Organic passivation layers for pentacene organic thin-film transistors

We studied the passivation layers for pentacene organic thin-film transistors (OTFTs) that were used to drive the active-matrix organic-light-emitting-diodes (AMOLEDs) fabricated by inkjet process. Conventional polyvinyl acetate (PVA) passivation layer could not protect OTFT channel from poly(3,4-ethylenedioxythiophene):poly(styrene sulfonic acid) (PEDOT:PSS) inkjet process so that the performance was degraded critically after the process. By applying PVA/PVA/photoacryl (PA) multi-passivation layers, we could get OTFT arrays with switching ratio over 10⁶ even after PEDOT:PSS process. Using these OTFTs, we could drive AMOLEDs made by inkjet process.     

Bias stress effect in low-voltage organic thin-film transistors

The bias stress effect in pentacene organic thin-film transistors has been investigated. The transistors utilize a thin gate dielectric based on an organic self-assembled monolayer and thus can be operated at low voltages. The bias stress-induced threshold voltage shift has been analyzed for different drain-source voltages. By fitting the time-dependent threshold voltage shift to a stretched exponential function, both the maximum (equilibrium) threshold voltage shift and the time constant of the threshold voltage shift were determined for each drain-source voltage. It was found that both the equilibrium threshold voltage shift and the time constant decrease significantly with increasing drain-source voltage. This suggests that when a drain-source voltage is applied to the transistor during gate bias stress, the tilting of the HOMO and LUMO bands along the channel creates a pathway for the fast release of trapped carriers.     

Surface modified polymeric gate insulators for pentacene organic thin-film transistors

This paper demonstrates effects of a surface modification of polymeric gate insulators on a performance of organic thin-film transistor (OTFT). Pentacene OTFTs were fabricated with three types of polymer gate insulators—poly(4-vinylphenol) (PVP, G1) with comparably high dielectric constant, polyimide (PI, G2) with n-octadecyl (C₁₈) side chain, which resulted in hydrophobicity and low dielectric constant, and surface modified PVP(G3). The G3 was prepared by a spin-coating the solution of G2 onto the G1 film. We found that the n-octadecyl group of the G3 protruded from the surface and made the PVP surface more hydrophobic. The less polar surface strongly improved the device performance. Subthreshold slope (s.s.) of the OTFT with G3 as the gate insulator decreased significantly to 2.7 V/dec, which was much smaller than that of OTFTs fabricated with G1 (4.0 V/dec). That is, thin layer with fewer C₁₈ group in contact with pentacene induced a good electrical property like lower s.s. Further the higher dielectric constant of the underlying layer resulted in higher mobility of the device. The mobility (0.50 cm² V⁻¹ s⁻¹) of the OTFT with G3 as the gate insulator showed a higher value compared to that (0.25 cm² V⁻¹ s⁻¹) of the OTFT with G2.     

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.     

Laser forward transfer of silver electrodes for organic thin-film transistors

Organic thin-film transistors (OTFTs) with top- and bottom-contact configurations were fabricated using silver nano-inks printed by laser forward transfer for the gate and source/drain electrodes with pentacene and poly-4-vinylphenol as the organic semiconductor and dielectric layers, respectively. The volume of the laser-printed Ag pixels was typically in the subpicoliter (0.2–0.4 pl) range. The top-contact OTFTs resulted in lower contact resistance compared to those obtained from the bottom-contact OTFTs, and showed improved overall device performance. The top-contact OTFTs exhibited field-effect mobilities of ∼0.16 cm² V⁻¹ s⁻¹ and on/off current ratios of ∼10⁵.     

A photosensitive copolymer for the gate insulator of organic thin-film transistors

A novel cross-linkable copolymer for the gate insulators of organic thin-film transistors (OTFTs) was synthesized by free radical copolymerization with methyl methacrylate and ethylene methylacrylate cinnamoylate. Copolymers of molecular weights (Mn: 109200–160000 g mol⁻¹) and polydispersities (1.59–2.24) were characterized by FTIR and NMR. Spin-coated thin films had smooth surfaces with the root-mean-square (RMS) surface roughness of 0.23 nm, 0.41 nm, respectively, before and after UV irradiation. Exposure of the copolymers to UV light produced cross-linking of the polymeric chains that could be confirmed by comparing the FTIR and UV spectra recorded prior and after irradiation. Moreover, the vanadyl-phthalocyanine (VOPc) OTFTs with the photosensitive copolymer as gate insulator were fabricated and found to exhibit a carrier mobility of 0.25 cm²/V s, an on/off ratio of 10⁴.     

Quantitative analysis of charge-carrier trapping in organic thin-film transistors from transfer characteristics

A dynamic method for quantifying the amount and mechanism of trapping in organic field effect transistors (OFETs) is proposed. It exploits transfer characteristics acquired upon application of a triangular waveform gate sweep V _G. The analysis of the transfer characteristics at the turning point V _G=−V _max between forward and backward gate sweeps, viz. around the maximum gate voltage V _max applied, provides a differential slope Δm which depends exclusively on trapping. Upon a systematic change of V _max it is possible to extract the initial threshold voltage, equivalent to one of the observables of conventional stress measurements, and assess the mechanism of trapping via the functional dependence on the current. The analysis of the differential logarithmic derivative at the turning point yields the parameters of trapping, as the exponent β and the time scale of trapping τ. In the case of an ultra-thin pentacene OFET we extract β=1 and τ=10²–10³ s, in agreement with an exponential distribution of traps. The analysis of the hysteresis parameter Δm is completely general and explores time scales much shorter than those involved in bias stress measurements, thus avoiding irreversible damage to the device.     

High-performance n-channel organic thin-film transistors based on the dual effects of heterojunction and surface modification

This paper presents two n-channel organic heterojunction transistors with modified insulator by using hexadecafluorophthalocyaninatocopper (F₁₆CuPc)/copper phthalocyanine (CuPc) and F₁₆CuPc/pentacene as the active layers. Compared with a single-layer device, it reports that an improved field-effect mobility and a 6-fold higher drain current are observed. The highest mobility of 0.081~cm²/(V.s) was obtained from F₁₆CuPc/CuPc heterojunction devices. This result is attributed to the dual effects of the organic heterojunction and interface modification. Furthermore, for two heterojunction devices, the performance of the F₁₆CuPc/CuPc-based transistor is better than that of F₁₆CuPc/pentacene. This is attributed to the morphologic match of two organic components.     

Tuning the contact resistance in organic thin-film transistors with an organic-inorganic hybrid interlayer

We demonstrated the tunable contact resistance in pentacene thin film transistor (TFT) by inserting an organic-inorganic hybrid interlayer between Au electrode and pentacene layer. The contact resistance of pentacene-TFT varies with concentration of pentacene-TFT varies with concentration of MoO_x in organic-inorganic hybrid interlayer. MoO_x in organic-inorganic hybrid interlayer. The contact resistance of the device with 55 wt% MoO_x doped pentacene interlayer is about 7.8 times smaller than that of device without interlayer at the gate voltage of −20 V. Comparing the properties of pentacene-TFT without interlayer, the performance of the pentacene-TFT with 55 wt% MoO_x doped pentacene was significantly improved: saturation mobility increased from 0.39 to 0.87 cm²/V s, threshold voltage reduced from −21.3 to −7.2 V, and threshold swing varied from 3.75 to 1.39 V/dec. Our results indicated that the organic-inorganic hybrid interlayer is an effective way to improve the performance of p-channel 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.     

Spatially resolved photoresponse measurements on pentacene thin-film transistors

A confocal setup with a spatial resolution in the submicron regime is employed for investigating the response of pentacene transistors to local illumination. The transistors show enhanced and inhomogeneous photoresponse in the proximity of the hole-injecting contact. These inhomogeneities represent contact areas of varying injection efficiency. Thus, this technique allows imaging of contact efficiencies with submicron resolution over large areas up to hundreds of microns. Drift–diffusion simulations including a photogeneration/recombination process have been performed to model the photoresponse. The simulations illustrate that the potential drop along the channel is dramatically reduced in the illuminated area due to photoconductance (i.e. photoinjection of excitons and subsequent dissociation). Also, the injection barrier for holes is reduced if the illumination is close to the hole-injecting electrode. The rapid decay of the photoresponse with increasing distance to the positively biased electrode is caused by the limited electron mean free path in our devices.     

High temperature characteristics of AlGaN/GaN high electron mobility transistors

Direct current (DC) and pulsed measurements are performed to determine the degradation mechanisms of AlGaN/GaN high electron mobility transistors (HEMTs) under high temperature. The degradation of the DC characteristics is mainly attributed to the reduction in the density and the mobility of the two-dimensional electron gas (2DEG). The pulsed measurements indicate that the trap assisted tunneling is the dominant gate leakage mechanism in the temperature range of interest. The traps in the barrier layer become active as the temperature increases, which is conducive to the electron tunneling between the gate and the channel. The enhancement of the tunneling results in the weakening of the current collapse effects, as the electrons trapped by the barrier traps can escape more easily at the higher temperature.     

Improvement in semiconductor laser printing using a sacrificial protecting layer for organic thin-film transistors fabrication

Laser-induced forward transfer (LIFT) has been used to deposit pixels of an organic semiconductor, distyryl-quaterthiophenes (DS4T). The dynamics of the process have been investigated by shadowgraphic imaging for the nanosecond (ns) and picosecond (ps) regime on a time-scale from the laser iradiation to 1.5 μs. The morphology of the deposit has been studied for different conditions. Intermediate sacrificial layer of gold or triazene polymer has been used to trap the incident radiation. Its role is to protect the layer to be transferred from direct irradiation and to provide a mechanical impulse strong enough to eject the material.     

High-performance inverters based on ambipolar organic-inorganic heterojunction thin-film transistors

This work reports on the integration of organic and inorganic semiconductors as heterojunction active layers for highperformance ambipolar transistors and complementary metal-oxide-semiconductor(CMOS)-like inverters. Pentacene is employed as a p-type organic semiconductor for its stable electrical performance, while the solution-processed scandium(Sc) substituted indium oxide(Sc In O) is employed as an n-type inorganic semiconductor. It is observed that by regulating the doping concentration of Sc, the electrical performance of the n-type semiconductor could be well controlled to obtain a balance with the electrical performance of the p-type semiconductor, which is vital for achieving high-performance inverters. When the doping concentration of Sc is 10 at.%, the CMOS-like logic inverters exhibit a voltage gain larger than 80 and a wide noise margin(53% of the theoretical value). The inverters also respond well to the input signal with frequency up to 500 Hz.     

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.     

Solution-processed ZnO nanoparticle-based semiconductor oxide thin-film transistors

We have prepared solution-processed oxide semiconductor thin-film transistors using ZnO nanoparticles with various particle shapes. Uniform, dense, thin films were produced by spin-coating ZnO nanoparticle dispersions containing either nanorods or nanospheres. The influence of annealing atmosphere on both nanoparticle-based TFT devices was investigated. XPS analysis revealed that the ZnO particles of the nanorod and nanosphere dispersions have distinct stoichiometries (i.e., molar ratios of Zn:O). The starting particles in turn predetermine the carrier concentration within the annealed ZnO films, which in turn determines whether the device is a semiconductor or metallic conductor, depending upon the annealing atmosphere. Grain structures of the channel layer also play an important role in determining the device performance of the nanoparticle derived ZnO TFTs.     

High efficient plastic substrate polymer white light emitting diode

White polymer light emitting diode (PLED) has attracted the interest of researchers by the advantage of having low cost, flexible light sources. One of the major advantages of PLED is that it can be able to fabricate in flexible plastic substrate instead of glass substrate. Generally PLED??s requires a substrate of high refractive index to enhance the amount of trapped light in the device, but the refractive index of flexible plastic substrate is low (n?<?1.6). In this paper, we present a white PLED on a flexible plastic substrate with a new enhancement method. In which the semi-transparent gold layer is sandwiched between the layers of tantalum oxide and molybdenum oxide which does not require a high refractive index substrate. Using this design, the extraction efficiency of the device is increased from 1.5 to 2.1 cw compared to that of the device using glass substrate.     

High current density in light-emitting transistors of organic single crystals

We measured the external electroluminescence quantum efficiency (eta(ext)) in light-emitting field-effect transistors (LETs) made of organic single crystals and found that, in the ambipolar transport region, eta(ext) is not degraded up to several hundreds A/cm(2) current-density range, which is 2 orders of magnitude larger than that achieved in conventional organic light-emitting diodes. The present result indicates the single-crystal organic LET is a promising device structure that is free from various kinds of nonradiative losses such as exciton dissociation near electrodes and exciton annihilations.