Showing papers in "Physica Status Solidi (a) in 2012"
TL;DR: In this paper, the indirect and direct optical bandgap energy of near-stoichiometric lithium niobate (nSLN) crystals is evaluated by optical absorption measurement.
Abstract: Energy of the indirect and direct optical bandgap of near-stoichiometric lithium niobate (nSLN) crystals is evaluated by optical absorption measurement. The value of the indirect bandgap (3.95 eV) is consistent with the earlier reports. However, the direct bandgap energy of 4.12 eV is higher than the previously reported experimental value (3.68 eV). The direct bandgap energy obtained here is closer to the recent theoretical value estimated by Thierfelder et al. [Phys. Status Solidi C 7, 362 (2010)] in comparison to the previously predicted value by Ching et al. [Phys. Rev. B 50, 1992 (1994)]. The phonons involved in the indirect allowed transitions have energies ∼85 meV (685 cm−1) and are identified as the E(TO9) and E(LO8) optical phonon modes from the Raman scattering measurement.
119 citations
TL;DR: In this paper, a self-limiting, constant growth rate per cycle temperature window (100-200°C) was established which is the major characteristic of an ALD process.
Abstract: Crystalline aluminum nitride (AlN) films have been prepared by plasma-enhanced atomic layer deposition (PEALD) within the temperature range from 100 to 500 °C. A self-limiting, constant growth rate per cycle temperature window (100–200 °C) was established which is the major characteristic of an ALD process. At higher temperatures (>225 °C), deposition rate increased with temperature. Chemical composition, crystallinity, surface morphology, mass density, and spectral refractive index were studied for AlN films. X-ray photoelectron spectroscopy (XPS) analyses indicated that besides main AlN bond, the films contained AlON, AlO complexes, and AlAl metallic aluminum bonds as well. Crystalline hexagonal AlN films were obtained at remarkably low growth temperatures. The mass density increased from 2.65 to 2.96 g/cm3 and refractive index of the films increased from 1.88 to 2.08 at 533 nm for film growth temperatures of 100 and 500 °C, respectively.
101 citations
TL;DR: In this article, a 3D model for charge transport and exciton generation in disordered organic semiconductors with a Gaussian DOS is presented, with a focus on applications to organic light-emitting diodes.
Abstract: In this review, we discuss recent advances in our understanding of charge transport and exciton generation in disordered organic semiconductors with a Gaussian DOS, with a focus on applications to organic light-emitting diodes (OLEDs). Three-dimensional (3D) modeling shows that the actual current density in OLEDs based on materials with a Gaussian electron and hole DOS is filamentary. However, it is possible to accurately calculate the average current density by solving a one-dimensional (1D) drift-diffusion equation, making use of compact expressions for the temperature, electric field, and carrier density dependent mobility which have been derived from 3D-modeling. For the cases of spatially uncorrelated energetic disorder and spatially correlated disorder due to random dipole fields, these models are called the extended Gaussian disorder model (EGDM) and extended correlated disorder model (ECDM), respectively. We discuss how the effects of trapping on guest molecules can be included, and how exciton generation is described. The application of these models to hole and electron transporting polymer and small molecule materials is discussed, with an emphasis on the modeling of the transport and emission of blue-emitting OLEDs based on a polyfluorene-derivative.
90 citations
TL;DR: In this paper, a characterization method of entropy-change measurement for the study of the electrocaloric effect (ECE) was demonstrated, after the specific heat capacity was measured under different applied fields using a differential scanning calorimeter (DSC) accompanied with DC power supply.
Abstract: We demonstrated a characterization method of entropy-change measurement for the study of the electrocaloric effect (ECE). After the specific heat capacity was measured under different applied fields using a differential scanning calorimeter (DSC) accompanied with DC power supply, the electrocaloric ΔS was calculated from the temperature integral of specific heat capacities based on the basic definition of entropy. The ΔS–T curve of BaTiO3 single crystal showed a sharp peak around Tc, which increased gradually and shifted to higher temperature with the rise of applied field. A high ECE with ΔS = 1.9 J/kg K and ΔT = 1.6 K is achieved under a quite low field of 10 kV/cm. The results agreed with the thermodynamic calculation. It provides a direct, precise, and time-efficient method for the electrocaloric studies.
85 citations
TL;DR: In this paper, the authors discuss the key aspects of excitons, following the excited state lifecycle, and discuss nonlinear effects at high excitation levels that, to date, limit the high brightness efficiency of phosphorescent OLEDs.
Abstract: Excited states of organic molecules (excitons) are the heart of any organic electroluminescent device. They mediate the conversion of injected charges – electrons and holes – into photons. Phosphorescent emission originating from triplet excitons is especially important, as it is to date the only general route to enable unity charge-to-photon conversion efficiencies. In this paper, we discuss the key aspects of excitons, following the excited state lifecycle. First, we review fundamentals of singlet and triplet exciton formation in organic semiconductors, followed by a discussion of concepts that aim to alter the singlet-to-triplet formation rates to enable higher electroluminescence yields in the fluorescence manifold. Subsequently, we focus on the exciton distribution within the organic semiconductor material during its lifetime. The processes involved ultimately determine organic light-emitting diode (OLED) performance and are especially key in the development of concepts for white emission, where precise balance of the exciton between different emitter species control the emitted color. We close this paper with discussion of non-linear effects at high excitation levels that, to date, limit the high brightness efficiency of phosphorescent OLEDs.
83 citations
TL;DR: Copper alloyed ZnS was investigated as a p-type, transparentconducting material composed of earth-abundant elements as mentioned in this paper, and the results showed that transparency and conductivity were comparable to some of the best reported P-type materialswithour bestfilmsexhibitingconductivitiesof54Scm.
Abstract: , Phone: þ011 1 510 486 6715, Fax: þ011 510 486 4995Copper alloyed ZnS was investigated as a p-type, transparentconducting material composed of earth-abundant elements.Thin films of Cu-alloyed ZnS were synthesized using pulsedlaserdepositionwithCucontentsintherangeofx¼0.06–0.27(Cu content x is reported as the fraction of cation present).Thermopower and Hall effect measurements show that thefilms are p-type. We find that transparency and conductivityare comparable to some of the best reported p-type materialswithourbestfilmsexhibitingconductivitiesof54Scm
81 citations
TL;DR: In this article, the defect densities in several high pressure high temperature (HPHT) and CVD diamond crystals were quantified and compared; in particular defects originating from polishing could be distinguished from extended defects inside the crystal.
Abstract: H2/O2 plasma treatments offer advantages over other etching processes of diamond as a technique to prepare the substrate surface prior to chemical vapor deposition (CVD) diamond growth. It allows removing defects induced on the surface by polishing, thus leading to an improved morphology and limiting the stress within the grown crystal. Moreover, they present the advantage to be performed in situ just before the CVD diamond growth. In this work, H2/O2 plasma treatments were performed so that threading dislocations and other defects are etched preferentially, thus leaving typical etch-pits. The defect densities in several high pressure high temperature (HPHT) and CVD diamond crystals were then quantified and compared; in particular defects originating from polishing could be distinguished from extended defects inside the crystal. Furthermore, the defect density was found to be of the order of 105/cm2 for HPHT crystals, which was approximately one order of magnitude lower than that measured in low cost commercial CVD monocrystals. The use of laser microscopy also allowed observing the morphology, size and depth of different etch-pits of 〈001〉-oriented and misoriented crystals and their evolution with etching time in order to get a better understanding of defect density and formation during CVD growth.
77 citations
TL;DR: In this paper, a solar cell fabricated with CZnSnS4 nanoparticles had a conversion efficiency of 0.25% using a one-pot synthesis method using microwave heating.
Abstract: Cu2ZnSnS4 (CZTS) nanoparticle inks were synthesized for the first time by a one-pot synthesis method using microwave heating. Precursor solutions were mixed and reacted at 190 °C for 30 min. Varying the initial concentration of the metal chlorides in ethylene glycol was necessary to avoid formation of copper rich and zinc poor nanoparticles. Analysis of the reaction supernatant indicates that Cu+ was fully utilized while significant amounts of Zn2+ remained unreacted. The CZTS nanoparticles were determined to be 7.6 ± 2.1 nm in diameter and were found to agglomerate into larger clusters. A solar cell fabricated with the CZTS nanoparticles had a conversion efficiency of 0.25%.
73 citations
TL;DR: In this article, the XRD and EDX results reveal that the lattice distortion exists in all the samples, possibly due to the defects and the intercalation of the residual sodium ions.
Abstract: Crystals of hexagonal tungsten oxides (hex-WO3) have been synthesized using hydrothermal method at 150 degrees C, assisted by the capping reagent of ammonium tartrate (AT). The XRD and EDX results reveal that the lattice distortion exists in all the samples, possibly due to the defects and the intercalation of the residual sodium ions. Different crystal shapes including plate-like, urchin-like, and particle structures were obtained by varying concentration of AT and pH values in the precursor solution. Beside the absorption action of the NH4+ and Na+ ions, the capping effect can be reinforced by the hydrogen bonding from the tartrate groups in the crystallization process. The bandgap energies were modulated by the size of the nanostructured hex-WO3 crystals due to quantum confinement effect, which increases from 2.74 to 3.04 eV. Based on the analysis of the photoluminescence and X-ray photoelectron spectroscopy, the enhancement of the blue emission of the nanocrystals is assigned as a result of a complex of the local intercalation of the residual sodium ions and the oxygen vacancies or defects. (C) 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
71 citations
TL;DR: In this paper, the loss of tin is investigated with respect to the influence of the selenization conditions such as substrate temperature and selenium partial pressure on the remaining decomposed layer.
Abstract: Cu2ZnSnSe4 (CZTSe) thin film solar cells are fabricated by a simple, non-vacuum deposition of metal salts dissolved in non-hazardous solvents followed by selenization in Se atmosphere. Despite a residual carbon-rich layer between the back contact and the CZTSe absorber layer, solar cells with up to 4.28% conversion efficiency are obtained for Cu-poor and Zn-rich CZTSe absorbers. A frequently reported problem, the loss of tin, is investigated with respect to the influence of the selenization conditions such as substrate temperature and selenium partial pressure. EDX point measurements directly confirm that the remaining decomposed layer consists of a mixture of binary ZnSe and Cu2−xSe phases if the substrate temperature is too high and not sufficient Se is supplied.
67 citations
TL;DR: The spinel ferrite systems NiFe2−xCrxO4 (with x = 0.0-1.0 in step of 0.2) were prepared by wet chemical co-precipitation method using sulphates of respective metal ions.
Abstract: The spinel ferrite systems NiFe2−xCrxO4 (with x = 0.0–1.0 in step of 0.2) were prepared by wet chemical co-precipitation method, using sulphates of respective metal ions. Formation of single phase spinel crystal structure of sample has been confirmed by X-ray diffraction method. Magnetic parameters such as coercivity and saturation magnetization are measured by employing vibrating samples magnetometer (VSM). The saturation magnetization and magneton number (nB) decreases with increase in Cr3+ content x. The Mossbauer spectra taken at 300 K and spectra are assigned to two strongly overlapping sextets corresponding to tetrahedral A sites and octahedral B sites. The cation distribution of ferrites system has been calculated by X-ray method, the behaviour of magnetization curve and Mossbauer spectra. The evaluation of cation distribution indicates that, in Cr-dominant region, few nickel ions occupies tetrahedral A sites which convert perfect inverse spinel to partially normal spinel structure.
TL;DR: In this paper, the authors investigated both electronic and optical properties of core-shell InGaN nanorods (NRs) for light emitting diodes (LEDs), and showed that the absence of piezoelectric fields on these nonpolar facets results in strong reabsorption of the emitted photons.
Abstract: In this theoretical study, we investigate both electronic and optical properties of core–shell InGaN nanorods (NRs) for light emitting diodes (LEDs). These structures feature active layers wrapped around high aspect ratio NR cores, thus allowing for an enormous increase in active area. Hence, efficiency may be increased by operating at lower carrier densities, mitigating efficiency droop known to limit conventional InGaN LEDs. Due to poor conductivity of the outer pGaN shell, current spreading along the entire NR length is challenging and requires an additional cover of transparent conductive oxide in order to fully exploit the active area. At the same time, quantum wells (QWs) on core–shell NRs are grown on m-plane rather than conventional c-plane GaN facets. The calculations show that the absence of piezoelectric fields on these non-polar facets results in strong reabsorption of the emitted photons. A comprehensive numerical model is applied, linking internal quantum efficiency (IQE) and light extraction via the process of photon recycling. In summary, increasing optical absorption and photon recycling losses limit extraction efficiency. While this partially compensates the gain in IQE with increasing active area, a net improvement of the external quantum efficiency (EQE) of +10% is achievable by the proposed design of a novel core–shell NR LED.
TL;DR: Label‐free electrical detection of consecutive deoxyribonucleic acid (DNA) hybridization/denaturation by means of an array of individually addressable field‐effect‐based nanoplate silicon‐on‐insulator (SOI) capacitors modified with gold nanoparticles (Au‐NP) is investigated.
Abstract: Label-free electrical detection of consecutive deoxyribonucleic acid (DNA) hybridization/denaturation by means of an array of individually addressable field-effect-based nanoplate silicon-on-insulator (SOI) capacitors modified with gold nanoparticles (Au-NP) is investigated. The proposed device detects charge changes on Au-NP/DNA hybrids induced by the hybridization or denaturation event. DNA hybridization was performed in a high ionic-strength solution to provide a high hybridization efficiency. On the other hand, to reduce the screening of the DNA charge by counter ions and to achieve a high sensitivity, the sensor signal induced by the hybridization and denaturation events was measured in a low ionic-strength solution. High sensor signals of about 120, 90, and 80 mV were registered after the DNA hybridization, denaturation, and re-hybridization events, respectively. Fluorescence microscopy has been applied as reference method to verify the DNA immobilization, hybridization, and denaturation processes. An electrostatic charge-plane model for potential changes at the gate surface of a nanoplate field-effect sensor induced by the DNA hybridization has been developed taking into account both the Debye length and the distance of the DNA charge from the gate surface.
TL;DR: In this article, X-ray diffraction measurements revealed that the polycrystalline films grew in (111 and (100) orientation on sapphire and in (100), orientation on MgO. Raman measurements indicated the presence of CuO inclusions.
Abstract: Cu2O thin films were grown on sapphire (0001) and MgO (100) substrates by chemical vapor deposition. The crystalline, vibrational and electrical properties of the layers and the amount of incorporated background impurities have been examined. X-ray diffraction measurements revealed, that the polycrystalline films grew in (111) and (100) orientation on sapphire and in (100) orientation on MgO. Raman measurements indicated the presence of CuO inclusions in the films. The electrical properties are dominated by an acceptor level located 150 meV above the valence band. This level may originate from unintentionally incorporated silicon impurities.
TL;DR: In this article, the authors employed 1.5'nm-thick AlGaN interlayer between an InGaN well layer and an upper GaN barrier layer to enhance the light output power of green light-emitting diodes.
Abstract: In order to enhance light output power of green light-emitting diodes (LEDs) on a sapphire (0001) substrate, we employ 1.5 nm-thick AlGaN interlayer between an InGaN well layer and an upper GaN barrier layer. AlGaN interlayer controls asymmetric band profile due to piezoelectric field, and thus it controls the quantum-confined Stark effect. Although both photoluminescence intensity and electroluminescence intensity of a conventional MQW decreases drastically as the emission wavelength becomes longer, the intensity drop at spectral range of 530–580 nm is suppressed by utilizing an AlGaN interlayer. The maximum light output power of 12 mW at 532 nm and external quantum efficiency of 25.4% have been achieved for the LED employing Al0.30Ga0.70N interlayer at a driving current of 20 mA. This result indicates that the band-engineering approach is effective for an IQE enhancement at pure green or longer wavelength, even if a polar (0001) face is utilized.
TL;DR: In this article, the anodic alumina films were amorphous and acted as dielectric material in a solid state capacitor with Au top electrode, and the capacitors characteristics were evaluated using IV curves and frequency domain measurements.
Abstract: Ultrapure aluminium was thermally evaporated onto various plastics (polyethylene 2.6-naphthalate, PEN; polyethylene terephthalate, PET; polyimide, PI and glass for comparison) and potentiostatically anodized in a citric buffer. The anodisation procedure was monitored coulometrically and each alumina film formed was characterized by impedance spectroscopy. The resulting anodic alumina films were amorphous (proven by X-ray diffraction, XRD) and acted as dielectric material in a solid state capacitor with Au top electrode. The capacitors characteristics were evaluated using IV curves and frequency domain measurements. The performance of the capacitors demonstrated low leakage currents and low dielectric losses. The contrary properties capacity and breakdown voltage could be chosen by selecting the anodisation voltage. For each substrate apparent oxide formation factors and capacities were determined coulometrically. The ratio between apparent formation factor and projected area allowed determining the surface roughness. This surface roughness together with the high purity aluminium films and the anodisation compression was responsible for the unexpected high mechanical stability of this composite material.
TL;DR: In this paper, the surface morphologies of ZnO nanoflakes at different precursor concentrations were studied using scanning electron microscopy (SEM) and combined studies of SEM, photoluminescence (PL), and Raman spectroscopy suggested that nanorods started to grow along with nanofsakes at a precursor concentration of 0.05 M and the density of the nanors significantly increased when the concentration was raised to 0.075 M.
Abstract: Two-dimensional (2D) ZnO nanoflakes were grown on thin aluminum layer, deposited on silicon substrate, using a sol-gel method. The surface morphologies of ZnO nanoflakes at different precursor concentrations were studied using scanning electron microscopy (SEM). Combined studies of SEM, photoluminescence (PL), and Raman spectroscopy suggested that nanorods started to grow along with nanoflakes at a precursor concentration of 0.05 M and the density of the nanorods significantly increased when the concentration was raised to 0.075 M. Both the UV-luminescence and Raman spectra were intensified and redshifted with the increment of precursor concentration. Spectral intensification suggests improvement in crystal qualities and better optical properties of the fabricated ZnO nanostructures. The structural defects at lower levels of precursor were probably due to the hypoxic environment, whereas, the redshift of PL and Raman spectra was due to the local heating of ZnO nanocrystals.
TL;DR: In this paper, the authors reported the computational modeling of localized surface plasmon effects arising on gold nanoparticles deposited on silicon nanohole (SiNH) textured surface for thin film silicon solar cells application.
Abstract: We report the computational modeling of localized surface plasmon effects arising on gold (Au) nanoparticles deposited on silicon nanohole (SiNH) textured surface for thin film silicon solar cells application. Detailed balance analysis is carried out for the limiting efficiency of an optimized SiNH array textured surface in combination with the surface and bottom-of-a-trench Au nanoparticle array described herein. We found that for the proposed geometry of the solar cell, the short circuit current density (JSC) and the power conversion efficiency (PCE) are 31.57 mA/cm2 and 25.42%, respectively, that compare favorably to the predicted JSC and PCE values of 25.45 mA/cm2 and 20.87% for an optimized SiNH textured surface without Au nanoparticles. We optimized the silicon dioxide/silicon nitride (SiO2/Si3N4) stack as a passivation layer, retaining the higher optical absorption. The scattering of incident radiation by the Au nanoparticles near their localized plasmon resonance is responsible for higher optical absorption and hence the higher predicted values for JSC and PCE.
TL;DR: In this paper, the authors show that resistive switching mechanism is consistent with the formation and rupture of conducting filaments and demonstrate the potential to fabricate high-density RRAM devices by using this material.
Abstract: Resistive random access memory (RRAM) devices are made by copper nitride films prepared by DC magnetron sputtering. After a forming process, the CuxN-based RRAM devices show bipolar character with low operation voltage and distinguishable resistance ratio. The fitting results for the electrical measurements and the conducting atomic force microscope (CAFM) analysis indicate that resistive switching mechanism is consistent with the formation and rupture of conducting filaments. Simultaneously, the distribution of conducting filaments measured by CAFM reveals a promising potential to fabricate high-density RRAM device by using this material. The mechanism of the formation of conducting filaments is discussed based on our results.
TL;DR: In this paper, the magnetic properties of rare earth (RE) doped ceria (CeO2) samples have been investigated and reported and the origin of magnetism in these samples can be related to oxygen vacancies and formation of fluorite crystal structure.
Abstract: Magnetic properties of rare earth (RE) doped ceria (RE = Nd, Sm, Gd, Tb, Er and Dy) samples have been investigated and reported in this paper. Room temperature ferromagnetism (FM) was observed in calcined powders as well as in sintered samples of Nd and Sm doped CeO2, whereas other RE dopants (Gd, Tb, Er and Dy) in CeO2 exhibit paramagnetic behaviour. The origin of magnetism in these samples can be related to oxygen vacancies and formation of fluorite crystal structure. Though the magnetization was found to be lower as compared to transition metal (TM) doped ceria, the segregation of metallic and secondary phases can be avoided in RE doped CeO2. CeO2 doped with Sm ions in different concentration (1, 5 and 15%) were also studied to see the dopant effects on magnetic properties. The origin of magnetism in these samples may be related to the oxygen vacancies created due to RE dopants, which was confirmed from the peak around 555 cm−1 in Raman spectra.
TL;DR: In this article, the effect of copper-doped TiO2 in dye-sensitized solar cells (DSSCs) was analyzed with several instrumental techniques, including X-ray diffraction (XRD), inductively coupled plasma atomic emission spectroscopy (ICP-AES), scanning transmission electron microscopy (STEM), and UV-Vis spectroscopic analysis.
Abstract: TiO2 doping has been widely used in photocatalysis and photovoltaic cells to improve the performance of this semiconductor. This paper studies the use of copper as a dopant in TiO2 in dye-sensitized solar cells (DSSC), analysing the effect on the photovoltaic properties of the cells of different concentrations of copper incorporated into the semiconductor. The copper-doped TiO2 semiconductor was characterized with several instrumental techniques, including X-ray diffraction (XRD), inductively coupled plasma atomic emission spectroscopy (ICP-AES), scanning transmission electron microscopy (STEM), and UV–Vis spectroscopy in order to know its structure, composition and band gap energies with different concentrations of the dopant. An analysis was also performed of the variations in open-circuit voltage depending on the concentration of copper. This showed that the presence of copper in DSSCs made with a standard configuration – using a ruthenium complex (N3) as a dye and the redox pair as the electrolyte with 3-methoxypropionitrile as a solvent – leads to improvements of up to 10% in the open-circuit voltage of DSSCs.
TL;DR: In this article, a method to tune the lateral and vertical dimensions of the etched crater structures by combined variations of ZnO layer thickness and etching time was presented. But the effect of these morphologies on the performance of the solar cells, prepared in both p-i-n and n-i−p configurations, was analyzed.
Abstract: ZnO layers with a wide range of surface morphologies were studied for applications in thin film silicon solar cells. The texture of ZnO layers, achieved by chemical etching, was measured by atomic force microscopy (AFM) and statistically analyzed in terms of the roughness, the crater depth, the crater diameter, and the crater opening angle. We present a method to independently tune the lateral and vertical dimensions of the etched crater structures by combined variations of ZnO layer thickness and etching time. The optical scattering properties of the textured ZnO layers are investigated. The effects of these morphologies on the performance of the solar cells, prepared in both p–i–n and n–i–p configurations are analyzed. For the latter cell type the roughness is introduced at the back side, thus the cell does not sense ZnO absorption and conductivity of the different morphologies. The link between the morphology, the optical properties of ZnO layers, and the solar cell performance is discussed.
TL;DR: In this article, the Boltzmann transport equation was used to calculate the electrical and thermal properties of nanostructured p-type SiGe alloys and the effect of various parameters of milling process and sintering conditions on the thermoelectric properties of the grown samples were studied.
Abstract: Nanostructured silicon germanium thermoelectric materials prepared by mechanical alloying and sintering method have recently shown large enhancement in figure-of-merit, ZT. The fabrication of these structures often involves many parameters whose understanding and precise control is required to attain large ZT. In order to find the optimum parameters for further enhancing the ZT of this material, we have grown and studied both experimentally and theoretically different nanostructured p-type SiGe alloys. The effect of various parameters of milling process and sintering conditions on the thermoelectric properties of the grown samples were studied. The electrical and thermal properties were calculated using Boltzmann transport equation and were compared with the data of nanostructured and crystalline SiGe. It was found that the thermal conductivity not only depends on the average crystallite size in the bulk material, but also it is a strong function of alloying, porosity, and doping concentration. The Seebeck coefficient showed weak dependency on average crystallite size. The electrical conductivity changed strongly with synthesis parameters. Therefore, depending on the synthesis parameters the figure-of-merit reduced or increased by ∼60% compared with that of the crystalline SiGe. The model calculation showed that the lattice part of thermal conductivity in the nanostructured sample makes ∼80% of the total thermal conductivity. In addition, the model calculation showed that while the room temperature hole mean free path (MFP) in the nanostructured sample is dominated by the crystallite boundary scattering, at high temperature the MFP is dominated by acoustic phonon scattering. Therefore, the thermal conductivity can be further reduced by smaller crystallite size without significantly affecting the electrical conductivity in order to further enhance ZT.
TL;DR: In this paper, a comparison between the use of graphene oxide (GO)/poly(ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) and reduced graphene oxide(rGO)/Poly(poly(stylenedioxymethyliophene)-PSS as a hole extraction layer (HEL) was made.
Abstract: A comparison was made between the use of graphene oxide (GO)/poly(ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) and reduced graphene oxide (rGO)/PEDOT:PSS as a hole extraction layer (HEL) in organic photovoltaic (OPV) cells. Hydrazine hydrate (HYD) and the thermal method were adopted to change the GO to rGO. The OPV cell with the GO (∼2 nm)/PEDOT:PSS HEL exhibits a power conversion efficiency (PCE) as high as 3.53% under 100 mW/cm2 illumination and air mass conditions, which is higher than that of the OPV cell without the HEL, viz. 1.78%. The device with the PEDOT:PSS/GO HEL shows a similar PCE of 3.48%. However, the PCE of the OPV cell with the rGO/PEDOT:PSS HEL is not high as those of the cells with the HYD-rGO/PEDOT:PSS and T-rGO/PEDOT:PSS, viz. 3.3 and 3.37%, respectively. The work function of GO was 4.7 eV, but those of HYD-rGO and T-rGO were 4.2 and 4.5 eV, respectively, suggesting that the decrease of the barrier height between GO and active materials is higher than that in rGO case.
TL;DR: In this article, the authors report on the results of experimental and theoretical study of a promising way for suppression of the efficiency droop with current in InGaN-based light emitting diodes.
Abstract: In this paper, we report on the results of experimental and theoretical study of a promising way for suppression of the efficiency droop with current in InGaN-based light emitting diodes. Simulations carried out using a drift-diffusion approach with quantum-mechanical corrections clearly show that non-radiative Auger recombination is the principal mechanism limiting the device performance at high-injection level. New design of LED heterostructure with short-period superlattice in the active region is proposed and assessed theoretically. Experimentally, the implementation of the structure design in high-power devices has resulted in substantial suppression of the efficiency droop compared to conventional multiquantum-well InGaN LEDs.
TL;DR: In this paper, the authors investigated the utility of mechanical rubbing for macroscopic scale alignment of colloidal semiconductor nanorods, exhibiting linearly polarized emission, are aligned by mechanical rubbing of a spin-coated glass substrate, the dragging force exerted by the rubbing fibers results in deflection and reorientation of the NRs along the rubbing direction.
Abstract: Large-scale lateral alignment of nanorods (NRs) is of interest for manifestation of their anistropic properties including polarized emission and directional electrical transport. This study investigates the utility of mechanical rubbing for macroscopic scale alignment of colloidal semiconductor NRs. CdSe/CdS seeded-rods, exhibiting linearly polarized emission, are aligned by mechanical rubbing of a spin-coated glass substrate. The dragging force exerted by the rubbing fibers results in deflection and reorientation of the NRs along the rubbing direction. The rubbed samples were characterized by various methods including absorption, polarized emission, optical fluorescence microscopy, atomic force microscopy, and ultra-high resolution scanning electron microscopy. The emission polarization contrast ratio (CR), defined as the ratio between emission intensities parallel and perpendicular to the rubbing direction, was used to characterize the rods alignment. The effects of substrate treatments on the CR were studied, showing that partially hydrophobic surface provides optimal conditions for alignment. Excess organic ligands added to the deposited NR solution strongly affect the extent of alignment. This was studied for a series of NR samples of different dimensions and an optimal additive ratio of ∼3 ligand molecules per 1 nm2 NR surface area was found to yield the highest CR. Average CR values of 3.5 were detected over the entire 6 cm2 substrate area, with local values exceeding 4.5. While samples of rubbed spherical quantum dots and spin-coated films of NRs show no emission polarization, the emission intensity from rubbed NR samples is polarized obeying Malus' law (wherein, the intensity is proportional to cos2(θ)). Mechanical rubbing, well known for its use in LC devices, may be considered as a method for large-scale alignment of NRs on substrates.
TL;DR: In this paper, the authors discuss how the charge injection into an organic semiconductors can be enhanced by the insertion of a thin interfacial layer or an electrically doped OS layer between an electrode and an undoped OS.
Abstract: Interfacial doping in organic semiconductors (OSs) is an important technique to achieve efficient organic electronic devices. In this paper, we discuss how the charge injection into an OS can be enhanced by the insertion of a thin interfacial layer or an electrically doped OS layer between an electrode and an undoped OS. We present that the vacuum level shift and Fermi level modification by the electrical doping is the origin of the efficient charge injection through a metal–organic junction and an organic–organic junction. Application to organic electronics such as organic light-emitting diodes (OLEDs) and organic photovoltaics (OPVs) is briefly summarized.
TL;DR: In this article, the in-plane transport properties were measured at room temperature, the thermopower was 130 µV K−1 for Sb2Te3 and −153 ǫVǫK −1 for Bi 2Te3 thin films.
Abstract: Sb2Te3 and Bi2Te3 thin films were grown at room temperature on SiO2 substrates using MBE and were subsequently annealed at 250 °C. The films were stoichiometric, polycrystalline, textured, and yielded strikingly low charge carrier densities of about 2.7 × 1019 cm−3. The in-plane transport properties were measured at room temperature, the thermopower was 130 µV K−1 for Sb2Te3 and −153 µV K−1 for Bi2Te3 thin films. The small charge carrier densities are explained by a reduced antisite defect density due to the low temperatures to which the thin films were exposed during annealing.
TL;DR: NDs have faceted surfaces that can mediate potent interactions with surrounding water molecules or therapeutic compounds that can mitigate premature drug release to prevent major side effects such as myelosuppression, while water molecule recruitment can increase the relaxivity of covalently conjugated gadolinium.
Abstract: A spectrum of materials has been explored as potential platforms for nanomedicine in the areas of drug delivery, imaging, and diagnostics/sensing. Depending upon the application required, the therapeutic compound being delivered, or the imaging agent being modified, certain nanoparticulate materials offer specific advantages that may uniquely enhance efficiency. Among the classes of particles being investigated, diamond-based platforms have emerged as promising vehicles for drug delivery and imaging following several recent studies that demonstrate their ability to enhance therapeutic efficacy, particularly for anthracyclines, mediate markedly improved magnetic resonance imaging contrast and photostable fluorescence, possess scalable processing parameters, and exhibit biocompatibility, among many other important attributes. More specifically, detonation NDs have faceted surfaces that can mediate potent interactions with surrounding water molecules or therapeutic compounds. This attribute can mitigate premature drug release to prevent major side effects such as myelosuppression, while water molecule recruitment can increase the relaxivity of covalently conjugated gadolinium. These properties of NDs and diamond-based devices will be discussed in this article, with a focus on therapeutic delivery in addition to insight on their use in imaging, devices, implants/coatings, and biocompatibility. Key areas of potential, where advances in biology and medicine can be realized through the continued development of this platform as well as requisite future studies, will also be highlighted.
TL;DR: In this paper, the authors used deep level transient spectroscopy (DLTS) to distinguish between isolated point defects, small precipitate complexes and decorated extended defects in single crystal and multi-crystalline solar silicon.
Abstract: Electronic grade Czochralski and float zone silicon in the as grown state have a very low concentration of recombination generation centers (typically <1010 cm−3). Consequently, in integrated circuit technologies using such material, electrically active inadvertent impurities and structural defects are rarely detectable. The quest for cheap photovoltaic cells has led to the use of less pure silicon, multi-crystalline material, and low cost processing for solar applications. Cells made in this way have significant extrinsic recombination mechanisms. In this paper we review recombination involving defects and impurities in single crystal and in multi-crystalline solar silicon. Our main techniques for this work are recombination lifetime mapping measurements using microwave detected photoconductivity decay and variants of deep level transient spectroscopy (DLTS). In particular, we use Laplace DLTS to distinguish between isolated point defects, small precipitate complexes and decorated extended defects. We compare the behavior of some common metallic contaminants in solar silicon in relation to their effect on carrier lifetime and cell efficiency. Finally, we consider the role of hydrogen passivation in relation to transition metal contaminants, grain boundaries and dislocations. We conclude that recombination via point defects can be significant but in most multi-crystalline material the dominant recombination path is via decorated dislocation clusters within grains with little contribution to the overall recombination from grain boundaries.