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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 73 records · Page 4
All sourcesNASA NTRSDOE OSTI
DOE OSTI2020

Boron phosphide as a p-type transparent conductor: Optical absorption and transport through electron-phonon coupling

Boron phosphide has recently been identified as a potential high-hole-mobility transparent conducting material. This promise arises from its low hole effective masses. However, boron phosphide has a relatively small, 2 eV, indirect bandgap which will affect its transparency. In this study, we computationally study both optical absorption across the indirect gap and phonon-limited electronic transport to quantify the potential of boron phosphide as a p-type transparent conductor. We find that phonon-mediated indirect optical absorption is weak in the visible spectrum and that the phonon-limited hole mobility is very high (around 900 cm 2 /Vs) at room temperature. This exceptional mobility comes from the combination of a low hole effective mass and very weak scattering by polar phonon modes. We rationalize the weak scattering by the less ionic bonding in boron phosphide compared to oxides. We suggest that this could be a general advantage of nonoxides for p-type transparent conducting applications. Using our computed properties, we assess the transparent conductor figure of merit of boron phosphide and show that it exceeds by one order of magnitude that of established p-type transparent conductors, confirming the potential of this material.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Complex quantum transport in a modulation doped strained Ge quantum well heterostructure with a high mobility 2D hole gas

The complex quantum transport of a strained Ge quantum well (QW) modulation doped heterostructure with two types of mobile carriers has been observed. The two dimensional hole gas (2DHG) in the Ge QW exhibits an exceptionally high mobility of 780 000 cm{sup 2}/Vs at temperatures below 10 K. Through analysis of Shubnikov de-Haas oscillations in the magnetoresistance of this 2DHG below 2 K, the hole effective mass is found to be 0.065 m{sub 0}. Anomalous conductance peaks are observed at higher fields which deviate from standard Shubnikov de-Haas and quantum Hall effect behaviour due to conduction via multiple carrier types. Despite this complex behaviour, analysis using a transport model with two conductive channels explains this behaviour and allows key physical parameters such as the carrier effective mass, transport, and quantum lifetimes and conductivity of the electrically active layers to be extracted. This finding is important for electronic device applications, since inclusion of highly doped interlayers which are electrically active, for enhancement of, for example, room temperature carrier mobility, does not prevent analysis of quantum transport in a QW.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Simulation of mixed-host emitting layer based organic light emitting diodes

‘SimOLED’ simulator is used in this work to investigate the efficiency of the mixed-host organic light emitting devices (MH-OLEDs). Tris-(8-hydroxyquinoline) aluminum(3) (Alq{sub 3}) and N,N-diphenyl-N,N-Bis(3-methylphenyl)-1,1-diphenyl-4,4-diamine (TPD) are used as the electron transport layer (ETL) material and hole transport layer (HTL) material respectively, and the indium-doped tin oxide (ITO) and aluminum (Al) as anode and cathode. Three MH-OLEDs, A, B and C with the same structure of ITO / HTM (15 nm) / Mixed host (70 nm) / ETM (10 nm) /Al, are stimulated with ratios TPD:Alq{sub 3} of 3:5, 5:5, and 5:3 respectively. The Poole-Frenkel model for electron and hole mobilities is employed to compute the current density-applied voltage-luminance characteristics, distribution of the electric field, carrier concentrations and recombination rate.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Temperature dependent van der Pauw-Hall measurements on sodium doped single crystalline cadmium telluride

In this report, results of the temperature dependent electrical conductivity measurements conducted on single crystalline cadmium telluride (CdTe), containing sodium (Na) impurities are presented and discussed. The electrical conductivity measurements were conducted using an apparatus that allowed the implementation of a standard van der Pauw-Hall effect technique through which the electrical resistivity, concentration of majority carriers, as well as the carrier mobility were determined for temperatures ranging between 24 K and 350 K. Over this temperature range, the electrical resistivity was observed to change by 7 orders of magnitude. Hall measurements showed that the hole concentration at 300 K was ∼3 × 10{sup 15 }cm{sup –3} and the hole mobility at the same temperature was ∼80 cm{sup 2}/V s. Measuring the concentration of holes as a function of the sample temperature enabled the estimation of the acceptor energy level with respect to the valence band maximum to be ∼60 meV. The same data also revealed the potential presence of a compensating donor level. Furthermore, the hole mobility was also analyzed over the entire temperature range and the data revealed that above 100 K, the carrier mobility was dominated by the scattering of holes from lattice vibrations.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2013

Monte Carlo modeling of transport in PbSe nanocrystal films

A Monte Carlo hopping model was developed to simulate electron and hole transport in nanocrystalline PbSe films. Transport is carried out as a series of thermally activated hopping events between neighboring sites on a cubic lattice. Each site, representing an individual nanocrystal, is assigned a size-dependent electronic structure, and the effects of particle size, charging, interparticle coupling, and energetic disorder on electron and hole mobilities were investigated. Results of simulated field-effect measurements confirm that electron mobilities and conductivities at constant carrier densities increase with particle diameter by an order of magnitude up to 5 nm and begin to decrease above 6 nm. We find that as particle size increases, fewer hops are required to traverse the same distance and that site energy disorder significantly inhibits transport in films composed of smaller nanoparticles. The dip in mobilities and conductivities at larger particle sizes can be explained by a decrease in tunneling amplitudes and by charging penalties that are incurred more frequently when carriers are confined to fewer, larger nanoparticles. Using a nearly identical set of parameter values as the electron simulations, hole mobility simulations confirm measurements that increase monotonically with particle size over two orders of magnitude.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2023

A Si-Substituted Spirobifluorene Hole-Transporting Material for Perovskite Solar Cells

Hybrid organic-inorganic perovskite solar cells (PSCs) have shown promise for next-generation photovoltaics. Here, this study presents a simple approach for enhancing the performance and stability of PSCs by substituting the center carbon atom of the common hole transport material (HTM) Spiro-MeOTAD with a silicon atom. This modification, termed Si-Spiro, results in an increased hole mobility. A density functional theory simulation indicates that the enhanced hole mobility is due to the structural change of Si-Spiro. Electron beam-induced current microscopy measurements indicate improved charged extraction at the Si-Spiro/perovskite interface. A power conversion efficiency of 22.5% is achieved in Si-Spiro-based PSCs, outperforming standard Spiro-MeOTAD. Additionally, Si-Spiro-based PSCs demonstrate enhanced stability, maintaining over 90% of performance over 120 h of one-sun operation. Depth-profiling X-ray photoelectron spectroscopy revealed that Si-Spiro effectively blocks metal ion migration, which contributes to its enhanced stability. The findings suggest that Si-Spiro could be promising a HTM for high-performing, stable PSCs.

14 SOLAR ENERGY↗
DOE OSTI2015

Electric field and substrate–induced modulation of spin-polarized transport in graphene nanoribbons on A3B5 semiconductors

In this work, we present the density functional theory calculations of the effect of an oriented electric field on the electronic structure and spin-polarized transport in a one dimensional (1D) zigzag graphene nanoribbon (ZGNR) channel placed on a wide bandgap semiconductor of the A3B5 type. Our calculations show that carrier mobility in the 1D semiconductor channel of the ZGNR/A3B5(0001) type is in the range from 1.7×10{sup 4} to 30.5×10{sup 4} cm{sup 2}/Vs and can be controlled by an electric field. In particular, at the critical value of the positive potential, even though hole mobility in an one-dimensional 8-ZGNR/h-BN semiconductor channel for spin down electron subsystems is equal to zero, hole mobility can be increased to 4.1×10{sup 5} cm{sup 2}/Vs for spin up electron subsystems. We found that band gap and carrier mobility in a 1D semiconductor channel of the ZGNR/A3B5(0001) type depend strongly on an external electric field. With these extraordinary properties, ZGNR/A3B5(0001) can become a promising materials for application in nanospintronic devices.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2019

Conductivity Tuning via Doping with Electron Donating and Withdrawing Molecules in Perovskite CsPbI 3 Nanocrystal Films

Doping of semiconductors enables fine control over the excess charge carriers, and thus the overall electronic properties, crucial to many technologies. Controlled doping in lead-halide perovskite semiconductors has thus far proven to be difficult. However, lower dimensional perovskites such as nanocrystals, with their high surface-area-to-volume ratio, are particularly well-suited for doping via ground-state molecular charge transfer. Here, the tunability of the electronic properties of perovskite nanocrystal arrays is detailed using physically adsorbed molecular dopants. Incorporation of the dopant molecules into electronically coupled CsPbI 3 nanocrystal arrays is confirmed via infrared and photoelectron spectroscopies. Untreated CsPbI 3 nanocrystal films are found to be slightly p-type with increasing conductivity achieved by incorporating the electron-accepting dopant 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F 4 TCNQ) and decreasing conductivity for the electron-donating dopant benzyl viologen. Time-resolved spectroscopic measurements reveal the time scales of Auger-mediated recombination in the presence of excess electrons or holes. Microwave conductance and field-effect transistor measurements demonstrate that both the local and long-range hole mobility are improved by F4TCNQ doping of the nanocrystal arrays. The improved hole mobility in photoexcited p-type arrays leads to a pronounced enhancement in phototransistors.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Study of charge transport properties in a ZnO/CdS/Cu(In,Ga)Se 2 solar cell via admittance spectroscopy

The authors investigate the charge carrier transport properties in a thin-film heterojunction Cu(In,Ga)Se 2 (CIGS) solar cell device via coordinated capacitance-voltage and admittance spectroscopic measurements. The CIGS absorber width was measured using temperature dependent depletion width in the freeze-out regime and its acceptor density extracted from conventional capacitance-voltage analysis in the non-freeze-out regime. The bias-dependent modified dielectric relaxation in the Cu(In,Ga)Se 2 solar cell was exploited to extract the absorber's resistivity, hole mobility, and their temperature dependency, all by admittance spectroscopy. In the temperature range of 133-300 K, the extracted hole mobility in the CIGS device increases from 7.55 x 10 -3 to 4.08 cm 2 /Vs with temperature. The temperature dependent resistivity and mobility show Arrhenius behavior with an activation energy ~120 meV. The thermally activated behavior of mobility is related to the potential fluctuation encountered by the holes during their transport, which enables admittance spectroscopy to directly measure the potential fluctuation in the CIGS polycrystalline material.

14 SOLAR ENERGY↗
DOE OSTI2014

Charge carrier transport properties in layer structured hexagonal boron nitride

Due to its large in-plane thermal conductivity, high temperature and chemical stability, large energy band gap (~ 6.4 eV), hexagonal boron nitride (hBN) has emerged as an important material for applications in deep ultraviolet photonic devices. Among the members of the III-nitride material system, hBN is the least studied and understood. The study of the electrical transport properties of hBN is of utmost importance with a view to realizing practical device applications. Wafer-scale hBN epilayers have been successfully synthesized by metal organic chemical deposition and their electrical transport properties have been probed by variable temperature Hall effect measurements. The results demonstrate that undoped hBN is a semiconductor exhibiting weak p-type at high temperatures (> 700 °K). The measured acceptor energy level is about 0.68 eV above the valence band. In contrast to the electrical transport properties of traditional III-nitride wide bandgap semiconductors, the temperature dependence of the hole mobility in hBN can be described by the form of μ ∝ (T/T{sub 0}){sup −α} with α = 3.02, satisfying the two-dimensional (2D) carrier transport limit dominated by the polar optical phonon scattering. This behavior is a direct consequence of the fact that hBN is a layer structured material. The optical phonon energy deduced from the temperature dependence of the hole mobility is ħω = 192 meV (or 1546 cm{sup -1}), which is consistent with values previously obtained using other techniques. The present results extend our understanding of the charge carrier transport properties beyond the traditional III-nitride semiconductors.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Direct probe of the nuclear modes limiting charge mobility in molecular semiconductors

Recent theories suggest that low frequency dynamic intramolecular and intermolecular motions in organic semiconductors (OSCs) are critical to determining the hole mobility. So far, however, it has not been possible to probe these motions directly experimentally and therefore no unequivocal and quantitative link exists between molecular-scale thermal disorder and macroscale hole mobility in OSCs. Here we use inelastic neutron scattering to probe thermal disorder directly by measuring the phonon spectrum in six different small molecule OSCs, which we accurately reproduce with first principles simulations. We use the simulated phonons to generate a set of electron–phonon coupling parameters. Using these parameters, the theoretical mobility is in excellent agreement with macroscopic measurements. Comparison of mobility between different materials reveals routes to improve mobility by engineering phonon and electron–phonon coupling.

36 MATERIALS SCIENCE↗
DOE OSTI2024

Ultrawide bandgap semiconductor heterojunction p–n diodes with distributed polarization-doped p-type AlGaN layers on bulk AlN substrates

Ultrawide bandgap heterojunction p–n diodes with polarization-induced AlGaN p-type layers are demonstrated using plasma-assisted molecular beam epitaxy on bulk AlN substrates. Current–voltage characteristics show a turn-on voltage of Vbi≈5.5 V, a minimum room temperature ideality factor of η≈1.63, and more than 12 orders of current modulation at room temperature. A stable current operation of the ultrawide bandgap semiconductor diode is measured up to a temperature of 300 °C. The one-sided n+–p heterojunction diode design enables a direct measurement of the spatial distribution of polarization-induced mobile hole density in the graded AlGaN layer from the capacitance–voltage profile. The measured average mobile hole density is p∼5.7×1017 cm−3, in close agreement with what is theoretically expected from distributed polarization doping. Light emission peaked at 260 nm (4.78 eV) observed in electroluminescence corresponds to interband radiative recombination in the n+ AlGaN layer. A much weaker deep-level emission band observed at 3.4 eV is attributed to cation-vacancy and silicon complexes in the heavily Si-doped AlGaN layer. These results demonstrate that distributed polarization doping enables ultrawide bandgap semiconductor heterojunction p–n diodes that have wide applications ranging from power electronics to deep-ultraviolet photonics. These devices can operate at high temperatures and in harsh environments.

Physics↗
DOE OSTI2015

Predicting a quaternary tungsten oxide for sustainable photovoltaic application by density functional theory

A quaternary oxide, CuSnW{sub 2}O{sub 8} (CTTO), has been predicted by density functional theory (DFT) to be a suitable material for sustainable photovoltaic applications. CTTO possesses band gaps of 1.25 eV (indirect) and 1.37 eV (direct), which were evaluated using the hybrid functional (HSE06) as a post-DFT method. The hole mobility of CTTO was higher than that of silicon. Further, optical absorption calculations demonstrate that CTTO is a better absorber of sunlight than Cu{sub 2}ZnSnS{sub 4} and CuIn{sub x}Ga{sub 1−x}Se{sub 2} (x = 0.5). In addition, CTTO exhibits rigorous thermodynamic stability comparable to WO{sub 3}, as investigated by different thermodynamic approaches such as bonding cohesion, fragmentation tendency, and chemical potential analysis. Chemical potential analysis further revealed that CTTO can be synthesized at flexible experimental growth conditions, although the co-existence of at least one secondary phase is likely. Finally, like other Cu-based compounds, the formation of Cu vacancies is highly probable, even at Cu-rich growth condition, which could introduce p-type activity in CTTO.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2014

Advantage of suppressed non-Langevin recombination in low mobility organic solar cells

Photovoltaic performance in relation to charge transport is studied in efficient (7.6%) organic solar cells (PTB7:PC{sub 71}BM). Both electron and hole mobilities are experimentally measured in efficient solar cells using the resistance dependent photovoltage technique, while the inapplicability of classical techniques, such as space charge limited current and photogenerated charge extraction by linearly increasing voltage is discussed. Limits in the short-circuit current originate from optical losses, while charge transport is shown not to be a limiting process. Efficient charge extraction without recombination can be achieved with a mobility of charge carriers much lower than previously expected. The presence of dispersive transport with strongly distributed mobilities in high efficiency solar cells is demonstrated. Reduced non-Langevin recombination is shown to be beneficial for solar cells with imbalanced, low, and dispersive electron and hole mobilities.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2019

The Effect of Ring Expansion in Thienobenzo[ b ]indacenodithiophene Polymers for Organic Field-Effect Transistors

A fused donor, thienobenzo[b]indacenodithiophene (TBIDT), was designed and synthesized using a novel acid-promoted cascade ring closure strategy, and then copolymerized with a benzothiadiazole (BT) monomer. The backbone of TBIDT is an expansion of the well-known indacenodithiophene (IDT) unit and was expected to enhance the charge carrier mobility by improving backbone planarity and facilitating short contacts between polymer chains. However, the optimized field-effect transistors demonstrated an average saturation hole mobility of 0.9 cm 2 V -1 s -1 , lower than the performance of IDT-BT (~1.5 cm 2 V -1 s -1 ). Mobilities extracted from time-resolved microwave conductivity measurements were consistent with the trend in hole mobilities in organic field-effect transistor devices. Scanning tunneling microscopy measurements and computational modeling illustrated that TBIDT-BT exhibits a less ordered microstructure in comparison to IDT-BT. This reveals that a regular side-chain packing density, independent of conformational isomers, is critical to avoid local free volume due to irregular packing, which can host trapping impurities. DFT calculations indicated that TBIDT-BT, despite containing a larger, planar unit, showed less stabilization of planar backbone geometries in comparison to IDT-BT. This is due to the reduced electrostatic stabilizing interactions between the peripheral thiophene of the fused core and the BT unit, resulting in a reduction of the barrier to rotation around the single bond. Finally, these insights provide a greater understanding of the general structure-property relationships required for semiconducting polymer repeat units to ensure optimal backbone planarization, as illustrated with IDT-type units, guiding the design of novel semiconducting polymers with extended fused backbones for high-performance field-effect transistors.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Tuning the Cross-Linker Crystallinity of a Stretchable Polymer Semiconductor

The cross-linking of conjugated polymers has been shown to be an effective strategy to improve its elastic properties to give deformable semiconductors for plastic electronics. Although there have been extensive studies of the structural requirements of the polymer host for good film ductility, no work to date has focused on the relevance of the structural design or chemistry of these cross-linker additives. In this study, urethane groups and tertiary carbon atoms are inserted into the alkyl backbone of perfluorophenyl azide-based cross-linkers to investigate the importance of cross-linker crystallinity with respect to polymer morphology and hence mechanical and electrical properties. Linear cross-linkers with hydrogen bonding from urethane groups readily phase separate and recrystallize in the polymer network to form cross-linked domains that obstruct the strain distribution of the polymer film. Branch cross-linkers with tertiary carbon on the other hand form an evenly cross-linked network in the polymer blend stemming from excellent miscibility and show a 4-fold increase in fracture strain. Furthermore, a stable hole mobility of 0.2 cm 2 V –1 s –1 is achieved up to ε = 100%, and a stable hole mobility of 0.1 cm 2 V –1 s –1 after 2000 cycles of ε = 25% on fully stretchable organic field-effect transistors.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Cofacial Versus Coplanar Arrangement in Centrosymmetric Packing Dimers of Dipolar Small Molecules: Structural Effects on the Crystallization Behaviors and Optoelectronic Characteristics

Two D-π–A-A molecules ( MIDTP and TIDTP ) composed of an electron-rich ditolylamino group (D) and an electron-deficient 5-dicyanovinylenylpyrimidine (A-A) fragment bridged together with indeno[1,2- b ]thiophene (IDT) were synthesized. These molecules provide an opportunity to examine in-depth the impact of side-chain variations (methyl vs p -tolyl) on the crystallization behaviors, solid-state morphology, physical properties, and optoelectronic characteristics relevant for practical applications. X-ray analyses on single-crystal structures indicate that methyl-substituted MIDTP forms “coplanar antiparallel dimers” via C–H···S interactions and organizes into an ordered slip-staircase arrays. In contrast, p -tolyl-bearing TIDTP shows “cofacial centrosymmetric dimers” via π–π interactions and packs into a less-ordered layered structures. The X-ray diffraction analyses upon thermal treatment are consistent with a superior crystallinity of MIDTP , as compared to that of TIDTP . This difference indicates a greater propensity to organization by introduction of the smaller methyl group versus the bulkier p -tolyl group. The increased propensity for order by MIDTP facilitates the crystallization of MIDTP in both solution-processed and vacuum-deposited thin films. MIDTP forms solution-processed single-crystal arrays that deliver OFET hole mobility of 6.56 × 10 –4 cm 2 V –1 s –1 , whereas TIDTP only forms amorhpous films that gave lower hole mobility of 1.34 × 10 –5 cm 2 V –1 s –1 . MIDTP and TIDTP were utilized to serve as donors together with C 70 as acceptor in the fabrication of small-molecule organic solar cells (SMOSCs) with planar heterojunction (PHJ) or planar-mixed heterojunction (PMHJ) device architectures. OPV devices based on higher crystalline MIDTP delivered power conversion efficiencies (PCEs) of 2.5% and 4.3% for PHJ and PMHJ device, respectively, which are higher than those of TIDTP -based cells. Finally, the improved PCEs of MIDTP -based devices are attributed to better hole-transport character.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Hole effective masses and subband splitting in type-II superlattice infrared detectors

We explore band structure effects to help determine the suitability of n-type type-II superlattice (T2SL) absorbers for infrared detectors. It is often assumed that the exceedingly large growth-direction band-edge curvature hole effective mass in n-type long wavelength infrared (LWIR) T2SL would lead to low hole mobility and therefore low detector collection quantum efficiency. We computed the thermally averaged conductivity effective mass and show that the LWIR T2SL hole conductivity effective mass along the growth direction can be orders of magnitude smaller than the corresponding band-edge effective mass. LWIR InAs/GaSb T2SL can have significantly smaller growth-direction hole conductivity effective mass than its InAs/InAsSb counterpart. For the InAs/InAsSb T2SL, higher Sb fraction is more favorable for hole transport. Achieving long hole diffusion length becomes progressively more difficult for the InAs/InAsSb T2SL as the cutoff wavelength increases, since its growth-direction hole conductivity effective mass increases significantly with decreasing band gap. However, this is mitigated by the fact that the splitting between the top valence subbands also increases with the cutoff wavelength, leading to reduced inter-subband scattering and increased relaxation time.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗