Pentacene Organic Thin-film Transistors Research Articles

High Performance Flexible Organic Thin Film Transistors (OTFTs) with Octadecyltrichlorsilane/Al2O3/Poly(4-vinylphenol) Multilayer Insulators

The incorporation of a thin, atomic layer deposited Al2O3 layer in between a spin-coated poly-4-vinyl phenol (PVP) organic layer and octadecyltrichlorsilane (OTS) in the multilayer gate dielectric for pentacene organic thin film transistors on a n(+)-Si substrate reduced the gate leakage current and thereby significantly enhanced the current on/off ratio up to 2.8 x 10(6). Addition of the OTS monolayer on the UV-treated Al2O3 improved the crystallinity of the pentacene layer, where the OTS/UV-treated Al2O3 surfaces increased their contact angles to 100 degrees. X-ray diffraction (XRD) analysis revealed a more intense (001) crystal reflectance of pentacene deposited on OTS/UV-treated Al2O3 surface than that on OTS/Al2O3 surface. Moreover, the improved pentacene layer contributed to the field effect mobility (0.4 cm2/Vs) and subsequently improved the electrical performances of organic thin film transistor (OTFT) devices. This PVP/UV treated Al2O3/OTS multilayer gate dielectric stack was superior to those of the device with the single PVP gate dielectrics due to the improved crystallinity of pentacene.

Impact of scaling of dielectric thickness on mobility in top-contact pentacene organic thin film transistors

The effect of scaling of poly(methylmethacrylate) (PMMA) and cross-linkable poly(4-vinylphenol) (PVP) polymer dielectric thickness on field effect mobility in top contact pentance organic thin film transistors was investigated. Mobility at a constant gate voltage improved significantly with reduction in thickness of both dielectrics. Analysis at a constant gate electric field or identical induced accumulation charge revealed that only a part of the improvement occurs due to simple scaling of dielectric thickness, and the remaining is due to improvement in interface quality. Atomic force microscopic analysis of the dielectric surface revealed that dielectric roughness reduced with reduction in dielectric thickness. A comparison of the two dielectrics also indicates that band mobility is significantly higher in pentacene/PMMA as compared to pentacene/PVP dielectric.

Effects of piezoresistivity of pentacene channel in organic thin film transistors under mechanical bending

In this report, the electrical properties of both pentacene thin-film and pentacene organic thin-film transistors (OTFTs) under static and dynamic bending were investigated. From the responses to the applied static strain, the piezoresistivity coefficient of pentacene thin-film was determined to be 12.5. Significant changes in the field-effect mobility, μ, and the channel current, which were ascribed to the piezoresistivity of the pentacene channel in OTFTs, were observed while other parameters, such as contact resistance and gate capacitance, appear invariant up to the bending radius of 4.3 mm. Hysteresis in cyclic bending, which was assumed to be caused by local defects of pentacene thin-film, was observed to be independent of bending speed during dynamic bending tests.

Pentacene-Gate Dielectric Interface Modification with Silicon Nanoparticles for OTFTs

We report on the properties of pentacene layers and OTFTs (Organic Thin Film Transistors) deposited on semiconductor-gate insulator interfaces covered with silicon nanoparticles (SiNPs) monolayer prepared by the Langmuir-Blodgett method compared to a reference sample (without SiNPs) prepared in an otherwise identical way. To analyse the structural quality, micro-Raman spectroscopy was employed and the correspondence between thin and bulk phase of the integral intensities peaks ratio (α) at 1154 and 1158 cm-1 (α = Int1154 /Int1158) was evaluated. The AFM analysis of the pentacene layers reveals that the different surface treatment of SiO2 gate insulator (hydrophobic or hydrophilic) before SiNPs monolayer deposition has a distinct influence on the formation of different pentacene grain size and morphology. We demonstrate the higher time stability of pentacene OTFT and increasing of saturation current (∼ 2.5 ×) behavior after storage time if the semiconductor-gate insulator interface is modified using a SiNP monolayer.

The application of a high-k polymer in flexible low-voltage organic thin-film transistors

Although much progress has been made in the study of high-mobility organic semiconductors in recent years, one major challenge for organic thin film transistors (OTFTs) in real applications is the relatively high operating voltage, which is mainly related to the gate dielectric of the OTFT. Here we show the application of a high-k poly(vinylidene fluoride-trifluoroethylene-chlorofloroethylene) (P(VDF-TrFE-CFE)) as the gate dielectric of flexible low-voltage pentacene OTFTs for the first time. The performance of the OTFTs is optimized by modifying the surface of P(VDF-TrFE-CFE) films with various thin polymer films by a solution process. The flexible OTFTs show excellent performance at the operating voltage of 4 V and good stability after 1000 bending tests. It is expected that P(VDF-TrFE-CFE) is a suitable gate dielectric for low-voltage OTFTs based on various small-molecule organic semiconductors because P(VDF-TrFE-CFE) terpolymer films can be easily modified with different thin polymer films by a solution process.

Thermal Expansion Coefficient Considerations on Field-Effect Mobility of Pentacene Organic Thin-Film Transistors With an AlN Gate Dielectric

The field-effect mobility of pentacene-based organic thin-film transistors (OTFTs) with AlN as the gate dielectric was strongly affected by the substrate temperature during pentacene deposition. The field-effect mobility μ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">FE</sub> extracted from the transfer characteristics for the substrate temperatures of 70 °C, 50 °C, and room temperature (RT) are 0.06, 0.18, and 1 cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> /Vs, respectively. The (001) peak position of the pentacene thin-film phase in the grazing-incidence X-ray diffraction spectra decreases from 5.75 ° (70 °C) to 5.65° (RT). The small angle reduction (-0.1°) indicates that the c-axis of the triclinic structure of pentacene is contracted due to the stress at the pentancene/AlN interface, which is supported by the shift of Raman peak at ~ 1373 cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> . A mechanism based on the difference of the coefficient of thermal expansion is proposed to explain the enhancement of the field-effect mobility of pentacene OTFTs at RT.

Self-Defined Short Channel Formation With Micromolded Separator and Inkjet-Printed Source/Drain Electrodes in OTFTs

We demonstrated fabrication of self-defined short channel using inkjet printing and micromolding in capillary (MIMIC) methods. Self-defined source/drain electrodes were successfully formed when silver ink drops were printed on a silicon dioxide/p+ heavily doped silicon substrate having a prepatterned narrow high-aspect-ratio hydrophobic teflon separator made by MIMIC. We fabricated well-defined channel region of pentacene organic thin film transistors (OTFTs) with 5 and 10 μm channel lengths, which showed good mobilities of 0.1 ~ 0.2 cm2/V · s. Characteristics of the fabricated OTFTs were also analyzed in accordance with channel length and surface wetting properties.

Morphology control of inkjet-printed small-molecule organic thin-film transistors with bank structures

Reported herein is the film morphology control of inkjet-printed 6,13-bis(triisopropylsilylethynyl) pentacene (TIPS-pentacene) organic thin-film transistors for the improvement of their performance and of the device-to-device uniformity. The morphology of the inkjetted TIPS-pentacene films was significantly influenced by the bank geometry such as the bank shapes and confinement area for the channel region. A specific confinement size led to the formation of uniform TIPS-pentacene channel layers and better electrical properties, which suggests that the ink volume and the solid concentration of the organic-semiconductor solutions should be considered in designing the bank geometry.

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 2000h). 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.

Low-Voltage Multilevel Memory Based on Organic Thin-Film Transistor

A low-voltage multilevel organic thin-film memory device is proposed and demonstrated for nonvolatile data storage. Charge carrier density in the storage field was tuned up to four discrete levels in pentacene organic thin-film transistor with poly(methyl methacrylate)-modified Al <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> film as the dielectric layer through light-assisted program procedures. Charges can be stored over 1 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sup> s at room temperature. The states can be clearly distinguished after 1 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sup> write-read cycles.

Stable Encapsulated Organic TFT With a Spin-Coated Poly(4-Vinylphenol-Co-Methyl Methacrylate) Dielectric

The influences of encapsulation on the hysteresis and the gate-bias-stress effects (both positive and negative gate bias stresses) of pentacene organic thin-film transistors (OTFTs) with poly(4-vinylphenol) and poly(4-vinylphenol-co-methyl methacrylate) (PVP-PMMA) gate dielectrics are investigated. The encapsulation and the use of less polar gate dielectrics like PVP-PMMA copolymers are both important to suppress moisture adsorption and to obtain a stable pentacene OTFT. Compared to the air-stable OTFT with a fluoropolymer dielectric, the stable encapsulated OTFT with a PVP-PMMA dielectric is a low-cost promising candidate for mass production consideration.

An integrated organic passive pixel sensor

We demonstrate an imaging passive pixel sensor circuit consisting of a bottom-gate, top-contact pentacene organic thin-film transistor (OTFT) integrated with a top-illuminated, inverted subphthalocyanine/C 60 organic photodetector (OPD). The vacuum-deposited OTFT utilizes parylene as the gate insulator, achieving a drain current ON/OFF ratio of 10 5. The transistor hole mobility is 0.09 ± 0.02 cm 2/V s. The inverted OPD has a dark current of 20 pA at a reverse bias of 1.5 V. By integrating the two components, a 12-bit dynamic range passive pixel sensor is achieved, with an OFF current of 31 ± 5 pA and a pixel readout time of 0.4 ± 0.05 ms, limited by the discharge time of the OTFT channel. The integrated pixel has potential for use in large-scale focal plane array imagers.

Transport mechanism in ambipolar pentacene organic thin film transistors with lithium fluoride gate dielectric

The electrical properties of organic thin film transistors (OTFTs) based on pentacene as the active layer and lithium fluoride (LiF) as the gate dielectric layer were investigated. Fabricated devices exhibited ambipolar behavior that can be controlled by the applied source, drain and gate biases. It was found that at low source-drain biases, multistep hopping is the dominant conduction mechanism, whereas in high voltage regimes, I-V data fits in the Fowler-Nordheim (FN) tunneling model. The relationship between the applied gate bias and the field enhancement factor, which is the ratio of the local surface electric field to the applied electric field, were determined from the slope of the FN plots. The dependency between the transition points in conduction mechanism upon gate bias has also been extracted from the FN plots. The transition points show more dependency on gate voltage for negative biases compared to the positive biases.

Aging control of organic thin film transistors via ion-implantation

One of the open issues in organic electronics is the long-term stability of devices based on organic materials, as oxidation is believed to be a major reason for early device failure. The focus of our research is to investigate 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). Despite the strong molecular structure modifications induced by ion implantation, we have observed that a controlled damage depth distribution preserves the functionality of the device. The electrical properties of the pentacene layer and of the OTFT have been investigated by means of current–voltage and photocurrent spectroscopy analyses. We have characterized the structural modification induced by ion implantation and we have monitored the effectiveness of this process in stabilizing the device carrier mobility and threshold voltage over a long time (over 2000 h). In particular, 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.

P‐120: High‐performance of Ink‐jet Printed Organic Thin‐film Transistors on Flexible Substrate using a Small Molecule‐polymer Blend

AbstractWe report the high performance, 6,13‐bis(triisopropyl silylethynyl) pentacene (TIPS‐pentacene) organic thin‐film transistors (OTFTs) by ink‐jet printing on a thin polymer layer. The OTFT with polymer/TIPS‐pentacene exhibited the saturation field‐effect mobility (μsat) of 1.11 cm2/Vs and threshold voltage (Vth) of −0.41 V. In contrast, the ink‐jet printed TIPS‐pentacene TFT without PS layer shows the μsat of 0.36 cm2/Vs and Vth of −0.62 V. The improved performance of polymer blended OTFT appears to be due to the improved crystallinity of the polymer blended TIPS‐pentacene layer.

A poly(styrene-co-methyl methacrylate)/room-temperature sputtered hafnium oxide bi-layer dielectrics as gate insulator for a low voltage organic thin-film transistors

In this study, we present a low voltage pentacene organic thin film transistor (OTFT) with poly(styrene-co-methyl methacrylate) grafted hafnium oxide (PS-r-PMMA/HfO x ) as gate dielectrics. The HfO x was sputtered at room temperature to approach low temperature and meet low cost requirements of organic electronics. The thickness of the PS-r-PMMA can be controlled to be extremely thin (<10 nm). Consequently, the gate overdriving voltage as low as 3 V was achieved for an OTFT. Furthermore, by applying PS-r-PMMA on top of the HfO x surface, the traps on the HfO x surface and possible pinholes of the HfO x can be passivated. Therefore, the gate dielectric properties in terms of interface states and leakage current were improved. The leakage current reduced to approximately two orders of magnitude and the interface states reduced to nearly one order of magnitude. The reduction of the interface state density was observed by capacitance–voltage measurement. As a result, the subthreshold swing and on/off current ratio were improved significantly from 1.01 V/dec. and 10 3 to 0.41 V/dec. and 10 5, respectively.

Light Sensing in a Photoresponsive, Organic-Based Complementary Inverter

A photoresponsive organic complementary inverter was fabricated and its light sensing characteristics was studied. An organic circuit was fabricated by integrating p-channel pentacene and n-channel copper hexadecafluorophthalocyanine (F16CuPc) organic thin-film transistors (OTFTs) with a polymeric gate dielectric. The F16CuPc OTFT showed typical n-type characteristics and a strong photoresponse under illumination. Whereas under illumination, the pentacene OTFT showed a relatively weak photoresponse with typical p-type characteristics. The characteristics of the organic electro-optical circuit could be controlled by the incident light intensity, a gate bias, or both. The logic threshold (V(M), when V(IN) = V(OUT)) was reduced from 28.6 V without illumination to 19.9 V at 6.94 mW/cm². By using solely optical or a combination of optical and electrical pulse signals, light sensing was demonstrated in this type of organic circuit, suggesting that the circuit can be potentially used in various optoelectronic applications, including optical sensors, photodetectors and electro-optical transceivers.

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.

Effects of Different Annealing Gases on Pentacene OTFT With HfLaO Gate Dielectric

Pentacene organic thin-film transistors (OTFTs) with HfLaO high-κ gate dielectric were fabricated. The dielectric was prepared by a sputtering method and then annealed in N2, NH3, O2, or NO at 400 °C. The carrier mobility of the NH3-annealed OTFT could reach 0.59 cm2/V · s, which is higher than those of the other three devices. Moreover, the NH3-annealed OTFT obtained the smallest subthreshold swing of 0.26 V/dec among them. Furthermore, 1/f noise measurement indicated that the NH3-annealed OTFT achieved the smallest 1/f noise. All these should be attributed to the improved interface between the gate dielectric and the organic semiconductor associated with the passivation effects of the NH3 annealing on the dielectric surface.

Study of the Characteristics of Organic Thin Film Transistors with Plasma-Polymer Gate Dielectrics

The effects of gate dielectrics material in organic thin film transistors (OTFTs) were investigated. The gate dielectrics were deposited by plasma enhanced chemical vapor deposition (PECVD) with cyclohexane and tetraethylorthosilane (TEOS) respectively used as organic and inorganic precursors. The gate dielectrics (gate insulators) were deposited as either organic plasma-polymer or organic–inorganic hybrid plasma-polymer thin film by using cyclohexane or cyclohexane with TEOS, respectively. Additionally, hydrogen and argon were used as precursor bubbler gases. A polyimide (PI) substrate was used in the fabrication of pentacene OTFTs with a plasma-polymer gate insulator, an Au source–drain (S/D), and Cu gate electrodes. Different gate dielectrics were investigated. The as-grown plasma-polymer thin films were first analyzed using Fourier-transform infrared (FT-IR) spectroscopy. Also, they were analyzed by nano-indentation and capacitance measurements. The electrical properties, such as mobility and threshold voltage of the pentacene field-effect transistors with the plasma-polymer gate-dielectrics were investigated. Transistor with cyclohexane gate dielectric had a higher field-effect mobility, µFET = 0.84 cm2 V-1 s-1, and a smaller threshold voltage, VT= -6.8 V, than the transistor with the hybrid gate-dielectric.