Enhanced Charge Mobility Research Articles

Systematic Doping Control of CVD Graphene Transistors with Functionalized Aromatic Self‐Assembled Monolayers

Recent reports have shown that self‐assembled monolayers (SAMs) can induce doping effects in graphene transistors. However, a lack of understanding persists surrounding the quantitative relationship between SAM molecular design and its effects on graphene. In order to facilitate the fabrication of next‐generation graphene‐based devices it is important to reliably and predictably control the properties of graphene without negatively impacting its intrinsic high performance. In this study, SAMs with varying dipole magnitudes/directions are utilized and these values are directly correlated to changes in performance seen in graphene transistors. It is found that, by knowing the z‐component of the SAM dipole, one can reliably predict the shift in graphene charge neutrality point after taking into account the influence of the metal electrodes (which also play a role in doping graphene). This relationship is verified through density functional theory and comprehensive device studies utilizing atomic force microscopy, X‐ray photoelectron spectroscopy, Raman spectroscopy, and electrical characterization of graphene transistors. It is shown that properties of graphene transistors can be predictably controlled with SAMs when considering the total doping environment. Additionally, it is found that methylthio‐terminated SAMs strongly interact with graphene allowing for a cleaner graphene transfer and enhanced charge mobility.

Significant Enhancement of Polymer Solar Cell Performance via Side-Chain Engineering and Simple Solvent Treatment

Rational design and synthesis of polymeric semiconductors is critical to the development of polymer solar cells (PSCs). In this work, a new series of benzodithiophene–difuranylbenzooxadiazole-based donor–acceptor co-polymers—namely, PBDT-DFBO, PBDTT-DFBO, and PBDTF-DFBO, with various side groups—have been developed for bulk-heterojunction PSCs. These side-group substituents provide the opportunity to tailor the opto-electrical properties of the polymers. In addition, we show that the reduction of the bandgap of polymers and the enhancement of charge mobility in the devices can be accomplished concurrently by substituting the alkylthienyl side group with its furan counterpart. In the preliminary investigation, one could obtain PSCs with a power conversion efficiency (PCE) of 2.1% for PBDT-DFBO with an alkoxyl side chain, 2.2% for PBDTT-DFBO with an alkylthienyl side group, and 3.0% for PBDTF-DFBO with an alkylfuranyl side group. Further optimizing the performance of the devices was conducted via a simple s...

Faceted shape-drive cathode particles using mixed hydroxy-carbonate precursor for mesocarbon microbeads versus LiNi1/3Mn1/3Co1/3O2 Li-ion pouch cell

Using a mixed hydroxy-carbonate (MHC) precursor, homogeneous LiNi1/3Mn1/3Co1/3O2 phase synthesized through a facile route yielded well dispersed, faceted, edge-blunted polyhedral particles (∼200 nm). These cathode particles revealed high degree of hexagonal ordering favorable for impressive electrochemical performance viz., high capacity 187 mAh g−1 in Li vs LiNi1/3Mn1/3Co1/3O2 cell, not fortuitously. Rather, can be clearly attributed to edge-blunted cathode particles that may facilitate enhanced charge mobility mimicking a process akin to corona discharge predominantly in charge concentrated regions which contrasts to moderate capacity from spherical particles explainable under the ambit of shape-drive hither to unknown. Li-ion pouch cell (∼500 mAh) fabricated with this activity enhanced cathode quite rugged enough to endure loads (0.12 Wh, 1.1 Wh), as-demonstrated.

Mobility improvement of P3HT thin film by high-voltage electrostatic field-assisted crystallization

High-temperature annealing, assisted by the in-plane application of a high-voltage electric field, was used to form thin films of poly(3-hexylthiophene) (P3HT), boasting increased crystallite size and enhanced charge mobility. In-plane crystallite growth and surface morphology were characterized by atomic force microscopy. X-ray diffraction analyses showed that the application of an electric field decreased film thickness and increased the dimensions of the crystalline domains by 25.6% during subsequent high-temperature annealing. Crystalline domains were 13.4% larger, and exhibited 70% higher mobility, than those obtained with thermal annealing in the absence of an electric field.

Real-time dynamic hologram in photorefractive ferroelectric liquid crystal with two-beam coupling gain coefficient of over 800 cm–1 and response time of 8 ms

The photorefractive effect in photoconductive ferroelectric liquid crystals (FLCs) that contain photoconductive chiral compounds was investigated. Terthiophene compounds with chiral structure were chosen as the photoconductive chiral compounds and mixed with an achiral smectic C liquid crystal. The mixture exhibited the ferroelectric chiral smectic C phase and photoconductivity. The photorefractivity of the mixture was investigated by two-beam coupling experiment. It was found that the FLC containing the photoconductive chiral compound exhibits a large gain coefficient of over 800 cm−1 and a fast response time of 8 ms. The high photorefractive performance was considered to be due to enhanced charge mobility.

Novel zinc‐rich epoxy paint coatings with hydrated alumina and carbon nanotubes supported polypyrrole for corrosion protection of low carbon steel: Part I: Inhibitor particles and their dispersions

AbstractIn part 1 of this work, preparation, structure, spectroscopic, and electrochemical characteristics of the polypyrrole (PPy) deposited alumina/multi‐walled carbon nanotubes (MWCNTs) inhibitor particles (PDAMIPs) are presented. TEM observations evidenced uniform deposition of thin PPy film on the functionalized nanotubes, whereas co‐deposition of PPy and polystyrene sulfonic acid (PSS) lead to thick polymer coverage on hydrated alumina. Modification with polymer complexes resulted in moderately dispersed PDAMIPs, which is due to the various degrees of aggregation and coalescence. FTIR revealed compact and dense PPy structure on the functionalized MWCNTs while it was not the case on alumina and the PSS modified nanotubes. Closer interaction of PPy with the MWCNTs resulted in enhanced charge mobility, whereas greater electroactivity and reversibility of PPy were noted to samples containing functionalized nanotubes and low amount of PSS. Rheological study verified moderate micron‐scale dispersity and the modification caused various degrees of aggregations of the PDAMIPs. These were recognized to be valid in the suspensions at a solid phase concentration with component contents similar to the corrosion tested hybrid coatings. Rheological percolation of the nanotubes (with anisotropic factor of ∼100) was confirmed at volume fractions of 3.30 × 10−3 and 6.0 × 10−4 which were under the dilute/semi‐dilute boundary type theoretical and experimental thresholds. This is related to the extensive interconnection of the nanotube‐supported filaments. Thus, overlapping of the nanotubes should contribute to the electrical percolation thereby galvanic corrosion prevention function of the zinc‐rich hybrid coatings, which is discussed in the 2nd part.

Strengthening π–π Interactions While Suppressing Csp2–H···π (T-Shaped) Interactions via Perfluoroalkylation: A Crystallographic and Computational Study That Supports the Beneficial Formation of 1-D π–π Stacked Aromatic Materials

The design and synthesis of aromatic crystalline materials with controllable crystal structure packing is of particular interest in organic semiconductor and optoelectronic devices, where 1-D π–π stacked structures that enhance charge mobility are the most beneficial. We report here that the π–π interactions between aromatic molecules can be strengthened and the Csp2–H···π (T–shape) interaction can be suppressed by perfluoroalkylation of corresponding aromatics. Both crystal structure data and ab initio calculations show that the π–π interaction is strengthened due to the electronic effects of perfluoroalkyl substituents, and the Csp2–H···π interaction is suppressed by the steric effects of the perfluoroalkyl substituents. The Csp3–F···F–Csp3 attractive interactions between perfluoroalkyl chains further stabilize the crystal structures. We also found that Csp3–F···π interaction can be eliminated if an optimal electron deficiency of the π system is tuned by adjusting the number of perfluoroalkyl substituen...

A ZnO/N-doped carbon nanotube nanocomposite charge transport layer for high performance optoelectronics

Metal oxide charge transport layers are widely used to promote the interfacial charge transport of organic optoelectronics. Nevertheless, frequently used wide-bandgap metal oxides with low electrical conductivities reveal inherent limitations in the charge transport enhancement. We present the remarkable electro-conductivity enhancement of solution processable ZnO charge transport layers upon dispersing a tiny amount (less than 0.1 wt%) of chemically doped CNTs and the corresponding device performance improvement of light-emitting diodes (OLEDs). Using various undoped or doped CNTs, whose work function was systematically tuned by substitutional doping of electron deficient B or electron rich N,N-doped CNT (N-CNT), the composite showed a lowered work function matching well with the conduction band of ZnO. Consequently, the ZnO/N-CNT nanocomposite transport layer with 0.08 wt% N-CNT showed a five-fold enhancement of electron mobility, while maintaining the intrinsic bandgap energy levels, optical transparency and solution processability of pure ZnO. The inverted OLEDs employing ZnO/N-CNT nanocomposite electron transport layers could facilitate well-balanced electron–hole injection and, thus, more than two-fold enhancement of maximum luminance (from 21 000 cd m−2 at 14.6 V to 46 100 cd m−2 at 14.0 V) and efficiency (from 6.9 cd A−1 at 13.4 V to 14.3 cd A−1 at 13.6 V). This highly effective charge mobility enhancement enabled by work function tunable, chemically doped CNTs would be beneficial for various organic and inorganic charge transport materials with different energy levels.

Optical and electrical characterization of (PEO+methyl violet) polymer electrolytes

AbstractOptical properties and electrical conductivity of polyethylene oxide (PEO) with methyl violet dopant film have studied. The complexation of the methyl violet dopant with PEO was confirmed by X‐ray diffraction (XRD) and Fourier transform infrared (FTIR) spectroscopic studies. The microstructure morphology have been analyzed by scanning electron microscope (SEM) for pure and dopant films. The UV‐absorption studies were made in the wavelength range 190–1100 nm for pure and doped films. The dc electrical conductivity data was collected using two probe technique in the temperature range 303–333 K. The UV–visible spectra showed the absorption band at 190 nm for pure PEO and doped from 208–224 nm region with different absorption intensities. The absorption edge, direct and indirect band gap were estimated using Mott and Davis Model. The optical activation energy can be determined using the Urbach rule, for pure PEO it was found 2.38 eV and 1.28–4.08 eV for doped films. The absorption band was shifted toward the higher frequency, the direct and indirect band gap decreases with increasing of dopant concentration, corresponds to the allowed inter band transition of electron. The dc electrical conductivity results shows that it increases with increasing dopant weight percentage and temperature which corresponds to the enhancement of charge mobility in these dye doped polymers. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012

Influence of Indium Doping on the Activity of Gallium Oxynitride for Water Splitting under Visible Light Irradiation

Visible-light-active indium-doped gallium oxynitrides with a wurtzite-like structure were synthesized from nitridation of In(OH)3-containing Ga(OH)3 under NH3 flow at 625 °C and used for photocatalytic water splitting. This synthesis method yielded a homogeneous In distribution in gallium oxynitride solid solutions for Ga replacement levels of up to 1%. An appropriate amount of In substitution for Ga, approximately 0.5%, significantly enhanced the activity of gallium oxynitride in the visible-light-induced evolutions of H2 and O2 gases from methanol and AgNO3 solutions, respectively. X-ray photoelectron spectroscopic analysis indicated that In doping increased the dispersion of hybridized orbitals in the valence band of gallium oxynitride. This can enhance charge mobility in gallium oxynitride and thus photocatalytic activity. A higher degree of In doping resulted in nucleation of InN-like oxynitride on the gallium oxynitride surface and degraded the photocatalytic activity. This study demonstrates that band structure engineering of gallium oxynitride powders with In doping is a facile procedure for obtaining visible-light sensitive photocatalysts with high activities.

Enhanced Charge Mobility in Polymer Nanocomposites Incorporating Donor–Acceptor Interfaces

Enhanced Charge Mobility in Polymer Nanocomposites Incorporating Donor–Acceptor Interfaces

Enhanced Charge Mobility in Polymer Nanocomposites Incorporating Donor–Acceptor Interfaces

Charge carrier transport in donor–acceptor (D–A) composites based on either poly(N-vinyl carbazole) or polyimide derivative incorporating either carbon single-walled nanotubes or nanocrystals of J-aggregated cyanine dyes is shown to exhibit a similar behavior. In the composite films, polymer/nanomaterial interface provides pathways of the high conductivity. Charge–transfer states (CTS) formed at the D–A interface are involved in the transport. The charge transport along the interface is suggested to arise due to the D–A integer charge transfer and strong interaction between adjacent opposite charges located on the donor and acceptor molecules. The approach based on the concept of sequence of charge carrier transfers through charge transfer states describes the increased electron and hole mobility in the composites. The approach predicts enhanced conductivity with reduced activation energy. Moreover, once the density of electron–hole pairs at the interface is rather high, significant part of the charge carriers can avoid hopping transport resulting in conductivity of metal type. The value of two-dimensional conductivity is estimated by numerical modeling.

Diethynylbenzene-Based Liquid Crystalline Semiconductor for Solution-Processable Organic Thin-Film Transistors

We report here the synthesis and characterization of novel diethynylbenzene-based liquid crystalline semiconductor (P1) for organic thin-film transistors (OTFTs). Compound P1 was synthesized by the Sonogashira coupling reaction between 2-bromo-5-(4-hexylthiophen-2-yl)thieno[3,2-b]thiophene and 1,4-bis(dodecyloxy)-2,5-diethynylbenzene. Top contact OTFTs were fabricated by spin casting with 2 wt% solution of P1 in chloroform and their best performance, which exhibited a hole mobility of 4.5 x 10(-5) cm2/Vs, was showed after annealing of the films at liquid crystalline temperature. Time-of-flight (TOF) mobility measured at liquid crystalline phase was observed to be 1.5 x 10(-6) cm2/Vs for both positive and negative carriers. These results indicate that the liquid crystallinity helps to improve the molecular packing and enhance charge mobility for P1. These advantages can be applicable to design and construct solution-processable OTFT materials for electronic applications.

Electrospinning of Poly(alkoxyphenylenevinylene) and Methanofullerene Nanofiber Blends

Poly(p-phenylenevinylene) (PPV) derivatives have long been studied because of their attractive opto- and electroluminescent properties and have potential applications for devices such as light-emitting diodes and photovoltaics. The ability to induce alignment of these PPV derivatives may lead to the enhancement of charge mobility and their efficiency. In this study, uniform nanofibers of poly[2,5-(2'-ethylhexyloxy)]-1,4-phenylenevinylene (BEH-PPV) have been fabricated through the method of electrospinning, and an induced alignment of the polymer fibers was observed through photoluminescence data. This study also focuses on the doping of these fibers with the fullerene derivative, 1-(3-methoxycarbonyl)-propyl-1-phenyl-(6,6)-C(61) (PCBM), to induce more incidence of donor/acceptor junctions. Composite fibers with up to a 1:2 weight ratio of PCBM/BEH-PPV have been fabricated and exhibited an ability to quench the photoluminescence of BEH-PPV, indicative of charge transfer.

Carbon nanotube-incorporated direct-patternable SnO 2 thin films formed by photochemical metal-organic deposition

The electrical property of SnO 2 thin film was improved by the incorporation of multi-wall carbon nanotubes (MWNTs). MWNTs were incorporated into SnO 2 photosensitive precursor solution and the direct-patternable film was prepared by photochemical metal-organic deposition. The incorporation of MWNTs into SnO 2 films slightly reduced the transmittance of SnO 2 films due to light scattering by the incorporated MWNTs. In addition, with incorporation of MWNTs, the crystallinity of the SnO 2 hybrid films decreased slightly and the film sheet resistance improved due to enhancement of charge mobility because of the π-bonding nature of MWNTs. Direct-patterning of SnO 2 hybrid films containing MWNTs was performed without photoresist or dry etching. These results suggest that a micro-patterned system can be simply fabricated at a low cost and the electrical properties of SnO 2 films can be improved by incorporating MWNTs.

Thermal treatment under reverse bias: Effective tool for polymer/fullerene bulk heterojunction solar cells

The effect of post-thermal treatment under reverse bias on the performance of bulk heterojunction photovoltaic cells based on poly[2-methoxy-5-(20-ethylhexyloxy)- p-phenylene vinylene] (MEH-PPV) and fullerene (C60) composites is investigated. The experimental results show that treated devices deliver increased short-circuit current density ( J sc) and conversion efficiency ( η p) values. Especially, for the device annealed under the bias of −6 V, the short-circuit current density ( J sc) and the conversion efficiency ( η p) are enhanced by a factor of 6 and 5.4, respectively. It is concluded that the enhancement of charge mobility induced by polymer chains modified orientation along the direction of electric field leads to a significant increase in the photovoltaic parameters.

Incorporation of carbon nanotube into direct-patternable ZnO thin film formed by photochemical solution deposition

Abstract Direct-patterning of ZnO hybrid films containing MWNT was realized without using photoresist and dry etching. Photosensitive 2-nitrobenzaldehyde was introduced into the solution precursors as a stabilizer and contributed to form a cross-linked network structure during photochemical reaction. According to the incorporation of multi-walled nanotube (MWNT) into ZnO films, the transmittance of ZnO hybrid film containing MWNT did not change but the sheet resistance was improved due to the enhancement of charge mobility due to π-bonding nature of MWNT. These results suggested a possibility that a micro-patterned system can be fabricated relatively easily and without high-cost processes, for example, by conventional etching procedure.

Effective hole transport layer structure for top-emitting organic light emitting devices based on laser transfer patterning

The design and preparation of top-emitting organic light emitting devices with thick hole transport layers that effectively smooth the substrate were performed. Bilayer transfer with laser thermal patterning method was used to simplify the fabrication of hole transport layers with various thicknesses, with the aim of optimizing the cavity effect in the top-emitting devices. By carrying out optical simulations and experiments, the optimal thicknesses of the hole transport layers for our structure of blue, green and red devices were found to be 140 nm, 160 nm and 230 nm, respectively. We have briefly illustrated the underlying device physics of top-emission in terms of the resonance and microcavity effect. The resulting top-emitting devices with thick hole transport layers exhibit better colour purity than bottom-emitting devices and satisfactory luminous efficiency. The relatively high operating voltages can be further improved by the use of transport materials with enhanced charge mobility.

Enhanced charge mobility in a molecular hole transporter via addition of redox inactive ionic dopant: Implication to dye-sensitized solar cells

Upon the addition of lithium salts to the hole-transporter matrix, 2,2′,7,7′-tetrakis(N,N-di-p-methoxypheny-amine)-9,9′-spirobifluorene (spiro-MeOTAD), the authors observe a 100-fold increase in conductivity through spiro-MeOTAD within a TiO2 mesoporous network. The authors demonstrate this to be a bulk effect and not due to improved injection at the electrodes. By testing “hole-only” diodes of pure spiro-MeOTAD and those doped with lithium salts, the authors calculate that the hole mobility increases from 1.6×10−4to1.6×10−3cm2∕Vs. The authors discuss the possible mechanisms for this significant enhancement in charge mobility and its implication to the dye-sensitized solar cell operation.

Morphological aspects and thermal behaviour of tri- p-tolylamine (TTA)-based model charge transport composites: A comparison with TPD

The morphology and thermal behaviour of model charge transport composites based on the hole transport molecule tri- p-tolylamine (TTA) in bisphenol-A polycarbonate (BPAPC), cyclohexyl polycarbonate (PCZ) and polystyrene (PS) were investigated. A very large decrease in the glass transition temperatures ( T g) of the host polymers is seen with the dispersion of TTA in the polymer matrix. The T g reduces almost close to ambient temperature. This is much more pronounced than in the case of TPD (a tetraphenyl diamine) based composites [F. Khan, A.-M. Hor, P.R. Sundararajan, J. Phys. Chem. B 108 (2004) 117–126]. It is found that the extent of T g depression is higher when the polymer is conformationally less flexible. FTIR study indicates a molecular level of interaction between TTA and the polymers used here. Upon annealing, phase separation and crystallization of TTA occurs at temperatures as low as 80 °C, and leads to a recovery of the T g of the polymer. The T g recovery is again much more significant than in the case of TPD. Sublimation of TTA occurs when the polycarbonate based composites are annealed at ≥120 °C. Re-plasticization upon annealing as suggested in the case of TPD/BPAPC is not observed with TTA/BPAPC. Due to such sublimation and the accompanying depletion of TTA, a photoreceptor, annealed at 140 °C failed to discharge. With polystyrene as the binder, sub-micron crystals of TTA are seen even with a low concentration of 30%. Chaining of such crystals is seen with a concentration of 50%, which could be a contributing factor for the enhanced charge mobility noted for polystyrene based charge transport composites.