐Shearing Coated Organic Semiconducting Thin Films

Recent improvements in solution‐coated organic semiconductors (OSCs) evidence their high potential for cost‐efficient organic electronics and sensors. Molecular packing structure determines the charge transport property of molecular solids. However, it remains challenging to control the molecular packing structure for a given OSC. Here, the application of alternating electric fields is reported to fine‐tune the crystal packing of OSC solution‐shearing coated at ambient conditions. First, a theoretical model based on dielectrophoresis is developed to guide the selection of the optimal conditions (frequency and amplitude) of the electric field applied through the solution‐shearing blade during coating of OSC thin films. Next, electric field‐induced polymorphism is demonstrated for OSCs with both herringbone and 2D brick‐wall packing motifs in 2,7‐dioctyl[1]benzothieno[3,2‐b][1]benzothiophene and 6,13‐bis(triisopropylsilylethynyl) pentacene, respectively. Favorable molecular packing can be accessible in some cases, resulting in higher charge carrier mobilities. This work provides a new approach to tune the properties of solution‐coated OSCs in functional devices for high‐performance printed electronics.     

Environmental effects on the electrical behavior of pentacene thin-film transistors with a poly(methyl methacrylate) gate insulator

The electrical properties of top-contact pentacene thin-film transistors (TFTs) with a poly(methyl methacrylate) (PMMA) gate dielectric were analyzed in air and vacuum environments. Compared to the vacuum case, the pentacene TFT in air exhibited lower drain currents and more pronounced shifts in the threshold voltage upon reversal of the gate voltage sweep direction, together with a decrease in the field-effect mobility. These characteristic variations were explained in terms of two distinctive actions of polar H₂O molecules in pentacene TFT. H₂O molecules were suggested to diffuse under the source and drain contacts and interrupt the charge injection into the pentacene film, whereas those that permeate at the pentacene/PMMA interface retard hole depletion in and around the TFT channel. The diffusion process was much slower than the permeation process. The degraded TFT characteristics in air could be recovered mostly by storing the device under vacuum, which suggests that the air instability of TFTs is due mainly to the physical adsorption of H₂O molecules within the pentacene film.     

A benchmark study of commercially available copper nanoparticle inks for application in organic electronic devices

A set of three commercial copper nanoparticle based inkjet inks has been benchmarked with respect to their potential to form conducting printed structures for future applications in organic electronic devices. Significant differences were observed in terms of jetting properties, spreading behaviour and line formation on a number of relevant substrates. The inks' stabilities against oxidation were investigated, inkjet printed patterns were subjected to photonic flash sintering and their electrical properties characterized. As a result, optimized conditions for printing and post-deposition processing were determined. Photonic flash sintering, which is a roll-to-roll compatible manufacturing process, allowed a significant reduction in sintering time. Flash sintering was performed in the presence of air, thereby excluding the necessity for processing under inert atmosphere. One product was identified which showed satisfactory performances regarding all tested features: stable jet formation, well-defined definition of the printed structures and high electrical conductivity (20% of the value of bulk Cu). The obtained results can be considered as a promising step towards the future application of Cu inks in organic electronic devices.     

Structural and Electrostatic Complexity at a Pentacene/Insulator Interface

The properties of organic‐semiconductor/insulator (O/I) interfaces are critically important to the operation of organic thin‐film transistors (OTFTs) currently being developed for printed flexible electronics. Here we report striking observations of structural defects and correlated electrostatic‐potential variations at the interface between the benchmark organic semiconductor pentacene and a common insulator, silicon dioxide. Using an unconventional mode of lateral force microscopy, we generate high‐contrast images of the grain‐boundary (GB) network in the first pentacene monolayer. Concurrent imaging by Kelvin probe force microscopy reveals localized surface‐potential wells at the GBs, indicating that GBs will serve as charge‐carrier (hole) traps. Scanning probe microscopy and chemical etching also demonstrate that slightly thicker pentacene films have domains with high line‐dislocation densities. These domains produce significant changes in surface potential across the film. The correlation of structural and electrostatic complexity at O/I interfaces has important implications for understanding electrical transport in OTFTs and for defining strategies to improve device performance.     

Enhanced Performance Consistency in Nanoparticle/TIPS Pentacene‐Based Organic Thin Film Transistors

In this study, inorganic silica nanoparticles are used to manipulate the morphology of 6,13‐bis(triisopropylsilylethynyl)‐pentacene (TIPS pentacene) thin films and the performance of solution‐processed organic thin‐film transistors (OTFTs). This approach is taken to control crystal anisotropy, which is the origin of poor consistency in TIPS pentacene based OTFT devices. Thin film active layers are produced by drop‐casting mixtures of SiO₂ nanoparticles and TIPS pentacene. The resultant drop‐cast films yield improved morphological uniformity at ～10% SiO₂ loading, which also leads to a 3‐fold increase in average mobility and nearly 4 times reduction in the ratio of measured mobility standard deviation (μ_Stdev) to average mobility (μ_Avg). Grazing‐incidence X‐ray diffraction, scanning and transmission electron microscopy as well as polarized optical microscopy are used to investigate the nanoparticle‐mediated TIPS pentacene crystallization. The experimental results suggest that the SiO₂ nanoparticles mostly aggregate at TIPS pentacene grain boundaries, and 10% nanoparticle concentration effectively reduces the undesirable crystal misorientation without considerably compromising TIPS pentacene crystallinity.     

Contact effects in organic thin film transistors with printed electrodes

The influence of contact effects on the performance of pentacene thin film transistors with printed electrodes was investigated. The electrodes of the transistor were realized by a combination of microcontact printing and selective dewetting/wetting. Printing of silane based self-assembled monolayers on glass or silicon substrates allows for the modulation of the surface energy, so that polymers or resists can be selectively deposited in the hydrophilic regions of the substrate, whereas the hydrophobic regions stay uncoated. A poly methyl methacrylate (PMMA) resist was selectively deposited in the hydrophilic regions. The resists structures were used as a template to pattern electrodes of pentacene thin film transistors by a lift-off process. The transistors exhibit charge carrier mobilities of 0.2 cm²/V s, low threshold voltages, and high on/off ratios of 10⁶. The pentacene transistors with printed drain and source electrodes were compared to devices patterned by optical lithography. In particular the influence of the drain and source contacts on the charge carrier mobility of the devices will be discussed. A simple model will be presented which takes the influence of contact effects into account when describing the electrical behavior of the transistors.     

Highly Sensitive Non‐Classical Strain Gauge Using Organic Heptazole Thin‐Film Transistor Circuit on a Flexible Substrate

A non‐classical organic strain gauge as a voltage signal sensor is reported, using an inverter‐type thin‐film transistor (TFT) circuit, which is able to sensitively measure a large quantity of elastic strain (up to ≈2.48%), which approaches an almost folding state. Novel heptazole‐based organic TFTs are chosen to be incorporated in this gauge circuit; organic solid heptazole has small domain size in general. While large crystal domain‐pentacene TFTs seldom show sufficient current variation upon mechanical bending for tensile strain, these heptazole TFTs demonstrate a significant variation for the same strain condition as applied to pentacene devices. In addition, the pentacene channel does not recover to its original electric state after bending but heptazole channels are very elastic and reversible, even after going through serious bending. More interesting is that the heptazole TFTs show only a little variation of signal current under horizontal direction strain, while they make a significant amount of current decrease under vertical direction strain. Utilizing the anisotropic response to the tensile bending strain, an ultrasensitive voltage output strain gauge composed of a horizontally and vertically oriented TFT couple is demonstrated.