Transport Lifetime Research Articles

Effect of charged line defects on conductivity in graphene: Numerical Kubo and analytical Boltzmann approaches

Charge carrier transport in single-layer graphene with one-dimensional charged defects is studied theoretically. Extended charged defects, considered an important factor for mobility degradation in chemically-vapor-deposited graphene, are described by a self-consistent Thomas-Fermi potential. A numerical study of electronic transport is performed by means of a time-dependent real-space Kubo approach in honeycomb lattices containing millions of carbon atoms, capturing the linear response of realistic size systems in the highly disordered regime. Our numerical calculations are complemented with a kinetic transport theory describing charge transport in the weak scattering limit. The semiclassical transport lifetimes are obtained by computing scattered amplitudes within the second Born approximation. The transport electron-hole asymmetry found in the semiclassical approach is consistent with the Kubo calculations. In the strong scattering regime, the conductivity is found to be a sublinear function of electronic density and weakly dependent on the Thomas-Fermi screening wavelength. We attribute this atypical behavior to the extended nature of one-dimensional charged defects. Our results are consistent with recent experimental reports.

Influence of surface states on quantum and transport lifetimes in high-quality undoped heterostructures

We present a comparison between experimental and theoretical values of transport ${\ensuremath{\tau}}_{t}$ and quantum ${\ensuremath{\tau}}_{q}$ scattering lifetimes in high-quality undoped Al${}_{0.34}$Ga${}_{0.66}$As/GaAs heterostructures. We obtain excellent agreement between the experimental and modeled scattering times using three scattering processes: background impurity, interface roughness, and remote ionized impurity scattering from surface states. We show that despite the high mobility (${\ensuremath{\mu}}_{\text{peak}}=5.6\ifmmode\times\else\texttimes\fi{}{10}^{6}\phantom{\rule{0.222222em}{0ex}}{\text{cm}}^{2}\phantom{\rule{0.16em}{0ex}}{\text{V}}^{\ensuremath{-}1}\phantom{\rule{0.16em}{0ex}}{\text{s}}^{\ensuremath{-}1}$), the quantum lifetime ${\ensuremath{\tau}}_{q}$ is significantly reduced by small-angle scattering from remote surface charge. We further show that in shallow devices scattering from surface charges will be a limiting factor for both transport and quantum lifetimes.

Equivalence of Single-Particle and Transport Lifetimes from Hybridization Fluctuations

Single band theories of quantum criticality successfully describe a single-particle lifetime with non-Fermi liquid temperature dependence, but they fail to obtain a charge transport rate with the same dependence unless the interaction is assumed to be momentum independent. Here we demonstrate that a quantum critical material, with a long-range mode that transmutes electrons between light and heavy bands, exhibits a quasilinear temperature dependence for both the single-particle and the charge transport lifetimes, despite the strong momentum dependence of the interaction.

Efficient and stable CH3NH3PbI3-sensitized ZnO nanorod array solid-state solar cells

We report for the first time the use of a perovskite (CH3NH3PbI3) absorber in combination with ZnO nanorod arrays (NRAs) for solar cell applications. The perovskite material has a higher absorption coefficient than molecular dye sensitizers, gives better solar cell stability, and is therefore more suited as a sensitizer for ZnO NRAs. A solar cell efficiency of 5.0% was achieved under 1000 W m(-2) AM 1.5 G illumination for a solar cell with the structure: ZnO NRA/CH3NH3PbI3/spiro-MeOTAD/Ag. Moreover, the solar cell shows a good long-term stability. Using transient photocurrent and photovoltage measurements it was found that the electron transport time and lifetime vary with the ZnO nanorod length, a trend which is similar to that in dye-sensitized solar cells, DSCs, suggesting a similar charge transfer process in ZnO NRA/CH3NH3PbI3 solar cells as in conventional DSCs. Compared to CH3NH3PbI3/TiO2 solar cells, ZnO shows a lower performance due to more recombination losses.

TiO2 Film Morphology, Electron Transport and Electron Lifetime in Ultra-fast Sintered Dye-sensitized Solar Cells

ABSTRACTWith the application of near-infrared radiation (NIR), TiO2 films for dye-sensitized solar cells (DSCs) on metallic substrates can be sintered in just 12.5 s. The photovoltaic performance of devices made with NIR sintered films match those devices made with conventionally sintered films prepared by heating for 1800 s. Here we characterise the electron transport, electron lifetime and phase-morphological properties of ultrafast NIR sintered films, using impedance spectroscopy, transient photovoltage decay and x-ray diffraction measurements. An important factor in NIR processing of TiO2 films is the peak metal temperature (PMT) and we show that during the 12.5 second heat treatment that a PMT of around 635 °C gives near identical electron transport, electron lifetime and morphological properties, as well comparable photovoltaic performance to a conventionally sintered (500 °C, 30 mins) film. What is perhaps most interesting is that the rapid heating of the TiO2 (to temperatures of up to 785°C) does not lead to a large scale rutile phase transition. As such photovoltaic performance of resultant DSC devices is maintained even though the TiO2 has been at temperatures which traditionally would have reduced cell photocurrents via anatase-to-rutile phase transition.

Dependence of Transport Rate on Area of Lithography and Pretreatment of Tip in Dip-Pen Nanolithography

This study examines the lithographic capacity of tips in dip-pen nanolithography (DPN). The dependence of the transport rate (R) decay on the area of lithography (A(lith)), the dependence of A(lith) on the lithographic time (t), and the effect of piranha cleaning on the lithographic capacity are considered herein. The dependencies in the line-drawing lithography process are studied using 16-mercaptohexadecanoic acid (MHA) ink. On the basis of the linear decay dependence discovered in the R-A(lith) dependence, piranha treatment can increase the lithographic capacity by up to 35.5-fold, an improvement that may originate from a change in the tip's surface chemistry. Moreover, a theoretical model is derived to describe the A(lith)-t dependence accurately and to predict the tips' lifetime. Furthermore, an experiment involving DPN-based nanostructure fabrication demonstrates the importance of monitoring the tips' transport rate and lifetime. In addition to shedding light on the physical and chemical principles behind DPN, this study provides a comprehensive model for a quantitative analysis of the tips' behavior.

Conveniently fabricated heterojunction ZnO/TiO2 electrodes using TiO2 nanotube arrays for dye-sensitized solar cells

TiO2 nanotube arrays with an inner average pore diameter of 83 nm and a length of 14 μm are grown on Ti foils by electrochemical anodization in ammonium fluoride–water–glycerol solution. ZnO is introduced into the TiO2 nanotube arrays by a convenient electrodeposition technique. ZnO/TiO2 nanocomposites supported on Ti substrate are used as the photo-anode electrode for dye-sensitized solar cells (DSSCs). The morphology, structure and electrochemical properties are investigated using field-emission scanning electron microscopy, transmission electron microscopy, X-ray diffraction, UV–vis diffusion reflection spectroscopy, X-ray photoelectron spectroscopy and cyclic voltammetry measurements. It is found that ZnO have been embedded in the TiO2 nanotube arrays, and changed some photoelectric properties. The conversion efficiency of the dye-sensitized solar cells is more than doubled, compared with that of bare TiO2 nanotube arrays with deposited 60 min. This improvement comes from the synergetic effect between ZnO and TiO2, which increases dye absorption, electron transport and electron lifetime.

Enhancing the solar cell efficiency through pristine 1-dimentional SnO2 nanostructures: Comparison of charge transport and carrier lifetime of SnO2 particles vs. nanorods

Efficiency of dye-sensitized solar cells (DSSC) fabricated with pristine SnO2 nanocrystals was reported to be less superior compared to DSSC based on mesoporous TiO2 nanoparticles though both oxides have comparable electrical and surface properties. Owing to inherent high charge recombination properties of SnO2 nanoparticles, photoanode fabricated with SnO2 nanoparticles resulting in unexpected low open circuit voltage (Voc) and fill factor (FF). To overcome inherent charge recombination in SnO2, we investigated pristine SnO2 nanorods and showed enhanced Voc, FF and overall conversion efficiency (η) for SnO2 nanorods. The photoanode made of SnO2 nanorods yields nearly a 2-fold improvement in fill factor, 5 fold increases in η and a greater than 2-fold increase in short-circuit current density with a moderate increase in open-circuit photovoltage. The effects appear to arise primarily from longer electron lifetimes and reduced charge recombination of SnO2 nanorod based solar cells compared to that of SnO2 particles owing to 1-D nature of SnO2 nanorod which were evaluated by open-circuit voltage decay (OCVD) and electrochemical impedance spectroscopy (EIS) methods.

Electron Transfer in Dye-Sensitized Nanocrystalline TiO2 Solar Cell

The dye-sensitized solar cells (DSSC) have been regarded as one of the most promising new generation solar cells. Tremendous research efforts have been invested to improve the efficiency of solar energy conversion which is generally determined by the light harvesting efficiency, electron injection efficiency and undesirable electron lifetime. In this review, various characteristics of dye- sensitized nanostructured TiO2 solar cells, such as working principle, electron transport and electron lifetime, were studied. The review avoids detailed mathematical and spectroscopic discussion, but rather tries to summarize the key conclusions relevant to materials design.

The Impact of Uncertainties in African Biomass Burning Emission Estimates on Modeling Global Air Quality, Long Range Transport and Tropospheric Chemical Lifetimes

The chemical composition of the troposphere in the tropics and Southern Hemisphere (SH) is significantly influenced by gaseous emissions released from African biomass burning (BB). Here we investigate how various emission estimates given in bottom-up BB inventories (GFEDv2, GFEDv3, AMMABB) affect simulations of global tropospheric composition using the TM4 chemistry transport model. The application of various model parameterizations for introducing such emissions is also investigated. There are perturbations in near-surface ozone (O3) and carbon monoxide (CO) of ~60–90% in the tropics and ~5–10% in the SH between different inventories. Increasing the update frequency of the temporal distribution to eight days generally results in decreases of between ~5 and 10% in near-surface mixing ratios throughout the tropics, which is larger than the influence of increasing the injection heights at which BB emissions are introduced. There are also associated differences in the long range transport of pollutants throughout the SH, where the composition of the free troposphere in the SH is sensitive to the chosen BB inventory. Analysis of the chemical budget terms reveals that the influence of increasing the tropospheric CO burden due to BB on oxidative capacity of the troposphere is mitigated by the associated increase in NOx emissions (and thus O3) with the variations in the CO/N ratio between inventories being low. For all inventories there is a decrease in the tropospheric chemical lifetime of methane of between 0.4 and 0.8% regardless of the CO emitted from African BB. This has implications for assessing the effect of inter-annual variability in BB on the annual growth rate of methane.

Effect of TiO2 morphology on photovoltaic performance of dye-sensitized solar cells: nanoparticles, nanofibers, hierarchical spheres and ellipsoid spheres

Different morphologies of anatase TiO2 from nanoparticles, nanofibers, and hierarchical spheres to ellipsoid spheres are successfully fabricated via solvothermal reactions of titanium n-butoxide and acetic acid. The variations of morphology, size and crystal phase of TiO2 micro-/nanostructures are investigated in detail by XRD, FTIR, SEM and TEM. Effects of different TiO2 morphologies on the photovoltaic performance of dye-sensitized solar cells (DSSCs) are also discussed based on I–V, IPCE, IMPS, IMVS and UV-vis absorption and diffuse spectra. The DSSC based on the hierarchical anatase TiO2 sphere photoelectrode shows an overall light-to-electricity conversion efficiency of 9.35% accompanying a short-circuit current density of 17.94 mA cm−2, an open-circuit voltage of 803 mV and fill factor of 0.65, which is much higher than that of nanoparticles (7.37%), nanofibers (8.15%) and ellipsoid TiO2 spheres (7.93%). The significant enhancement of short-circuit current density and power conversion efficiency for the hierarchical sphere-based DSSC compared to other nanostructures is mainly attributed to the larger dye loading, superior light scattering ability, and/or faster electron transport and longer electron lifetime.

High-performance dye-sensitized solar cells based on hierarchical yolk–shell anatase TiO2beads

Yolk–shell TiO2 beads consisting of nanoparticles are synthesized by using a facile hydrothermal method. The influences of hydrothermal reaction temperature on the size of TiO2 particles and the photovoltaic performance of dye-sensitized solar cells (DSSCs) have been investigated in detail. A DSSC based on the yolk–shell TiO2 bead photoelectrode (∼15 μm) exhibits an energy conversion efficiency of 9.05%, which is much higher than that of P25 (7.56%). This can be mainly attributed to the fast electron transport and long electron lifetime resulting in a larger electron diffusion coefficient and higher charge collection efficiency for the former, which are confirmed by intensity-modulated photocurrent spectroscopy (IMPS), intensity-modulated photovoltage spectroscopy (IMVS), and electrochemical impedance spectroscopy (EIS). In addition, the superior light scattering ability of hierarchical yolk–shell TiO2 beads compared to P25 film measured by UV-vis diffuse spectrum also contributes to higher photocurrent and photovoltaic performance.

Electronic transport within a quasi-two-dimensional model for rubrene single-crystal field effect transistors

Spectral and transport properties of the quasi two-dimensional adiabatic Su-Schrieffer-Heeger model are studied adjusting the parameters in order to model rubrene single-crystal field effect transistors with small but finite density of injected charge carriers. We show that, with increasing temperature $T$, the chemical potential moves into the tail of the density of states corresponding to localized states, but this is not enough to drive the system into an insulating state. The mobility along different crystallographic directions is calculated including vertex corrections which give rise to a transport lifetime one order of magnitude smaller than spectral lifetime of the states involved in the transport mechanism. With increasing temperature, the transport properties reach the Ioffe-Regel limit which is ascribed to less and less appreciable contribution of itinerant states to the conduction process. The model provides features of the mobility in close agreement with experiments: right order of magnitude, scaling as a power law $T^{-\gamma}$, with $\gamma$ close or larger than two, and correct anisotropy ratio between different in-plane directions. Due to a realistic high dimensional model, the results are not biased by uncontrolled approximations.

Magnetotransport properties of two-dimensional electron gas in AlGaN/AlN/GaN heterostructures

Magnetotransport measurements are carried out on the AlGaN/AlN/GaN in an SiC heterostructure, which demonstrates the existence of the high-quality two-dimensional electron gas (2DGE) at the AlN/GaN interface. While the carrier concentration reaches 1.32×1013 cm−2 and stays relatively unchanged with the decreasing temperature, the mobility of the 2DEG increases to 1.21×104 cm2/(V·s) at 2 K. The Shubnikov—de Haas (SdH) oscillations are observed in a magnetic field as low as 2.5 T at 2 K. By the measurements and the analyses of the temperature-dependent SdH oscillations, the effective mass of the 2DEG is determined. The ratio of the transport lifetime to the quantum scattering time is 9 in our sample, indicating that small-angle scattering is predominant.

Enhanced efficiency of dye-sensitized solar cells through TiCl4-treated, nanoporous-layer-covered TiO2 nanotube arrays

TiCl4-treated, nanoporous-layer-covered TiO2 (Type II) nanotube arrays are fabricated through a two-step anodization process followed by treatment with TiCl4. A dye-sensitized solar cell (DSSC) with TiCl4-treated, nanoporous-layer-covered Type II TiO2 nanotubes is compared with other DSSCs based on untreated Type II and both untreated and TiCl4-treated, conventional TiO2 (Type I) nanotube arrays. The TiCl4 surface treatment's effects on dye adsorption, charge transport, and electron lifetime are dependent on the morphologies of the TiO2 nanotubes. The TiCl4-treated Type I nanotubes allow higher dye adsorption, whilst the TiCl4-treated Type II nanotubes provide much faster electron transport and enhanced electron lifetime. This is because there are fewer defect traps in the nanostructure well-aligned without bundling, which contributes to the significantly improved cell performance over the DSSC with the TiCl4-treated Type I nanotubes.

Investigation of Ionic Conductivity and Long-Term Stability of a LiI and KI Coupled Diphenylamine Quasi-Solid-State Dye-Sensitized Solar Cell

In this work, enhancement of ionic conductivity and long-term stability through the addition of diphenylamine (DPA) in poly(ethylene oxide) (PEO) is demonstrated. Potassium iodide (KI) is adopted as the crystal growth inhibitor, and DPA is used as a charge transport enhancer in the electrolyte. The modified electrolyte is used with titanium dioxide (TiO2) nanoparticles, which is systematically tuned to obtain high surface area. The dye-sensitized solar cell (DSSC) showed a photocurrent of 14 mAcm2 with a total conversion efficiency of 5.8% under one sun irradiation. DPA enhances the interaction of the TiO2 nanoparticle film and the I-/I3- electrolyte leading to high ionic conductivity (3.5 × 10-3 Scm-1), without compromising on the electrochemical and mechanical stability. Electrochemical impedance spectroscopy (EIS) studies show that electron transport and electron lifetime are enhanced in the DPA added electrolyte due to reduced sublimation of iodine. The most promising feature of the electrolyte is increased device stability with 89% of the overall efficiency preserved even after 40 days.

Transient photocurrent and photovoltage studies on charge transport in dye sensitized solar cells made from the composites of TiO2 nanofibers and nanoparticles

Herein, we report the transient photocurrent and photovoltage studies on the charge transport in dye sensitized solar cells (DSSCs) made from the composites of electrospun titanium oxide (TiO2) nanofibers, and conventional TiO2 nanoparticles. The results on charge transport parameters (including the charge recombination lifetime, the electron transport lifetime, and the diffusion length) indicated that the addition of TiO2 nanofibers into composite photoanodes led to substantial improvement on the efficiency of charge collection, suggesting that the composites of nanoparticles and one-dimensional nanostructures are promising materials for the development of DSSCs with high efficiency.

Quantum lifetimes and momentum relaxation of electrons and holes in Ga0.7In0.3N0.015As0.985/GaAs quantum wells

Electronic transport in n- and p-type modulation-doped Ga0.7In0.3N0.015As0.985/GaAs quantum wells are investigated using magneto-transport measurements in the temperature range between T = 1.8 and 32 K and at magnetic fields up to B = 11 T. The momentum relaxation and the quantum lifetimes (τq ) of electrons and holes are obtained directly from the temperature and magnetic field dependencies of the SdH oscillation amplitudes, respectively. A detailed analysis of quantum and transport life times indicates that the momentum relaxation of holes is forward displaced in k-space, while a large angle-scattering mechanism is prominent for the electrons. This discrepancy is believed to be due to scattering of electrons with nitrogen complexes and to the lack of such a mechanism for holes.

Poroelectroelastic theory of water sorption and swelling in polymer electrolyte membranes

This paper presents a novel poroelectroelastic model of equilibrium water sorption and swelling in polymer electrolyte membranes. These phenomena determine transport properties, electrochemical performance, durability and lifetime of the membrane in a polymer electrolyte fuel cell. Based on a consistent treatment of thermodynamic equilibrium conditions, involving capillary, osmotic and elastic effects, we establish the equation of state of water in a single membrane pore. It relates the charge density at the pore walls to a microscopic swelling parameter. Extended to the water sorption equilibrium in a pore ensemble, the model reconciles microscopic swelling in a single pore with macroscopic swelling of the membrane. Theoretical relations are developed that rationalize the impact of external conditions, statistical distribution of anionic head groups and elastic properties of the polymer on water sorption and swelling. The model resolves Schroder's paradox and other unexplained phenomena related to water sorption equilibria, pressure distribution, and transport properties in PEM.

Optical spin generation/detection and spin transport lifetimes

Abstract We generate electron spins in semiconductors by optical pumping. The detection of them is also performed by optical technique using time-resolved pump-probe photoluminescence polarization measurements in the presence of an external magnetic field perpendicular to the generated spin. The spin polarization in dependences of the pulse length, pump-probe delay and external magnetic field is studied. From the dependence of spin-polarization on the delay of the probe, the electronic spin transport lifetimes and the spin relaxation frequencies as a function of the strength of the magnetic field are estimated. The results are discussed based on hyperfine effects for interacting electrons.