Electron Hole Conduction Research Articles

Novel bipolar carbazole-containing polyphenylquinoxalines: Synthesis and photophysical properties

39 In recent time, conjugated electroluminescent polymers have attracted much attention from both the scientific and practical viewpoint due to their application as electroactive materials for light emitting diodes [1–3]. It has been found that intense electroluminescence (EL) in polymer requires high electron–hole conduction of a medium related to the mobility of charge carriers and a balance of injection of carriers of both signs from opposite electrodes into the emission poly meric layer. However, the majority of EL polymers are hole transport materials, which leads to the imbalance of charge injection and a poor quantum yield. A search for new efficient electron transport polymers is required to improve performance of electrooptical devices, which is achieved in particu lar by introducing into macromolecules electron withdrawing fragments: triazole [4], oxadiazole [5], quinoline [6], and quinoxaline [7]. The latter is the least studied. One can suppose that polyphenylqui noxalines (PPQs) will show higher electron with drawing properties owing to the presence of four electron withdrawing heteroatoms. A promising approach to improve the quantum yield is provided not only by the balance of the electron and hole conduction but also by design of multilayer bipolar (donor–acceptor) emission polymers. In this work, to solve this problem, we have used a combination of carbazole and quinoxaline contain ing fragments in PPQ macromolecules. Such mole cules are known to show high thermal and oxidative stability, good mechanical and film forming proper ties, and solubility in organic solvents [8]. In continu ation of our studies [9–11], we have prepared and studied novel bipolar carbazole containing PPQs as efficient electron–hole and transport materials; for this purpose, we obtained previously unknown bis(α diketone) III. A common method for the synthesis of bis(α diketone)s is the oxidation of bis(phenylethy nyl)arenes obtainable from aromatic dibromides and arylacetylene. We have proposed a new method for the synthesis of bis(α diketone) III by the nucleophilic substitution reaction of fluorodiketone II, already containing bis(α dicarbonyl) groups, with dicarbazole I accord ing to Scheme 1. CHEMISTRY

Proton conductivity in Sm 2Sn 2O 7 pyrochlores

The electrical conductivity of the pyrochlore systems, Sm 2Sn 2O 7, Sm 1.92Ca 0.08Sn 2O 7 − δ and Sm 2Sn 1.92Y 0.08O 7 − δ was studied using impedance spectroscopy under wet and dry gas (O 2 and Ar) in the temperature range 150–1000 °C. Enhancements of the bulk conductivity of all samples at temperatures up to ~ 550 °C were observed for wet conditions consistent with significant levels of proton conduction. The presence of dissolved protons in the acceptor-doped materials, Sm 1.92Ca 0.08Sn 2O 7 − δ and Sm 2Sn 1.92Y 0.08O 7 − δ , is supported by infrared spectroscopy and thermogravimetric analysis. Proton conduction was confirmed by isotope effects under heavy water (O 2/D 2O and Ar/D 2O). The A-site substituted sample Sm 1.92Ca 0.08Sn 2O 7 − δ yielded the highest levels of proton conduction and displayed mixed ionic and electronic conduction under dry oxidising conditions. Electron hole conduction dominates in dry oxygen for Sm 2Sn 1.92Y 0.08O 7 − δ and Sm 2Sn 2O 7. For the A-site doped sample bulk and grain boundary conduction could be separated. The specific grain boundary conduction was calculated using the brick layer model and was found to be two orders of magnitude lower compared to the bulk conductivity. The unexpected increase in conductivity seen for the undoped sample under wet gas is discussed in the context of structural disorder and possible filling of the un-occupied anion site in the pyrochlore structure by OH-groups.

Control of mixed protonic and electronic conductivity by mixing rare-earth ortho-borates

In order to develop mixed protonic and electronic conductors, we proposed a novel concept for material design that enables to control partial conductivities by fabricating solid solutions of protonic and electronic conductors. In this work, Sr-doped LaBO 3 and Sr-doped CeBO 3 were chosen as model compounds conducting protons and electron holes, respectively. Solid solutions of the above borates, Sr-doped La 1 − x Ce x BO 3, were prepared, and their electrical conductivities were investigated in 8.5 × 10 2–4.2 × 10 3 Pa of p(H 2O) and 1.0 × 10–1.0 × 10 5 Pa of p(H 2) at 1073 K. From the experimental results of the gas partial pressure dependences of the conductivities, major charge carrier species were identified as a function of x. It was found that proton was the major charge carrier when x < 0.2 while the contribution of the electron hole conduction became remarkable as x increased above 0.2. The contribution of the electron hole conduction can be interpreted by the percolation model.

Electrical conduction and mass transport properties of SrZr 0.99Fe 0.01O 3 − δ

The electrical conductivity of SrZr 0.99Fe 0.01O 3 − δ was evaluated by a four-probe ac technique. The measurements were conducted in hydrogen and in oxygen containing atmospheres at 823 ≤ T / K ≤ 1273. It was found from the X-ray absorption spectroscopic measurements that Fe in the oxide is trivalent both in hydrogen and in oxygen. In order to determine the major carrier in the oxide, gas partial pressure dependences and isotope effect of hydrogen and deuteron on the electrical conductivity was investigated. In humidified hydrogen, it was found that proton conduction is predominant in the lower temperature region while oxide ion conduction starts to contribute to the total by increasing temperature of above 1173 K. In oxygen containing gas, the protonic conduction is found to be predominant at lower temperatures while the contribution of the electron hole conduction is significant at higher temperatures. Hydrogen evolution property was evaluated using the SrZr 0.99Fe 0.01O 3 − δ disc as a solid electrolyte. Hydrogen evolution rate obeyed the Faraday's law in humidified hydrogen at 1173 K, suggesting that the transport number of ionic species is unity.

ChemInform Abstract: Electrical Conductivity of the High-Temperature Proton Conductor BaZr0.9Y0.1O2.95.

The impedance of the cubic perovskite BaZr0.9Y0.1O3-δ has been systematically investigated in dry and wet atmospheres at high and low oxygen partial pressures. In the grain interior, conductivity contributions from oxygen ions, electron holes, and protons can be identified. Below 300°C, proton conduction dominates and increases linearly with the frozen-in proton concentration. The proton mobility, with an activation energy of 0.44 ± 0.01 eV is among the highest ever reported for a perovskite-type oxide proton conductor. For dry oxygen atmos-pheres, electron hole conduction dominates with an activation energy of ∼0.9 eV. At temperatures <500°C, the grain-boundary conductivity can be separated and increases upon incorporation of protons. The high electrical conductivity and chemical stability make acceptor-doped barium zirconate a good choice for application as a high-temperature proton conductor.

A general approach to the space–charge field solution in photorefractive materials in aTWM geometry

A general approach to the approximate analytical solution of photorefractive transportequations for arbitrary fringe contrast is presented. The method based partially on theperturbative scheme permits us to find the stationary space–charge field distribution insidea photorefractive crystal for a two-wave mixing (TWM) geometry with a DC electric field.The method can be employed for various band transport models. In the presentwork two distinct electron–hole conduction models are investigated. The solutionsare compared with the results of numerical calculations and good agreement isfound.

Recovery of hydrogen from gas mixture by an intermediate-temperature type proton conductor

The characteristics of electrochemical hydrogen pump using one-end closed tube type BaZr 0.955Y 0.03Co 0.015O 3− α , which has higher proton conductivity among other zirconate series proton conductor at the intermediate temperature, were demonstrated by a direct current method in the temperature range from 623 to 873 K. Under water vapor electrolysis condition, mixed proton and electron–hole conduction were exhibited and proton transport number was decreased less than 0.37. On the other hand, under hydrogen pumping from a mixture gas contained with hydrogen gas, proton transport number has attained more than 0.85. However, not only proton and electron–hole conductions but also oxide-ion conduction was exhibited even at lower temperature below 700 K. Thus, the growth of electrochemical failure induced by oxide ion migration was observed. From the standpoint of high proton transport number and low oxide-ion conduction, a candidate proton conductor for nuclear fusion engineering applications was proposed to compare with hydrogen pump characteristics among the zirconate series proton conductor.

Effects of rock-salt layer on electronic and oxide ionic mixed conductivity in strontium titanate, SrO(SrTiO 3) n ( n = 1, 2, ∞)

Electronic conductivity and oxygen permeation properties of analogous compounds of SrO(SrTiO 3) n ( n = 1, 2) and SrTiO 3 were investigated. Electronic conductivity and oxide ion conductivity decrease with increasing n, and SrTiO 3 shows much higher conductivity. The effects of Co doping on oxide ion conductivity and electronic conductivity were also examined and it is observed that it is effective for increasing electrical conductivity and the oxygen permeation rate. Hall effect measurements suggests that the low electron-hole conductivity in Co-doped SrO(SrTiO 3) n ( n = 1, 2) can be assigned to a low mobility, which may result from the low electron and atomic density in the Sr–O rock-salt layer. Quantum mechanical calculations of the electron density also indicate low electron density in the rock-salt layer. This study reveals that the rock-salt layer consisting of Sr–O disturbs electron transfer in SrTiO X oxide and that the perovskite layer has much higher electronic and oxide ionic conductivity.

Combustion synthesis and properties of highly phase-pure perovskite electrolyte Co-doped La 0.9Sr 0.1Ga 0.8Mg 0.2O 2.85 for IT-SOFCs

The highly phase-pure perovskite electrolyte, La 0.9Sr 0.1Ga 0.8Mg 0.115Co 0.085O 2.85 (LSGMCO), was prepared by means of glycine–nitrate process (GNP) for intermediate-temperature solid oxide fuel cells (IT-SOFCs). The perovskite phase evolution, sintering, electrical conductivity and electrochemical performance of LSGMCO were investigated. The results show that the highly phase-pure perovskite electrolyte LSGMCO can be obtained after calcining at 1150 °C. The sample sintered at 1450 °C for 20 h in air exhibited a better sinterability, and the relative density of LSGMCO was higher than 95%. The stoichiometric indexes of the elements in the sintered sample LSGMCO determined experimentally by EDS were in good agreement with the nominal composition. The electrical conductivities of the sample were 0.094 and 0.124 S· cm −1 at 800 °C and 850 °C in air, respectively. The ionic conduction of the sample was dominant at high temperature with the higher activation energies. While at lower temperature the electron hole conduction was predominated with the lower activation energies. The maximum power densities of the single cell fabricated with LSGMCO electrolyte with Ce 0.8Sm 0.2O 1.9 (SDC) interlayer, SmBaCo 2O 5+ x cathode and NiO/SDC anode achieved 643 and 802 mW cm −2 at 800 °C and 850 °C, respectively.

Structure and conductivity in CeNb3O9-δ

Abstract In the search of new materials for solid oxide fuel cells, a study of the structure and electrical conductivity of Mg-doped and nominally pure CeNb3O9-δ was undertaken. This material exhibits an orthorhombic crystal structure as determined by Rietveld refinement. Through a combined study of 4-point DC and AC impedance spectroscopy, it was determined that the material presents oxygen ion conductivity, electron conductivity and electron-hole conductivity according to the partial pressure of oxygen and temperature in agreement with a simple defect chemistry model. Finally, some experiments seem to indicate the presence of proton conduction.

High temperature humidity sensors based on sputtered Y-doped BaZrO 3 thin films

Sputter deposited Y-doped BaZrO 3 thin films (BaZr x Y 1− x O 3− y/2 , x = 0.2, y > 0), were investigated as to their viability for reliable, long-term stable humidity or water vapor microsensors operating at high operating temperatures ( T > 400 °C). Reliable exhaust gas composition measurements at these temperatures could allow optimization of power plant efficiency and reduce emissions. Electrical conductivity and gas sensitivity tests on bulk Y-doped BaZrO 3 in environments with varying humidity suggest that this material could be suitable as a highly selective humidity sensor with sensitivities between 2 atm −1 and 26 atm −1 in the temperature range of 500–700 °C, based on an ionic proton conduction mechanism [W. Wang, A.V. Virkar, Ionic and electron–hole conduction in BaZr 0.93Y 0.07O 3− δ by 4-probe dc measurements, J. Power Sources 142 (2005) 1–9; W. Wang, A.V. Virkar, A conductimetric humidity sensor based on proton conducting perovskite oxides, J. Sens. Actuators, B, Chem. 98 (2004) 282–290]. Thin films of BaZrO 3:Y were investigated for their potential to significantly reduce sensor response time, increase sensitivity, and allow integration in microsensor platforms. Unlike other sensors that are based on adsorption, the sensing mechanism presented here at temperatures >400 ̊C should be based on absorption of H 2O, which dissolves into the BaZrO 3:Y lattice, “fills in” the oxygen vacancies, and releases protons. Since protons have a higher mobility than oxygen ions, they would dominate the electric conductivity. XRD spectra from all as-deposited and annealed BaZrO 3:Y films show that the material changed from amorphous to crystalline at annealing temperatures >500 °C. After annealing, the samples exhibit an increasing particle size from 70 nm to 167 nm and an increase of root mean squared (RMS) roughness (from 1.3 nm to 4.2 nm) with increasing film thickness (200–500 nm). The film conductivity increases as a function of temperature (from 100 °C to 400 °C) and upon exposure to a humid atmosphere, supporting our hypothesis of a proton conduction based conduction and sensing mechanism. The water vapor sensitivity was measured using 0.058 atm partial pressure of water at 400 °C. Sensitivity values ranging from 22 atm −1 to 62 atm −1 with a response time of τ 60 ≈ 6 s were achieved. We have demonstrated Y-doped BaZrO 3 thin film humidity sensors that operate at high temperatures.

Mixed Protonic/Electronic Conductor Cathodes for Intermediate Temperature SOFCs Based on Proton Conducting Electrolytes

The electrical properties of doped and were studied as a function of temperature, and oxygen and water vapor partial pressure using electrochemical impedance spectroscopy. Rare-earth elements, such as Yb, Eu, and Sm, were used as cation dopants because of their multivalent oxidation state, low third ionization potential, and comparable ionic radius with cerium. Higher conductivity values were recorded doping and oxides with Yb. Partial conductivities and transport numbers were calculated from the conductivity measurements at different oxygen partial pressures using an established defect model. In an oxidizing condition, electron–hole conduction was dominant at high temperatures and decreased with decreasing the temperature and increasing the water vapor content in the gas atmosphere. presented the highest conductivity among the tested samples both in dry and wet oxygen atmospheres. Moreover, showed a mixed conduction of ions and holes at temperatures above , suggesting that this perovskite-type oxide could be a possible cathode material in a solid oxide fuel cell (SOFC) based on a proton conductor electrolyte.

Performance of the electrochemical hydrogen pump of a proton-conducting oxide for the tritium monitor

To improve the sensitivity of the tritium monitor, we have proposed a new type of tritium monitor and constructed a prototype system equipped with a closed loop and a proton-conducting oxide as an electrochemical hydrogen pump. To assess the performance of the hydrogen pump using CaZr 0.9In 0.1O 3− α , hydrogen extraction by water vapor electrolysis is demonstrated at a constant current in the temperature range from 873 to 1073 K. The hydrogen extraction rate reached 1.8 cm 3/min at 0.5 A and 973 K. The present cell exhibited mixed proton and electron hole conduction and the transport number of protons decreased at relatively high temperatures. This suggested that high temperatures are not required for a hydrogen pump operating under water vapor electrolysis conditions. Furthermore, as an initial feasibility study, a concentration test was performed on hydrogen gas using the closed loop system and hydrogen pump apparatus. It was found that the concentration rate of hydrogen in the closed loop reached five times that of water vapor in the anode compartment over a period of 50 min.

Mixed Conduction in BaCe0.8Pr0.2O3- Ceramic

BaCe0.8Pr0.2O3- ceramic was synthesized by high temperature solid-state reaction. The structural characteristics and the phase purity of the crystal were determined using powder X-ray diffraction analysis. By using the methods of AC impedance spectroscopy, gas concentration cell and electrochemical pumping of hydrogen, the conductivity and ionic transport number of BaCe0.8Pr0.2O3- were measured, and the electrical conduction behavior of the material was investigated in different gases in the temperature range of 500-900 C. The results indicate that the material was of a single perovskite-type orthorhombic phase. From 500 C to 900 C, electronic-hole conduction was dominant in dry and wet oxygen, air or nitrogen, and the total conductivity of the material increased slightly with increasing oxygen partial pressure in the oxygen partial pressure range studied. Ionic conduction was dominant in wet hydrogen, and the total conductivity was about one or two orders of magnitude higher than that in hydrogen-free atmosphere (oxygen, air or nitrogen).

On the possibility of the electron-hole plasma conductivity in diamond being negative

The mobilities of charge carriers in boron-doped diamond at low temperatures are estimated by numerically solving the Boltzmann equation for a steady-state nonequilibrium velocity distribution function of the carriers with allowance for their scattering by phonons. Estimates show that, at temperatures of up to 100 K, charge carriers with a negative mobility exist over a fairly broad (on the order of 100 K) range of their energies. In the steady-state case, the absolute mobility integrated over the distribution function turns out to be positive, but there are grounds to suppose that, in the case of an unsteady pulsed source of charge carriers, it may become negative.

The enhanced electronic transference number at the grain boundaries in Sr-doped LaGaO 3

The partial electron-hole conductivity of the bulk and the grain boundary of 1 mol% Sr-doped LaGaO 3 has been measured separately using a dc-polarization method employing an oxygen ion-blocking cell. The electronic transference number for the electron-hole at the grain boundary was found to be greater than that in the bulk. This is because the oxygen vacancy depletes more severely in the grain boundary area than the electron-hole does to form the space charge zone, due to its higher effective charge compared to that of the electron-hole. Such a local variation in the electronic transfer number thus confirms that the conduction across the grain boundary in 1 mol% Sr-doped LaGaO 3 is controlled by the space charge. These results are consistent with those previously reported based on the ac impedance studies.

High-temperature thermoelectric properties of Nb-doped MNiSn (M=Ti, Zr) half-Heusler compound

Abstract The temperature dependence of electrical conductivity, Seebeck coefficient, Hall coefficient, and thermal conductivity of Nb-doped MNiSn (M = Ti, Zr) half-Heusler compounds were investigated at different temperature, ranging from room temperature to 1000 K. The power factor reached 4 × 10−3 Wm−1 K−2 above 600 K for both systems. The power factor for TiNiSn-based samples decreased above 700 K due to the narrower band gap. The Hall mobility was relatively small; however, estimated carrier effective mass was larger by one order of magnitude than that for conventional thermoelectric material. The thermal conductivity increased above 700 K due to the ambipolar diffusion effect. The ambipolar diffusion effect depended on the band gap width and the ratio of electron–hole conductivity. Heavy carrier-doping effectively suppressed the ambipolar diffusion effect, i.e. restrain the increase of thermal conductivity at high temperature. The maximum ZT value of 0.6 at 800 K was obtained for Zr0.98Nb0.02NiSn.

Molecular Design of High Performance Poly-Fused Thiophene Radical Cations: A Density Functional Theory Study

Hybrid density functional theory (DFT) calculations have been carried out for neutral and radical cation species of a fused thiophene oligomer, denoted by T(n), where n represents the number of thiophene rings in the oligomer, to elucidate the electronic structures at ground and low-lying excited states. A polymer of fused thiophene was also investigated using one-dimensional periodic boundary conditions (PBCs) for comparison. It was found that the reorganization energy of a radical cation of T(n) from a vertical hole trapping point to its relaxed structure is significantly small. Also, the reorganization energy decreased gradually with increasing n, indicating that T(n) has an effective hole transport property. It was found that the radical cation species of T(n) has a low energy band in the near-IR region, which is strongly correlated to hole conductivity. The relationship between the electronic states and hole conductivity was discussed on the basis of theoretical calculations.

Molecular design of high performance fused heteroacene radical cations: A DFT study

Hybrid density functional theory (DFT) calculations have been carried out for neutral and radical cation of fused furan oligomer, denoted by F( n) where n means number of furan rings in the oligomer, to elucidate the electronic structures at ground and low-lying excited states. A polymer of fused furan was also investigated using one-dimensional periodic boundary condition (PBC) for comparison. It was found that the reorganization energy of radical cation of F( n) from vertical hole trapping point to its relaxed structure is significantly small. Also, the reorganization energy decreased gradually with increasing n, indicating that F( n) has an effective hole transport property. It was found that the cation radical of F( n) has a low energy band at near IR region, which is strongly correlated to hole conductivity. The relation between the electronic states and hole conductivity was discussed on the basis of theoretical calculations.

Role of protons in the electrical conductivity of acceptor-doped BaPrO 3, BaTbO 3, and BaThO 3

We present results from ongoing and former, hitherto unpublished, studies of the role of protons in the electrical conductivity of three acceptor-substituted perovskites; BaMO 3 (M = Pr, Tb, Th). Acceptor-doped BaPrO 3 becomes dominated by electron holes at low temperatures in oxidising atmospheres, making it a p-type electronic conductor, leaving hydration and proton conduction barely measurable. It is unstable towards even mild reduction, as is also BaTbO 3. Acceptor-doped BaTbO 3 and BaThO 3 exhibit mixed proton and electron hole conduction, and the hydration reaction and proton mobility have been studied through conductivity variations with T, p(O 2) and p(H 2O). Modeling of the data yielded enthalpies of hydration and activation enthalpy of proton mobility for BaTb 0.9Yb 0.1O 3–δ of Δ H hyd 0 = − 90 kJ mol − 1 H 2O and Δ H m, H + = 53 kJ mol − 1 while the corresponding values for BaTh 0.9Nd 0.1O 3–δ were Δ H hyd 0 = − 128 kJ mol − 1 H 2O and Δ H m,H + = 67 kJ mol − 1 . The parameters are in qualitative agreement with literature for comparable perovskites, and with a correlation proposed between hydration enthalpy and difference between B- and A-site occupants' electronegativities. The BaTbO 3 and BaThO 3 materials had grain boundary resistances comparable to those of the grain interiors, suggesting that these materials with relatively high p-type electronic conduction generally exhibit smaller grain boundary resistances than typical BaZrO 3-based materials, which are purer proton conductors.