Stabilization of organic thin film transistors by ion implantation

We report on the effects of low energy ion implantation (N and Ne) in the reduction and control of the degradation of pentacene organic thin film transistors (OTFTs) due to the exposure to atmosphere (i.e. oxygen and water). We have observed that a controlled damage depth distribution preserves the functionality of the devices, even if ion implantation induces significant molecular structure modifications, in particular a combination of dehydrogenation and carbonification effects. No relevant changes in the pentacene thin film thickness have been observed. The two major transport parameters that characterize OTFT performance are the carrier mobility and the threshold voltage. We have monitored the effectiveness of this process in stabilizing the device by monitoring the carrier mobility and the threshold voltage over a long time (over 2000 h). Finally, we have assessed by depth resolved X-ray Photoemission Spectroscopy analyses that, by selectively implanting with ions that can react with the hydrocarbon matrix (e.g. N⁺), it is possible to locally modify the charge distribution within the organic layer.     

Fabrication of 6,13‐bis(triisopropyl‐silylethynyl)–pentacene thin‐film transistors with the silver ink transfer method using a polymer stamp

We fabricated 6,13‐bis(triisopropylsilylethynyl)–pentacene (TIPS–pentacene) thin film transistors using a direct metal transfer method. Using different metals, such as Au and Ag ink, electrode patterns are formed from the relief region of the polymer mold. TIPS–pentacene TFTs using the Ag ink transfer method show a similar performance to those using the Au metal transfer method. This method has advantages over the Au metal transfer method because it does not require vacuum equipment and a dry etching process. The self‐assembled monolayer (SAM) treated device exhibits a carrier mobility of 9.5 × 10^–2 cm²/V · s, and an on/off ratio of 4.6 × 10⁴. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Lowering the driving voltage and improving the luminance of blue fluorescent organic light-emitting devices by thermal annealing a hole injection layer of pentacene

We chose pentacene as a hole injection layer (HIL) to fabricate the high performance blue fluorescent organic light-emitting devices (OLEDs). We found that the carrier mobility of the pentacene thin films could be efficiently improved after a critical annealing at temperature 120 °C. Then we performed the tests of scanning electron microscopy, atomic force microscopy, and Kelvin probe to explore the effect of annealing on the pentacene films. The pentacene film exhibited a more crystalline form with better continuities and smoothness after annealing. The optimal device with 120 ℃ annealed pentacene film and n-doped electron transport layer (ETL) presents a low turn-on voltage of 2.6 V and a highest luminance of 134800 cd/m² at 12 V, which are reduced by 26% and improved by 50% compared with those of the control device.     

对以并五苯和酞菁铜为不同有源层的有机薄膜晶体管特性研究

通过扫描电镜和X射线衍射对SiO₂衬底上生长并五苯和酞菁铜薄膜的表面形貌进行表征,并得到在SiO₂衬底上生长的并五苯薄膜是以岛状结构生长,其大小约为100nm,且薄膜有较好的结晶取向,呈多晶态存在. 酞菁铜薄膜则没有表现出明显的生长机理,其呈非晶态存在. 还对通过掩膜的方法制作得以酞菁铜和并五苯为有源层的顶栅极有机薄膜晶体管的特性进行了研究. 有源层的厚度为40nm,绝缘层SiO₂的厚度为250nm,器件的沟道宽长比(W/关键词： 有机薄膜晶体管 并五苯薄膜 酞菁铜薄膜 μ_EF)')" href="#">场效应迁移率(μ_EF)     

Performance improvement in pentacene organic thin film transistors by inserting a C60 ultrathin layer

The contact effect on the performances of organic thin film transistors is studied here. A C₆₀ ultrathin layer is inserted between Al source--drain electrode and pentacene to reduce the contact resistance. By a 3 nm C₆₀ modification, the injection barrier is lowered and the contact resistance is reduced. Thus, the field-effect mobility increases from 0.12 to 0.52 cm²/(V·s). It means that inserting a C₆₀ ultra thin layer is a good method to improve the organic thin film transistor (OTFT) performance. The output curve is simulated by using a charge drift model. Considering the contact effect, the field effect mobility is improved to 1.15 cm²/(V·s). It indicates that further reducing the contact resistance of OTFTs should be carried out.     

Comparative study of field-effect mobility with different expressions in organic thin film transistors

In organic thin film transistors (OTFTs), mobility generally exhibits field-effect dependence, and is strongly related to the disorder property of organic semiconductors used in OTFTs. Here, we compared three typical field-effect mobility models and used them to simulate the output characteristics of pentacene-based OTFTs. From the comparison of the theory and experiment, an analytic expression for the field-effect mobility to exactly describe the electrical characteristics of OTFTs was obtained. The better fit to the output characteristics of poly(9,9-dioctyl-fluorene-co-N-(4-butylphenyl)-diphenylamine) (TFB)-based OTFT and copper phthalocyanine (CuPc)-based OTFT by using the obtained analytic expression of the field-effect mobility further extended its applicability in OTFTs. This supplies a valuable manner to derive the field-effect mobility of carriers and understand the transport characteristics of carriers in OTFTs.     

Enhanced electrical properties of pentacene-based organic thin-film transistors by modifying the gate insulator surface

A reliable surface treatment for the pentacene/gate dielectric interface was developed to enhance the electrical transport properties of organic thin-film transistors (OTFTs). Plasma-polymerized fluorocarbon (CFx) film was deposited onto the SiO₂ gate dielectric prior to pentacene deposition, resulting in a dramatic increase of the field-effect mobility from 0.015 cm²/(V s) to 0.22 cm²/(V s), and a threshold voltage reduction from −14.0 V to −9.9 V. The observed carrier mobility increase by a factor of 10 in the resulting OTFTs is associated with various growth behaviors of polycrystalline pentacene thin films on different substrates, where a pronounced morphological change occurs in the first few molecular layers but the similar morphologies in the upper layers. The accompanying threshold voltage variation suggests that hole accumulation in the conduction channel-induced weak charge transfer between pentacene and CFx.     

Effect of molecular structure on bias stress effect in organic thin-film transistors

Two thiophene-phenylene semiconductors, bis(2-phenylethynyl) end-substituted oligothiophenes (diPhAc-nTs, n = 2, 3), were studied as active layers in organic thin film transistors (OTFTs). Structural and electrical properties of such high vacuum evaporated thin films were compared to pentacene. All three oligomers behave as p-type semiconducting layers into OTFTs. In the same preparation and measurement conditions, diPhAc-3T possesses two of incontrovertible attributes of OTFTs for low cost applications, a high air-stable mobility at low substrate temperature (T_sub), i.e. typically 25 °C together with a reduced bias stress effect compared to the well-known pentacene semiconductor. This study brings to light on the role of the molecular structure involved in the active layer in thin-film devices and describes effects as thin film morphology as important parameters when optimizing the structure of OTFTs.     

内嵌CuO薄膜对并五苯薄膜晶体管性能的改善

制备了基于内嵌氧化物铜(CuO)薄膜的并五苯薄膜晶体管器件. 将3 nm CuO薄膜内嵌入到并五苯(pentacene)中, 作为空穴注入层, 降低电极与并五苯之间的空穴注入势垒. 相对于纯并五苯薄膜晶体管器件, 研制的晶体管的迁移率、阈值电压(V_TH)、电流开关比(I_on/I_off) 等参数都有明显改善. X射线光电子能谱数据表明, 这种空穴注入势垒的降低源自并五苯向CuO的电子转移.     

Room-temperature and solution-processed vanadium oxide buffer layer for efficient charge injection in bottom-contact organic field-effect transistors

We introduce a room temperature and solution-processible vanadium oxide (VO_x) buffer layer beneath Au source/drain electrodes for bottom-contact (BC) organic field-effect transistors (OFETs). The OFETs with the VO_x buffer layer exhibited higher mobility and lower threshold voltages than the devices without a buffer layer. The hole mobility with VO_x was over 0.11 cm²/V with the BC geometry with a short channel length (10 μm), even without a surface treatment on SiO₂. The channel width normalized contact resistance was decreased from 98 kΩ cm to 23 kΩ cm with VO_x. The improved mobility and the reduced contact resistance were attributed to the enhanced continuity of pentacene grains, and the increased work function and adhesion of the Au electrodes using the VO_x buffer layer.     

Optimization and improvement of TIPS–pentacene transistors (OTFT) with UV–ozone and chemical treatments using an all-step solution process

We fabricated an organic thin-film transistor (OTFT) using an all-step solution process. The printed layers, in which the electrode (silver), dielectric layer (BaTiO₃–PMMA), source–drain layer, and semiconductor 6,13-Bis(triisopropylsilylethynyl)pentacene(TIPS–pentacene), were optimized using roll-to-roll, an inkjet printer, and drop-casting. After coating the source–drain layer, we applied ultraviolet (UV)–ozone and self-assembled monolayer (SAM) treatments to the composite layer. The OTFTs treated with the UV–ozone and SAM treatments were found to exhibit excellent performance and good properties in comparison to silicon-based OTFTs.     

不同绝缘层上生长的并五苯薄膜及其OTFT器件性能的研究

分别以SiO₂和PMMA为绝缘层材料制备了底栅顶接触结构的OTFT器件,得到以PMMA为绝缘层的器件具有更好的性能,其场效应迁移率为0.207 cm2·Vs-1,开关电流比为4.93×103,阈值电压为-4.3 V;而以SiO₂为绝缘层的器件,其场效应迁移率仅为0.039 cm2·Vs-1,开关电流比为5.98×102,阈值电压为-5.4 V。为分析器件性能差异的原因,测得了SiO₂和PMMA薄膜表面的AFM图谱及其上沉积并五苯薄膜后的AFM和XRD图谱。通过AFM图谱发现PMMA表面较SiO₂表面粗糙度小,其表面粗糙度的均方根值为0.216 nm,而二氧化硅薄膜表面粗糙度的均方根值为1.579 nm;且发现在PMMA上生长的并五苯薄膜的成膜质量优于在SiO₂,具有较大的晶粒尺寸和较少的晶粒间界。通过XRD图谱发现在PMMA上生长的并五苯薄膜具有明显的衍射峰,进一步证明了在PMMA上生长的并五苯薄膜具有更好的结晶状况,将更有利于载流子的传输。     

Mobility Enhancement of P3HT‐Based OTFTs upon Blending with Au Nanorods

The mobility enhancement of organic thin‐film transistors based on poly(3‐hexylthiophene) (P3HT) by incorporating gold nanorods (Au NRs) is reported. Through varying the doping concentration and surface modifier of the Au NRs in P3HT matrix, the P3HT/Au composite with 0.5 mg mL^?1 pyridine‐capped Au NRs exhibits a hole mobility of 0.059 cm² V^?1 s^?1, this value is seven times higher than that of pristine P3HT. This remarkable improvement of mobility originates from the enhanced crystallinity and optimized orientation of P3HT after doping with Au NRs. In addition, the appropriate surface modification can produce more‐efficient hole conduction of Au NRs.     

Studies on morphology and molecular arrangement of pentacene on different substrates

Pentacene (C₂₂H₁₄) thin films with different thicknesses were fabricated to study the dynamic growth process and morphology of pentacene on different substrates. A discontinuous monomolecular layer was observed when a pentacene thin film is about 0.5 nm thick on native oxide silicon wafer. The terraced islands and dendritic structure gradually formed with increasing pentacene thin film thickness. The height of each layer is about 1.4 nm which corresponds well with the length of the long axis of the pentacene molecule at 1.45 nm. Experimental results show that the pentacene molecule is perpendicular to the silicon wafer surface with a slight tilted angle. However, the pentacene molecular orientation on a polymer pre-covered indium tin oxide coated substrate could not give any indication on the scale of nanometers. The surface roughness of substrates strongly influences pentacene molecular diffusion and the morphology of pentacene thin films.     

有机半导体薄膜生长原位实时测量方法的研究

针对原位实时监测有机半导体薄膜生长情况的需求, 提出了差分反射光谱法与场效应晶体管法结合的光电联合测量方法, 设计研制了测量系统. 以并五苯有机分子为例, 通过自制底栅底接触式场效应管微结构, 实验测试了热蒸发法生长导电膜层过程中光电信号的演变与相互关联. 光谱信号显示, 并五苯以薄膜态结构进行生长, 光谱随生长进程变化显著. 实验数据与四相结构模型仿真结果的良好吻合, 表明因薄膜增厚引起干涉条件的改变是光谱变化的主因, 由此推算出薄膜生长速率为0.23 nm/min. 当薄膜等效厚度达到28 nm时, 场效应管的导电性显著增强, 标志着并五苯有效传输层的形成. 此后, 薄膜厚度持续增加, 但测试电流增长缓慢, 说明该结构进入电学特性饱和区. 光电联合法不仅有助于研究有机半导体薄膜的光谱信息、电学特性和薄膜结构之间的相互对应关系, 也为发展原位监测有机半导体薄膜制备过程, 探索最佳工艺提供了新的研究手段.     

The interaction of oxygen and ozone with pentacene

We use ultraviolet photoemission spectroscopy (UPS) to investigate the effect of oxygen and air exposure on pentacene thin film electronic structure. It is found that O₂ and water do not react noticeably with pentacene on the timescale of several hours, whereas a mixture of oxygen atoms, singlet oxygen and ozone readily oxidizes the organic compound. We obtain no evidence for irreversible intercalation of oxygen into pentacene or considerable p-type doping after re-evacuation. Infrared spectroscopy and atomic force microscopy are used to study the oxidation of pentacene thin films. Our data suggest the oxidation of pentacene with reactive oxygen species to yield highly volatile reaction products as evidenced by significant mass-losses of the films.     

Exploring the magnetic and organic microstructures with photoemission electron microscope

We present photoemission electron microscopy (PEEM) studies on geometrically constrained ferromagnetic, organic, and organics–ferromagnet hybrid structures. Powered by an elliptically polarized undulator, the PEEM at Taiwan Light Source (TLS) is capable of recording polarization enhanced X-ray images and has been employed to examine the domain configurations in a lithographically patterned permalloy film as well as the orientations of pentacene molecules adsorbed on self-assembled monolayers (SAMs) modified gold surfaces. In addition, magnetic images acquired on cobalt/pentacene and pentacene/cobalt bilayers reveal that in hybrid structures the order of thin film deposition can lead to distinct domain configurations. Spectroscopic evidence further suggests that there is significant orbital hybridization at the interface where metallic cobalt was deposited directly on organic pentacene.     

Study on characteristics of a double-conductible channel organic thin-films transistor with an ultra-thin hole-blocking layer

The properties of top-contact organic thin-film transistors (TC-OTFTs) using ultra-thin 2, 9-dimethyl-4, 7-diphenyl-1, 10-phenanthroline (BCP) as a hole-blocking interlayer have been improved significantly and a BCP interlayer was inserted into the middle of the pentacene active layer. This paper obtains a fire-new transport mode of an OTFT device with double-conductible channels. The accumulation and transfer of the hole carriers are limited by the BCP interlayer in the vertical region of the channel. A huge amount of carriers is located not only at the interface between pentacene and the gate insulator, but also at the two interfaces of pentacene/BCP interlayer and pentacene/gate insulator, respectively. The results suggest that the BCP interlayer may be useful to adjust the hole accumulation and transfer, and can increase the hole mobility and output current of OTFTs. The TC-OTFTs with a BCP interlayer at V_DS=-20~V showed excellent hole mobility μ_FE and threshold voltage V_TH of 0.58~cm²/(V\cdots) and --4.6~V, respectively.     

Electronic non-equilibrium conditions at C60–pentacene heterostructures

The electronic structure of vacuum-sublimed layered organic heterostructures of pentacene (PEN) and fullerene (C₆₀) on conducting polymer substrates was investigated using ultraviolet photoelectron spectroscopy (UPS). The conditions at the PEN/C₆₀interface changed from thermodynamic non-equilibrium (i.e. the onset of the PEN highest occupied molecular orbital above the substrate Fermi-energy) for thin PEN coverages on C₆₀ to thermodynamic equilibrium for thicker PEN coverages (i.e. Fermi-level pinning of PEN). This finding is attributed to a coverage-dependent pinhole connection of PEN through the C₆₀ layer with the substrate. The experiments demonstrate the importance of organic thin film morphology for UPS measurements to assess the energy level alignment at organic/organic heterointerfaces.     

Electric field confinement effect on charge transport in organic field-effect transistors

While it is known that the charge-carrier mobility in organic semiconductors is only weakly dependent on the electric field at low fields, the experimental mobility in organic field-effect transistors using silylethynyl-substituted pentacene is found to be surprisingly field dependent at low source-drain fields. Corroborated by scanning Kelvin probe measurements, we explain this observation by the severe difference between local conductivities within grains and at grain boundaries. Redistribution of accumulated charges creates very strong local lateral fields in the latter regions. We further confirm this picture by verifying that the charge mobility in channels having no grain boundaries, made from the same organic semiconductor, is not significantly field dependent. We show that our model allows us to quantitatively model the source-drain field dependence of the mobility in polycrystalline organic transistors.