Pentacene Active Layer Research Articles

The Performance of Organic Field Effect Transistor Affected by Different Thickness of Active Semiconductor Layer and Thickness of Gate Insulator

The purpose of this paper is to fabricate organic field effect transistor and to investigate the effect of the thickness of the pentacene active layer and the thickness of gate insulator layer on MOSFET performance. The fabricated structure is top-contact. When the thickness of the insulator gate layer increases from 10 nm to 30 nm, the magnitude of the drain source current, when VGS = VDS = -4 V, decreases from 1813 nA to 214 nA and then the threshold voltage shifts from -1.4 V to -2.4 V. When the thickness of the pentacene increases from 9 nm to 40 nm, the threshold voltage voltage shifts slightly in the negative direction from -1.4 V to -1.6 V for SiO2 thickness of 10 nm. In case of SiO2 thickness of 20 nm, the threshold voltage voltage shifts from -1.9 V to -2.2 V. In case of SiO2 thickness of 30 nm, the threshold voltage voltage shifts from -2.4 V to -2.9 V. Besides that, the mobility decreases from around 0.31 cm2/(Vs) to 0.15 cm2/(Vs) when the pentacene thickness increases from 9 nm to 40 nm.

Behavioral modeling for low-voltage pentacene-based OTFTs and their implementations for organic logic circuits

We present a behavioral compact model and its implementation for low-voltage pentacene-based organic thin film transistors (OTFTs) using industry standard BSIM4 (Berkeley Short-channel IGFET Model) in LTspice platform. First, we simplify the number of BSIM4 model parameters according to the device structure. On the other hand, we adapt the BSIM4 mobility equation to OTFT and also involve the off-current using two behavioral current sources. Then, we perform metaheuristics-based optimization to extract the related BSIM4 model parameters together with additional four parameters, introduced in this work, fitting the modified model to the experimental data of an OTFT, using pentacene active layer. Finally, we implement the behavioral compact model in LTspice and validate its usage for low-voltage pentacene-based OTFTs comparing the experimental and modeled current–voltage curves. In this study, we also successfully simulate OTFT implementations of a Pseudo-E inverter, a Pseudo-E NAND gate, and a 1-bit full adder circuit based on NAND gates using our behavioral OTFT model and then investigate the static and dynamic performance parameters of these circuits. The supply voltage is 3 V and maximum frequency of the inputs is 500 Hz in the simulations.

UV/Ozone Based Facile Surface Modulation Technique for Efficient Organic Thin Film Transistor

The capacity to actualize a comprehensive range of electronic functions based on organic devices is important to the development of fully functional organic electronic circuits. Specifically low voltage, quick switching transistors, in particular, have previously been demonstrated but a critical function, i.e. dependable data storage, is currently lacking. Organic thin-film transistors (OTFTs) have received a lot of interest for possible use in memory, flexible electronics such as sensors, and light-emitting diodes because of their inexpensive production costs, lightweight, mechanical flexibility, and other advantages.The most often utilised active ingredient in organic TFTs is pentacene (C22H14). The electrical properties of organic TFTs based upon pentacene, such as field-effect mobility, are equivalent to those of amorphous silicon thin film transistors (a-Si TFTs), which are typically employed for the backplane of liquid crystal displays (LCDs), but far inferior to those of generic inorganic TFTs. As a result, electrical properties for high-performance flexible electronics must be improved. The interfacial properties of active layer and gate insulator layer plays a very crucial role in determining the electrical properties of organic TFTs. Therefore in this work we report an UV/ozone treatment on the surface of gate dielectric layer so as to improve the interfacial properties.At first the glass substrate was cleaned in an ultrasonic cleaner with acetone, ethanol, isopropyl alcohol and DI water. Then by using thermal evaporation technique a gate electrode with thickness of approximately 80 nm was deposited on the substrate. Later a solution of Poly 4-vinylphenol (PVP) was spin coated with a thickness of 140 nm, this organic layer was deposited as the bottom insulator layer. Now an ozone treatment (wavelength = 253nm) for 5,10 and 20 min was done over the PVP layer. Subsequently a layer of PMMA was coated over the UV treated layer. Finally with a coating of 70nm thick pentacene active layer 100 nm thick silver drain/source electrodes with channel length of 50µm were also deposited using thermal deposition.Further the morphology and topography was studied using AFM images and element analysis was done from XPS spectra. The device performance (output and transfer characteristics) was characterized using Kiethley 4200 SCS semiconductor parameter analyser. The changes in performance of transistor devices before and after UV/ozone treatment could be explained using XPS spectra, AFM images and water contact angle measurement. It is evident from the AFM images that the grain size of pentacene on UV/Ozone treated dielectric surface are larger than the untreated surface, thus reducing the boundary scattering and enhancing the field effect mobility. Thus surface treatment with UV/Ozone could acts as simple, low cost method to enhance the performance of a organic thin film transistors.

Reactivity of an air-stable dihydrobenzoimidazole n-dopant with organic semiconductor molecules

Summary 1,3-Dimethyl-2-(4-(dimethylamino)phenyl)-2,4-dihydro-1H-benzoimidazole, N-DMBI-H, is widely used as an air-stable n-dopant for organic semiconductors. Here, its reactivity is investigated with a variety of imide- and amide-containing semiconductor molecules that have reduction potentials in the range −0.54 to −1.10 V versus ferrocene. Reaction rates correlate poorly with these potentials. The more reactive of the imides form the corresponding radical anions cleanly, but kinetic isotope studies using N-DMBI-D indicate that the reaction proceeds via an initial hydride- or hydrogen-transfer step. For an amide- and ester-rigidified bis(styryl)benzene derivative the hydride-reduced product is stable under an inert atmosphere and can be observed directly; the radical anion can only be obtained in sub-stoichiometric yield and under certain reaction conditions. On the other hand, (N-DMBI)2 rapidly reduces all the imides and amides examined to the corresponding radical anions. The implications of these findings for dopant selection and use are discussed.

The Electrical Properties of Fabricated Pentacene Based Phototransistor with Polystyrene Gate Insulator

In this study, the fabrication of top contact pentacene based phototransistor having polystyrene gate dielectric has been carried out. To analyze the surface morpholgy of polystyrene insulator and pentacene active layer, scanning electron microscopy (SEM) has been used. The electrical characterization of pentacene based phototransistor and also the effect of illumination on the output characteristics have been investigated. The obtained mobility value and on/off ratio of the transistor are 5×10-3 cm2/Vs and ~102, respectively. The increase of the drain current with increasing illumination intensity indicates that the light acts as an additional terminal. Also, this fabricated device behaves as a phototransistor because of its reaction to the illumination.

Temperature effects on the electrical properties of ambipolar organic complementary-like inverters

Abstract Efficient and balanced pentacene-based ambipolar organic thin-film transistors (AmOTFTs) were prepared and ready for use to achieve simple fabrication of complementary-like inverters with high gains. We examined the effect of temperature on the electrical characteristics of pentacene-based AmOTFTs and corresponding complementary-like inverters. Such complementary-like inverters can perform nonpolar operations, such as first and third quadrant operations, and can work normally up to nearly 110 °C. In situ measurements demonstrated the excellent thermal stability of pentacene active layer at the corresponding temperatures. The operating principle of the ambipolar-based complementary-like inverters was also discussed. Given the ambipolar nature of pentacene active channel, the dual-carrier recombination and release processes governed the temperature-dependent switch behaviors of the inverters. A temperature-dependent linearity function was derived using the hysteresis switch voltage, thereby highlighting that pentacene-based ambipolar complementary-like inverters could potentially be employed in temperature sensors.

An origin of the irreproducibility of hole injection barrier from Au top-contact electrodes and its influence on device performance in top-contact organic field-effect transistors

Abstract To understand the origin of the irreproducibility of the hole injection barrier at the interface between Au top-contact electrodes and organic semiconductors, we performed a systematic study by focusing on the deposition rate of Au. As a benchmarking material system, we fabricated field-effect transistors with pentacene active layers and Au top-contact electrodes evaporated with the deposition rate from 0.5 to 10 A/s. From the contact potential differences (ΔCPDs) between Au and pentacene measured with Kelvin-probe force microscopy, cross-sectional structures with scanning transmission electron microscopy, crystallographic structures with X-ray diffraction, and Au depth profiles with secondary ion mass spectrometry, we concluded that the variation of ΔCPD is due to embedded Au nanoclusters in pentacene with the average diameter of 4–6 nm. These Au nanoclusters, of which depth significantly varies by the Au deposition rate, modulate the ΔCPD and interfere with the reproducibility of hole injection barrier. As a result, gate threshold voltage and field-effect mobility also become irreproducible.

Bending Performance of Flexible Organic Thin-Film Transistors With/Without Encapsulation Layer

Flexible pentacene-based organic thin-film transistors (TFTs) were fabricated and their performance was investigated as a function of the bending radius and the thickness of the polydimethylsiloxane (PDMS) encapsulation layer. The TFTs were fabricated on a flexible polyimide film (film thickness: 75 $\mu \text{m}$ ), and encapsulated by a PDMS layer. Degradation of the device performance during application of a tensile bending stress of 3 mm was minimized by utilizing an encapsulation layer thickness of 75 $\mu \text{m}$ , because the mechanical strain on the pentacene layer was almost off-set (tensile strain was applied to the bottom layer of the pentacene, and compressive strain was applied to the top layer of pentacene). At the tensile bending stress of 3 mm, the performance of the non-encapsulated TFT was degraded, whereas the encapsulated device showed great stability. This flexibility and bending stability were enabled by the use of the 75- $\mu \text{m}$ PDMS encapsulation layer, due to the location of the pentacene active layer in the neutral region position. A mechanical reliability test was performed for 120 min with a bending radius of 3 mm, demonstrating that only the device with the 75- $\mu \text{m}$ thick encapsulation layer showed stable device performance over a stress time of 120 min.

Flexible and Transparent Organic Phototransistors on Biodegradable Cellulose Nanofibrillated Fiber Substrates

AbstractThis work demonstrates flexible, transparent phototransistors that can detect visible light with nontoxic organic active materials on biodegradable substrates toward environment‐friendly electronics. The molybdenum trioxide (MoO3)‐buffered indium zinc oxide as high‐performance hole injector and transparent electrodes is applied for the first time to organic phototransistors on cellulose nanofibrillated fiber substrates to achieve more than 70% of transmittance in the visible range (400–750 nm) while showing high conductivity under multiple bendings. Excellent electrical switching characteristics are obtained from transistors using a pentacene active layer with a saturation mobility value of 1.40 cm2 V−1 s−1. The phototransistors, which can detect visible light and perform in two operation modes, exhibit a maximum responsivity of 54.8 A W−1 and a photosensitivity of 24.4 under white light illumination at an intensity of 0.12 mW cm−2. Moreover, the devices show a stable operation during mechanical bending tests with radii ranging from 100 to 5 mm and cyclic bending tests of up to 2000 cycles at a fixed radius of 5 mm. The results suggest that these flexible phototransistors with properties of transparency and biodegradability have considerable potential for use in low‐cost and eco‐friendly disposable sensor systems.

Radical polymers improve the metal-semiconductor interface in organic field-effect transistors

Modifying the organic-metal interface in organic field-effect transistors (OFETs) is a critical means by which to improve device performance; however, to date, all of the interfacial modifying layers utilized in these systems have been closed-shell in nature. Here, we introduce open-shell oxidation-reduction-active (redox-active) macromolecules, namely radical polymers, in order to serve as interfacial modifiers in pentacene-based OFETs. Through careful selection of the chemistry of the specific radical polymer, poly(2,2,6,6-tetramethylpiperidine-1-oxyl methacrylate) (PTMA), the charge transport energy level of the interfacial modifying layer was tuned to provide facile charge injection and extraction between the pentacene active layer and the gold source and drain electrodes of the OFET. The inclusion of this radical polymer interlayer, which was deposited in through straightforward inkjet printing, led to bottom-contact, bottom-gate OFETs with significantly increased mobility and ON/OFF current ratios relative to OFETs without the PTMA interlayer. The underlying mechanism for this improvement in device performance is explained in terms of the charge transport capability at the organic-metal interface and with respect to the pentacene grain growth on the radical polymer. Thus, this effort presents a new, open-shell-based class of materials for interfacial modifying materials, and describes the underlying physics behind the practical operation of these materials.

Mechanism of Label-Free DNA Detection Using the Floating Electrode on Pentacene Thin Film Transistor

The analysis of DNA in the posthuman genome project era has become an ever-expanding branch of research and is thus routinely employed in the majority of biochemical laboratories. This work discusses the mechanism and label-free detection of DNA using a pentacene thin film transistor with a gold floating electrode on the active layer. Thiolated polynucleotide probes were used, which form self-aligned monolayers on the floating electrode over the pentacene active layer. The immobilization of the DNA on floating electrode increased the work function and raised the Schottky barrier of the device, resulting in a charge screening effect. Hence, the negative charge of the DNA caused a positive shift in the threshold voltage of the transistor. Based on the change in the electrical output, synthesized DNA and the viral DNA were detected.

Pentacene organic ferroelectric transistors with [P(VDF-TrFE)] gate by Langmuir-Blodgett process

We report the fabrication and electrical properties of pentacene-based ferroelectric organic field-effect transistors (FeOFETs) with ultrathin poly(vinylidene fluoride trifluoroethylene) [P(VDF-TrFE)] gate insulators. A ultrathin and uniform P(VDF-TrFE) film was successfully deposited by Langmuir-Blodgett (LB) deposition with well-defined ferroelectric microdomains at the interface between P(VDF-TrFE) films and pentacene active layers. The P(VDF-TrFE) films derived by LB deposition significantly enhance the crystallization of the upper pentacene channel films and thus the performance of our FeOFETs. Our FeOFET device achieves a threshold voltage shift of 8.56 V induced by ferroelectric polarization under different voltage sweeping directions and such enhancement indicates a great potential for future organic nonvolatile memory applications.

A Comparative Study on the Optimization of a Ternary P3HT:PCBM:Pentacene Active Layer in Bulk Heterojunction Organic Solar Cells

This work shows a comparative and processing optimization study of the recent ternary bulk heterojunction (BHJ) of P3HT: PCBM:pentacene (1:0.9:0.1 wt. ratio) against the well-known active layer of poly(3-hexylthiophene) (P3HT) blended to phenyl C61 butyric acid methyl ester (PCBM) (1:1 wt. ratio). Initially, monochlorobenzene (MCB) was compared and later replaced by dichlorobenzene (DCB) as active layer solvent. The following optimization step related to thermal annealing effects on solar cell power conversion efficiency (PCE) and fill-factor (FF). Ternary junction slow drying at room temperature without a postproduction thermal treatment proved to be the most suitable to improve BHJ morphology at nanoscale. Finally, ternary composite mass concentration ranged from 20 to 60 mg/ml in DCB to achieve an efficiency near 3% at 40 mg/ml. An observed improvement of at least 15% in photovoltaic efficiency and a practically constant open-circuit voltage (i.e., just 1.2% variation) compared to the already-established P3HT:PCBM blend corroborates the role of pentacene in the active layer to balance mobility from both charge carrier types.

Performance enhancement of p-type organic field-effect transistor through introducing organic buffer layers

Top contact organic field-effect transistors (OFETs) based on pentacene active layer, which employed the organic buffer layers of subphthalocyanine, triphenyldiamine derivative, and 4,4′,4″-tris[3-methylphenyl(phenyl)amino] triphenylamine (m-MTDATA) as the hole injection layers were fabricated. The results showed that the electrical performance of these OFETs, including the saturation current, the field-effect mobility, the on/off current ratio, and the threshold voltage, were all significantly improved by introducing the organic hole injection buffer layers. By optimizing the film thickness of these organic buffer layers to the appropriate thickness, the charge injection from gold source/drain electrodes to pentacene film could be effectively improved. Also, the interfacial properties and the contact resistance between gold source/drain electrodes and pentacene film was analyzed, and the results indicated that the interface property was significantly improved. Moreover, it was found that OFET with m-MTDATA hole injection layer exhibited the best performance compared to other two kinds of materials, and the intrinsic reason was further revealed.

High mobility organic field-effect transistor based on water-soluble deoxyribonucleic acid via spray coating

High mobility organic field-effect transistors (OFETs) by inserting water-soluble deoxyribonucleic acid (DNA) buffer layer between electrodes and pentacene film through spray coating process were fabricated. Compared with the OFETs incorporated with DNA in the conventional organic solvents of ethanol and methanol: water mixture, the water-soluble DNA based OFET exhibited an over four folds enhancement of field-effect mobility from 0.035 to 0.153 cm2/Vs. By characterizing the surface morphology and the crystalline structure of pentacene active layer through atomic force microscope and X-ray diffraction, it was found that the adoption of water solvent in DNA solution, which played a key role in enhancing the field-effect mobility, was ascribed to both the elimination of the irreversible organic solvent-induced bulk-like phase transition of pentacene film and the diminution of a majority of charge trapping at interfaces in OFETs.

Reversibility of humidity effects in pentacene based organic thin-film transistor: Experimental data and electrical modeling

P-type organic thin-film transistors (OTFTs), in which the active semiconductor is made of pentacene with silicon dioxide as a gate insulator, were fabricated and characterized. The effects of humidity on the electrical characteristics of pentacene based thin-film transistors (pentacene-TFTs) in the linear and saturation regimes were investigated. We report the variation of the electrical parameters by relative humidity (RH) extracted from the experimental electrical characteristics current–voltage of pentacene-TFT devices. We show that the diffusion of water molecules (H2O), the creation of acceptor states due to the presence of oxygen and the formation of clusters in the pentacene active layer considerably affect the stability and the performances of pentacene-TFTs. The degradation of electrical parameters of the pentacene-TFTs under relative humidity (RH) can be recovered with a simple pumping under vacuum (3×10−5 to 5×10−5mbar). We also show that the changes introduced by the effects of humidity are reversible. Moreover the pentacene-TFT presents an intensive response for a high relative humidity (RH=57%), which could be used for a humidity detection device technology.

Enhanced sensing performance of nitrogen dioxide sensor based on organic field-effect transistor with mechanically rubbed pentacene active layer

Organic field-effect transistor (OFET)-based nitrogen dioxide (NO2) sensors with mechanically rubbed pentacene active layer were fabricated by utilizing polystyrene as the dielectric. Compared with those of the reference device, the sensing properties, including sensitivity and response time, were significantly enhanced in OFETs with the pentacene film rubbed parallel to the source/drain electrodes, while the device with pentacene film rubbed perpendicular to the source/drain electrodes showed sensing properties lower than those of the reference device. Atomic force microscope was used to analyze the morphologies of pentacene film with or without rubbing, and much smaller grains of pentacene were observed after mechanical rubbing. Consequently, more grain boundaries which were beneficial to the diffusion of analytes were formed, and NO2 diffused to the channel of OFET with parallel rubbed pentacene would modify the hole density more effectively as the charge flow in such device was forced to travel through the rubbed pentacene.

Effect of UV/ozone treatment on polystyrene dielectric and its application on organic field-effect transistors

The influence of UV/ozone treatment on the property of polystyrene (PS) dielectric surface was investigated, and pentacene organic field-effect transistors (OFETs) based on the treated dielectric was fabricated. The dielectric and pentacene active layers were characterized by atomic force microscopy, X-ray photoelectron spectroscopy, and scanning electron microscopy. The results showed that, at short UVO exposure time (<10 s), the chemical composition of PS dielectric surface remained the same. While at long UVO exposure time (>60 s), new chemical groups, including alcohol/ether, carbonyl, and carboxyl/ester groups, were formed. By adjusting the UVO exposure time to 5 s, the hole mobility of the OFETs increased to 0.52 cm2/Vs, and the threshold voltage was positively shifted to -12 V. While the time of UVO treatment exceeded 30 s, the mobility started to shrink, and the off-current was enlarged. These results indicate that, as a simple surface treatment method, UVO treatment could quantitatively modulate the property of PS dielectric surface by controlling the exposure time, and thus, pioneered a new way to modulate the characteristics of organic electronic devices.

High sensitivity and air stability in an organic transistor-based biosensor by inserting a CuPc layer

The improved label-free DNA sensor based on pentacene organic thin-film transistors (OTFTs) with high sensitivity and air stability was fabricated by inserting a copper phthalocyanine (CuPC) layer between metal electrodes and pentacene active layer. Compared to the electric signals of devices with bare pentacene layer, one can reasonably infer that the single-stranded DNA (ssDNA) molecules may be well immobilized onto the CuPc/pentacene film surface, as well as not degrading the performance of the DNA sensor. Furthermore, the optimized thickness of the CuPc layer for the electrical response to the ssDNA immobilization was also obtained. As a result, the device with a CuPc layer shows that the sensitivity is slightly improved, and there is no significant initial performance drop after exposure to the ambient for 30 days. CuPc is used as the interfacial layer on pentacene film to enhance the sensitivity and air stability of the OFET-based sensors.

Correlating optimal electrode buffer layer thickness with the surface roughness of the active layer in organic phototransistors

Inserting a C60 buffer layer between Au source/drain electrodes and pentacene active layer has been proved to improve the performances of pentacene organic phototransistors (PENT-OPTs) in our previous study. Buffer layer certainly has an optimal thickness with which the modified device can achieve the best performance. Based on the surface morphology analysis of different thickness C60 buffer layer on pentacene film, we further optimized the thickness of C60 buffer layer for best performance of PENT-OPTs and investigated its physical origins. Studies on PENT-OPTs with different pentacene surface morphology realized by different substrate temperatures indicate that the optimal thickness of C60 buffer layer directly related to the surface roughness of pentacene active layer and it is found that the optimized buffer layer thickness increases with the roughness of pentacene layer. Besides, we found that the photogenerated current of OPTs increases with the increasing of gate electric bias and then gradually reach saturation. An approximate analytical expression for gate voltage dependence of the photogenerated current was derived and used to fit the experiment data. An important parameter, saturated photoresponsivity, was introduced for better comparing the performances of OPTs.