Top-contact OTFTs Research Articles

Study of the Performance Enhancement of MoO3/Au Bilayer Source–Drain Electrode for Top-Contact Pentacene-Based OTFT

Study of the Performance Enhancement of MoO3/Au Bilayer Source–Drain Electrode for Top-Contact Pentacene-Based OTFT

Conjugated Polymer Controlled Morphology and Charge Transport of Small-Molecule Organic Semiconductors

In this study, we report an effective approach to tune the crystallization, microstructure and charge transport of solution-processed organic semiconductors by blending with a conjugated polymer additive poly(3-hexylthiophene) (P3HT). When 6,13-bis(triisopropylsilylethynyl) pentacene (TIPS pentacene) was used as a model semiconductor material to mix with different amount of P3HT, their intermolecular interactions led to distinctive TIPS pentacene film morphologies, including randomly-oriented crystal ribbons, elongated needles with enhanced long-range order, and grass-like curved microwires with interlinkages. Each type of morphology was found to further correlate to considerably different charge transport and device performance. As compared to pristine TIPS pentacene devices, bottom-gate, top-contact OTFTs with 2% in weight P3HT additive showed a 2-fold and 5-fold improvement of average field-effect mobility and performance consistency (defined as the ratio of average mobility to the standard deviation), respectively. The improvement in transistor electrical performance can be attributed to the combined effect of enhanced crystal orientation and uniformity, as well as increased areal coverage. This work can be applied beyond the particular example demonstrated in this study and to tune the charge transport of other small-molecule organic semiconductors in general.

3D reconstruction of pentacene structural organization in top-contact OTFTs via resonant soft X-ray reflectivity

Herein, we describe the use of soft X-ray reflectivity at the carbon K-edge to study the molecular organization (orientation, structure, and morphology) of pentacene active films in a top-contact transistor geometry. This technique is not affected by sample charging, and it can be applied in the case of insulating substrates. In addition, the sampling depth is not limited to the near-surface region, giving access to buried device interfaces (metal/organic and dielectric/organic). Spectral lineshape simulations, based on ab-initio calculations using a realistic 3D layer-by-layer model, allow us to unravel the details of the molecular organization in all the specific and crucial areas of the active film, overcoming the limitations of conventional approaches. The tilt angle of the long molecular axis in the whole film is found to progressively decrease with respect to the substrate normal from 25° to 0° with the increasing film thickness. A full vertical alignment, optimal for in-plane charge hopping, is reached only after the complete formation of the first five monolayers. Remarkably, starting from the first one in contact with the dielectric substrate, all the monolayers in the stack show a change in orientation with the increasing thickness. On the other hand, at the buried interface with a gold top-contact, the molecules assume a flat orientation that only propagates for two or three monolayers into the organic film. Top-contact devices with the highest performances can thus be obtained using films of at least ten monolayers. This explains the observed thickness dependence of charge mobility in pentacene transistors.

An analytical approach for parameter extraction in linear and saturation regions of top and bottom contact organic transistors

This paper analyzes the behaviour of top contact (TC) and bottom contact (BC) OTFTs through charge drift model modifying in terms of device series resistance. Subsequently, the drain and the gate voltages take into account the potential drop across the respective contacts. The mobility is modified in terms of overdrive voltage $$(V_{GS}-V_{T})$$(VGS-VT) and mobility enhancement factor as well. Additionally, the mathematical models are employed to extract the contact resistance including other parameters, such as, field dependent mobility, threshold voltage, mobility enhancement factor and drain current, separately for linear and saturation regimes. The model includes straightforward differential mathematics accounting of gate bias dependent contact resistance to evaluate the parameters analytically that too with single device only This resolves the complexity for realizing several devices with exactly same dimensions (except L) and moreover with same physical fabrication parameters. Besides this, the simulation is performed for four different TC and BC OTFTs using Atlas-Silvaco to comprehensively understand the device physics. Finally, the model is validated in terms of output characteristics and performance parameters against the experiment and simulation results. Analytically extracted mobility along with current for all four OTFTs is in a close harmony to the simulation results with an average error of 4 and 2.7 % in linear and saturation regions, respectively; whereas $$V_{T}$$VT is reasonably matched with 2 and 3 % average deviation.

High performance p-type organic thin film transistors with an intrinsically photopatternable, ultrathin polymer dielectric layer

A high-performing bottom-gate top-contact pentacene-based oTFT technology with an ultrathin (25–48nm) and electrically dense photopatternable polymeric gate dielectric layer is reported. The photosensitive polymer poly((±)endo,exo-bicyclo[2.2.1]hept-5-ene-2,3-dicarboxylic acid, diphenylester) (PNDPE) is patterned directly by UV-exposure (λ=254nm) at a dose typical for conventionally used negative photoresists without the need for any additional photoinitiator. The polymer itself undergoes a photo-Fries rearrangement reaction under UV illumination, which is accompanied by a selective cross-linking of the macromolecules, leading to a change in solubility in organic solvents. This crosslinking reaction and the negative photoresist behavior are investigated by means of sol–gel analysis. The resulting transistors show a field-effect mobility up to 0.8cm2V−1s−1 at an operation voltage as low as −4.5V. The ultra-low subthreshold swing in the order of 0.1Vdec−1 as well as the completely hysteresis-free transistor characteristics are indicating a very low interface trap density. It can be shown that the device performance is completely stable upon UV-irradiation and development according to a very robust chemical rearrangement. The excellent interface properties, the high stability and the small thickness make the PNDPE gate dielectric a promising candidate for fast organic electronic circuits.

Conjugated Polymers Based on a S- and N-Containing Heteroarene: Synthesis, Characterization, and Semiconducting Properties

Three alternating conjugated polymers (P-bTC10, P-bTC12, and P-bTC16) comprising a novel S- and N-containing heteroarene, i.e., N-dodecyldithieno[2,3-b;7,6-b]carbazole, and 4,4′-dialkyl-2,2′-bithiophenes have been designed and synthesized. The nitrogen atom in the heteroarene allows introducing an additional alkyl substituent in each repeating unit of the polymers. The resulting polymers are thermally stable with decomposition temperature of ∼400 °C and highly soluble in various organic solvents, such as tetrahydrofuran (THF), toluene, chloroform, and chlorobenzene, etc. Bottom gate, top contact OTFT devices were fabricated by solution casting. Mobility of the devices increased with an increase of the alkyl chain length in bithiophene units, and P-bTC10, P-bTC12, and P-bTC16 exhibited the mobilities up to 0.0064, 0.022, and 0.035 cm2/(V s), respectively. The mobility of P-bTC16 is comparable to that of regioregular poly(3-hexylthiophene) (rr-P3HT) reference devices, indicating that this heteroarene is a potential building block for high-mobility conjugated polymers.

Solution processable ter-anthrylene-ethynylenes semiconductors: thin film transistor properties and STM study on HOPG and Au(111)

The article reports the properties of new solution processable 9,10-ter-anthrylene-ethynylenes functionalised with n-butyl (B3A) n-octyl (O3A) and 2-ethyl-hexyl (E3A) functional groups. The characterization of the oligomers as organic semiconductors in solution processed top-contact bottom-gate OTFT devices reveal that E3A shows the best figures of merit (average μ of 1.1 ± 0.1 × 10−2 cm2 V−1 s−1 and on/off ratios of 105). Spontaneous physisorption of all the molecules on both highly ordered pyrolytic graphite (HOPG) and reconstructed Au(111) surfaces allowed the study of their self assembling behaviour at the liquid–solid interface by scanning tunnelling microscopy. STM revealed a peculiar behaviour for B3A, bearing shorter n-butyl chains, that arranges in a quasi hexagonal packing on HOPG, differently from the other molecules showing the formation of lamellae rows on this substrate. Due to the stronger interactions with the ter-anthrylene-ethynylene cores, on Au(111) surfaces, all structures self assemble in closer lamellar rows with very similar geometric parameters, independently from the alkyl chain length that are assumed to be tilted off the surface.

Synthesis and characterization of α,ω-disubstituted quaterthiophenes functionalized with polar groups for solution processed OTFTs

A series of soluble quaterthiophenes ( 4Ta– g) bearing ester groups in the α,ω-terminal positions separated from the quaterthiophene core by ethylene ( 4Ta– c), vinylene ( 4Td– f) or ethynylene ( 4Tg) spacers was synthesized by means of a Pd-catalyzed homocoupling of bithiophenes proceeding via C–H bond activation. The synthetic approach gave satisfying yields of 4Ta– f but resulted in only 3% yield of 4Tg due to the competitive hydrofluorination of the triple bond. The quaterthiophenes 4Ta– g were characterized by NMR, FTIR, UV–vis, PL spectroscopies, HRMS, TGA and CV. Thin-films of 4Ta– g were deposited either by spin-coating or by thermal evaporation on Si/SiO 2 for the fabrication of top-contact OTFTs. The devices prepared using 4Ta– c bearing the ester functional group separated from the quaterthiophene core by an ethylene spacer showed average hole field-effect mobility up to 2.7×10 −3 cm 2 V −1 s −1 and up to 6×10 −3 cm 2 V −1 s −1 for solution processed and for thermally evaporated OTFTs, respectively. The remarkably high solubility of 4Ta– c, along with their respectable performances in OTFTs render these molecules promising for practical applications as active layers in chemically-sensitive devices.

Thieno[3,2-b]thiophene oligomers and their applications as p-type organic semiconductors

This study describes the synthesis, characterization and electronic properties of a novel series of soluble thieno[3,2-b]thiophene oligomers (1a and b and 2a and b) for thin film transistor (TFT) applications. All the compounds were synthesized in high yield using Pd-catalyzed Stille or Suzuki coupling reactions and were substituted by two dodecyl groups at the 3- or 4-position of the thiophene unit to ensure the solubility for facile device fabrication. Aryl units such as phenyl and naphthyl were used for ‘end-capping’ to provide stability against oxidation. The design of these materials has focused on their self-assembly and solution processability. All the compounds have been characterized by 1H, 13C NMR, and elemental analysis. Their electronic and optical properties were investigated using UV-Vis and photoluminescence spectroscopy, cyclic voltammetry, thermal gravimetric analysis (TGA), and differential scanning calorimetry (DSC). High-resolution STM images of 1a and 2a adsorbed on HOPG revealed highly ordered self-organized domains. Two-dimensional wide-angle X-ray scattering (2D-WAXS) was used to study the solid state packing of 1a and 2a. Top-contact OTFT devices from 1a were prepared by spin coating and showed promising behaviour with mobilities up to 3.11 × 10−2 cm2V−1 s−1 and on/off ratios up to 104.

9,10-Ter-anthrylene-ethynylene: a new molecular architecture for solution processed anthracene-based thin film transistors

This study deals with the properties of a new molecular architecture for organic thin film transistors obtained by introducing 10-decylanthr-9-yl-ethynyl moieties at the anthracene 9,10 positions. The resulting structure, formally a ter-anthrylene-ethynylene (D3ANT), is stable and endowed with sufficient solubility to guarantee the formation of good quality films. The molecule was characterised by 1H NMR, mass spectrometry, IR, UV–vis and photoluminescence both in solution and in the solid state. Cyclic voltammetry measurements, combined with solution UV–vis, permitted the evaluation of HOMO and LUMO energy levels. A series of D3ANT-based OTFT devices were built both in bottom and top contact configurations by a spin coating technique. Top contact OTFT devices exhibited the best semiconducting characteristics, showing an average mobility of 1.2 × 10−2 cm2 V−1 s−1 with on–off ratios higher then 104, while the highest mobility obtained was 0.055 cm2 V−1 s−1. AFM thin-film characterization showed evidence of a granular structure. High-resolution STM images of D3ANT monolayers adsorbed on Au(111) and HOPG reveal highly-ordered self-organized domains with molecules lying flat on the substrates, but, remarkably, no reflections were recorded by out-of plane GIXRD of spin coated films on SiO2.

Grain size effects on contact resistance of top-contact pentacene TFTs

Multiple top-contact OTFTs with various channel lengths ( L c) were successfully scaled-down to the L c of 1.8 μm by using the membrane shadow mask and the interface between the evaporated Au and pentacene was analyzed based on the channel resistance method. For large grain pentacene (S-80) deposited at 80 °C, the parasitic resistance ( R p) at V GS = −20 V has 1.8 ± 0.2 kΩ cm, whereas for small grain pentacene (S-20) deposited at 20 °C has 4.2 ± 0.2 kΩ cm, which means that R p depends on the grain size of pentacene. The grain size and grain boundary trap density for pentacene can be possibly origins to determine R p, which is critically correlated with bulk transport in pentacene. The grain boundary trap density ( N t) for S-80 and S-20 was extracted as (5.6 ± 0.5) × 10 11 and (1.2 ± 0.3) × 10 12 cm −2 from the Levinson plots, respectively. In addition, activation energy of R p for S-80 is in the range from 42 to 48 meV, whereas for S-20 is from 72 to 108 meV.

Performance improvement of scaled-down top-contact OTFTs by two-step-deposition of pentacene

When scaling down to the channel length (L/sub c/) of 1.8 /spl mu/m using a membrane shadow mask, top-contact pentacene thin-film transistors (TFTs) show that grain size dependency on the anomalous leakage current becomes conspicuous as L/sub c/ is comparable to the grain size. For scaled-down OTFTs with large and small grain, the obvious difference of off-current in the depletion regime can be attributed to various reasons such as pentacene conductivity, parasitic resistance, locally ill-defined source/drain edge, and Au interdiffusion. To improve mobility as well as I/sub on//I/sub off/ ratio for scaled-down OTFTs, two-step-deposition (TSD) technique that enables us to control the channel conductivity in the depletion and accumulation regime as well as to improve the film continuity was proposed. To the best of our knowledge, the I/sub on//I/sub off/ ratio of 10/sup 7/ and the mobility of 0.20 cm/sup 2//V/spl middot/s for OTFTs with L/sub c/ of 1.8 /spl mu/m deposited by using the TSD technique was one of the best results in the literature.

Study on the characteristics of metal–organic interface for organic thin-film transistors

In this study, the interfacial characteristics between pentacene and Au layers were investigated with varying of the deposition rate of Au layer from 1.0 to 15.0 Å/s. For the devices with the structure of bottom-Au/pentacene/top-Au, it was observed that the electrical characteristics could be improved by increasing the deposition rate of top-Au, and the highest electrical conductivity value, 1.5 × 10 −6 S/cm, was obtained for the device with the top-Au-deposited at 15.0 Å/s. AES results showed that the integrated atomic content of Au in top-Au layer is substantially increased with the deposition rate of top-Au, but there was no critical difference in the depth profile of Au atoms regardless of the deposition rate of top-Au. And also, we fabricated pentacene-based Schottky diodes and measured the hole injection barrier heights from Au electrode into pentacene layer using Fowler-Nordheim theory. Upon the investigations, it was observed that the hole injection barrier was reduced with increasing the deposition rate of Au electrode and the lowest value of 0.12 eV was obtained for the device with the Au electrode deposited at 15.0 Å/s. As a result, the performance of top-contact OTFT could be improved with increasing the deposition rate of Au electrodes (source and drain).

P-18: Performance Improvement of Scaled-down Top-contact OTFTs by Two-Step-Deposition of Pentacene

Top-contact pentacene TFTs were successfully scaled down from 20 μm to 1.8 μm for the channel length (LC) by using silicon nitride membrane shadow mask. When LC was reduced to 1.8 μm, pentacene TFTs with the grain size of a few micrometer showed poor on/off current ratio (Ion/Ioff) of 103 and high mobility of 0.25 cm2/Vsec, whereas pentacene TFTs with the grain size of sub-micrometer resulted in low mobility of 0.08 cm2/Vsec, but good Ion/Ioff of 5.7×106. To obtain both high mobility and good Ion/Ioff for the scaled-down OTFTs at the same time, two-step-deposition (TSD) technique was proposed and resulted in Ion/Ioff of 107 as well as mobility of 0.19 cm2/Vsec even for the TFT with LC of 1.8 μm. To our best knowledge, this result is one of the best electrical characteristics among the reported electrical performance of OTFTs scaled down to LC of 1.8 μm.