Efficient and bright polymer light emitting field effect transistors

Light emitting field effect transistors (LEFETs) are emerging as a multi-functional class of optoelectronic devices. LEFETs can simultaneously execute light emission and the standard logic functions of a transistor in a single architecture. However, current LEFET architectures deliver either high brightness or high efficiency but not both concurrently, thus limiting their use in technological applications. Here we show an LEFET device strategy that simultaneously improves brightness and efficiency. The key step change in LEFET performance arises from the bottom gate top-contact device architecture in which the source/drain electrodes are semitransparent and the active channel contains a bi-layer comprising of a high mobility charge-transporting polymer, and a yellow–green emissive polymer. A record external quantum efficiency (EQE) of 2.1% at 1000 cd/m² is demonstrated for polymer based bilayer LEFETs.     

Morphology transformations of pentacene on an UV-patternable polymer insulator and their effect on performance of flexible organic thin-film transistors

This work presents a low temperature with high resolution capability UV-patternable polymer, i.e. mr-UVCur06, for use as a gate insulator in OTFTs, by investigating the morphology transformation of pentacene deposited on the mr-UVCur06. The device structure is polyethylene terephthalate (PET)/indium-tin oxide (ITO)/mr-UVCur06/pentacene/Au (source/drain). In addition to its solution-processable capability, mr-UVCur06 is directly patterned by UV light in a low-temperature process. UV/ozone post-treatment of the patternable mr-UVCur06 can illuminate organic contaminants from its surface and increases surface energy. Experimental results indicate that a high surface energy existing at the mr-UVCur06 surface has produced in a larger ratio of I_{thin film phase}/I_{triclinic bulk phase} in pentacene. Then, the distance of pentacene molecular crystal structure, which is arranged in the thin film phase, is shorter than that in triclinic bulk phase. The performance of pentacene-based OTFTs can be enhanced with few contaminants and a high surface energy on the UV-patternable gate insulator. By performing UV/ozone post-treatment on the mr-UVCur06 insulator surface for 60 s, the OTFTs demonstrate a mobility, on/off drain current ratio, and V_T of 0.34 cm²/V s, 5.5 × 10⁴, and 2.5 V, respectively. Furthermore, the low-temperature photopatternable polymer dielectric paves the way for a relatively easy and low-cost fabrication of an OTFT array without expensive and complicated photolithography and dry etching.     

Silver nanowire-polymer composite electrode for high performance solution-processed thin-film transistors

Silver nanowires (AgNWs)/poly-(3,4-ethylenedioxythiophene/polystyrene sulphonate) (PEDOT:PSS) composite films as conductive electrode for OTFTs were prepared, and their optical and electrical properties were investigated. The conductive composite films used in this study afforded low sheet resistance of <140 Ω/sq and transmittance as high as 70% in the visible region. For the composite film with 0.1 wt.% of AgNWs, contact resistance as low as 2.7 × 10⁴ Ω cm was obtained, as examined by Transfer length model (TLM) analysis, and work function of the corresponding film was 5.0 eV. Furthermore, the composite films were employed as source and drain electrodes for top-gate/bottom-contact organic thin-film transistors (OTFTs) based on solution-processed 5,11-bistriethylsilylethynyl anthradithiophene (TES-ADT) as organic semiconductor, and the resulting device showed high electrical performance with carrier mobility as high as 0.21 cm²/V s.     

Electric field induced ferroelectric-surface modification for high mobility organic field effect transistors

Inherent spontaneous polarization in ferroelectric-dielectric polymer PVDF-TrFE (Poly[(vinylidenefluoride-co-trifluoroethylene]) and an external electric field induced surface modification procedure are utilized to enhance organic field effect transistor (OFET) characteristics. The increase in the carrier mobility of the electric-field (EF) treated device correlates with the EF magnitude and evolution of dielectric microstructure and exhibits an enhancement beyond 300%. The enhanced interfacial transport property appears to have its origin in the dipolar orientation and nanostructure evolution at the interface.     

High-resolution direct-writing of metallic electrodes on flexible substrates for high performance organic field effect transistors

We report on organic field-effect transistors (OFETs) with sub-micrometer channels fabricated on plastic substrates with fully direct-written electrical contacts. In order to pattern source and drain electrodes with high resolution and reliability, we adopted a combination of two digital, direct writing techniques: ink-jet printing and femtosecond laser ablation. First silver lines are deposited by inkjet printing and sintered at low temperature and then sub-micrometer channels are produced by highly selective femtosecond laser ablation, strongly improving the lateral patterning resolution achievable with inkjet printing only. These direct-written electrodes are adopted in top gate OFETs, based on high-mobility holes and electrons transporting semiconductors, with field-effect mobilities up to 0.2 cm²/V s. Arrays of tens of devices have been fabricated with high process yield and good uniformity, demonstrating the robustness of the proposed direct-writing approach for the patterning of downscaled electrodes for high performance OFETs, compatibly with cost-effective manufacturing of large-area circuits.     

Bottom-contact small-molecule n-type organic field effect transistors achieved via simultaneous modification of electrode and dielectric surfaces

Low-voltage, n-type organic field effect transistors (OFETs) with simultaneously modified bottom-contact (BC) electrodes and dielectric were compared to their top-contact (TC) counterparts. The devices modified with 6-phenoxyhexylphosphonic acid (Ph6PA) self-assembled monolayer (SAM) showed similar performance, morphology, and contact resistance. Electron mobility of C₆₀ devices were 0.212 and 0.320 cm² V⁻¹ s⁻¹ and [6,6]-phenyl-C₆₁-butyric acid methyl ester (PCBM) devices were 0.04 and 0.06 cm² V⁻¹ s⁻¹ for TC and BC devices, respectively. Low contact resistance between 11 and 45 kΩ cm was found regardless of device architecture or n-type semiconductor used. This work shows it is possible to fabricate solution processable low-voltage bottom-contact devices with performance that is similar or better than their top-contact counterparts without the addition of complex and time-consuming processing steps.     

Improved pentacene growth continuity for enhancing the performance of pentacene-based organic thin-film transistors

This study proposes an alternative planar bottom-contact (pBC) structure to enhance the electrical performance of pentacene-based organic thin-film transistors (OTFTs). This pBC structure uses a bilayer dielectric to control planarization with a precise etch depth and introduces a bilayer photoresist lift-off method to ensure that planarization produces an optimum flatness. Because of the improved growth continuity of pentacene near the edge of the source/drain electrodes, the contact resistance between the source/drain and the pentacene was reduced significantly, thereby enhancing the electrical performance of OTFTs. The mechanism for the enhanced performance was also verified by a physics-based numerical simulation.     

A process for high yield and high performance carbon nanotube field effect transistors

Carbon nanotube field effect transistors (CNTFETs) have been considered as one of the potential candidates for nanoelectronics beyond Si CMOS. However, it is not easy to have high performance CNTFETs with high yield currently. In this work, we proposed a local bottom-gate (LBG) CNTFETs combined with a novel device concept and optimized process technologies. High performance of CNTFET with low subthreshold swing of 139 mV/dec, high transconductance of 1.27 μS, and high I_on/I_off ratio of 10⁶ can be easily obtained with Ti source/drain contact after a post annealing process. Record high yield of 74% has been demonstrated. On the basis of the proposed process, lots of high performance CNTFETs can be obtained easily for advanced study on the electrical characteristics of CNTFETs in the future.     

Molecular-scale charge trap medium for organic non-volatile memory transistors

In this work, we introduce a molecular-scale charge trap medium for an organic non-volatile memory transistor (ONVMTs). We use two different types of small molecules, 2,3,6,7,10,11-hexahydroxytriphenylene (HHTP) and 2,3,6,7,10,11-hexamethoxytriphenylene (HMTP), which have the same triphenylene cores with either hydroxyl or methoxy end groups. The thickness of the small molecule charge trap layer was sophisticatedly controlled using the thermal evaporation method. X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared (FTIR) analysis revealed that there were negligible differences in the chemical structures of both small molecules before and after thermal deposition process. The ONVMTs with a 1-nm-thick HHTP charge trap layer showed a large hysteresis window, approximately 20 V, under a double sweep of the gate bias between 40 V and −40 V. The HMTP-based structure showed a negligible memory window, which implied that the hydroxyl groups affected hysteresis. The number of trapped charges on the HHTP charge trap layer was measured to be 4.21 × 10¹² cm⁻². By varying the thickness of the molecular-scale charge trap medium, it was determined that the most efficient charge trapping thickness of HHTP charge trap layer was approximately 5 nm.     

Enhancing the performance of solution-processed organic thin-film transistors by blending binary compatible small molecule semiconductors

Physical blending is a facile and effective way to improve the performance of solution processed organic thin-film transistors (OTFTs). Blending small molecule semiconductors with soluble polymers has been extensively studied in recent years. However, blending between binary small molecule semiconductors is rare due to the difficulty to obtain ideal thin films. Herein, we systematically investigate the blending effects on the morphologies of thin films and their field-effect performance by using two small molecule semiconductors, 2-phenyl[1]benzothieno[3,2-b][1]benzothiophene (Ph-BTBT) and 2-(4-dodecylphenyl) [1]benzothieno[3,2-b]benzothiophene, (C12-Ph-BTBT), which have the same aromatic skeleton. Molecular ordering and better crystallinity are observed in most of spin-coated blend thin films, thanks to the enhanced molecular interaction after blending. As a result, OTFTs based on blend thin films exhibit improved performance in most cases, with the highest average hole mobility about 1.5 cm² V⁻¹ s⁻¹ demonstrated. Further device performance improvements are demonstrated by blending polystyrene with Ph-BTBT and C12-Ph-BTBT blends. The results here indicate that blending between small molecule semiconductors with compatible fused ring structures may be a promising strategy to enhance the performance of organic transistors.     

Optimizing the fabrication process for high performance graphene field effect transistors

As an emerging material, graphene has attracted vast interest in solid-state physics, materials science, nanoelectronics and bioscience. Graphene has zero bandgap with its valence and conduction bands are cone-shaped and meet at the K points of the Brillouin zone. Due to its high intrinsic carrier mobility, large saturation velocity, and high on state current density, graphene is also considered as a promising candidate for high-frequency devices. To improve the reliability of graphene FETs, which include shifting the Dirac point voltage toward zero, increasing the channel mobility and decreasing the source/drain contact resistance, we optimized the device fabrication process. For CVD grown graphene, the film transfer and the device fabrication processes may produce interfacial states between graphene and the substrate and make graphene p or n-type, which shift the fermi level far away from the Dirac point. We have found that after graphene film transfer, an annealing process at 400 °C under N₂ ambient will shift Dirac point toward zero gate voltage. Ti/Au, Ni, and Ti/Pd/Au source/drain structures have been studied to minimize the contact resistance. According to the measured data, Ti/Pd/Au structure gives the lowest contact resistance (～500 ohm μm). By controlling the process of graphene growth, transfer and device fabrication, we have achieved graphene FETs with a field effective mobility of 16,000 cm²/V s after subtraction of contact resistance. The contact resistivity was estimated in the range of 1.1 × 10^?6 Ω cm² to 8.8 × 10^?6 Ω cm², which is close to state of the art III–V technology. The maximum transconductance was found to be 280 mS/mm at V_D = 0.5 V, which is the highest value among CVD graphene FETs published to date.     

Carboxylic acid mediated self-assembly of small molecules for organic thin film transistors

A p-type small molecule bearing dicarboxylic acid functional group (–COOH) is synthesized and evaluated for field-effect transistor properties. We discover and report for the first time, that the –COOH groups assist in the passivation of surface traps on the dielectric layer and simultaneously facilitate the self-assembly of the molecules via inter-molecular hydrogen bonding resulting in crystalline active channels. A 9-fold decrease in the threshold voltage was observed for the transistors made using the –COOH functionalized molecule, QT-DA, compared to its ester analogue, QT-ES, providing an evidence of surface passivation. This resulted in an increase in the hole mobility of QT-DA by up to 2 orders of magnitude. It was shown that QT-DA adopts a vertical alignment with respect to the substrate due to preferential interaction between the –COOH groups and the SiO₂ surfaces.     

A small molecule with selenophene as the central block for high performance solution-processed organic solar cells

A solution-processable A–D–A structure small molecule donor material called DRCN7T-Se with selenophene as the central block was synthesized. Conventional bulk-heterojunction solar cell devices based on DRCN7T-Se and PC₇₁BM were optimized by thermal annealing and an excellent power conversion efficiency of 8.30% was achieved under AM 1.5G irradiation (100 mW cm⁻²).     

Numerical analysis of capacitance compact models for organic thin-film transistors

Analytical expressions for the gate-voltage dependence of the channel capacitance and the gate-to-contacts overlap capacitances in top-contact organic thin-film transistors (OTFTs) are derived and implemented in an organic compact capacitance model. The resulting modified model is verified by experimental data of transistors with constant mobility. The same model is analyzed by numerical simulations for OTFTs with a voltage-dependent mobility. The simulation results indicate that the quasistatic model describes well the simulated capacitances. In accumulation, the modeled values are slightly overestimated because of the generally accepted assumption of the charge-sheet model. It is also demonstrated that the quasistatic regime occurs at lower frequencies because of the reduced mobility at lower charge carrier concentrations.     

A new dithienosilole-based oligothiophene with methyldicyanovinyl groups for high performance solution-processed organic solar cells

A new linear dithienosilole-based oligothiophene end-capped with methyl and electron-withdrawing dicyanovinyl groups, DTS(Oct)₂-(2T-DCV-Me)₂, was prepared in good yield. This oligomer exhibited broad absorption spectra in bulk down to the near-IR region with the optical edge at 900 nm, resulting in an initially high power conversion efficiency of 5.44% in solution-processed organic solar cells using PC₇₁BM as an acceptor.     

Optimization of electrohydrodynamic-printed organic electrodes for bottom-contact organic thin film transistors

In this study, we investigate the optimization of printed (3,4-ethylenedioxythiophene):poly(4-styrenesulfonate) (PEDOT:PSS) as source/drain electrodes for organic thin film transistors (OTFTs) through electrohydrodynamic (EHD) printing process. The EHD-printed PEDOT:PSS electrodes should fulfill the prerequisites of not only high conductivity but also optimum surface tension for successful jetting. The conductivity of PEDOT:PSS was dramatically enhanced from 0.07 to 352 S/cm by the addition of dimethylsulfoxide (DMSO). To use the DMSO-treated PEDOT:PSS solution in the EHD printing process, its surface tension was optimized by the addition of surfactant (Triton X-100), which was found to enable various jetting modes. In the stable cone-jet mode, the patterning of the modified PEDOT:PSS solution was realized on the surface-functionalized SiO₂ substrates; the printed line widths were in the range 384 to 81 μm with a line resistance of 8.3 × 10³ Ω/mm. In addition, pentacene-based OTFTs employing the EHD-printed PEDOT:PSS as source and drain electrodes were found to exhibit electrical performances superior to an equivalent vacuum-deposited Au-based device.     

Dialkylated dibenzotetrathienoacene derivative as semiconductor for organic field effect transistors

A novel semiconductor material based on dialkylated thienoacene is designed and synthesized. The dihexyl-substituted dibenzotetrathienoacene derivative C6-DBTTA exhibits high stability which is evidenced by thermogravimetric analysis (TGA), UV–vis spectroscopy and electrochemistry. X-ray diffraction measurements of the vacuum-evaporated thin films show strong diffraction and indicate that the molecules are stacked nearly perpendicular to the substrate. AFM images reveal that the morphology of thin films depended on the deposition temperature. Thin film FETs devices based on C6-DBTTA were constructed and showed high mobility up to 0.48 cm² V⁻¹ s⁻¹ and an on/off ratio over 10⁷. These results suggest that this new dihexylated thienoacene is an important organic semiconductor for field effect transistors.     

Design and modeling of self-aligned nano-imprinted sub-micrometer pentacene-based organic thin-film transistors

Sub-micrometer channel length (0.5 μm) organic thin-film transistors (OTFTs) are demonstrated using a process flow combining nano-imprint lithography (NIL) and self-alignment (SA). A dedicated test structure was designed and samples were fabricated on 4-in. plastic foils using a p-type sublimated small molecule (pentacene) as semiconductor. Field-effect mobilities, in saturation, between 0.1 and 1 cm²/Vs were obtained not only for the supermicron OTFTs but also for the submicron OTFTs. Those devices were used to select a model based on the “TFT Generic Charge Drift model” which works well for a broad range of channel lengths including the submicron OTFTs. We show that these OTFTs can be accurately modeled, thus giving access to complex circuit simulations and design.     

Carbon nanotube field effect transistors for high performance analog applications: An optimum design approach

There is a need to explore circuit designs in new emerging technologies for their rapid commercialization to extend Moore’s law beyond 22 nm technology node. Carbon nanotube based transistor (CNFET) has significant potential to replace CMOS in the future due to its better electrostatics and higher mobility. This paper presents a complete optimal design of an inverting amplifier in CMOS, CNFET and hybrid technologies. We investigate and conceptually explain the performance measure of the amplifier at 32 nm technology node in terms of operating voltage, number of carbon nanotubes (CNT), diameter and pitch (inter-nanotube distance) variations of carbon nanotubes in a CNFET transistor in pure and hybrid technologies for area, power and performance optimization. This paper also explores the scope, possibilities and challenges associated with pure CNFET and hybrid amplifiers. We have found that pure CNFET amplifier provided good amplification while hybrid pCNFET-nMOS amplifier offered excellent frequency response and pMOS-nCNFET amplifier gave better transient performance compared with planar CMOS.     

Effects of P3HT concentration on the performance of organic field effect transistors

Top-contact organic field effect transistors(OFETs) based on poly(3-hexylthiophene)(P3HT) with different concentrations in chloroform(CHCl3) are fabricated.The output characteristics indicate that the P3HT concentration has significant influence on the OFET devices.The performance of the devices firstly is enhanced with increasing the P3HT concentration,and then decreases.The optimized devices with the P3HT concentration of 2 mg/mL show the best performance.The fieldeffect mobility is up to 1.4×10-2 cm2/Vs,...