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Results for “HOLE MOBILITY”

Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 91 records · Page 5
All sourcesNASA NTRSDOE OSTI
DOE OSTI2018

Te-doped perovskite NaTaO3 as a promising photocatalytic material for hydrogen production from water splitting driven by visible light

Highlights: • S, Se and Te-doped NaTaO{sub 3} is a promising material for photocatalytic water splitting. • Enhanced absorption coefficient in the visible light range is identified for Te-doped NaTaO{sub 3}. • Te-doped NaTaO{sub 3} exhibits higher electron and hole mobility than pristine NaTaO{sub 3}. - Abstract: The photocatalytic hydrogen production from water splitting can be achieved by using the pristine NaTaO{sub 3} experimentally, but the reaction cannot be driven by the visible light because of the wide band gap of NaTaO{sub 3}. The present work focuses on tuning the photocatalytic performance of NaTaO{sub 3} into the visible light range by doping S, Se, and Te elements. The obvious decrease in band energy gaps (E{sub g}s) and noticeable red-shift of the absorption edges are observed for the doped structures. The valence band maximum and conduction band minimum of the doped structures satisfy the requirement of the water splitting. Moreover, a significant enhancement of absorption coefficient in the visible light range is observed for the doped structure of Te@O. The calculated carrier mobility is used to evaluate the charge spatial separation of Te@O and the unusual increases and the obvious differences of the mobility of the electrons and holes are observed.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2013

Investigation of electronic properties of the thienoacenes using first principles methods

In the present work, an examination of the electronic properties of pentacene, dinaphtho[2,3-b:2',3'-f]thieno[3,2-b]thiophene (DNTT) and dianthra[2,3-b:2',3'-f]thieno[3,2-b]thiophene (DATT) using ab initio methodology is reported. According to the results, the herringbone packing and π-stack orientation play an important role in the hole mobilities for DNTT and DATT. Band structure of pentacene crystals is more anisotropic than DNTT and DATT and the highest values for the dispersion are found in the herrigbone plane. For the band transport regime, it is demonstrated that the DATT must have carrier mobilities larger than pentacene and smaller than that found in DNTT.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2016

Photo-electrical and transport properties of hydrothermal ZnO

We performed the studies of optical, photoelectric, and transport properties of a hydrothermal bulk n-type ZnO crystal by using the contactless optical techniques: photoluminescence, light-induced transient grating, and differential reflectivity. Optical studies revealed bound exciton and defect-related transitions between the donor states (at ∼60 meV and ∼240 meV below the conduction band) and the deep acceptor states (at 0.52 eV above the valence band). The acceptor state was ascribed to V{sub Zn}, and its thermal activation energy of 0.43 eV was determined. A low value of carrier diffusion coefficient (∼0.1 cm{sup 2}/s) at low excitations and temperatures up to 800 K was attributed to impact the recharged deep acceptors. Electron and hole mobilities of 140 and ∼80 cm{sup 2}/Vs, correspondently, were determined at room temperature. The decrease of carrier lifetime with excitation was ascribed to increasing rate of radiative recombination at low temperatures and nonradiative recombination above the room temperature.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2024

Organic Nano-Junctions: Linking Nanomorphology and Charge Transport in Organic Semiconductor Nanoparticles for Organic Photovoltaic Devices

In this study, innovative nanoscale devices are developed to investigate the charge transport in organic semiconductor nanoparticles. Using different steps of lithography techniques and dielectrophoresis, planar organic nano-junctions are fabricated from which hole mobilities are extracted in a space charge-limited current regime. Subsequently, these devices are used to investigate the impact of the composition and morphology of organic semiconductor nanoparticles on the charge mobilities. Pure donor nanoparticles and composite donor:acceptor nanoparticles with different donor compositions in their shell are inserted in the nanogap electrode to form the nano-junctions. The results highlight that the hole mobilities in the composite nanoparticles decrease by two-fold compared to pure donor nanoparticles. However, no significant change between the two kinds of composite nanoparticle morphologies is observed, indicating that conduction pathways for the holes are as efficient for donor proportion in the shell from 40% to 60%. Organic photovoltaic (OPV) devices are fabricated from water-based colloidal inks containing the two composite nanoparticles (P3HT:eh-IDTBR and P3HT:o-IDTBR) and no significant change in the performances is observed in accordance with the mobility results. Through this study, the performance of OPV devices have been successfully correlated to the transport properties of nanoparticles having different morphology via innovative nanoscale devices.

42 ENGINEERING↗
DOE OSTI2014

Interface modified thermally stable hole transporting layer for efficient organic light emitting diodes

Electrical transport in thermally stable 2, 7-bis [N, N-bis (4-methoxy-phenyl) amino]-9, 9-spirobifluorene (MeO-Spiro-TPD) thin films has been investigated as a function of temperature and organic layer thickness. ITO/MeO-Spiro-TPD interface was found to be injection limited and has been studied in detail to find barrier height for hole injection. The thickness of tetra-fluoro-tetracyano-quinodimethane thin films were optimized to be used as hole injection buffer layer which resulted in switching of charge transport mechanism from injection limited to space charge limited conduction above a critical thickness of 3 nm. Hole mobility has been measured using transient space charge limited conduction (SCLC), field dependent SCLC, and top contact transistor characteristics. The charge carrier transport in interface modified hole only devices was analysed using Gaussian disorder model. The thermal stability of MeO-Spiro-TPD has been investigated by atomic force microscopy and X-ray diffraction studies. The study indicates a thermally stable and highly efficient hole transport material for application in organic semiconductor based devices.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2019

Repurposing DNA-binding agents as H-bonded organic semiconductors

Abstract Organic semiconductors are usually polycyclic aromatic hydrocarbons and their analogs containing heteroatom substitution. Bioinspired materials chemistry of organic electronics promises new charge transport mechanism and specific molecular recognition with biomolecules. We discover organic semiconductors from deoxyribonucleic acid topoisomerase inhibitors, featuring conjugated backbone decorated with hydrogen-bonding moieties distinct from common organic semiconductors. Using ellipticine as a model compound, we find that hydrogen bonds not only guide polymorph assembly, but are also critical to forming efficient charge transport pathways along π−conjugated planes when at a low dihedral angle by shortening the end-to-end distance of adjacent π planes. In the π−π stacking and hydrogen-bonding directions, the intrinsic, short-range hole mobilities reach as high as 6.5 cm 2 V −1 s −1 and 4.2 cm 2 V −1 s −1 measured by microwave conductivity, and the long-range apparent hole mobilities are up to 1.3 × 10 –3 cm 2 V −1 s −1 and 0.4 × 10 –3 cm 2 V −1 s −1 measured in field-effect transistors. We further demonstrate printed transistor devices and chemical sensors as potential applications.

59 BASIC BIOLOGICAL SCIENCES↗
DOE OSTI2022

Benzimidazolone-Dioxazine Pigments-Based Conjugated Polymers for Organic Field-Effect Transistor

Molecules based on benzimidazolone-dioxazine are known as blue/violet pigments and have been commercialized for decades. However, unfavorable solubility limits the application of these structures as building blocks of conjugated polymers despite their low band gaps. Herein, a series of donor–acceptor conjugated polymers containing soluble benzimidazolone-dioxazine structures as the acceptors and oligothiophene as donors are synthesized and investigated. With increasing numbers of thiophene rings, the steric hindrance diminishes and high molecular weight polymers can be achieved, leading to an improved performance in organic field effect transistor devices. The hole mobility of polymers with three to six thiophene units is in the order of 10 -1 cm2 V -1 s -1 . Among all the polymers, polymer P3 with three thiophene units between benzimidazolone-dioxazine structures shows the best hole mobility of 0.4 cm2 V -1 s -1 . Grazing-incidence wide-angle X-ray scattering results reveal that the high mobility of organic field-effect transistors (OFETs) can be accredited by matched donor–acceptor packing in the solid thin films.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2017

Complementary Semiconducting Polymer Blends: Influence of Side Chains of Matrix Polymers

The concept of complementary semiconducting polymer blends (c-SPBs) has been recently proposed to achieve enhanced solution processability and/or melt-processing capability for organic electronics. In the previous study, we demonstrated the impact of conjugation-break spacers of matrix polymers. In the current work, we explore the influence of the side chains of the matrix polymer on the physical properties of the pure polymers and their corresponding c-SPBs, including electrical properties and phase transition behaviors. Specifically, six diketopyrrolopyrrole (DPP)-based polymers with pentamethylene conjugation-break spacers (CBSs) and various side chains, including branched-alkyl, triethylene glycol (TEG), and siloxaneterminated side chains, were synthesized and characterized. The UV–vis spectra show that the side chains have a noticeable impact on the intermolecular interactions in the solid states. In addition, side chains also have a significant influence on the thermal behaviors of the polymers. Polymers with asymmetric side chains attached to the same DPP unit exhibit lower melting points compared to the congeners with symmetric side chains. The polymer with both branched-alkyl and TEG side chain exhibits the lowest melting point of 104 °C. As for charge transport properties, polymers with branched-alkyl and/or siloxaneterminated side chains give hole mobilities on the same order of magnitude, whereas the polymers with TEG side chains exhibits much lower mobilities. When c-SPBs with a fully conjugated polymer with branched-alkyl side chains are concerned, the c-SPBs of all polymers, except for the polymer with only TEG side chains (TEG-DPP-C5), show hole mobilities 2 orders of magnitude higher than the corresponding pure matrix polymers. In contrast, TEG-DPP-C5 merely presents an improvement of 20 times, which resulted from the incompatibility of TEG side chains from the matrix polymer and the alkyl side chains from the tie chain polymer. These results provide new insights into structural design for semiconducting materials with both high performance and better processability.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2025

Understanding ERE and iVOC Metrics for Graded CdSeTe Absorbers

PL-based external radiative efficiency (ERE) and implied open-circuit voltage (iVOC) metrics were introduced for thin-film solar absorbers to better understand the voltage deficit and diagnose losses in solar cells. Traditionally, elevated ERE and iVOC measurements are associated with diminished recombination within the solar device, a rationale heavily reliant on the assumption of a uniform bandgap and high carrier mobilities in the absorber. Recently, very low mobilities in CdSeTe absorbers (< 1 cm2/(V.s)) were measured using the light-induced transient grading technique. In this study, we use a detailed numerical model of iVOC to investigate the possible reasons of elevated iVOC in realistic CdSeTe absorbers with a graded Se profile. In particular, we examine how the bandgap nonuniformity and the reduced hole mobility in graded CdSeTe absorbers affect iVOC measurements. We show that high iVOC may result from inflated quasi-Fermi level splitting in the high-Se region in the front part of a CdSeTe absorber with slow hole transport. We reproduce the experimentally reported 360 mV increase in iVOC-VOC gap with reduced doping using a model with sub-1 cm2/(V.s) hole mobility in the high-Se region. Based on our results, we conclude that the iVOC metric (or ERE metric) should not be used as a sole metric of CdSeTe absorber quality. We discuss possible ways to extract useful information from the iVOC-VOC gap by supplementing the front-side illumination measurements with back-side illumination measurements.

14 SOLAR ENERGY↗
DOE OSTI2016

Highly anisotropic electronic transport properties of monolayer and bilayer phosphorene from first principles

The intrinsic carrier transport dynamics in phosphorene is theoretically examined. Utilizing a density functional theory treatment, the low-field mobility and the saturation velocity are characterized for both electrons and holes in the monolayer and bilayer structures. The analysis clearly elucidates the crystal orientation dependence manifested through the anisotropic band structure and the carrier-phonon scattering rates. In the monolayer, the hole mobility in the armchair direction is estimated to be approximately five times larger than in the zigzag direction at room temperature (460 cm{sup 2}/V s vs. 90 cm{sup 2}/V s). The bilayer transport, on the other hand, exhibits a more modest anisotropy with substantially higher mobilities (1610 cm{sup 2}/V s and 760 cm{sup 2}/V s, respectively). The calculations on the conduction-band electrons indicate a comparable dependence while the characteristic values are generally smaller by about a factor of two. The variation in the saturation velocity is found to be less pronounced. With the anticipated superior performance and the diminished anisotropy, few-layer phosphorene offers a promising opportunity particularly in p-type applications.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2022

Unravelling the Role of Electron Acceptors for the Universal Enhancement of Charge Transport in Quinoid-Donor-Acceptor Polymers for High-Performance Transistors

The quinoid-donor-acceptor (Q-D-A) strategy has recently emerged as a promising approach for constructing high mobility semiconducting polymers. In order to fully explore the potential of this strategy in improving the charge transport and elucidating the structure-property-performance relationships in Q-D-A polymers, a series of new polymers with different electron acceptor units and backbone coplanarity have been synthesized and characterized. In this work, all of the resulting Q-D-A polymers exhibit much more planar backbone conformations in comparison to their donor-acceptor (D-A) counterparts. Moreover, organic field-effect transistors based on Q-D-A polymers exhibit excellent effective hole mobilities in a range of 0.44 to 3.35 cm 2 V –1 s –1 , most of which are orders of magnitude higher than those of their corresponding D-A polymers. Notably, the hole mobility of 3.35 cm 2 V –1 s –1 is among the highest for the quinoidal-aromatic polymers characterized by conventional spin-coating methods. Furthermore, the role of electron acceptors in Q-D-A polymers has been comprehensively investigated. Polymers with stronger acceptor units are more inclined to deliver edge-on lamellas, high film crystallinity, small effective hole masses, and decent operational stability. The detailed structure-property-device performance relationship will pave the way toward high performance semiconducting polymers using the potent Q-D-A strategy.

36 MATERIALS SCIENCE↗
NASA NTRS1989

Highly mobile oxygen holes in magnesium oxide

High-purity MgO exhibits an unexpected giant anomaly of the apparent static dielectric constant and a positive surface charge of the order of 5 x 10 to the 21st/cu cm in the top 15 nm. It is postulated that the MgO matrix contains traces of peroxy defects, O2(2-), associated with Mg(2+) vacancies. Above approximately 400 C the O2(2-) dissociates to vacancy bound O(-) and highly mobile O(-) states, which diffuse to the surface, giving rise to a high surface conductivity.

Freund, Minoru M.↗
DOE OSTI2015

Annealing effect on donor-acceptor interface and its impact on the performance of organic photovoltaic devices based on PSiF-DBT copolymer and C{sub 60}

In this work, poly[2,7-(9,9-bis(2-ethylhexyl)-dibenzosilole)-alt-4,7-bis(thiophen-2-yl) benzo-2,1,3-thiadiazole] (PSiF-DBT) was used as active layer in bilayer solar cell with C{sub 60} as electron acceptor. As cast devices already show reasonable power conversion efficiency (PCE) that increases to 4% upon annealing at 100 °C. Space charge limited measurements of the hole mobility (μ) in PSiF-DBT give μ ∼ 1.0 × 10{sup −4} cm{sup 2}/(V s) which does not depend on the temperature of the annealing treatment. Moreover, positron annihilation spectroscopy experiments revealed that PSiF-DBT films are well stacked even without the thermal treatment. The variations in the transport of holes upon annealing are then small. As a consequence, the PCE rise was mainly induced by the increase of the polymer surface roughness that leads to a more effective interface for exciton dissociation at the PSiF-DBT/fullerene heterojunction.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2017

Single Crystal Microwires of p -DTS(FBTTh 2 ) 2 and Their Use in the Fabrication of Field-Effect Transistors and Photodetectors

Single crystal microwires of a well–studied organic semiconductor used in organic solar cells, namely p –DTS(FBTTh 2 ) 2 , are prepared via a self–assembly method in solution. The high level of intermolecular organization in the single crystals facilitates migration of charges, relative to solution–processed films, and provides insight into the intrinsic charge transport properties of p –DTS(FBTTh 2 ) 2 . Field–effect transistors based on the microwires can achieve hole mobilities on the order of ≈1.8 cm 2 V –1 s –1 . Furthermore, these microwires show photoresponsive electrical characteristics and can act as photoswitches, with switch ratios over 1000. Here, these experimental results are interpreted using theoretical simulations using an atomistic density functional theory approach. Based on the lattice organization, intermolecular couplings and reorganization energies are calculated, and hole mobilities for comparison with experimental measurements are further estimated. These results demonstrate a unique example of the optoelectronic applications of p –DTS(FBTTh 2 ) 2 microwires.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Chalcogenide Perovskite YScS 3 as a Potential p-Type Transparent Conducting Material

Transparent conducting materials (TCMs) have been widely used in optoelectronic applications such as touchscreens, flat panel displays and thin film solar cells. These applications of TCMs are currently dominated by n-type doped oxides. High-performance p-type TCMs are still lacking due to their low hole mobility or p-type doping bottleneck, which impedes efficient device design and novel applications such as transparent electronics. Here, based on first-principles calculations, we propose chalcogenide perovskite YScS 3 as a promising p-type TCM. According to our calculations, its optical absorption onset is above 3 eV, which allows transparency to visible light. Its hole conductivity effective mass is 0.48m 0 , which is among the smallest in p-type TCMs, suggesting enhanced hole mobility. It could be doped to p-type by group-II elements on cation sites, all of which yield shallow acceptors. Here, combining these properties, YScS 3 holds great promise to enhancing the performance of p-type TCMs toward their n-type counterparts.

36 MATERIALS SCIENCE↗
DOE OSTI2025

(NH 3 (CH 2 ) 7 NH 3 ) 2 Sn 3 I 10 , a Vacancy-Ordered Three-Dimensional Tin(II) Perovskite-Derived Semiconductor

Ordering vacancies in hybrid Sn(II) halide semiconductors provides a strategy for preventing uncontrolled oxidation and formation of mobile holes. In this study, we report the structure and optical and electronic properties of (NH 3 (CH 2 ) 7 NH 3 ) 2 Sn 3 I 10 , a vacancy-ordered perovskite derivative with three-dimensional inorganic connectivity. The crystal structure resembles that of a Dion–Jacobson layered perovskite derivative, but with [SnI 5 ] square pyramids bridging the layers. UV–vis diffuse reflectance spectroscopy reveals a sharp onset of light absorption at 1.86(1) eV with the photoluminescence emission maximum at 1.90(1) eV. However, the maximum excitation occurs from 3.42 to 3.81 eV (325 to 370 nm), revealing a significant Stokes shift of 1.3 eV. The electronic properties determined from dark and time-resolved microwave conductivity measurements reveal a minimum carrier mobility of 4.3 × 10 –2 cm 2 V –1 s –1 and a maximum carrier density of 5.96 × 10 16 cm –3 , a uniquely low value for a hybrid Sn(II) halide semiconductor. The transport behavior in combination with first-principles calculations of the electronic band structure and dielectric permittivity suggest polaron-mediated electronic transport, yet the photogenerated carriers have a fast and fluence-dependent nonradiative recombination rate, suggestive of localized “defect-like” states at the band edge. The observed photoluminescence is most consistent with single-ion-like behavior of an asymmetric Sn(II) environment. Together, these results suggest that defect ordering presents a strategy for the reduction of mobile charge carriers at equilibrium.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2014

Anomalous spectral dependence of optical polarization and its impact on spin detection in InGaAs/GaAs quantum dots

We show that circularly polarized emission light from InGaAs/GaAs quantum dot (QD) ensembles under optical spin injection from an adjacent GaAs layer can switch its helicity depending on emission wavelengths and optical excitation density. We attribute this anomalous behavior to simultaneous contributions from both positive and negative trions and a lower number of photo-excited holes than electrons being injected into the QDs due to trapping of holes at ionized acceptors and a lower hole mobility. Our results call for caution in reading out electron spin polarization by optical polarization of the QD ensembles and also provide a guideline in improving efficiency of spin light emitting devices that utilize QDs.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗