329 results found
Semple J, Georgiadou DG, Wyatt-Moon G, et al., 2017, Flexible diodes for radio frequency (RF) electronics: a materials perspective, Semiconductor Science and Technology, Vol: 32, ISSN: 0268-1242
Over the last decade, there has been increasing interest in transferring the research advances in radiofrequency (RF) rectifiers, the quintessential element of the chip in the RF identification (RFID) tags, obtained on rigid substrates onto plastic (flexible) substrates. The growing demand for flexible RFID tags, wireless communications applications and wireless energy harvesting systems that can be produced at a low-cost is a key driver for this technology push. In this topical review, we summarise recent progress and status of flexible RF diodes and rectifying circuits, with specific focus on materials and device processing aspects. To this end, different families of materials (e.g. flexible silicon, metal oxides, organic and carbon nanomaterials), manufacturing processes (e.g. vacuum and solution processing) and device architectures (diodes and transistors) are compared. Although emphasis is placed on performance, functionality, mechanical flexibility and operating stability, the various bottlenecks associated with each technology are also addressed. Finally, we present our outlook on the commercialisation potential and on the positioning of each material class in the RF electronics landscape based on the findings summarised herein. It is beyond doubt that the field of flexible high and ultra-high frequency rectifiers and electronics as a whole will continue to be an active area of research over the coming years.
Panidi J, Paterson AF, Khim D, et al., 2017, Remarkable Enhancement of the Hole Mobility in Several Organic Small-Molecules, Polymers, and Small-Molecule:Polymer Blend Transistors by Simple Admixing of the Lewis Acid p-Dopant B(C6F5)(3), Advanced Science, Vol: 5, ISSN: 2198-3844
Improving the charge carrier mobility of solution-processable organic semiconductors is critical for the development of advanced organic thin-film transistors and their application in the emerging sector of printed electronics. Here, a simple method is reported for enhancing the hole mobility in a wide range of organic semiconductors, including small-molecules, polymers, and small-molecule:polymer blends, with the latter systems exhibiting the highest mobility. The method is simple and relies on admixing of the molecular Lewis acid B(C6F5)3 in the semiconductor formulation prior to solution deposition. Two prototypical semiconductors where B(C6F5)3 is shown to have a remarkable impact are the blends of 2,8-difluoro-5,11-bis(triethylsilylethynyl)anthradithiophene:poly(triarylamine) (diF-TESADT:PTAA) and 2,7-dioctyl-benzothieno[3,2-b]benzothiophene:poly(indacenodithiophene-co-benzothiadiazole) (C8-BTBT:C16-IDTBT), for which hole mobilities of 8 and 11 cm2 V−1 s−1, respectively, are obtained. Doping of the 6,13-bis(triisopropylsilylethynyl)pentacene:PTAA blend with B(C6F5)3 is also shown to increase the maximum hole mobility to 3.7 cm2 V−1 s−1. Analysis of the single and multicomponent materials reveals that B(C6F5)3 plays a dual role, first acting as an efficient p-dopant, and secondly as a microstructure modifier. Semiconductors that undergo simultaneous p-doping and dopant-induced long-range crystallization are found to consistently outperform transistors based on the pristine materials. Our work underscores Lewis acid doping as a generic strategy towards high performance printed organic microelectronics.
Pattanasattayavong P, Promarak V, Anthopoulos TD, 2017, Electronic Properties of Copper(I) Thiocyanate (CuSCN) (vol 3, 1600378, 2017), Advanced Electronic Materials, Vol: 3, ISSN: 2199-160X
Wijeyasinghe N, Regoutz A, Eisner F, et al., 2017, Copper(I) Thiocyanate (CuSCN) Hole-Transport Layers Processed from Aqueous Precursor Solutions and Their Application in Thin-Film Transistors and Highly Efficient Organic and Organometal Halide Perovskite Solar Cells, ADVANCED FUNCTIONAL MATERIALS, Vol: 27, ISSN: 1616-301X
This study reports the development of copper(I) thiocyanate (CuSCN) hole-transport layers (HTLs) processed from aqueous ammonia as a novel alternative to conventional n-alkyl sulfide solvents. Wide bandgap (3.4–3.9 eV) and ultrathin (3–5 nm) layers of CuSCN are formed when the aqueous CuSCN–ammine complex solution is spin-cast in air and annealed at 100 °C. X-ray photoelectron spectroscopy confirms the high compositional purity of the formed CuSCN layers, while the high-resolution valence band spectra agree with first-principles calculations. Study of the hole-transport properties using field-effect transistor measurements reveals that the aqueous-processed CuSCN layers exhibit a fivefold higher hole mobility than films processed from diethyl sulfide solutions with the maximum values approaching 0.1 cm2 V−1 s−1. A further interesting characteristic is the low surface roughness of the resulting CuSCN layers, which in the case of solar cells helps to planarize the indium tin oxide anode. Organic bulk heterojunction and planar organometal halide perovskite solar cells based on aqueous-processed CuSCN HTLs yield power conversion efficiency of 10.7% and 17.5%, respectively. Importantly, aqueous-processed CuSCN-based cells consistently outperform devices based on poly(3,4-ethylenedioxythiophene) polystyrene sulfonate HTLs. This is the first report on CuSCN films and devices processed via an aqueous-based synthetic route that is compatible with high-throughput manufacturing and paves the way for further developments.
Isakov I, Faber H, Grell M, et al., 2017, Exploring the Leidenfrost Effect for the Deposition of High-Quality In2O3 Layers via Spray Pyrolysis at Low Temperatures and Their Application in High Electron Mobility Transistors, ADVANCED FUNCTIONAL MATERIALS, Vol: 27, ISSN: 1616-301X
Fei Z, Chen L, Han Y, et al., 2017, Alternating 5,5-Dimethylcyclopentadiene and Diketopyrrolopyrrole Copolymer Prepared at Room Temperature for High Performance Organic Thin-Film Transistors, Journal of the American Chemical Society, Vol: 139, Pages: 8094-8097, ISSN: 1520-5126
We report that the inclusion of nonaromatic 5,5-dimethylcyclopentadiene monomer into a conjugated backbone is an attractive strategy to high performance semiconducting polymers. The use of this monomer enables a room temperature Suzuki copolymerization with a diketopyrrolopyrrole comonomer to afford a highly soluble, high molecular weight material. The resulting low band gap polymer exhibits excellent photo and thermal stability, and despite a large π–π stacking distance of 4.26 Å, it demonstrates excellent performance in thin-film transistor devices.
Fei Z, Han Y, Gann E, et al., 2017, Alkylated selenophene-based ladder-type monomers via a facileroute for high performance thin-film transistor applications, Journal of the American Chemical Society, Vol: 139, Pages: 8552-8561, ISSN: 1943-2984
We report the synthesis of two new selenophene-containing ladder-type monomers, cyclopentadiselenophene (CPDS) and indacenodiselenophene (IDSe), via a 2-fold and 4-fold Pd-catalyzed coupling with a 1,1-diborylmethane derivative. Copolymers with benzothiadiazole were prepared in high yield by Suzuki polymerization to afford materials which exhibited excellent solubility in a range of nonchlorinated solvents. The CPDS copolymer exhibited a band gap of just 1.18 eV, which is among the lowest reported for donor–acceptor polymers. Thin-film transistors were fabricated using environmentally benign, nonchlorinated solvents, with the CPDS and IDSe copolymers exhibiting hole mobility up to 0.15 and 6.4 cm2 V–1 s–1, respectively. This high performance was achieved without the undesirable peak in mobility often observed at low gate voltages due to parasitic contact resistance.
Faber H, Das S, Lin Y-H, et al., 2017, Heterojunction oxide thin-film transistors with unprecedented electron mobility grown from solution., Science Advances, Vol: 3, ISSN: 2375-2548
Thin-film transistors made of solution-processed metal oxide semiconductors hold great promise for application in the emerging sector of large-area electronics. However, further advancement of the technology is hindered by limitations associated with the extrinsic electron transport properties of the often defect-prone oxides. We overcome this limitation by replacing the single-layer semiconductor channel with a low-dimensional, solution-grown In2O3/ZnO heterojunction. We find that In2O3/ZnO transistors exhibit band-like electron transport, with mobility values significantly higher than single-layer In2O3 and ZnO devices by a factor of 2 to 100. This marked improvement is shown to originate from the presence of free electrons confined on the plane of the atomically sharp heterointerface induced by the large conduction band offset between In2O3 and ZnO. Our finding underscores engineering of solution-grown metal oxide heterointerfaces as an alternative strategy to thin-film transistor development and has the potential for widespread technological applications.
Petti L, Pattanasattayavong P, Lin Y-H, et al., 2017, Solution-processed p-type copper(I) thiocyanate (CuSCN) for low-voltage flexible thin-film transistors and integrated inverter circuits, APPLIED PHYSICS LETTERS, Vol: 110, ISSN: 0003-6951
Khim D, Lin Y-H, Nam S, et al., 2017, Modulation-Doped In2O3/ZnO Heterojunction Transistors Processed from Solution, Advanced Materials, Vol: 29, ISSN: 0935-9648
This paper reports the controlled growth of atomically sharp In2O3/ZnO and In2O3/Li-doped ZnO (In2O3/Li-ZnO) heterojunctions via spin-coating at 200 °C and assesses their application in n-channel thin-film transistors (TFTs). It is shown that addition of Li in ZnO leads to n-type doping and allows for the accurate tuning of its Fermi energy. In the case of In2O3/ZnO heterojunctions, presence of the n-doped ZnO layer results in an increased amount of electrons being transferred from its conduction band minimum to that of In2O3 over the interface, in a process similar to modulation doping. Electrical characterization reveals the profound impact of the presence of the n-doped ZnO layer on the charge transport properties of the isotype In2O3/Li-ZnO heterojunctions as well as on the operating characteristics of the resulting TFTs. By judicious optimization of the In2O3/Li-ZnO interface microstructure, and Li concentration, significant enhancement in both the electron mobility and TFT bias stability is demonstrated.
Casey A, Green JP, Shakya Tuladhar P, et al., 2017, Cyano substituted benzotriazole based polymers for use in organic solar cells, Journal of Materials Chemistry A, Vol: 5, Pages: 6465-6470, ISSN: 2050-7496
A new synthetic route to the electron accepting di-cyano substituted benzo[d][1,2,3]triazole (BTz) monomer 2-(2-butyloctyl)-4,7-di(thiophen-2-yl)-2H-benzotriazole-5,6-dicarbonitrile (dTdCNBTz) is reported. The cyano substituents can be easily introduced to the BTz unit in one step via the nucleophilic aromatic substitution of the fluorine substituents of the fluorinated precursor 2-(2-butyloctyl)-4,7-di(thiophen-2-yl)-2H-benzotriazole-5,6-difluoro (dTdFBTz). Co-polymers were prepared with distannylated benzo[1,2-b:4,5-b′]dithiophene (BDT) monomers containing either 2-ethylhexylthienyl (T-EH) side chains or 2-butyloctylthienyl (T-BO) side chains via Stille coupling to yield the novel medium band gap polymers P1 and P2 respectively. Whilst the organic photovoltaic (OPV) performance of P1 was limited by a lack of solubility, the improved solubility of P2 resulted in promising device efficiencies of up to 6.9% in blends with PC61BM, with high open circuit voltages of 0.95 V.
Pattanasattayavong P, Promarak V, Anthopoulos TD, 2017, Electronic Properties of Copper(I) Thiocyanate (CuSCN), Advanced Electronic Materials, Vol: 3, ISSN: 2199-160X
With the emerging applications of copper(I) thiocyanate (CuSCN) as a transparent and solution-processable hole-transporting semiconductor in numerous opto/electronic devices, fundamental studies that cast light on the charge transport physics are essential as they provide insights critical for further materials and devices performance advancement. The aim of this article is to provide a comprehensive and up-to-date report of the electronic properties of CuSCN with key emphasis on the structure–property relationship. The article is divided into four parts. In the first section, recent works on density functional theory calculations of the electronic band structure of hexagonal β-CuSCN are reviewed. Following this, various defects that may contribute to the conductivity of CuSCN are discussed, and newly predicted phases characterized by layered 2-dimensional-like structures are highlighted. Finally, a summary of recent studies on the band-tail states and hole transport mechanisms in solution-deposited, polycrystalline CuSCN layers is presented.
Semple J, Wyatt-Moon G, Georgiadou DG, et al., 2017, Semiconductor-Free Nonvolatile Resistive Switching Memory Devices Based on Metal Nanogaps Fabricated on Flexible Substrates via Adhesion Lithography, IEEE TRANSACTIONS ON ELECTRON DEVICES, Vol: 64, Pages: 1973-1980, ISSN: 0018-9383
Dellis S, Isakov I, Kalfagiannis N, et al., Rapid laser-induced photochemical conversion of sol-gel precursors to In2O3 layers and their application in thin-film transistors, Journal of Materials Chemistry C, ISSN: 2050-7534
We report the development of indium oxide (In2O3) transistors via a singlestep laser-induced photochemical conversion process of a sol-gel metal oxideprecursor. Through careful optimization of the laser annealing conditions wedemonstrated successful conversion of the precursor to In2O3 and its subsequentimplementation in n-channel transistors with electron mobility up to 13 cm2/Vs.Importantly, the process does not require thermal annealing making itcompatible with temperature sensitive materials such as plastic. On the otherhand, the spatial conversion/densification of the sol-gel layer eliminatesadditional process steps associated with semiconductor patterning and hencesignificantly reduces fabrication complexity and cost. Our work demonstratesunambiguously that laser-induced photochemical conversion of sol-gel metaloxide precursors can be rapid and compatible with large-area electronicsmanufacturing.
Isakov I, Paterson AF, Solomeshch O, et al., 2016, Hybrid complementary circuits based on p-channel organic and n-channel metal oxide transistors with balanced carrier mobilities of up to 10 cm(2)/Vs, Applied Physics Letters, Vol: 109, ISSN: 1077-3118
We report the development of hybrid complementary inverters based on p-channel organic andn-channel metal oxide thin-film transistors (TFTs) both processed from solution at <200 C. For theorganic TFTs, a ternary blend consisting of the small-molecule 2,7-dioctylbenzothieno[3,2-b]benzothiophene, the polymer indacenodithiophene-benzothiadiazole (C16IDT-BT) and the p-typedopant C60F48 was employed, whereas the isotype In2O3/ZnO heterojunction was used for the nchannelTFTs. When integrated on the same substrate, p- and n-channel devices exhibited balancedcarrier mobilities up to 10 cm2/Vs. Hybrid complementary inverters based on these devices show highsignal gain (>30 V/V) and wide noise margins (70%). The moderate processing temperaturesemployed and the achieved level of device performance highlight the tremendous potential of the technologyfor application in the emerging sector of large-area microelectronics.
Tetzner K, Isakov I, Regoutz A, et al., 2016, The impact of post-deposition annealing on the performance of solution-processed single layer In2O3 and isotype In2O3/ZnO heterojunction transistors, JOURNAL OF MATERIALS CHEMISTRY C, Vol: 5, Pages: 59-64, ISSN: 2050-7526
Nam S, Han H, Seo J, et al., 2016, Ambipolar Organic Phototransistors with p-Type/n-Type Conjugated Polymer Bulk Heterojunction Light-Sensing Layers, ADVANCED ELECTRONIC MATERIALS, Vol: 2, ISSN: 2199-160X
Fallon KJ, Wijeyasinghe N, Manley EF, et al., 2016, Indolo-naphthyridine-6,13-dione Thiophene Building Block for Conjugated Polymer Electronics: Molecular Origin of Ultrahigh n-Type Mobility, CHEMISTRY OF MATERIALS, Vol: 28, Pages: 8366-8378, ISSN: 0897-4756
Nam S, Seo J, Han H, et al., 2016, > 10% Efficiency Polymer: Fullerene Solar Cells with Polyacetylene-Based Polyelectrolyte Interlayers, ADVANCED MATERIALS INTERFACES, Vol: 3, ISSN: 2196-7350
Casey A, Han Y, Gann E, et al., 2016, Vinylene-linked oligothiophene–difluorobenzothiadiazole copolymer for transistor applications, ACS Applied Materials & Interfaces, Vol: 8, Pages: 31154-31165, ISSN: 1944-8244
The synthesis of the novel donor-acceptor monomer 4,7-bis[(E)-2-(5-bromo-3-dodecylylthiophen-2-yl)ethenyl]-5,6-difluoro-2,1,3-benzothiadiazole (FBT-V2T2) is reported. Polymerization with 4,4'-ditetradecyl-5,5'-bistrimethylstannyl-2,2'-bithiophene afforded a highly crystalline polymer that aggregated strongly in solution. Polymer films were well ordered resulting in high performance field-effect transistors with low onset voltages, negligible hysteresis, high channel current on/off ratios and peak hole mobilities of up to 0.5 cm2V-1s-1. Notably the transistors exhibited close to ideal behavior with extracted mobilities almost independent of gate of voltage.
Han Y, Barnes G, Lin Y-H, et al., 2016, Doping of large ionization potential indenopyrazine polymers via Lewis acid complexation with tris(pentafluorophenyl)borane: a simple method for improving the performance of organic thin-film transistors, Chemistry of Materials, Vol: 28, Pages: 8016-8024, ISSN: 1520-5002
Molecular doping, under certain circumstances, can be used to improve the charge transport in organic semiconductors through the introduction of excess charge carriers which can in turn negate unwanted trap states often present in organic semiconductors. Here, two Lewis basic indenopyrazine copolymers with large ionization potential (5.78 and 5.82 eV) are prepared to investigate the p-doping efficiency with the Lewis acid dopant, tris(pentafluorophenyl)borane, using organic thin-film transistors (OTFTs). The formation of Lewis acid–base complex between the polymer and dopant molecules is confirmed via optical spectroscopy and electrical field-effect measurements, with the latter revealing a dopant-concentration-dependent device performance. By adjusting the amount of p-dopant, the hole mobility can be increased up to 11-fold while the OTFTs’ threshold voltages are reduced. The work demonstrates an alternative doping mechanism other than the traditional charge transfer model, where the energy level matching principle can limit the option of dopants.
Mukhopadhyay T, Puttaraju B, Senanayak SP, et al., 2016, Air-Stable n-type Diketopyrrolopyrrole-Diketopyrrolopyrrole Oligomers for High Performance Ambipolar Organic Transistor, ACS Applied Materials & Interfaces, Vol: 8, Pages: 25415-25427, ISSN: 1944-8244
N-type organic semiconductors are prone to oxidation upon exposed to ambient conditions. Herein, we report design and synthesis of diketopyrrolopyrrole (DPP) based oligomers for ambipolar organic thin film transistors (OFETs) with excellent air and bias stability at ambient conditions. The cyclic voltammetry measurements reveal exceptional electrochemical stability during the redox cycle of oligomers. Structural properties including aggregation, crystallinity and morphology in thin film were investigated by UV-visible spectroscopy, atomic force microscopy (AFM), thin film X-ray diffraction (XRD) and grazing incidence small angle X-ray scattering (GISAXS) measurements. AFM reveals morphological changes induced by different processing conditions whereas GISAXS measurements show increase in the population of face-on oriented crystallites in films subjected to a combination of solvent and thermal treatments. These measurements also highlight the significance of chalcogen atom from sulphur to selenium on the photophysical, optical, electronic and solid-state properties of DPP-DPP oligomers. Charge carrier mobilities of the oligomers were investigated by fabricating top-gate bottom-contact (TG-BC) thin-film transistors by annealing the thin films under various conditions. Combined solvent and thermal annealing of DPP-DPP oligomer thin films results in consistent electron mobilities as high as ~0.2 cm2V-1s-1 with an on/off ratio exceeding 104. Field-effect behaviour was retained for up to ~ 4 week which illustrates remarkable air and bias stability. This work paves the way towards the development of n-type DPP-DPP based oligomers exhibiting retention of field effect behaviour with superior stability at ambient conditions.
Kryvchenkova O, Abdullah I, Macdonald JE, et al., 2016, Nondestructive Method for Mapping Metal Contact Diffusion in In2O3 Thin-Film Transistors, ACS Applied Materials & Interfaces, ISSN: 1944-8244
The channel width-to-length ratio is an important transistor parameter for integrated circuit design. Contact diffusion in to the channel during fabrication or operation alters the channel width and this important parameter. A novel methodology combining atomic force microscopy and scanning Kelvin probe microscopy (SKPM) with self-consistent modelling is developed for the non-destructive detection of contact diffusion on active devices. Scans of the surface potential are modelled using physically-based Technology Computer Aided Design (TCAD) simulations when the transistor terminals are grounded and under biased conditions. The simulations also incorporate the tip geometry to investigate its effect on the measurements due to electrostatic tip-sample interactions. The method is particularly useful for semiconductor- and metal- semiconductor interfaces where the potential contrast resulting from dopant diffusion is below that usually detectable with scanning probe microscopy.
Fei Z, Han Y, Martin J, et al., 2016, Conjugated Copolymers of Vinylene Flanked Naphthalene Diimide, Macromolecules, Vol: 49, Pages: 6384-6393, ISSN: 0024-9297
We report the synthesis of a novel naphthalene diimide (NDI) monomer containing two (tributylstannyl)vinyl groups. The utility of this building block is demonstrated by its copolymerization with five different electron-rich comonomers under Stille conditions. The resulting high molecular weight polymers show red-shifted optical absorptions in comparison to the analogous polymers without the vinylene spacer and a significant increase in the intensity of the low-energy intramolecular charge transfer band. The polymers all exhibit ambipolar behavior in bottom-gate, top-contact organic thin-film transistors. The insertion of a solution-processed barium hydroxide layer between the polymer and the gold electrode led to unipolar behavior with improved electron mobilities.
Semple J, Rossbauer S, Anthopoulos TD, 2016, Analysis of Schottky Contact Formation in Coplanar Au/ZnO/Al Nanogap Radio Frequency Diodes Processed from Solution at Low Temperature, ACS Applied Materials & Interfaces, Vol: 8, Pages: 23167-23174, ISSN: 1944-8244
Much work has been carried out in recent years in fabricating and studying the Schottky contact formed between various metals and the n-type wide bandgap semiconductor zinc oxide (ZnO). In spite of significant progress, reliable formation of such technologically interesting contacts remains a challenge. Here, we report on solution-processed ZnO Schottky diodes based on a coplanar Al/ZnO/Au nanogap architecture and study the nature of the rectifying contact formed at the ZnO/Au interface. Resultant diodes exhibit excellent operating characteristics, including low-operating voltages (±2.5 V) and exceptionally high current rectification ratios of >106 that can be independently tuned via scaling of the nanogap’s width. The barrier height for electron injection responsible for the rectifying behavior is studied using current–voltage–temperature and capacitance–voltage measurements (C–V) yielding values in the range of 0.54–0.89 eV. C–V measurements also show that electron traps present at the Au/ZnO interface appear to become less significant at higher frequencies, hence making the diodes particularly attractive for high-frequency applications. Finally, an alternative method for calculating the Richardson constant is presented yielding a value of 38.9 A cm–2 K–2, which is close to the theoretically predicted value of 32 A cm–2 K–2. The implications of the obtained results for the use of these coplanar Schottky diodes in radio frequency applications is discussed.
Bovo G, Bräunlich I, Caseri WR, et al., 2016, Room temperature dielectric bistability in solution-processed spin crossover polymer thin films, Journal of Materials Chemistry C, Vol: 4, Pages: 6240-6248, ISSN: 2050-7534
The spin crossover (SCO) phenomena are a remarkable example of spin-state switching at the molecular level. The low- and high-spin states can be reversibly selected through application of external stimulus-often simply a variation in temperature. Since the particular spin-state embodies optical, electronic and structural characteristics, the spin switching can be readily detected or probed using a variety of techniques. In this regard, SCO phenomena show great promise for a range of devices. The key to this uptake is the preparation of high-quality, thin-films capable of retaining SCO properties, and solution-based materials, in particular, provide further opportunities for integration or blending with other functional materials. The present work examines SCO behaviour from two iron(ii)-triazole polymers, with short and long side chains, prepared and investigated in a variety of formats-from bulk powders to thick and thin films. Magnetic, optical and electronic techniques all verify the SCO behaviour is faithfully maintained for all formats. The two materials serve to highlight the impact on key properties arising from the different density of Fe atoms. The results, all from solution-based materials, are extremely promising and clearly emphasise the growing capabilities and processing advances associated with SCO compounds.
Hunter S, Mottram AD, Anthopoulos TD, 2016, Temperature and composition-dependent density of states in organic small-molecule/polymer blend transistors, Journal of Applied Physics, Vol: 120, ISSN: 1089-7550
The density of trap states (DOS) in organic p-type transistors based on the small-molecule 2,8-difluoro-5,11-bis(triethylsilylethynyl) anthradithiophene (diF-TES ADT), the polymer poly(triarylamine) and blends thereof are investigated. The DOS in these devices are measured as a function of semiconductor composition and operating temperature. We show that increasing operating temperature causes a broadening of the DOS below 250 K. Characteristic trap depths of ∼15 meV are measured at 100 K, increasing to between 20 and 50 meV at room-temperature, dependent on the semiconductor composition. Semiconductor films with high concentrations of diF-TES ADT exhibit both a greater density of trap states as well as broader DOS distributions when measured at room-temperature. These results shed light on the underlying charge transport mechanisms in organic blend semiconductors and the apparent freezing-out of hole conduction through the polymer and mixed polymer/small molecule phases at temperatures below 225 K.
Bottacchi F, Bottacchi S, Späth F, et al., 2016, Nanoscale Charge Percolation Analysis in Polymer-Sorted (7,5) Single-Walled Carbon Nanotube Networks, Small, Vol: 12, Pages: 4211-4221, ISSN: 1613-6810
The current percolation in polymer-sorted semiconducting (7,5) single-walled carbon nanotube (SWNT) networks, processed from solution, is investigated using a combination of electrical field-effect measurements, atomic force microscopy (AFM), and conductive AFM (C-AFM) techniques. From AFM measurements, the nanotube length in the as-processed (7,5) SWNTs network is found to range from ≈100 to ≈1500 nm, with a SWNT surface density well above the percolation threshold and a maximum surface coverage ≈58%. Analysis of the field-effect charge transport measurements in the SWNT network using a 2D homogeneous random-network stick-percolation model yields an exponent coefficient for the transistors OFF currents of 16.3. This value is indicative of an almost ideal random network containing only a small concentration of metallic SWNTs. Complementary C-AFM measurements on the other hand enable visualization of current percolation pathways in the xy plane and reveal the isotropic nature of the as-spun (7,5) SWNT networks. This work demonstrates the tremendous potential of combining advanced scanning probe techniques with field-effect charge transport measurements for quantification of key network parameters including current percolation, metallic nanotubes content, surface coverage, and degree of SWNT alignment. Most importantly, the proposed approach is general and applicable to other nanoscale networks, including metallic nanowires as well as hybrid nanocomposites.
Paterson AF, Treat ND, Zhang W, et al., 2016, Small Molecule/Polymer Blend Organic Transistors with Hole Mobility Exceeding 13 cm V−1 s−1, Advanced Materials, Vol: 28, Pages: 7791-7798, ISSN: 1521-4095
A ternary organic semiconducting blend composed of a small-molecule, a conjugated polymer, and a molecular p-dopant is developed and used in solution-processed organic transistors with hole mobility exceeding 13 cm(2) V(-1) s(-1) (see the Figure). It is shown that key to this development is the incorporation of the p-dopant and the formation of a vertically phase-separated film microstructure.
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