Hole Mobility Ratio Research Articles

Unraveling carrier distribution in far-UVC LEDs by temperature-dependent electroluminescence measurements

The hole transport and the carrier distribution in AlGaN-based far-ultraviolett (UVC) light emitting diodes (LEDs) emitting around 233 nm was investigated. Temperature-dependent electroluminescence measurements on dual wavelength AlGaN multiple quantum well (MQW) LEDs show a strong shift in the spectral power distribution from 250 to 233 nm with decreasing temperature. Comparing experimental data with simulation shows that the hole mobility and the electron to hole mobility ratios have a significant influence on the carrier injection efficiency (CIE) and that the change in the spectral power distribution is originating from a change in the hole distribution in the MQWs. Poor hole injection and charge carrier confinement in the AlGaN MQW active region was identified as one of the main reasons for the low CIE in far-UVC LEDs.

The Impact of Polymorphism on Charge Transport Properties for Pyrene-FxTCNQ Cocrystals

Research on the charge transport properties of organic semiconductor materials is crucial for the development of organic sensors. In this paper, we investigated the charge transport properties of pyrene-TCNQ, pyrene-F2TCNQ, and pyrene-F4TCNQ cocrystals using the quantum nuclear tunneling model and Monte Carlo simulations to calculate charge mobility. At room temperature, we observed hole mobility values is 0.658 cm2V-1s-1 and electron mobility values is 1.492 cm2V-1s-1, respectively, for pyrene-TCNQ. As the number of F atoms in TCNQ increases, we observed an increase in both hole and electron mobility. Our results showed that the electron to hole mobility ratio was approximately 2.2 for pyrene-TCNQ, 0.6 for pyrene-F2TCNQ, and 0.5 for pyrene-F4TCNQ. Our findings suggest that the introduction of F atoms causes changes in the TCNQ molecular structure, cocrystal stacking mode, adjacent molecular distance, and intermolecular polarization. These changes lead to a modification of the reorganization energy and transfer integrals, ultimately affecting charge transport. Additionally, our results showed that transport along the π-π direction in the three cocrystals exhibited strong anisotropic characteristics, which were much larger than those observed in the other two directions.

Comparison between optical and electrophysical data on hole concentration in zinc doped p-GaAs

Optical and electrophysical properties of Cz-grown zinc doped p-GaAs samples have been investigated. Middle-infrared reflection spectra of ten p-GaAs samples have been obtained. Galvanomagnetic Van der Pau measurements have been made on these samples also, and the values of resistivity and Hall coefficient have been calculated. All experiments have been carried out at room temperature.Reflection spectra have been processed by Kramers–Kronig relations. The spectral dependences of real and imaginary parts of complex dielectric permittivity have been obtained and loss function has been calculated. The value of characteristic wave number corresponding to high-frequency plasmon-phonon mode has been determined by loss function maximum position.The theoretical calculations have been made, and the dependence has been obtained which gave the possibility to determine heavy hole concentration value at T = 295K by the value of characteristic wave number. Then by comparison of optical and Hall data the values of light hole mobility to heavy hole mobility ratio have been determined. This mobility ratio has been shown to be equal to (1.9–2.8) which is considerably less, than predicted theoretical value based on assumption that both light and heavy holes are scattered by optical phonons. It has been suggested that scattering mechanisms of light and heavy holes might be quite different.

Comparison between optical and electrical data on hole concentration in zinc-doped p-GaAs

The optical and electrical properties of zinc-doped Cz p-GaAs have been studied. Reflection spectra of ten p-GaAs specimens have been taken in the mid-IR region. Van der Pau galvanomagnetic, electrical resistivity and Hall coefficient measurements have been carried out for the same specimens (all the measurements were carried out at room temperature). The reflection spectra have been processed using the Kramers–Kronig relations, spectral dependences of the real and imaginary parts of the complex dielectric permeability have been calculated and loss function curves have been plotted. The loss function maximum position has been used to calculate the characteristic wavenumber corresponding to the high-frequency plasmon-phonon mode frequency. Theoretical calculations have been conducted and a calibration curve has been built up for determining heavy hole concentration in p-GaAs at T = 295 K based on known characteristic wavenumber. Further matching of the optical and Hall data has been used for determining the light to heavy hole mobility ratio. This ratio proves to be in the 1.9–2.8 range which is far lower as compared with theoretical predictions in the assumption of the same scattering mechanism for light and heavy holes (at optical phonons). It has been hypothesized that the scattering mechanisms for light and heavy holes differ.

Anisotropy in colossal piezoelectricity, giant Rashba effect and ultrahigh carrier mobility in Janus structures of quintuple Bi2X3 (X = S, Se) monolayers

Due to the asymmetric structures, two-dimensional Janus materials have gained significant attention in research for their intriguing piezoelectric and spintronic properties. In the present work, quintuple Bi2X3 (X = S, Se) monolayers (MLs) have been modified to create stable Janus Bi2X2Y (X ≠ Y = S, Se) MLs that display piezoelectricity in both the planes along with Rashba effect. The out-of-plane piezoelectric constant (d 33) is 41.18 (−173.14) pm V−1, while the in-plane piezoelectric constant (d 22) is 5.23 (6.21) pm V−1 for Janus Bi2S2Se (Bi2Se2S) ML. Including spin–orbit coupling in the Janus MLs results in anisotropic giant Rashba spin splitting (RSS) at the Γ point in the valence band, with RSS proportional to d 33. The Rashba constant along the Γ–K path, , is 3.30 (2.27) eV Å, whereas along Γ–M, is 3.58 (3.60) eV Å for Janus Bi2S2Se (Bi2Se2S) ML. The MLs exhibit ultrahigh electron mobility (∼5442 cm2V−1s−1) and have electron to hole mobility ratio of more than 2 due to their tiny electron-effective masses. The flexibility of the MLs allows for a signification alteration in its properties, like band gap, piezoelectric coefficient, and Rashba constant, via mechanical (biaxial) strain. For the MLs, band gap and d 33 value are enhanced with compressive strain. The d33 value of Janus Bi2Se2S reaches 4886.51 pm V−1 under compressive strain. The coexistence of anisotropic colossal out-of-plane piezoelectricity, giant RSS, and ultrahigh carrier mobilities in Janus Bi2S2Se and Bi2Se2S MLs showcase their tremendous prospects in nanoelectronic, piezotronics, and spintronics devices.

Determining spectral response of the National Ignition Facility particle time of flight diagnostic to x rays.

The Particle Time of Flight (PTOF) diagnostic is a chemical vapor deposition diamond detector used for measuring multiple nuclear bang times at the National Ignition Facility. Due to the non-trivial, polycrystalline structure of these detectors, individual characterization and measurement are required to interrogate the sensitivity and behavior of charge carriers. In this paper, a process is developed for determining the x-ray sensitivity of PTOF detectors and relating it to the intrinsic properties of the detector. We demonstrate that the diamond sample measured has a significant non-homogeneity in its properties, with the charge collection well described by a linear model ax + b, where a = 0.63 ± 0.16V-1mm-1 and b = 0.00 ± 0.04V-1. We also use this method to confirm an electron to hole mobility ratio of 1.5 ± 1.0 and an effective bandgap of 1.8eV rather than the theoretical 5.5eV, leading to a large sensitivity increase.

Bandgap engineered 1.48 eV GaAs0.95P0.05 solar cell with enhanced efficiency using double BSF layer

One of the most important criteria to design more than 30% efficient III–V compound/Si based dual junction solar cell is that we must design atleast 20% efficient III–V compound material top cell. In this regard, we designed a bandgap engineered GaAs0.95P0.05 single junction solar cell with reduced bandgap of (E g ) = 1.48 eV. Reducing the bandgap from 1.72 eV to 1.48 eV for GaAs0.95P0.05 cell leads to generate higher short circuit current, while having the tradeoff with the open circuit voltage. Due to small change in lattice constant of GaAs0.95P0.05 cell, some recombination is observed near the junction area. Although the minimal degradation is observed in open circuit voltage, the higher short circuit current drives the overall efficiency of the GaAs0.95P0.05 single junction solar cell. The designed solar cell provides an extended internal absorption for longer wavelength of spectrum. The high electron mobility of 8500 cm2 V–S−1 was observed with very high electron to hole mobility ratio of 21.25. The optimization of the cell is done using two back surface field layers (AlInP and AlGaInP) of higher bandgap material. The high short circuit current density of J SC = 25.93 mA cm−2 with V OC = 1.1635 V achieved by the designed cell with the highest efficiency of η = 25%. The solar cell is irradiated under 1-Sun solar irradiation in the AM1.5 G environment providing 1000 W m−2 of power spectral density. The External and Internal Quantum efficiency of more than 95% is achieved by the designed solar cell.

Magneto-Seebeck effect in bismuth

Thermoelectricity was discovered almost two centuries ago in bismuth. The large and negative Seebeck coefficient of this semimetal remains almost flat between 300 K and 100 K. This striking feature can be understood by considering the ratio of electron and hole mobilities and the evolution of their equal densities with temperature. The large and anisotropic magneto-Seebeck effect in bismuth, on the other hand, has not been understood up to the present day. Here, we report on a systematic study of the thermopower of bismuth from room temperature down to 20 K upon application of a magnetic field of 13.8 T in the binary-bisectrix plane. The amplitude of the Seebeck coefficient depends on the orientation of the magnetic field and the anisotropy changes sign with decreasing temperature. The magneto-Seebeck effect becomes non-monotonic at low temperatures. When the magnetic field is oriented along the binary axis, the Seebeck coefficient is not the same for positive and negative fields. This so-called Umkehr effect arises because the high symmetry axes of the Fermi surface ellipsoids are neither parallel to each other nor to the high symmetry axes of the lattice. The complex evolution of thermopower can be accounted for in a large part of the ($T,B,\Theta$)-space by a model based on semiclassical transport theory and incorporating Landau quantization. The employed energy dependence of the scattering time is compatible with electron-acoustic phonon scattering. We find that the transverse Nernst response plays an important role in setting the amplitude of the longitudinal magneto-Seebeck effect. Furthermore, Landau quantization significantly affects thermoelectricity up to temperatures as high as 120 K.

In-Plane Optical and Electrical Anisotropy of 2D Black Arsenic.

Low-symmetry two-dimensional (2D) semiconductors have attracted great attention because of their rich in-plane anisotropic optical, electrical, and thermoelectric properties and potential applications in multifunctional nanoelectronic and optoelectronic devices. However, anisotropic 2D semiconductors with high performance are still very limited. Here, we report the systematic study of in-plane anisotropic properties in few-layered b-As that is a narrow-gap semiconductor, based on the experimental and theoretical investigations. According to experimental results, we have come up with a simple method for identifying the orientation of b-As crystals. Meanwhile, we show that the maximum mobility of electrons and holes was measured in the in-plane armchair (AC) direction. The measured maximum electron mobility ratio is about 2.68, and the hole mobility ratio is about 1.79.

Evaluating the ratio of electron and hole mobilities from a single bulk sample using Photo-Seebeck effect

When a semiconductor is under photoexcitation, the voltage response to a temperature gradient is the photo-Seebeck effect. Here we study this effect, focusing on the contribution from transport of photo-excited carriers. We demonstrate that by combining photo-Seebeck with photoconductivity measurements, one can determine the ratio between electron and hole mobilities, and hence both of them when one is known. This is found for the case of defect-free samples, where no detail on the absorbance, carrier lifetime or recombination is necessary. Our method reported here does not require chemical doping, which could introduce defects and is often not feasible. It applies to both thin film and bulk samples. Experiment-wise, photo-Seebeck effect is relatively easy to implement, or added to existing systems. In a broader context, for semiconductors with significant influence from defects, our result suggests that the photo-Seebeck behavior can still be understood. In this case another photo-transport property is necessary, in order to identify the mobilities of carriers and information regarding the defects. This framework integrates the information from photoexcitation and thermal gradients to provide a general method to determine fundamental electronic properties of materials.

Performance analysis of organic solar cells: Opto-electrical modeling and simulation

In this paper, the effect of mobility and active layer thickness on the properties of bulk heterojunction solar cells have been investigated using the drift-diffusion model. It was observed that open circuit voltage and power conversion efficiency are affected by two loss mechanisms. The recombination of charge carriers in wrong contacts destroys photogenerated electron-hole pairs, and the bulk recombination increases the act of coupling between electron-hole pairs with different binding energies and consequently lowers the open circuit voltage. The first loss mechanism rises as the mobility of each of the two carriers increases. The bulk recombination not only depends on the slow carrier mobility but also affected by the mobility balance of two carriers. Thus, it generates two optimal points for power conversion efficiency at none unity electron to hole mobility ratio. In the active layer, light does not get absorbed uniformly, and the profile of the photogenerated excitons depends on the device thickness. Therefore, similar changes in the electron and hole mobility do not bring about the same changes in power conversion efficiency. Also, this study indicates that with the simultaneous increase of mobility and thickness, considerable enhancement in the efficiency of the bulk heterojunction solar cells would be achieved.

Electroluminescence of a Polymer Film with a Polymer/Polymer Interface

The effect of a two-dimensional quantum-confined structure formed at a polymer/polymer interface on the radiative recombination of excitons (i.e., electroluminescence) is investigated. It is shown that the implementation of such a structure in a polymer film leads to an increase in the emission intensity by more than two orders of magnitude and a twofold decrease in the voltage threshold for double injection. It is established that the polymer/polymer interface serves as the front of exciton recombination, and its position inside the film affects the electroluminescence intensity. The position of the two-dimensional structure within the polymer film corresponding to the highest intensity of radiative recombination is a function of the ratio of electron and hole mobilities.

Organic Solar Cells Based on High Hole Mobility Conjugated Polymer and Nonfullerene Acceptor with Comparable Bandgaps and Suitable Energy Level Offsets Showing Significant Suppression of Jsc–Voc Trade‐Off

Herein, a high‐mobility polymer (Si25) pairing a nonfullerene acceptor (O‐IDTBR) is introduced to construct active layers of organic solar cells (OSCs). The OSCs based on Si25 and O‐IDTBR with comparable bandgaps of 1.61 eV show high open‐circuit voltage (Voc) of 1.03 V. Suitable energy level offsets between the donor and acceptor as well as sufficient photon absorbance by a 400 nm thick active layer afford a notable short‐circuit current (Jsc) of 21.11 mA cm−2, indicating a significantly suppressed trade‐off between Jsc and Voc among OSCs. In addition, notable high power conversion efficiency (PCE) between 10.2% and 11.54% can be achieved with thick blend films from 210 to 560 nm, a thickness range beneficial to pin‐hole free printing. The maximum PCE of 11.54% corresponds to a 400 nm thick blend film, which is a rare thickness for high‐efficiency nonfullerene‐based OSCs. The corresponding fill factors (FFs) are between 51.59% and 53.33%. The inferior FF is due to a very low electron–hole mobility ratio, offering space for future FF elevation. The results highlight the high Voc and Jsc potentials for thick‐film nonfullerene OSCs based on a high hole mobility donor as well as looking forward to a high electron mobility nonfullerene acceptor.

Mobility Anisotropy in Black Phosphorus MOSFETs With HfO2 Gate Dielectrics

Precise measurements of the mobility anisotropy along high-symmetry crystal axes in black phosphorus (BP) MOSFETs are reported. Locally back-gated BP MOSFETs with 13-nm HfO2 dielectric and channel length ranging from 0.3 to 0.7 $\mu \text{m}$ are fabricated. A single BP flake of a uniform thickness is exfoliated and etched along armchair (AC) and zigzag (ZZ) crystal axes, and the orientations are confirmed using optical and transmission electron microscopy analyses. The hole and electron mobilities along each direction are extracted using the transfer length method. The AC-to-ZZ hole mobility ratio is found to increase from 1.4 (1.5) to 2.0 (2.9) as the sheet concentration increased from $5.1\times 10^{\textsf {11}}$ to $1.9\times 10^{\textsf {12}}$ cm−2 at room temperature (77 K). The room-temperature electron mobility anisotropy is found to be similar to that for holes with an AC-to-ZZ mobility ratio increasing from 1.4 to 2.1 from $5.1\times 10^{\textsf {11}}$ to $1.9\times 10^{\textsf {12}}$ cm−2 though electrons showed only a very weak temperature dependence. A Boltzmann transport model is used to explain the concentration- and temperature-dependent mobility anisotropies which can be well described using a charge center scattering model.

Field-Effect Transistors Based on van-der-Waals-Grown and Dry-Transferred All-Inorganic Perovskite Ultrathin Platelets.

Nowadays, the research on perovskite transistors is still in its infancy, despite the fact that perovskite-based solar cells and light-emitting diodes have been widely investigated. Two major hurdles exist before obtaining reliable perovskite-based transistors: the processing difficulty for their sensitivity to polar solvents and unsatisfactory perovskite quality on the transistor platform. Here, for the first time, we report on high-performance all-inorganic perovskite FETs profiting from both van der Waals epitaxial boundary-free ultrathin single crystals and completely dry-processed transfer technique without chemical contaminant. These two crucial factors ensure the unprecedented high-quality perovskite channels. The achieved FET hole mobility and on-off ratio reach 0.32 cm2 V-1 s-1 and 6.7 × 103, respectively. Moreover, at the low temperature, the mobility and on-off ratio can be enhanced to be 1.04 cm2 V-1 s-1 and 1.3 × 104. This work could open the door for the FET applications based on perovskite single crystals.

Comparison of conventional and inverted organic photovoltaic devices with controlled illumination area and extraction layers

The impact of device architecture on organic photovoltaic device performance has been problematic to quantify due to extraction layer materials and device testing issues. In particular, published reports have used different pairs of hole and electron extraction layer materials for conventional versus inverted devices. The origins of the large apparent discrepancy in device J–V characteristics become difficult to understand, arising from differences in built-in potentials and in the extraction layers’ conductivities that can affect collection of stray current outside of device area. Here, we show that by using an identical pair of hole and electron extraction layers, and by precisely defining illuminated device area with a shadow mask, most of the differences in the performance of conventional and inverted P3HT:PCBM organic photovoltaic devices are eliminated. The remaining difference in short-circuit current density and EQE spectral shape can be explained primarily by the ratio of hole and electron mobility that leads to more efficient carrier extraction in the inverted devices, with a minor contribution from unintentional p-type doping of the active layer that influences the drift field profile.

A New D-A conjugated polymer P(PTQD-BDT) with PTQD acceptor and BDT donor units for BHJ polymer solar cells application

A novel donor–acceptor (D–A) copolymer comprising of weak electron donating BDT moiety and strong 9-(2-octyldodecyl)−8H-pyrrolo[3,4-b] bisthieno[2,3-f:3',2'-h] quinoxaline-8,10(9H)-dione (PTQD) unit denoted as P(PTQD-BDT) was synthesized as donor material for polymer solar cells. P(PTQD-BDT) shows a broad visible-near-infrared absorption band with an optical bandgap of 1.74 eV and possesses a relatively low-lying HOMO level at −5.28 eV. Bulk-heterojunction polymer solar cell with the optimized blend of 1:2 (weight ratio) P(PTQD-BDT):PC71BM (processed with chloroform) shows an open circuit voltage of 0.92 V, a short circuit current density of 7.84 mA/cm2, and a fill factor of 0.50, achieving a power conversion efficiency (PCE) of 3.61%. The PCE has been further improved to 5.55% (Jsc = 10.34 mA/cm2, Voc = 0.88V and FF = 0.61), when 3% vol 1,8-diio-dooctane (DIO) was used as solvent additive for the processing of P(PTQD-BDT):PC71BM blended film. The enhancement in Jsc is as a result of the appropriate morphology and efficient exciton dissociation into free charge carrier. The increase in PCE has been attributed to the favorable nanoscale morphology for efficient exciton dissociation and charge transport (reduction in the electron to hole mobility ratio). © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2015, 53, 2390–2398

A general design rule to manipulate photocarrier transport path in solar cells and its realization by the plasmonic-electrical effect.

It is well known that transport paths of photocarriers (electrons and holes) before collected by electrodes strongly affect bulk recombination and thus electrical properties of solar cells, including open-circuit voltage and fill factor. For boosting device performance, a general design rule, tailored to arbitrary electron to hole mobility ratio, is proposed to decide the transport paths of photocarriers. Due to a unique ability to localize and concentrate light, plasmonics is explored to manipulate photocarrier transport through spatially redistributing light absorption at the active layer of devices. Without changing the active materials, we conceive a plasmonic-electrical concept, which tunes electrical properties of solar cells via the plasmon-modified optical field distribution, to realize the design rule. Incorporating spectrally and spatially configurable metallic nanostructures, thin-film solar cells are theoretically modelled and experimentally fabricated to validate the design rule and verify the plasmonic-tunable electrical properties. The general design rule, together with the plasmonic-electrical effect, contributes to the evolution of emerging photovoltaics.

Correlation between charge carriers mobility and nanomorphology in a blend of P3HT/PCBM bulk heterojunction solar cell: Impact on recombination mechanisms

Sandwich configurations consisting of hole-only and electron-only devices are employed to study the transport properties of polymer/fullerene blended material. Upon thermal annealing at 150°C and subsequent device characterisations a direct correlation between solar cell device power conversion efficiency and charge carrier mobilities is identified. The highest power conversion efficiency reported herein resulted from the electron to hole mobility ratio of 4.78. This ratio further reduces to 0.357 at 175°C as a result of the deterioration of electron mobility at the same time as enhancement in hole carrier mobility with no further improvement in PCE. Effects of thermal annealing on trap density reveal that the trap assisted recombination is more dominant as compared to the bimolecular recombination. Furthermore such a mechanism enhances as a result of increasing trap density at higher annealing temperatures. The effect of light intensity on the open circuit voltage VOC further verifies that the dominant recombination process is through trap assisted recombination.

A Contribution of Thermoelectric Properties of the Quaternary Chalcogenide Compound Tl_2GaInSe_4 Crystal

Thermoelectric transport measurements were made on single crystal samples of Tl2GaInSe4. The crystal was prepared by a special design based on the Bridgman technique. Measurements of thermoelectric power were carried out in a special high vacuum-tight calorimeter when the direction of temperature gradient is perpendicular to the cleavage plane. The measurements covered a temperature range extending from 300 to 725 K. The results indicate P -type conductivity for our investigated samples. At room temperature the value of thermoelectric power was 735 μV/deg. The electron to hole mobility ratio was found to be 1.35. The e ective mass of holes at room temperature was evaluated as 4.635×10−29 kg, while for electron was equal to 8.468×10−31 kg. The relaxation time of majority and minority carriers was estimated as τp = 2.968×10−10 s and τn = 7.326×10−12 s, respectively. Also, the di usion coe cient of holes and electrons at room temperature was calculated and found to be 265.132 cm/s and 358.139 cm/s, respectively. The di usion length of holes and electrons are found to be Lp = 2.805× 10−4 cm and Ln = 5.122 × 10−5 cm. In addition to these pronounced parameters, the e ciency of thermoelectric element ( gure of merit) was evaluated which leads to better applications in many elds.