Electron Transport Length Research Articles

Correlation of Nanomorphology with Structural and Spectroscopic Studies in Organic Solar Cells

The nanomorphology of bulk heterojunction (BHJ) blends based on poly[[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b′]dithiophene-2,6-diyl][3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4-b]thiophenediyl]] (PTB7) blended with [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) is systematically varied by using the volume fraction of the solvent additive 1,8-diiodooctane (DIO) from 0 to 20 vol % in chlorobenzene to prepare organic solar cells (OSCs). Blends prepared without DIO forms large phase-separated domains of PC71BM which are suppressed on addition of 3 vol % DIO, resulting in improved nanoscale features. Addition of 20 vol % DIO results in a finer interconnected morphology along with increased roughness due to polymer aggregation, which contrasts with previous reports. With increase in addition of DIO the photoluminescence (PL) from the blend is reduced; however, a relative increase in PL from 750 nm onward is observed for blends with 20 vol % DIO. As quenching of the blend PL is related to the donor/acceptor interface, structural characterizations in real-space (microscopy) and k-space (scattering) are performed to unravel the nanomorphology and correlate it with photophysical and charge transport processes. Electron-transport length scales measured by scanning photocurrent microscopy are found to increase with the addition of up to 3 vol % DIO associated with the breakup of the large PC71BM agglomerates, while the hole-transport length is found to increase on adding DIO up to 20 vol % due to aggregation of polymer chains. Hence, this work represents a unique set of results systematically examining the effect of nanomorphology on structural and solar cells properties of BHJ blends, which can have a direct implication on better understanding of the emerging high-efficiency OSC systems.

Metal-organic frameworks derived In-based nanoparticles encapsulated by carbonaceous matrix for highly efficient energy storage

The design and fabrication of novel material architectures via versatile strategy become one of the most fascinating research topics due to their fantastic properties. Specifically, the popular metal-organic-framework provides a rational strategy to overcome intrinsic limitations of existing synthetic methods for nanostructures. Herein, an intriguing kind of hollow In2O3/C porous hexagonal nanorods is presented for the application in lithium ion battery anodes, by a controlled one-step annealing process. The structural advantages of these materials are that the one-dimensional hollow porous nanorod structures can provide decreased lithium ions diffusion and electron transport lengths, more favorable pathways for efficient penetration of electrolyte, the numerous mesopores that could act as reservoirs for storing Li+, and also the ability to release the large volume expansion during charge/discharge. Additionally, carbon coating formed in situ can minimize the volume changes of In2O3, and act as an efficient electrical conducting pathway. This proposed anode delivers outstanding Li-storage capacity of 1078 mA h g−1 and rate capability even to the current density of 2000 mA g−1. The outstanding long-term stability (425 mA h g−1 after 1000 cycles at 1000 mA g−1) further demonstrates its prominent properties.

Improving Photovoltaic Performance of ZnO Nanowires Based Colloidal Quantum Dot Solar Cells via SnO2 Passivation Strategy

Colloidal quantum dot solar cells (CQDSCs) based on one-dimensional metal oxide nanowires (NWs) as electron transport layer (ETL) have attracted much attention due to its larger ETL/colloidal quantum dots (CQDs) contact area and longer electron transport length than other structure CQDSCs, such as planar CQDSCs. However, it is known that defect states in NWs would increase the recombination rate because of the high surface area of NWs. Here, the defect species on the ZnO NWs surface which resulted in the surface recombination and SnO2 passivation effects were investigated. Comparing with the solar cells using pristine ZnO NWs, the CQDSCs which based on SnO2 passivated ZnO NWs electrode exhibited a beneficial band alignment to charge separation, and the interfacial recombination at ZnO/CQDs interface was reduced, eventually resulted in a 40% improvement of power conversion efficiency (PCE). Overall, these findings indicate that surface passivation and the reduction of deep level defect in ETLs could contribute to improve the PCE of CQDSCs.

Synthesis of Li4 Ti5 O12 with Tunable Morphology Using l-Cysteine and Its Enhanced Lithium Storage Properties.

Nitrogen and sulfur co-doped carbon-coated Li4 Ti5 O12 (denoted as LTO/NSC) was developed to enhance the electrochemical performance of LTO material. l-Cysteine served as both the carbon source and the heteroatom doping source. The morphology of LTO was tuned by Ti-C bond formation during carbonation process, accompanied by a change in the original orientation growth of the LTO lattice plane. Consequently, LTO transformed from nanosheets to nanoparticles. SEM data proved that the structure of LTO/NSC nanoparticles was more stable than that of LTO nanosheets after hundreds of charge/discharge process. The N,S co-doped carbon layer can moderate particle aggregation and may help to shorten the electron transport length and enhance lithium storage capacity. The structural superiority and the N,S co-doped carbon layer endows LTO/NSC particles with high reversible specific capacity (183 mA h g-1 at 0.1 C), significantly enhanced rate capability (122 mA h g-1 at 10 C) and excellent cycling stability (capacity retention of 96.3 % after 200 cycles) relative to these features of LTO nanosheets. Thus, LTO/NSC is a promising anode material for high-performance lithium ion batteries.

Broadband Epsilon-near-Zero Reflectors Enhance the Quantum Efficiency of Thin Solar Cells at Visible and Infrared Wavelengths.

The engineering of broadband absorbers to harvest white light in thin-film semiconductors is a major challenge in developing renewable materials for energy harvesting. Many solution-processed materials with high manufacturability and low cost, such as semiconductor quantum dots, require the use of film structures with thicknesses on the order of 1 μm to absorb incoming photons completely. The electron transport lengths in these media, however, are 1 order of magnitude smaller than this length, hampering further progress with this platform. Herein, we show that, by engineering suitably disordered nanoplasmonic structures, we have created a new class of dispersionless epsilon-near-zero composite materials that efficiently harness white light. Our nanostructures localize light in the dielectric region outside the epsilon-near-zero material with characteristic lengths of 10-100 nm, resulting in an efficient system for harvesting broadband light when a thin absorptive film is deposited on top of the structure. By using a combination of theory and experiments, we demonstrate that ultrathin layers down to 50 nm of colloidal quantum dots deposited atop the epsilon-near-zero material show an increase in broadband absorption ranging from 200% to 500% compared to a planar structure of the same colloidal quantum-dot-absorber average thickness. When the epsilon-near-zero nanostructures were used in an energy-harvesting module, we observed a spectrally averaged 170% broadband increase in the external quantum efficiency of the device, measured at wavelengths between 400 and 1200 nm. Atomic force microscopy and photoluminescence excitation measurements demonstrate that the properties of these epsilon-near-zero structures apply to general metals and could be used to enhance the near-field absorption of semiconductor structures more widely. We have developed an inexpensive electrochemical deposition process that enables scaled-up production of this nanomaterial for large-scale energy-harvesting applications.

Three-Dimensional Graphene Sponge with Hierarchically Porous Structure for Electrochemical Supercapacitors

Supercapacitors is an electrochemical energy storage device typically used as sustainable power supply (such as electric vehicles and mobile electronic devices) due to their higher power density than batteries, higher energy density than conventional dielectric capacitors, and long life cycles. [1,2] Graphene, a two-dimensional (2D) one-atom-thick single layer of sp2-bonded carbon, is considered as an ideal electrode material in supercapacitors because of its superior electrical conductivity and exceptionally large specific surface area. Graphene sheets, unless well separated from each other, tend to form irreversible agglomerates through strong π-π stacking and van der Waals interaction.[3] Meanwhile, the electrochemical binders and additives are normally required to make graphene-based electrodes, which would have an adverse effect on the specific capacitance of the resulting electrodes. To tackle these challenges, three dimensional (3D) graphene networks with highly electrical conductivity and large specific surface area, short diffusion pathways for electrolyte ions and fast transport channels for electrons are very promising supercapacitor materials. To these aims, we have developed ways to synthesize a novel macroporous/nanoporous graphene sponge with tunable pore sizes. This 3D porous graphene sponge has i) an interconnected electrolyte-filled macroporous network that enables increase of contact surface between 3D network and electrolytic solution, and rapid ion transport, ii) short ion and electron transport lengths due to nanoporous structures, iii) a large electrode specific surface area and (iv) high electron conductivity in the electrode assembly. The structure characterization and capacitive properties of the 3D macroporous/nanoporous graphene sponge was investigated and discussed, indicating 3D porous graphene sponge as an ideal electrode materials for supercapacitors. Keywords：Graphene, 3D porous materials, supercapacitors, electrochemical energy

Al and/or Ni-doped nanomanganese dioxide with anisotropic expansion and their electrochemical characterisation in primary Li–MnO2 batteries

A variety of MnO2 nanorods containing one or two transition metals (M) (with M = Al and/or Ni) have been successfully synthesised via a facile hydrothermal synthesis route. The physical–chemical properties and electrochemical performance of manganese oxide were analysed by X-ray diffraction (XRD), inductively coupled plasma atomic emission spectrometry (ICP-OES), Fourier transform infrared spectrometer (FT-IR), scanning electron microscopy (SEM), Brunauer–Emmett–Teller method (BET), galvanostatic discharge and cyclic voltammetry (CV). The result indicated that α-type MnO2 was obtained, and a small quantity of Al and/or Ni were embedded into the crystal lattice of manganese oxide instead of the partial Mn ion, which resulted in anisotropic expansion of the MnO2 unit cell. The doping of Al can strengthen Mn–O bonds in the [MnO6] octahedral and increases the specific surface area of the modified material (i.e., Al–MnO2 is 119 m2 g−1). Interestingly, MnO2 electrode co-doped with equimolar Al and Ni exhibited the highest specific capacity of 169 mAh g−1 at 0.05 mA cm−2. The substantial enhancement of the electrochemical lithium storage capacity was due to the ameliorating of integrative factors, such as high specific surface area, excellent lattice parameters and lower electrical resistance, as well as short Li+ and electron transport length. In addition, a more stable host skeleton also guaranteed an endurable Li+ intercalation behaviour during the discharge process.

Ultra microelectrodes increase the current density provided by electroactive biofilms by improving their electron transport ability

Electroactive biofilms were formed from garden compost leachate on platinum wires under constant polarisation at −0.2 V vs.SCE and temperature controlled at 40 °C. The oxidation of 10 mM acetate gave maximum current density of 7 A m−2 with the electrodes of largest diameters (500 and 1000 μm). The smaller diameter wires exhibited an ultra-microelectrode (UME) effect, which increased the maximum current density up to 66 A m−2 with the 25 μm diameter electrode. SEM imaging showed biofilms around 75 μm thick on the 50 μm diameter wire, while they were only 25 μm thick on the 500 μm diameter electrode. Low scan cyclic voltammetry (CV) curves were similar to those already reported for biofilms formed with pure cultures of G. sulfurreducens. Concentrations of the redox molecules contained in the biofilms, which were derived from the non-turnover CVs, were around 0.4 to 0.6 mM, which was close to the value of 1 mM extracted from literature data for G. sulfurreducens biofilms. A numerical model was designed, which demonstrated that the microbial anodes were not controlled here by microbial kinetics. Introducing the concept of average electron transport length made the model well fitted with the experimental results, which indicates rate control by electron transport through the biofilm matrix. According to this model, the UME effect improved the electron transport network in the biofilm, which allowed the biofilm to grow to greater thickness.

Spin-dependent resonant quantum tunneling between magnetic nanoparticles on a macroscopic length scale

Macroscopic quantum phenomena are common features observed in superconductors, superfluid helium, and Bose-Einstein condensates. However, most of quantum transport studies are based on a small number of dots and are not in long-range electron transport length scale. Here we show that spin-dependent resonant quantum tunneling is achieved in the macroscopic length scale (a few millimeters) corresponding to an array of up to ${10}^{4}$ junctions in a series consisting of Co nanoparticles embedded in an oxygen-deficient TiO${}_{2}$ matrix. This phenomenon is observed by magnetoresistance measurements at 5 K in a Coulomb blockade regime. We further present a model based on resonant spin-polarized quantum tunneling of electrons of Co particles. It occurs through resonant continuous spin-polarized defect band states located near the Fermi level of the defective TiO${}_{2}$, which acts as a magnetic tunnel barrier. These results might be potentially useful for future designs of spintronic quantum devices.

Electron transport in back contact dye-sensitized solar cells

The electron transport properties of a back contact dye-sensitized solar cell (BCDSC) were investigated in comparison with a conventional DSC. It was found that the BCDSC had a lower short circuit current density (JSC) at the same thickness of TiO2 film and that JSC was not proportional to the thickness of the TiO2 film. Calculation of electron transport length in the TiO2 film suggested that the injected electron travels a longer distance to the electrode in the BCDSC than in the DSC. TiCl4 treatment of the TiO2 film produced a marked improvement of the JSC value in the BCDSC due to an increase in the electron diffusion coefficient of the TiO2 film, whereas the JSC of the DSC remained almost unchanged. It is clear that the value of JSC in the BCDSC is more dependent on the electron transport properties of the TiO2 film than in the DSC. Under standard AM 1.5 irradiation (100 mW cm−2), a BCDSC with N719 dye yielded an overall conversion efficiency of 8.0%.

Generation of THz signals based on quasi-ballistic electron reflections in double-heterojunction structures

The generation of THz signals by the periodic quasi-ballistic resonant motion of electrons on the basis of the combined action of electron acceleration in a potential well and reflection at the heterointerface is demonstrated by a Monte Carlo simulation. The electron dynamics in In0.52Al0.48As/In0.53Ga0.47As/In0.52Al0.48As heterostructures is investigated for different well widths and doping densities under the influence of fundamental-wave signals which conveniently can also be of square shape of 100 GHz and 200 GHz. It is found that the resulting quasi-ballistic electron motion produces oscillations within these wells which generate particularly high odd harmonics in the terahertz frequency range. Simulation results of this new type of resonance phenomenon show that the amplitude of the THz radiation strongly depends on the well width and voltage level of the square wave signal. This study shows that double-heterojunction structures with well width of ballistic electron transport length are promising candidates for the design of efficient THz sources.

The photorefractive sensitivity and the effective electron transport length in two-centre holographic recording

In this paper the photorefractive sensitivity defined for single-centre holographic recording is modified to adapt two-centre holographic recording. Based on the time analytic solution of Kukhtarev equations for doubly doped crystals, the analytical expression of photorefractive sensitivity is given. For comparison with single-centre holographic recording and summing the electron competition effects between the deeper and shallower traps, an effective electron transport length is proposed, which varies with the intensity ratios of recording light to sensitive light. According to analyses in this paper, the lower photorefractive sensitivity in two-centre holographic recording is mainly due to the lower concentration of unionized dopants in the shallower centre and the lower effective electron transport length.

Carbon foil thickness dependent electron yields induced by swift highly charged ions

We measured electron yields from the beam entrance and exit surfaces of thin carbon foils ( d ≈ 4–700 μg/cm 2) bombarded with swift ( E p M = 13.6 MeV/u ) highly charged ( q = 16–18) Ar ions. The dependence of the electron yields on target thickness d is analyzed within the framework of a phenomenological semiempirical model. We obtained electron transport lengths of high energy ( E ⪢ 100 eV) δ-electrons and diffusion lengths of slow electrons ( E ⪡ 100 eV). The projectile velocity dependence of these transport lengths is investigated by comparing our present results for 13.6 MeV/u Ar with results obtained with 0.1 MeV/u Ne, 1 MeV/u C, 3.9 MeV/u S and 74 MeV/u Ni. We also discuss the relation between electron yields and the electronic stopping power.

Target-thickness-dependent electron emission from carbon foils bombarded with swift highly charged heavy ions

We have measured electron yields from the beam entrance and exit surfaces of thin carbon foils (d\ensuremath{\approxeq}4--700 \ensuremath{\mu}g/${\mathrm{cm}}^{2}$) bombarded with swift (13.6 MeV/u) highly charged (q=16--18) argon ions. The dependence of the electron yields on target thickness and charge state of the ions is analyzed within the framework of an extended semiempirical model. Due to the high velocity of the ions, it is possible to distinguish electron production in primary ionization (related to the stopping power and the effective charge of the ions) from secondary electron production due to the transport of so-called \ensuremath{\delta} electrons (cascade multiplication). By combining the experimental results with numerical simulations of electron transport in matter by a Monte Carlo method, we have obtained electron transport lengths of high energy (E\ensuremath{\gg}100 eV) \ensuremath{\delta} electrons parallel and perpendicular to the ion trajectory, as well as diffusion lengths of slow electrons (E\ensuremath{\ll}100 eV). In order to study the velocity dependence of these transport lengths, we have not only investigated 13.6 MeV/u Ar ions, but also 1 MeV/u C and 3.9 MeV/u S, for which experimental results are available [Koschar et al., Phys. Rev. A 40, 3632 (1989)]. We discuss the origin of electron yield reductions (compared to a simple scaling with the square of the nuclear charge) with heavy ions and present measurements of double differential energy and angular electron distributions of 13.6 MeV/u ${\mathrm{Ar}}^{17+}$ ions.

Particle penetration through frozen gases

The energy deposition of swift charged particles penetrating solids is accompanied by such processes as particle (electrons, atoms, photons …) production or phase transition along the particle track. The electron yield obtained with thin copper foils covered with solid CO2 (or Xe) and the erosion of frozen CO2 (or Xe) by transmission of protons (1 MeV) was measured quantitatively. The obtained CO2 (or Xe) thickness was related 1. to the yield of convoy electrons (Ee ≃ 540eV) being emitted along the beam direction (Θ=O°) with a vector velocity ve equal to the projectile vector velocity vp and 2. to the intensity of low energy “true” secondary electrons (Ee < 50eV). The adsorbate thickness dependence of both electron yields is found to be independent of their electron transport lengths 1(Ee.

Electron emission and desorption from frozen gases under fast (MeV) ion bombardment

First results of the adsorbate-thickness dependence of electron yields obtained with thin copper foils covered with solid CO2 are presented. Absolute values of the CO2 coverage were determined by Rutherford forward scattering. The erosion of frozen CO2 by transmission of protons (1 MeV) was observed for the first time. The obtained CO2 thickness was related to the yield of convoy electrons (Ee ≈ 540 eV) being emitted along the beam direction (Θ = 0°) with a velocity νc equal to the projectile velocity νp and to the intensity of low energy “true” secondary electrons (Ee < 50 eV). Surprisingly, the adsorbate thickness dependence of both electron yields is not affected by different electron transport lengths λ(Ee). Our alternative interpretation is based on nonuniform erosion, which generates a modificaton of the bombarded and the contaminated target area with increasing irradiation time.

Models for the bulk photovoltaic effect in ferroelectric crystals

Three models for the bulk photovoltaic effect in electrooptic crystals are discussed. All models produce the same results. First, the photocurrent responsible for hologram storage is spatially shifted relative to the optical pattern that produced it. Second, the efficiency of hologram recording decreases with increases in the spatial frequency of the optical interference pattern. Third, the efficiency of hologram recording decreases for crystals exhibiting a strong bulk photovoltaic effect. The effect of finite electron transport length on volume-phase hologram storage is analyzed. It is shown that the resulting holograms are spatially shifted relative to the optical-intensity pattern that produced it. This phase shift affects beam coupling during hologram reading; it can be an aid in studying the bulk photovoltaic effect and the relative importance of the various mechanisms of hologram writing. The analysis also points out that the finite electron transport length reduces the ability of the crystal to store holograms if the spatial frequency of the optical interference pattern is increased.

Hologram writing in electro-optic materials with gaussian illumination

An analysis of hologram writing in electro-optic materials is carried out for the case of circular Gaussian beams incident on a crystal with finite dark conductivity and short electron transport length. It is shown that the ratio of light-to-dark carrier concentration affects the spatial uniformity of the hologram component that results due to the electron drift mechanism, viz. the smaller this ratio the more uniform the hologram. The metallic electrodes present at the two ends of the crystal to electrically control the hologram storage process are shown to slightly affect the written hologram because of the electrical charges that appear on them.

Holographic grating formation in photorefractive crystals with arbitrary electron transport lengths

The time development of optically induced refractive-index hologram gratings in photrefractive electro-optic crystals such as lithium niobate and bismuth silicon oxide is analyzed allowing arbitrary electron-transport lengths and nonsinusoidal grating shapes. In this analysis, diffusion and drift in applied and space-charge fields as well as the bulk photovoltaic effect (when applicable) are included. Results for the photoinduced space-charge field are presented for a range of realistic experimental conditions. It is shown that the spatial phase of the space-charge field and therefore the beam coupling is strongly dependent on the electron-transport lengths, whereas the magnitude of the field and therefore the diffraction efficiency is not dependent. It is shown that for paraelectric materials (such as BSO) the results of short- and long-transport-length analyses must converge for long recording times (saturation).