High Charge Transport Research Articles

Enhanced Thermoelectric Performance of Carbon Nanotubes/Polyaniline Composites by Multiple Interface Engineering.

Here, we put forward an effective strategy to regulate the interface structure of carbon nanotubes/polyaniline (CNTs/PANI) composite films and improve their thermoelectric (TE) properties by sequential dedoping-redoping treatment. Dedoping induces conductive resistance-undoped PANI to enhance the energy barrier between CNTs and PANI, leading to a greatly increased Seebeck coefficient and deteriorated conductivity. Subsequently, upon the redoping process, the electrical conductivity is dramatically improved owing to the generated conductive PANI chains, while Seebeck coefficient is maintained at 90% of the dedoped composites. This yields a significantly improved power factor of 407 μW m-1 K-2 from the as-prepared composites (234 μW m-1 K-2), which is the highest value among those of all the reported CNTs/PANI composites. The outstanding TE performanceis probably ascribed to the multiple interface structure of the PANI composite generated from incomplete dedoping and redoping processes, contributing to the enhanced carrier-filtering effect to retain a relatively high Seebeck coefficient and efficient charge transport to improve conductivity. Furthermore, the flexible TE device generates a high power of 1.5 μW at ΔT = 50 K, demonstrating the applicability of this composite for energy-harvesting electronic devices.

Open-Shell and Closed-Shell Quinoid-Aromatic Conjugated Polymers: Unusual Spin Magnetic and High Charge Transport Properties.

While quinoidal moieties are considered as emerging platforms showing efficient charge transport and interesting open-shell diradical characteristics, whether these properties could be changed by extension to the conjugated polymer structure remains as a fundamental question. Here, we developed and characterized two conjugated polymers incorporating quinoids with different lengths, which have a stable close- and open-shell diradical character, respectively, namely, poly(quinoidal thiophene-thienylene vinylene) (PQuT-TV) and poly(quinoidal bithiophene-thienylene vinylene) (PQuBT-TV). A longer length of a quinoidal core led to enhanced diradical characteristics. Therefore, the longer core length of QuBT was favorable for the formation of an open-shell diradical structure in its monomer and in the quinoidal polymer. PQuBT-TV exhibited high spin characteristics observed by the strong ESR signal, a low band gap, and improved electrochemical stability. On the other hand, as QuT maintained a closed-shell quinoid structure, PQuT-TV exhibited high backbone coplanarity and strong intermolecular interaction, which was beneficial for charge transport and led to high hole mobility (up to 2.40 cm2 V-1 s-1) in organic field-effect transistors. This work successfully demonstrated how the control of the closed/open-shell character of quinoidal building blocks changes charge transport and spin properties of quinoidal conjugated polymers via quinoid-aromatic interconversion.

Effect of electrolyte for back contact transparent conducting oxide-less dye-sensitized solar cells: iodine versus cobalt

Gratzel solar cells are reported in a transparent conducting oxide-less (TCO-less) back-contact dye-sensitized solar cell (BC-DSC) architecture using a stainless steel mesh-protected working electrode along with nanoporous TiO2 semiconductor and metal-free D205 dye. Liquid electrolytes play a significant role for the dye regeneration in the working operation of TCO-less BC-DSCs; therefore, we report the effectiveness of two different commonly utilized electrolytes (iodine- and cobalt-based redox shuttles) for the construction and performance of TCO-less dye-sensitized solar cells (DSCs). Differential performance of DSCs thus fabricated was interpreted utilizing impedance spectral and lifetime analysis. It was found that although utilization of cobalt bipyridyl complex-based electrolyte was able to harvest higher photons in the lower wavelength region (330 to 430 nm) as compared to its iodine electrolyte counterparts, hampered dye regeneration due to reduced driving force and slower ion diffusion in combination with higher charge transport resistance at TiO2 / dye / electrolyte was responsible for relatively hampered photovoltaic performance at peak absorption.

Reduction-induced surface reconstruction to fabricate cobalt hydroxide/molybdenum oxide hybrid nanosheets for promoted oxygen evolution reaction

Electrocatalysts for oxygen evolution reaction (OER) have received substantial interests owing to its critical role in boosting water electrolysis for hydrogen production, and designing highly efficient and durable OER catalysts based on earth-abundant elements is of great importance in replacing noble metal-based catalysts. Herein, a facile reduction-induced surface reconstruction process was proposed for fabricating cobalt hydroxide/molybdenum oxide hybrid nanosheets with high porosity, rough surface and facile charge transport behavior. Benefitted from the structural merits, the highly porous α-Co(OH)2/Mo9O26 hybrid nanosheets exhibit an ultralow overpotential of 279 mV for achieving a 10 mA cm−2 current density and a high current density of 225.2 mA cm−2 at 1.8 V vs. RHE. In addition, a remarkable normalized current density of 11.9 A F-1 can be identified at η = 400 mV, which is much larger than previously reported Co-based catalysts and even benchmarking RuO2 and IrO2. Moreover, for CoMoO-r displays stable OER activity towards long-term operation with no performance degradation but obvious enhancement even for 72 h. The high OER activity combined with the excellent stability makes the hybrid catalyst a promising candidate for commercial water splitting.

A charge-separated interfacial hole transport semiconductor for efficient and stable perovskite solar cells

Abstract Perovskite solar cells (PSCs) with high efficiency and high stability are still a challenge to produce although remarkable successes have been achieved since they were first reported in 2009. One strategy to effectively improve both the performance as well as the stability is to introduce an interfacial layer between perovskite and hole transport material. Herein, we report a charge-separated (CS) organic semiconductor as the interfacial layer that forms cascaded energy levels between perovskite and hole transportation material. This CS semiconductor displays high hole and electron mobilities by converting long-lived CS states in solution into permanent polarons (charged carriers) in films. Doping with iodinehydride is able to improve the surface morphology of the CS semiconductor layer. Our devices with an iodinehydride-doped CS semiconductor layer exhibit an efficiency of 17.87%, which is increased by ~25% in comparison with 14.24% of the reference devices that have no interfacial layer. This additional CS semiconductor layer also enhances the unsealed device stability by maintaining 90% of initial PCE, while the reference devices degraded by 35% at a relative humidity of 20–30%, temperature of 25 °C and ambient light for 240 h. This result reveals that the utilization of CS states is an alternative approach to construct high charge transport organic semiconductors. An interfacial semiconductor with proper energy level and a matching hole transport mobility can improve the hole extraction, speed up hole transport and suppress charge recombination of PSCs, and thus may be an effective strategy to improve their efficiency and stability.

Impedance analysis of charge transfer upon nickel doping in Tio2-based flexible dye-sensitized solar cell

Flexible dye-sensitized solar cells (FDSSCs) have promoted interest in plastic industries as they are lightweight, flexible, and mechanically robust to accelerate production and reduce cost. However, the plastic substrates have temperature limitations in producing the TiO2 photoanode and degrade the performance of FDSSC. The main reason for this degradation is the low charge transfer in the photoanode layer. Although there is plenty of research on low-temperature fabrication methods, they indirectly increase the operational cost. Therefore, a new approach is necessary for charge transfer improvement without affecting the temperature in a low-cost platform. In this study, we present a photoanode that improves the charge transfer by doping nickel (Ni) in the TiO2 layer. A low amount of Ni doping (15%) exhibited Rct >> Rt, indicating a high charge transport and low electron recombination rate (120.84 s−1). On the other hand, higher amount Ni doping (>> 45%) has Rct << Rt which deteriorates the performance of the cell by causing severe agglomeration issues, indicating a high electron recombination rate (369.75 s−1). Moreover, the high charge transfer in (TiO2)85-Ni15-based FDSSC facilitates the electron lifetime of the cell up to 8.28 ms. Therefore, an optimum doping of Ni in TiO2-based FDSSC is studied in this work.

Adsorption-photocatalytic degradation and kinetic of sodium isobutyl xanthate using the nitrogen and cerium co-doping TiO2-coated activated carbon

At present, the excessive use of sodium isobutyl xanthate (SIBX) in mineral processing has caused serious environmental problems, drawing ever-growing concern in China. A nitrogen and cerium co-doped TiO2-coated activated carbon (Ce/N–TiO2@AC) heterojunction were prepared through the sol-gel method to address these problems. The photocatalyst was characterized using XRD, TEM, SEM-EDS, PL, UV–Vis, XPS and a series of photoelectrochemical techniques. The results show that Ce/N–TiO2@AC photocatalyst possess a stable anatase phase, narrow band gap energy (2.24–2.61 eV) and high charge transport process. The photocatalytic activity of the photocatalyst was evaluated based on photodegradation kinetic studies of SIBX in aqueous solution, and it is found that it followed the Langmuir-Hinshelwood model very well. The Ce/N–TiO2@AC photocatalyst with 2% Ce appears to be the highest removal rate with 96.3% of SIBX and an apparent rate constant of 78.4 × 10−3 min−1. The reusability experiment for its potential applications was studied, and the removal rate of SIBX reached 95.8% after the fifth cycle. Besides, the proposed mechanism and degradation routes of SIBX were systematically studied, and certificate the concentration of SO2-4 ions in the final water sample was 95.9 mg/L, which was basically consistent with the theoretical value.

Nickel cobalt oxide nanowires with iron incorporation realizing a promising electrocatalytic oxygen evolution reaction

Developing cost-effective electrocatalysts for water electrolysis is a promising strategy to enhance conversion and storage efficiency of sustainable energy. Transition metal oxides have been considered as alternative oxygen evolution reaction (OER) catalysts to replace noble metal-based catalysts. Here, we report a series of Fe-doped NiCo2O4 (NCO) nanowires with different Fe-doped concentrations, synthesized by a facile solvothermal and calcinations process, as high-efficiency electrocatalysts for OER. Due to abundant catalytically active sites, high-charge transport capability and specific surface area, these as-obtained NCO nanowires exhibit low overpotential and small Tafel slope. Specifically, NCO-0.1 shows the outstanding OER performance with a low overpotential of 297 mV at a current density of 10 mA cm−2 and a small Tafel slope of about 68 mV dec−1 in 1.0 M KOH. This study offers a promising electrocatalyst for the OER in water splitting.

Naphthalene diimide based near-infrared luminogens with aggregation-induced emission characteristics for biological imaging and high mobility ambipolar transistors

Organic conjugated materials combining high conductivity with strong solid-state emission are highly desired for organic electronic applications, yet still rather rare. Herein, a novel luminogen (TEN) comprised by linking naphthalene diimides and tri-phenyl ethylene with vinyl bridges is reported. TEN exhibits aggregation-induced emission (AIE) behavior of a strong near-infrared fluorescence over 700 nm and the efficiency above 60.5% in the solid state, while also shows promising application in vivo bio-imaging with good permeability and extremely low background. Single crystal of TEN reveals intra- and inter-molecular C–H···O hydrogen bonds, contributing to an inclined molecular stacking along the a -axis of the cell, creating a 1D charge carrier transporting channel under a short π-π interaction distance of 3.42 A, which might benefit the solid emission and charge transport ability simultaneously. Solution processed bottom contact, top gate organic field effect transistors based on TEN reveal a high ambipolar charge transport ability with the hole mobility up to 0.13 cm2 V−1 s−1 and electron mobility up to 0.010 cm2 V−1 s−1. Further atomic force microscopy and X-ray diffraction analysis on TEN thin film confirm the existence of the 1D π-π stacking channel, suggesting the stacking geometry revealed in crystal crucial for facilitating high charge carrier mobility while preserving the strong solid emission at the same time.

Fine-tuning of charge transport properties of porphyrin donors for organic solar cell

To address the enhancing request of appropriate photovoltaic materials for application in renewable energy resources, an attempt has been made herein to design and investigate the novel porphyrin-based donors with higher transport properties. The interesting furan-linked porphyrin donor (FPD) and thiophene-linked porphyrin donor (TPD) containing Zn metal atom have been previously synthesized and applied into the organic solar cell. Inspired by this fascinating finding, we suggest a new set of porphyrin-based donors with high charge transport properties via designing molecular scaffolds with different numbers of coordinated nitrogen atoms (FPD-Nx and TPD-Nx, x = 0–4) and various types of metal atoms (FPD-M and TPD-M, M = Fe, Co, Ni, Cu, and Zn). Then, the structural, electronic, optical, and charge transport properties of the designed donors have been studied and compared with the available experimental results. Our calculations reveal that the FPD-N2, TPD-N2, FPD-Co, and TPD-Co porphyrins possess the planar structures, proper energy levels in reference to the PCBM acceptor, high open-circuit voltage (VOC), and excellent charge transport properties which make them ideal donors that are used in organic solar cells. Accordingly, our predicted donors represented the improvement in the structural and transport properties which may lead to organic solar cells with high power conversion efficiency. Consequently, this approach can be useful for further improving the efficiency of porphyrin donors in organic solar cells.

Tuning Biradical Character to Enable High and Balanced Ambipolar Charge Transport in a Quinoidal π-System.

Herein, two bis(dicyanomethylene)-substituted quinoidal molecules QBDT and QTBDT-3H were designed and synthesized to explore the open-shell effect on tuning the charge transport behavior of organic π-functional materials. The biradical character of QTBDT-3H was confirmed by DFT calculation, variable-temperature NMR, electron spin resonance (ESR), and superconducting quantum-interfering device (SQUID). The open-shell character enables QTBDT-3H an ambipolar characteristic under ambient conditions with highly balanced electron and hole mobilities of 0.32 and 0.16 cm2 V-1 s-1, respectively.

Electrode Design for High-Performance Sodium-Ion Batteries: Coupling Nanorod-Assembled Na3V2(PO4)3@C Microspheres with a 3D Conductive Charge Transport Network.

As one of the most promising cathodes for sodium-ion batteries, the polyanionic compounds still suffer from unsatisfactory capacity and rate performance resulting from poor electron conductivity. Furthermore, the charge-transfer kinetics, especially for Na+, becomes limiting as the mass loading increases. Herein, a robust free-standing electrode coupling optimal porous Na3V2(PO4)3@C microspheres with a bicontinuous charge transport network is designed and prepared by a simple casting method. In the design, the optimal porous carbon-coated microspheres, composed of some continuous nanorods, along with interwoven carbon nanofiber networks offer efficient electron transport and facile ion diffusion. Such an elaborate design enables impressive electron/ion conductivity, contributing to remarkable rate performance (116.1 mA h g-1 at 0.2 C; 96 mA h g-1 at 30 C) and outstanding cycling stability (90% capacity retention in 500 cycles at 1 C; 80% capacity retention in 5000 cycles at 10 C), which has surpassed other similar Na3V2(PO4)3-based free-standing electrodes as reported. More importantly, when mass loading extends to 8 mg cm-2, an excellent capacity retention of 75% at 10 C can be obtained. The research offers a new avenue into the rational design of porous microspheres electrode with high conductive charge transport network, indicating its superiority in practical applications.

Enhanced Vertical Charge Transport of Homo- and Blended Semiconducting Polymers by Nanoconfinement.

The morphology of conjugated polymers has critical influences on electronic and optical properties of optoelectronic devices. Even though lots of techniques and methods are suggested to control the morphology of polymers, very few studies have been performed inducing high charge transport along out-of-plane direction. In this study, the self-assembly of homo- and blended conjugated polymers which are confined in nanostructures is utilized. The resulting structures lead to high charge mobility along vertical direction for both homo- and blended conjugated polymers. Both semicrystalline and amorphous polymers show highly increased population of face-on crystallite despite intrinsic crystallinity of polymers. They result in more than two orders of magnitude enhanced charge mobility along vertical direction revealed by nanoscale conductive scanning force microscopy and macroscale IV characteristic measurements. Moreover, blends of semicrystalline and amorphous polymers, which are known to show inferior optical and electrical properties due to their structural incompatibility, are formed into harmonious states by this approach. Assembly of blends of semicrystalline and amorphous polymers under nanoconfinement shows charge mobility in out-of-plane direction of 0.73 cm2 V-1 s-1 with wide range of absorption wavelength from 300 to 750 nm demonstrating the synergistic effects of two different polymers.

High-performance tungsten carbide electrocatalysts for the hydrogen evolution reaction

High charge transport efficiency and a large specific surface are two critical factors for the hydrogen evolution reaction.

High Hole Mobility and Efficient Ambipolar Charge Transport in Heterocoronene-Based Ordered Columnar Discotics.

Heterocoronene, a new redox-active core fragment, is utilized for the synthesis of room-temperature columnar discotic liquid crystals (DLCs). Three wedge-shaped side chains having different lengths of alkyl tails are introduced at the periphery of the heterocoronene core to prepare three kinds of discotic molecules, 1 (R = C10H21), 2 (R = C12H25), and 3 (R = C14H29). X-ray diffraction (XRD) analysis confirmed the packing variation in the columnar lattices regulated by alkyl chains of discrete length and steric bulk. When used in space charge limited current devices, compound 1 exhibits a high hole mobility value of 8.84 cm2/V s at ambient temperature, whereas compounds 2 and 3 show efficient ambipolar charge transport behavior with maximum hole (μh) and electron (μe) mobilities of 0.70 and 3.59 cm2/V s, respectively, for compound 3. The mobility values (μh = 8.84 cm2/V s for 1 and μe = 3.59 cm2/V s for 3) are remarkable and the highest ever disclosed for any DLC-based organic semiconductor, promising to deliver a good balance between mobility and processability in devices. The grazing incidence small- and wide-angle X-ray scattering experiments are employed to quantify the extent of alignment in the film state, which correlates with the observed trend of mobility values.

Hydrothermally Synthesized CuCo2S4 Nanosheets as an Easily Accessible and Convenient Heterogeneous Catalyst for the Sonogashira Cross-Coupling Reactions

A high surface area and the possibility of high charge separation and transportability make the two-dimensional (2D) nanomaterials very special in many applications, particularly in the field of catalysis. The usefulness of 2D nanomaterial is positively demonstrated in this study by employing it as an efficient heterogeneous catalyst in Sonogashira cross-coupling reaction, where the later one is well known for the formation of carbon–carbon bonds. 2D nanosheets of CuCo2S4 are synthesized using a hydrothermal synthesis and the average thickness of the sheets is found to be in the range of 10−15 nm. The transparent nanosheets are interconnected to form thick sheets and then aggregated to form a highly open three-dimensional hierarchical structure. This study explores the Sonogashira reaction very efficiently in water/ethanol at room temperature using hydrothermally synthesized CuCo2S4 nanosheets as catalyst for the C-C coupling reaction for the first time. The developed method is found to be economic, green, high yields, low catalyst loading and reusability of the catalysts. This present CuCo2S4 nanosheet heterogeneous catalyst afford the Sonogashira coupling of various aryl halides with terminal alkynes, giving a variety of aryl acetylenes in excellent yields and can be recovered by a simple filtration and reused for at least ten consecutive reactions with almost constant yield and activity.

Mixed Lead Halide Passivation of Quantum Dots.

Infrared-absorbing colloidal quantum dots (IR CQDs) are materials of interest in tandem solar cells to augment perovskite and cSi photovoltaics (PV). Today's best IR CQD solar cells rely on the use of passivation strategies based on lead iodide; however, these fail to passivate the entire surface of IR CQDs. Lead chloride passivated CQDs show improved passivation, but worse charge transport. Lead bromide passivated CQDs have higher charge mobilities, but worse passivation. Here a mixed lead-halide (MPbX) ligand exchange is introduced that enables thorough surface passivation without compromising transport. MPbX-PbS CQDs exhibit properties that exceed the best features of single lead-halide PbS CQDs: they show improved passivation (43 ± 5 meV vs 44 ± 4 meV in Stokes shift) together with higher charge transport (4 × 10-2 ± 3 × 10-3 cm2 V-1 s-1 vs 3 × 10-2 ± 3 × 10-3 cm2 V-1 s-1 in mobility). This translates into PV devices having a record IR open-circuit voltage (IR Voc ) of 0.46 ± 0.01 V while simultaneously having an external quantum efficiency of 81 ± 1%. They provide a 1.7× improvement in the power conversion efficiency of IR photons (>1.1 µm) relative to the single lead-halide controls reported herein.

Ultrathin Co3O4 nanosheets anchored on multi-heteroatom doped porous carbon derived from biowaste for high performance solid-state supercapacitors

Low-cost multi-heteroatom doped porous carbons derived from agricultural waste were prepared as the base material. Then, the cobalt oxides nanosheets were anchored to the surface of the carbon material to obtain the composites. It was found that the adoption of porous carbon is of great significance for the improvement of materials properties. The presence of porous structure derived from carbon materials not only increases the stability of the material, but also facilitates the diffusion and transfer of electrolyte ions. Meanwhile, the flaky Co3O4 endows the material with high energy density and high charge transport efficiency. The resulted composite materials exhibit high specific surface area with low electrochemical impedance. Both aqueous and solid-state SCs were assembled separately. The energy density of aqueous SC can reach up to 42.5 Wh kg−1 at a power density of 746 W kg−1 and even with power density up to 30 kW kg−1 as the energy density still maintained 25 Wh kg−1. As for solid-state SC, the energy density can reach 40.6 μWh cm−2. The retention of solid-state SC can maintain at 87.1% even after 3000 cycle numbers. The excellent performance of composites enables them to be promising electrode materials for energy storage.

Au/TiO2 nanotube array based multi-hierarchical architecture for highly efficient dye-sensitized solar cells

Control over balanced high light-harvesting and efficient charge transport is of crucial importance for photoelectrochemical applications but still remains great challenge. We develop here single-wall TiO2 nanotube array with in-situ deposited Au nanoparticles. Based on this Au–TiO2 hybrid, we further build a “cigar-like” Au/TiO2-nanotube-array/TiO2-nanoparticles multi-hierarchical architecture through a novel vacuum-assisted colloid-filling approach for dye-sensitized solar cell application. In this elaborate structure, Au nanoparticles welding on nanotube walls play dual roles of direct injecting hot electrons into TiO2 to elevate the conductivity of TiO2 film by about 4 times (under illumination) for fast electron transport, and introducing a plasmonic effect to improve light-harvesting as well. Meanwhile, the thick TiO2 nanoparticulate decoration provides the photoanode vast surface area for ~3.2 times the dye-loading amount of the pristine TiO2 nanotube array based electrode, which will further boost the light-harvesting remarkably. The resulting dye-sensitized solar cell yields an impressively high power conversion efficiency of 8.93%, which is 190.4% that of the pristine TiO2 nanotube array based device. The technology presented in this work provides also a promising prospect for the preparation of multi-hierarchical structures for a wide range of applications, such as photocatalysis and gas sensors.

V-rich Bi2S3 nanowire with efficient charge separation and transport for high-performance and robust photoelectrochemical application under visible light

Abstract Bi2S3 is a n-type (negative type) semiconductor that has a narrow band gap and an ideal band edge position for hydrogen generation. However, high charge recombination rate, low photoelectrochemical (PEC) efficiency and poor stability are of major concerns of Bi2S3 as a photoelectrode. Herein, we report an efficient Vanadium-rich Bi2S3 nanowires (atomic ratio of V to Bi is 1/22) that have been prepared on fluorine-doped tin oxide (FTO) substrate by an in-situ solution-processed reaction with BiVO4 film as precursor. The resulting V-rich photoelectrode demonstrates notably high charge separation efficiency and fast charge transport with respect to pristine Bi2S3 nanowires. As a result, the photocurrent density of ca. 10 mA·cm−2 at -0.2 V vs Ag/AgCl is obtained under visible light illumination. In addition, V-rich Bi2S3 generates IPCE (incident photocurrent-to-current conversion efficiency) of 50% at 460 nm and 45% at 760 nm, demonstrating a 2.0 and 2.8-fold increase, respectively, when compared with pristine Bi2S3. This enhancement is probably due to increased light absorption, less charge recombination, and faster charge transfer. More importantly, in contrast to pristine Bi2S3 sample that suffers detrimental photocorrosion in sulfide containing electrolyte, V-rich Bi2S3 nanowires retain 84.6% of its initial photocurrent over the course of 1 h. These findings are expected to shed light on engineering high-performance and robust metal sulfide materials for photoelectrochemical application.