Top-contact Configuration Research Articles

Effect of concentration of DH6T on the performance of photoconductor fabricated using blends of P3HT and DH6T

The influence of small molecule (SM) α, ω-di-hexyl-sexithiophene (DH6T) concentration, in the blends of conjugated polymer (CP) poly (3-hexylthiophene) (P3HT) and DH6T, was investigated in terms of barrier potential reduction and improved photoresponse of the fabricated photoconductor using these blends. Barrier potential in Au/P3HT:DH6T/Au device structure, occurring at the interface of Au (top)/P3HT:DH6Twas estimated by Fowler Nordheim (FN) tunneling model-based analysis of I-V characteristics. The barrier potential of the fabricated device was observed to decrease upon addition of a small quantity of DH6T in comparison to the pristine polymer-based device. This reduction in barrier was attributed to the improved ordering and morphology of the polymer chains upon blending it with an SM. The variation in the ordering of the polymer chains was further confirmed with Photoluminescence spectroscopy, Absorption spectroscopy, and XRD data. Subsequently, it was also observed that only up to a definite SM concentration (25% in this study) ordering of polymer chains improved causing a reduction in barrier potential and subsequent improvement in the photoresponse of the fabricated devices. Finally, it was observed that the optimized blending of CP and SM could to be useful in reducing the effect of penetration of Au inside the CP matrix in the top contact configuration thereby resulting in the reduction of a barrier for carrier injection which is generally lower in the bottom contact configurations. These studies are critical from the point of view of the development of photoconductors and photosensitive top contact organic field effect transistors (OFETs).

Investigation of pentacene growth on SiO2 gate insulator after photolithography for nitrogen-doped LaB6 bottom-contact electrode formation

Nitrogen-doped (N-doped) LaB6 is a candidate material for the bottom-contact electrode of n-type organic field-effect transistors (OFETs). However, the formation of a N-doped LaB6 electrode affects the surface morphology of a pentacene film. In this study, the effects of surface treatments and a N-doped LaB6 interfacial layer (IL) were investigated to improve the pentacene film quality after N-doped LaB6 electrode patterning with diluted HNO3, followed by resist stripping with acetone and methanol. It was found that the sputtering damage during N-doped LaB6 deposition on a SiO2 gate insulator degraded the crystallinity of pentacene. The H2SO4 and H2O2 (SPM) and diluted HF treatments removed the damaged layer on the SiO2 gate insulator surface. Furthermore, the N-doped LaB6 IL improved the crystallinity of pentacene and realized dendritic grain growth. Owing to these surface treatments, the hole mobility improved from 2.8 × 10−3 to 0.11 cm2/(V·s), and a steep subthreshold swing of 78 mV/dec for the OFET with top-contact configuration was realized in air even after bottom-contact electrode patterning.

Growth of alkyl-monosubstituted thiophene/phenylene co-oligomer crystals and their device application

We synthesized a novel small-molecule organic semiconductor, which is soluble in organic solvents at room temperature under normal pressure. We demonstrated that its high-quality crystalline films can be directly grown on substrates using various solution techniques such as solution casting, slow evaporation, and edge casting. We applied crystals to FETs with a bottom- or top-contact configuration, revealing that the carrier mobility ranged from ∼10−4 to ∼10−2 cm2 V−1 s−1.

Importance of active layer positioning on gate electrode in organic thin-film transistors

ABSTRACTWe describe the importance of active layer positioning for a gate electrode in organic thin-film transistors (TFTs). For this study, we utilizea numerical simulation based on 2-D Atlas, which is a two-dimensional technology computer aided design (so called 2D TCAD) software tool created by Silvaco. Variation in the electrical characteristics of pentacene TFTs is systematically explored by changing the mismatch length (LM) between the active layer and gate electrode in the bottom-gate top-contact configuration. It is found that as the LM increases, the electrical performance of pentacene TFTs is exponentially degraded in terms of drain current on/off characteristics. In particular, we explain this phenomenon by examining variations in charge distribution and gate electric field in the TFT channel region byincreasing the LM.

π-Extended hexadeca-substituted cobalt phthalocyanine as an active layer for organic field-effect transistors.

Due to their flexibility and solution applicability, organic field-effect transistors (OFETs) are considered prominent candidates for application in flexible and low-cost devices. A soluble phthalocyaninato cobalt(ii) complex was designed and synthesized based on a hexadeca-substitution pattern by introducing peripheral phenylethynyl groups and non-peripheral n-butoxy groups. The cobalt phthalocyanine derivative was characterized using a wide range of spectroscopic and electrochemical methods, as well as single-crystal X-ray diffraction analysis. An OFET device was fabricated using a spin-coated film of soluble 1,4,8,11,15,18,22,25-octakisbutoxy-2,3,9,10,16,17,23,24-octakis-ethynyl phenyl phthalocyaninato cobalt(ii) with a bottom-gate top-contact device configuration. The transfer and output characteristics were investigated to evaluate the charge carrier mobility. The mechanisms of the leakage current through the gate dielectric were also investigated, which revealed that the dominant leakage current mechanism is Fowler-Nordheim tunneling.

Organic crystal light-emitting transistors with top-gate configuration

We applied the top-gate configuration to organic light-emitting transistors (OLETs) having an organic semiconductor crystal and an aluminum-doped zinc oxide (AZO) layer. The AZO layer was inserted between a quartz substrate and the organic crystal. The devices had the top-contact configuration where gold and an alloy of magnesium and silver were used for the contacts. Silver and Parylene C® were used for a gate contact and a gate insulator for the top-gate configuration, respectively. These OLETs showed more efficient current injection at low voltages than both the devices without a gate contact and the devices having the bottom-gate configuration. The present results indicate that the top-gate configuration is effective for improving the crystal OLET characteristics.

Influence of active layer thickness, device architecture and degradation effects on the contact resistance in organic thin film transistors

We analyze the influence of three combined effects on the contact resistance in organic- based thin film transistors: a) the active layer thickness, b) device architecture and c) semiconductor degradation. Transfer characteristics and parasitic series resistance were analyzed in devices with three different active layer thicknesses (50, 100 and 150 nm) using top contact (TC) and bottom contact (BC) thin film transistor (TFT) configurations. In both configurations, the lowest contact resistance (2.49 × 106 ?) and the highest field-effect mobility (4.8 × 10-2 cm2/V·s) was presented in devices with the thicker pentacene film. Top contact thin film transistors presented field-effect mobility values one order of magnitude higher (4.8 × 10-2 cm2/V·s) than bottom contact ones (1 × 10-3 cm2/V·s). Threshold voltage for top-contact thin film transistors was -3.1 V. After 2 months, performance in the devices degraded and presented an increase of one order of magnitude (105 - 106 ?) for BC-TFTs and two orders of magnitude (106 - 108 ?) for TC-TFTs in contact resistance.

The role of the electrode configuration on the electrical properties of small-molecule semiconductor thin-films

This paper presents a systematic analysis of the electrode configuration influence on the electrical properties of organic semiconductor (OSC) thin-film devices. We have fabricated and electrically characterized a set of planar two-terminal devices. The differences in I-V characteristics between the top and bottom contact structures are presented and analyzed. Top-contact configurations have a linear current vs. electric field behavior, while the bottom-electrode devices display a transition from ohmic to space-charge-limited conduction regime. The transition is temperature- and thickness-dependent. Finite-element calculations show that when the OSC film is connected using top electrodes, the current flows through the OSC bulk region. On the other hand, the bottom-electrode configuration allows most of the current to flow near the OSC/substrate interface. The current probes interfacial states resulting in a space-charge conduction regime. The results shed some light on the so-called “contact effects” commonly observed in organic thin-film transistors. The findings presented here have implications for both the understanding of the charge transport in OSC films and the design of organic semiconductor devices.

Electrode configurations for pentacene-deposited organic thin film transistors

ABSTRACTWhen a pentacene OTFT is fabricated with a bottom gate structure, the top-contact configuration of gold electrodes is preferred because it results in better electrical performance than the bottom-contact configuration. Pentacene molecules are deposited only on the SiO2 dielectric layer in the top-contact configuration, and thus all pentacene molecules form a vertical orientation that induces efficient charge transport between molecules. However, in an OTFT device with the bottom-contact configuration, pentacene molecules adopt a random tilted orientation at the boundary of the gold electrode, resulting in poor charge transport between molecules.

Effect of morphological and physicochemical properties of dielectric-organic semiconductor interfaces on photoresponse of organic phototransistors

We investigate the effect of the interfacial properties between a polymer dielectric and an organic semiconductor (OS) layers on the photoresponse properties of an organic phototransistor (OPT). Three different polymer materials having different interfacial properties, poly(methyl methacrylate), amorphous fluoropolymer CYTOP, and poly(4-vinylphenol), were used as gate insulators for the OPTs in a bottom-gate top-contact configuration. For the trapping and detrapping processes of minority carriers, the physicochemical nature and the density of functional group of the dielectric material were found to more dominantly govern the photoresponsive properties of the OPT than the morphological effect. Our molecular level description provides a useful guideline to select a proper dielectric material, being used as a gate insulator, for optical applications of the OPTs such as optical memory devices and optical sensors.

Organic devices based on pentacene and perylene by the neutral cluster beam deposition method

In this study, on the basis of p-type pentacene and n-type N,N′-dioctyl-perylene-3,4,9,10-tetracarboxylic acid diimide (PTCDI-C8) organic field-effect transistors (OFETs) and two-input complementary NAND logic gates in the top-contact device configuration were produced and characterized. The organic active layers were deposited on hydroxyl-free polymethylmethacrylate (PMMA)-modified indium tin oxide (ITO) glass gate substrates by the neutral cluster beam deposition (NCBD) method. The morphological and structural properties of the organic semiconducting active layers on the PMMA substrates were examined using atomic force microscopy, X-ray diffraction and contact-angle goniometry. Based on the growth of high-quality, well-packed crystalline films on the PMMA dielectric-modified ITO gate substrates, the p- and n-type transistors exhibited hole and electron mobilities of 0.247 and 7.23×10−2cm2/Vs, respectively, in the air without encapsulation. The trap density and activation energy were also derived from the transport characteristics for the temperature dependence of the mobilities in the temperature range 20−300K for the first time. Because of the well balanced p- and n-type OFETs in the devices, the complementary metal-oxide semiconductor (CMOS) NAND logic circuits exhibited a high voltage gain and a large noise margin with slight hysteresis.

Microcrystallization of a Solution-Processable Organic Semiconductor in Capillaries for High-Performance Ambipolar Field-Effect Transistors

We report on the use of microcrystallization in capillaries to fabricate patterned crystalline microstructures of the low-bandgap ambipolar quinoidal quaterthiophene derivative (QQT(CN)4) from a chloroform solution. Aligned needle-shaped QQT(CN)4 crystals were formed in thin film microstructures using either open- or closed- capillaries made of polydimethylsiloxane (PDMS). Their charge transport properties were evaluated in a bottom-gate top-contact transistor configuration. Hole and electron mobilities were found to be as high as 0.17 and 0.083 cm(2) V(-1) s(-1), respectively, approaching the values previously obtained in individual QQT(CN)4 single crystal microneedles. It was possible to control the size of the needle crystals and the microline arrays by adjusting the structure of the PDMS mold and the concentration of QQT(CN)4 solution. These results demonstrate that the microcrystallization in capillaries technique can be used to simultaneously pattern organic needle single crystals and control the microcrystallization processes. Such a simple and versatile method should be promising for the future development of high-performance organic electronic devices.

Ultraflexible and ultrathin polymeric gate insulator for 2 V organic transistor circuits

We have developed a high-yield process for fabricating organic transistors with ultraflexible and ultrathin polymeric (parylene) insulators. In a top-contact bottom-gate configuration, an oxygen plasma treatment for a Au gate surface before parylene deposition significantly improved the yield of transistors, enabling the parylene thickness to be reduced to 18 nm. Taking full advantage of the ultraflexible and ultrathin insulator, we have demonstrated 2 V ring oscillator circuits, where the yield was 97% for 360 transistors inside the area of 7 × 7 cm2. The highly reliable ultrathin insulator is useful for large-area circuits with low-voltage organic transistors.

Numerical simulation study of organic nonvolatile memory with polysilicon floating gate

A polysilicon-based organic nonvolatile floating-gate memory device with a bottom-gate top-contact configuration is investigated, in which polysilicon is sandwiched between oxide layers as a floating gate. Simulations for the electrical characteristics of the polysilicon floating gate-based memory device are performed. The shifted transfer characteristics and corresponding charge trapping mechanisms during programing and erasing (P/E) operations at various P/E voltages are discussed. The simulated results show that present memory exhibits a large memory window of 57.5 V, and a high read current on/off ratio of ≈ 103. Compared with the reported experimental results, these simulated results indicate that the polysilicon floating gate based memory device demonstrates remarkable memory effects, which shows great promise in device designing and practical application.

Electrical Instability in Pentacene Transistors with Mylar and PMMA/Mylar Gate Dielectrics Transferred by Lamination Process

This study deals with electrical instability under bias stress in pentacene-based transistors with gate dielectrics deposited by a lamination process. Mylar film is laminated onto a polyethylene terephthalate (PET) substrate, on which aluminum (Al) gate is deposited, followed by evaporation of organic semiconductor and gold (Au) source/drain contacts in bottom gate top contact configuration (Device 1). In order to compare the influence of the semiconductor/dielectric interface, a second organic transistor (Device 2) which is different from the Device 1 by the deposition of an intermediate layer of polymethyl methacrylate (PMMA) onto the laminated Mylar dielectric and before evaporating pentacene layer is fabricated. The critical device parameters such as threshold voltage (VT), subthreshold slope (S), mobility (μ), onset voltage (Von) and Ion/Ioff ratio have been studied. The results showed that the recorded hysteresis depend on the pentacene morphology. Moreover, after bias stress application, the electrical parameters are highly modified for both devices according to the regimes in which the transistors are operating. In ON state regime, Device 1 showed a pronounced threshold voltage shift associated to charge trapping, while keeping the μ, Ioff current and S minimally affected. Regardless of whether Device 2 exhibited better electrical performances and stability in ON state, we observed a bias stress-induced increase of depletion current and subthreshold slope in subthreshold region, a sign of defect creation. Both devices showed onset voltage shift in opposite direction.

Investigation of Gold and Poly(3-Alkylthiophene) interface in top and bottom contact structures

In this report we present the variations in the degree of crystalline order in π-conjugated polymer based thin films occurring at the interface between Gold (Au) and Poly(3-Alkylthiophene)s (P3ATs). For the purpose of study two possible electrode configurations were considered, i.e. the top contact (Au on top of Polymer) and the bottom contact (Polymer on top of Au). The current-voltage characteristics of Au(top)/P3AT/Au(bottom) devices revealed a near ohmic hole injection from bottom electrode but a non-ohmic hole injection from top electrode. Furthermore, the presence of barrier at the Au(top)/P3AT interface was estimated from the first order derivative of lnJ and lnV characteristics of the device under the low electric field biasing. It was observed that the process of coating metal (Au) itself led to significantly decreased polymer crystallinity and that the effect was more profound in top contact configuration compared to bottom contact ones. These observations were further verified through some non-contact mode characterization techniques such as absorption, photoluminescence, Raman spectroscopy and scanning electron microscopy. The difference in the hole injection current from the top and bottom contact electrodes in a Au/P3AT/Au device has been attributed to the creation of distorted π-conjugation due to Au diffusion effect in polymer films in top contact configuration.

High mobility ambipolar organic field-effect transistors with a nonplanar heterojunction structure

Ambipolar organic field-effect transistors (OFETs) based on a bilayer structure of highly crystalline small molecules, n-type α,ω-diperfluorohexylquaterthiophene (DFH-4T) and p-type dinaphtho[2,3-b:2′,3′-f]thieno[3,2-b]thiophene (DNTT), are investigated. By employing DFH-4T/DNTT as the bottom/top layers and appropriate high work function (WF) electrodes in a bottom-gate, top-contact configuration, the superior ambipolar characteristics with matched electron and hole mobilities of 1–1.1 cm2 V−1 s−1 are achieved. Intriguingly, this high-performance device exhibits a unique feature of an extremely rough, nonplanar heterojunction in the DFH-4T/DNTT combination and a large electron injection barrier from the high WF electrodes to DFH-4T, suggesting some underlying mechanisms for the effective charge transport and injection. The electrical and structural analyses reveal that the crystal packing of the bottom DFH-4T layer supports the growth of a high-quality DNTT crystal network for high-mobility hole transport upon the nonplanar heterojunction, and also enables the formation of an enlarged organic/metal contact surface for efficient electron injection from the high WF electrodes, as the key attributes leading to an overall excellent ambipolar behavior. The effect of intrinsic charge accumulation at the heterojunction interface on the ambipolar conduction is also discussed. Furthermore, a complementary-like inverter constructed with two DFH-4T/DNTT ambipolar OFETs is demonstrated, which shows a gain of 30.

Thermal behaviour and thin film properties of a bis-pyrene compound for organic thin film transistor applications

Following the standard Sonogashira cross-coupling conditions, the reaction between 1,4-diiodo-2,5-bis(octyloxy)benzene with 2.2equiv. of 1-ethynylpyrene afforded a yellowish bis-pyrene derivative, namely 1,4-bis(octyloxy)-2,5-bis(ethynylpyrene)benzene, that exhibits typical fluorescence and semiconducting properties. Thermal analysis studies are discussed, with emphasis made on the oxidative decomposition and thermal effects. Further on, thin films have been grown by vacuum thermal evaporation, on silicon and quartz substrates. The temperature's influence on the thin film's morphology and electrical properties is investigated. Finally, top-contact thin film transistor configurations are presented and the influence of substrate temperature is discussed with respect to the semiconducting properties.

Schottky barrier heights and band alignments in transition metal dichalcogenides

The Schottky barrier heights (SBHs) of metals on the layered transition metal dichalcogenides (TMDs) like MoS2 are calculated by density functional theory using supercell models. Despite the presence of van der Waals bonding between the layers, the metals are found to bond quite strongly to the chalcogen sites in the top contact configuration, without disturbing the intralayer covalent bonding. This allows the SBHs to follow the metal induced gap state (MIGS) model that applies to regular 3D semiconductors, and gives a pinning factor S ∼0.3. Additional pinning is caused by chalcogenide site vacancies. MoS2 is found to favor n-type devices, because the pinning levels are in its upper gap. Other compounds like MoSe2, WS2 or WSe2 have pinning levels around midgap, allowing ambipolar behavior.

Fabrication of a wafer-scale uniform array of single-crystal organic nanowire complementary inverters by nanotransfer printing

Abstract We report the fabrication and electrical characterization of a wafer-scale array of organic complementary inverters using single-crystal 6,13-bis(triisopropylsilylethynyl) pentacene (TIPS-PEN) and fullerene (C60) nanowires as p- and n-channels, respectively. Two arrays of single-crystal organic nanowires were generated consecutively on desired locations of a common substrate with a desired mutual alignment by a direct printing method (liquid-bridge-mediated nanotransfer molding). Another direct printing of silver micron scale structures, as source and drain electrodes, on the substrate with the two printed nanowire arrays produced an array of complementary inverters with a bottom gate, top contact configuration. Field-effect mobilities of single-crystal TIPS-PEN and C60 nanowire field-effect transistors (FETs) in the arrays were uniform with 1.01 ± 0.14 and 0.10 ± 0.01 cm2V−1 s−1, respectively. A wafer-scale array of complementary inverters produced all by the direct printing method showed good performance with an average gain of 25 and with low variations among the inverters.