Comparative frequency-resolved photoconductivity studies of amorphous semiconductors

Comparative frequency-resolved photoconductivity measurements in amorphous (a-) semiconductors, such as a-Si:H p–i–n junction, a-SiGe:H and a-chalcogenides (a-Se, a-As₂Se₃, a-As₂Te₃, a-SeTe, a-As₂S₃, etc.) are reported. In particular, photoconductivity lifetimes as a function of light intensity and temperature were determined by using the quadrature frequency-resolved spectroscopy method. The activation energies from the temperature-dependent lifetime and photocurrent were determined and compared in different materials. The exponent ν in the power-law relationship (I_ph∝G^ν) between generating flux and photocurrent was also obtained at different excitation wavelengths. The results were compared with the predictions of multiple-trapping (MT) and distant-pair (DP) models developed for photoconductivity of a-semiconductors at high and low temperatures, respectively.     

Surface passivation of crystalline silicon wafer via hydrogen plasma pre-treatment for solar cells

The carrier lifetime of crystalline silicon wafers that were passivated with hydrogenated silicon nitride (SiN_x:H) films using plasma enhanced chemical vapor deposition was investigated in order to study the effects of hydrogen plasma pre-treatment on passivation. The decrease in the native oxide, the dangling bonds and the contamination on the silicon wafer led to an increase in the minority carrier lifetime. The silicon wafer was treated using a wet process, and the SiN_x:H film was deposited on the back surface. Hydrogen plasma was applied to the front surface of the wafer, and the SiN_x:H film was deposited on the hydrogen plasma treated surface using an in-situ process. The SiN_x:H film deposition was carried out at a low temperature (<350 °C) in a direct plasma reactor operated at 13.6 MHz. The surface recombination velocity measurement after the hydrogen plasma pre-treatment and the comparison with the ammonia plasma pre-treatment were made using Fourier transform infrared spectroscopy and secondary ion mass spectrometry measurements. The passivation qualities were measured using quasi-steady-state photoconductance. The hydrogen atom concentration increased at the SiN_x:H/Si interface, and the minority carrier lifetime increased from 36.6 to 75.2 μs. The carbon concentration decreased at the SiN_x:H/Si interfacial region after the hydrogen plasma pre-treatment.     

The circuit point of view of the temperature dependent open circuit voltage decay of the solar cell

The open circuit voltage decay (OCVD) technique has been used to determine the minority carrier lifetime. In this study, an experimental and analytical method is described for determination of minority carrier lifetime at porous Si based solar cell by photo induced OCVD technique. The cell is illuminated by a monochromatic light source (λ = 658 nm) in the open circuit configuration, and the decay of voltage is measured after abruptly terminating the excitation. For the analysis of the OCVD characteristic of solar cell device, equivalent electrical circuit has been proposed in which the diffusion capacitance is connected in series with the contribution of the solar cell interface. Exact minority carrier lifetimes at low (50-170 K) and high (190-330 K) temperature regions have been obtained as 28.9 and 2.65 μs from the temperature dependent OCVD measurements by using an alternative extraction technique.     

Temperature dependence of open-circuit voltage and recombination processes in polymer-fullerene based solar cells

In this article, we have studied the temperature and illumination dependence of open-circuit voltage (V_OC) in polymer-fullerene based solar cells. It has been observed that V_OC at higher illumination intensities gets converged at 0 K which gives information about maximum achievable V_OC in a particular donor-acceptor blend. Besides this, recombination processes have been studied by transient open-circuit voltage decay (TOCVD) and the transition between recombination regimes has been observed for the first time. At low V_OC carrier lifetime exhibits a constant value around 500 μs, which is interpreted in terms of a monomolecular recombination regime. At higher V_OC carrier lifetime decreases as derived from a bimolecular relaxation law. The method allows estimating the recombination coefficient, which results in 2×10⁻¹³ cm³ s⁻¹. The results have been explained by considering Gaussian density-of-states (DOS) for highest-occupied molecular orbital (HOMO) and lowest-unoccupied molecular orbital (LUMO).     

Iron oxide (n-Fe2O3) nanowire films and carbon modified (CM)-n-Fe2O3 thin films for hydrogen production by photosplitting of water

Iron oxide n-Fe₂O₃ nanowire photoelectrodes were synthesized by thermal oxidation of Fe metal sheet (Alfa Co. 0.25 mm thick) in an electric oven then tested for their photoactivity. The photoresponse of the n-Fe₂O₃ nanowires was evaluated by measuring the rate of water splitting reaction to hydrogen and oxygen, which is proportional to photocurrent density, J_p. The optimized electric oven-made n-Fe₂O₃ nanowire photoelectrodes showed photocurrent densities of 1.46 mA cm⁻² at measured potential of 0.1 V/SCE at illumination intensity of 100 mW cm⁻² from a Solar simulator with a global AM 1.5 filter. For the optimized carbon modified (CM)-n-TiO₂ synthesized by thermal flame oxidation the photocurrent density for water splitting was found to increase by two fold to 3.0 mA cm⁻² measured at the same measured potential and the illumination intensity. The carbon modified (CM)-n-Fe₂O₃ electrode showed a shift of the open circuit potential by −100 mV/SCE compared to undoped n-Fe₂O₃ nanowires. A maximum photoconversion efficiency of 2.3% at applied potential of 0.5 V/E_aoc was found for CM-n-Fe₂O₃ compared to 1.69% for n-Fe₂O₃ nanowires at higher applied potential of 0.7 V/E_aoc. These CM-n- Fe₂O₃ and n- Fe₂O₃ nanowires thin films were characterized using photocurrent density measurements under monochromatic light illumination, UV-Vis spectra, X-ray diffraction (XRD) and scanning electron microscopy (SEM).     

Effect of oxygen nonstoichiometry on photo-electrochemical properties of TiO2−x

Polycrystalline TiO_2−x pellets were equilibrated at 1220–1420 K in a flow of Ar + 7 vol.% H₂ gas mixture. The resulting deviation x from stoichiometric composition of titanium dioxide was determined gravimetrically. X-ray diffractometry revealed coexistence of rutile with Ti_nO_2n−1 Magnéli phases. Optical reflectance and photocurrent spectroscopies served as experimental methods of the band gap determination. Polycrystalline TiO_2−x were used as photoanodes in a photoelectrochemical cell. The photocurrent response to the applied voltage was studied. It was found that TiO_2−x with x ca. 0.006 exhibited the best photoelectrochemical performance.     

Numerical study of the effects of non-equilibrium plasma on the ignition delay of a methane–air mixture using detailed ion chemical kinetics

Researches on non-equilibrium plasmas in ignition and combustion processes have drawn attention of many scientists, because a non-equilibrium plasma-assisted approach provides a useful method to ignite a combustible mixture and stabilize the combustion process. The ignition delay times of methane–air mixtures have been investigated experimentally and numerically; however, the influence of non-equilibrium plasma on the ignition of argon-free methane–air mixtures has seen relatively little discussion. Here, we investigate the ignition delay time of methane–air mixtures via numerical analysis using detailed chemical kinetics. Discharge process and following ignition process are simulated separately, because of significant differences in their time scales and mechanisms. Data on the concentration of atoms and radicals produced in the discharge processes were used as the initial input data to determine the subsequent ignition process because they play an important role in the subsequent ignition process. We focus on the effects of the strength of the reduced electric field, the discharge duration, and the initial temperature on the ignition delay time for zero-dimensional and axisymmetric one-dimensional models. The simulation results showed that the reduced electric field was important in promoting chemical reactions for both the one-dimensional model and the zero-dimensional model; for a constant reduced electric field, longer discharge durations provided more energy to excite the nitrogen, leading to a larger mole fraction of excited nitrogen species during discharge; the gaps between ignition delay times for E/N = 0 and E/N ? 50 Td were very small at high initial temperatures; however they became very large at low initial temperatures.     

The combined effects of temperature and electric fields on formaldehyde emission from building materials: Experiment and molecular dynamics simulation

An experimental and molecular dynamics simulation study concerning the emission of formaldehyde from building materials in different temperature and electric fields was conducted. Formaldehyde concentration was tested at the temperature of 298 K, 303 K, 308 K, 313 K and electric field of 0 kV, 12 kV, 18 kV, 24 kV. A new calculation method for solving three key parameters of formaldehyde emission was proposed and verified by experimental results. It was concluded that building materials emitted more formaldehyde at higher temperature and electric field. With the increase of temperature and electric field, experimental results indicated that both initial concentration (C₀) and diffusion coefficient (D_m) increased while partition coefficient (K_m) decreased. In addition, the temperature made larger contribution than electric field not only on formaldehyde emission and three key parameters by experimental results, but also on adsorption potential in the molecular dynamics simulation. What's more, the adsorption potential decreased while diffusion coefficient increased with greater temperature and electric field in the molecular dynamics simulation system, corresponding well with experimental results. The results of this study confirmed the contribution of electric field and temperature for the reduction of formaldehyde in the man-made boards.     

Experimental and simulation studies of primary vacuum freeze-drying process of random solids at microwave heating

This paper concerns experimental and theoretical studies of freeze-drying process at microwave heating. Two kinds of random solids were dried: material which are assumed to have no internal porosity (ground glass), as well as one containing internal porosity (Sorbonorit 4 activated carbon). Formulated one-dimensional two-region model of freeze-drying process at microwave heating takes into account unknown a priori sublimation temperature T_s(t) and mass concentration of water vapor C_s(t) at moving ice front. Steady capacity of internal heat source is correlated with electric field strength E and dissipation coefficient K(T) in both regions of the material to be dried. Linear temperature dependency of dissipation coefficient is assumed and described by two regression parameters: μ_1i and μ_2i for dry (i = I) and frozen (i = II) bed, respectively. A correlation between both measured and calculated temperatures of the sample and actual electric field strength was observed. Fairly good agreement between experimental and simulated results was stated.     

Influence of temperature on corrosion behavior of PbCaSnCe alloy in 4.5 M H2SO4 solution

The temperature effect on corrosion behaviors of PbCaSnCe alloy in 4.5 M H₂SO₄ solution was investigated by using potentiodynamic curve, electrochemical impedance spectra (EIS), Mott-Schottky plot and photocurrent response methods. It was found that PbCaSnCe alloy was in passive state in sulfuric acid solution, a passive film can be formed on alloy surface. The compositions of passive films formed at 0.9 V for 2 h under different temperatures were detected by X-ray photoelectron spectroscopy (XPS). The results showed that the film resistance and the transfer resistance decreased with the increment of the solution temperature. Mott-Schottky analysis and the photocurrent response revealed that the passive film exhibited n-type semi-conductive character, the donor density of the passive film decreased with increasing the solution temperature. Photocurrent response revealed that the photocurrent increased with increasing temperature. XPS results indicated that the PbO₂ content in passive films may increase with increasing the solution temperature.     

Factors limiting minority carrier lifetime in solar grade silicon produced by the metallurgical route

Solar grade, p-type multicrystalline silicon wafers with large grains from different parts of silicon ingots produced by the metallurgical route (SoG-Si) at ELKEM Solar were studied using a number of complementary methods such as microwave photoconductivity decay, deep level transient spectroscopy, transmission and scanning electron microscopy, X-ray fluorescence, and secondary ion mass spectroscopy. Wafers from the top of the ingots have uniform spatial distributions of both minority carrier lifetime (average lifetime τ=3.2 μs) and concentrations of illumination-sensitive recombination centers (N_rc=3×10¹⁰−2×10¹¹ cm⁻³) over the whole wafers. Wafers from the bottom of the ingots have regions of very low lifetimes (τ=0.3 μs) and high concentrations of illumination-sensitive recombination centers (N_rc=2×10¹² cm⁻³). In the top part of the ingots the observed DLTS peaks can be attributed to copper-related extended defects, and the DLTS results from grains and grain boundaries are not significantly different. The main factors limiting the lifetime in the high lifetime regions are concluded to be illumination-sensitive recombination centers such as Fe-B pairs, B-O complexes, and Cu-related extended defects. The low lifetimes in the bottom part of the ingots are explained by a combination of several factors including high concentrations of illumination-sensitive recombination centers and of some deleterious elements (S, Na and Al), and a large amount of structural defects.     

Heat transfer in the magnetohydrodynamic flow of a power-law fluid past a porous flat plate with suction or blowing

An analysis is made of the steady magnetohydrodynamic flow of a power-law fluid past an infinite porous flat plate subjected to suction or blowing. A uniform transverse magnetic field is applied normal to the plate. It is shown that for small magnetic field parameter M, the steady solutions for velocity distribution exist for a pseudoplastic (shear-thinning) fluid for which the power-law index n satisfies 1/2 < n ≤ 1 provided that there is suction at the plate. For blowing at the plate the steady solutions for velocity distribution exist only when n is of the form p/q, where p is an odd positive integer and q is an even positive integer provided 1/2 < n < 1. Velocity at a point is found to increase with increase in M. The solution of the energy equation governing temperature distribution in the flow of a pseudoplastic fluid past an infinite porous plate subjected to uniform suction reveals that the temperature at a given point increases with increase in M.     

A Peclet number based analysis of mixed convection for lid-driven porous square cavities with various heating of bottom wall

Mixed convection flows in a lid-driven square cavity filled with porous medium are studied numerically using penalty finite element analysis for uniform and non-uniform heating of bottom wall. The relevant parameters in the present study are Darcy number (Da = 10^− 5− 10^− 5), Grashof number (Gr = 10³− 10⁵), Prandtl number (Pr = 0.026−10) and Reynolds number (Re = 1−10²). The influence of convection is analyzed with Peclet number (Pe = Re.Pr). It is observed that the temperature profiles are symmetric for low values of Pe or Pr even in the presence of asymmetric flow fields irrespective of Da. The flow distribution affects significantly temperature distributions at high Pe irrespective of Da. Effect of Peclet numbers have been further investigated for both natural convection and forced convection dominant regimes at high Da. Strong coupling between flow fields and temperature are observed at high Pe. It is interesting to observe that large isothermal mixing zone at Pr = 10 reduces the overall flow strength compared to Pr = 0.026 case. Local Nusselt numbers show almost uniform and low values for low Peclet numbers and localized enhanced heat transfer rates are observed for high Peclet numbers at Da = 10^− 3.     

Enhanced photoelectrochemical properties of WO3 thin films fabricated by reactive magnetron sputtering

Polycrystalline WO₃ thin films were fabricated by reactive magnetron sputtering at a substrate temperature of 350 °C under different Ar/O₂ gas pressures. In order to study the thickness dependence of photoelectrochemical (PEC) behavior of WO₃, the thickness-gradient films were fabricated and patterned using a micro-machined Si-shadow mask during the deposition process. The variation of the sputter pressure leads to the evolution of different microstructures of the thin films. The films fabricated at 2 mTorr sputter pressure are dense and show diminished PEC properties, while the films fabricated at 20 mTorr and 30 mTorr are less dense and exhibit enhanced water photooxidation efficiency. The enhanced photooxidation is attributed to the coexistence of porous microstructure and space charge region enabling improved charge carrier transfer to the electrolyte and back contact. A steady-state photocurrent as high as 2.5 mA cm⁻² at 1 V vs. an Ag/AgCl (3 M KCl) reference electrode was observed. For WO₃ films fabricated at 20 mTorr and 30 mTorr, the photocurrent increases continuously up to a thickness of 600 nm.     

High-temperature tensile and creep properties of a ferritic stainless steel for interconnect in solid oxide fuel cell

The purpose of this study is to investigate the high-temperature mechanical properties of a ferritic stainless steel (Crofer 22 APU) for use as an interconnect material in planar solid oxide fuel cells (pSOFCs). Tensile properties of the Crofer 22 APU steel are evaluated at temperatures of 25-800 °C. Creep properties are evaluated by constant-load tests at 650-800 °C. Several creep lifetime models are applied to correlate the creep rupture time with applied stress or minimum creep rate. Experimental results show the variation of yield strength with temperature can be described by a sigmoidal curve for different deformation mechanisms. The creep stress exponent, n, has a value of 5 or 6, indicating a power-law creep mechanism involving dislocation motion. The apparent activation energy for such a power-law creep mechanism is estimated as 393 kJ mol⁻¹ through some thermally activated relations. Creep rupture time of the Crofer 22 APU steel can be described by a Monkman-Grant relation with a time exponent, m = 1.11. The relation between creep rupture time and normalized stress is well fitted by a universal simple power law for all of the given testing temperatures. Larson-Miller relationship is also applied and shows good results in correlating the creep rupture time with applied stress and temperature for the Crofer 22 APU steel. Fractographic and microstructural observations indicate most of the creep cavities are nucleated along grain boundaries and a greater amount of cavities are formed under high stresses.     

Brush electrodeposited CdSexTe1−x thin films and their properties

CdSe_xTe_1−x thin films were brush plated on titanium and conducting glass substrates from the precursors at different substrate temperatures in the range of 30-80 °C. X-ray diffraction studies indicated the films to possess hexagonal structure irrespective of composition. The strain and dislocation density decrease with increase of substrate temperature. The crystallite size increased from 30 to 100 nm as the substrate temperature increased. The resistivity of the films decreased with increase of substrate temperature. The carrier density and mobility increased with substrate temperature. Optical band gap of the films varied in the range of 1.45-1.72 eV. Higher photosensitivity was obtained compared to earlier reports.     

Hydrogen generation under sunlight by self ordered TiO2 nanotube arrays

Highly ordered TiO₂ nanotube arrays generate a considerable interest for hydrogen generation by an electrochemical photocell, since ordered architecture of nanotube arrays provides a unidirectional electric channel for electron's transport. Here, we report the hydrogen generation by highly ordered TiO₂ nanotube arrays under actual sunlight in KOH electrolyte. The two-electrode electrochemical cell included an adjustable anode compartment capable of tracing the trajectory of the sun and a set of alkaline batteries connected with a rheostat for application of external bias. The results showed that the photocurrent responses of nanotube arrays match well with the intensity of solar irradiance on a clear summer day. Addition of ethylene glycol into KOH electrolyte as a hole scavenger enhanced the rate of hydrogen generation. A maximum photocurrent density of 31 mA/cm² was observed at 13:30 h, by focusing the sunlight with an intensity of 113 mW/cm² on the surface of the TiO₂ nanotube arrays in 1 M KOH electrolyte with 10 vol% ethylene glycol under an applied bias of 0.5 V. The observed hydrogen generation rate was 4.4 mL/h cm² under the focalized solar irradiance with an intensity between 104 mW/cm² and 115 mW/cm² from 10:00 to 14:20 h.     

Incorporating carbon nanotube in a low-temperature fabrication process for dye-sensitized TiO2 solar cells

Dye-sensitized solar cells (DSSCs) incorporating TiO₂ porous films, prepared at a low temperature (150 °C), along with multi-wall carbon nanotubes (MWCNTs) were studied using two different electrolytes, namely LiI and THI. Electrochemical impedance spectroscopy (EIS) was employed to quantify the charge transport resistance and electron lifetime (τ_e) under different levels (wt%) of MWCNTs and electrolytes. The charge transport resistance at the TiO₂/dye/electrolyte interface (R_ct2) increased as a function of the MWCNT concentration, which ranged 0.1-0.5 wt%, due to a decrease in the surface area and decreased dye adsorption. The characteristic peak shifted to a lower frequency at 0.1 wt% of MWCNT, indicating a longer electron lifetime. The DSSC with the TiO₂ electrode containing 0.1 wt% of MWCNT resulted in a higher short-circuited current density (J_SC) of 9.08 mA/cm², an open-circuit voltage (V_OC) of 0.781 V, and a cell conversion efficiency of 5.02%. EIS was also conducted under dark conditions. The large value at a middle frequency represented electron transport at the TiO₂/dye/electrolyte interface (R_rec). The R_rec for 0.1 wt% MWCNT/TiO₂ was found to be 114 Ω, and for those with 0.3 and 0.5 wt% were 35 and 30 Ω, respectively. The significantly higher value of R_rec suggested that the charge recombination between injected electrons and electron acceptors in the redox electrolyte, I₃⁻, was remarkably retarded. Finally, electrolytes with LiI and THI were used to compare the cell conversion performance under the same conditions. It was found that more electrons were injected in the TiO₂ electrode and the electron recombination reaction was faster in the DSSC with THI than that with LiI.     

Efficient photocatalytic hydrogen generation by Pt modified TiO2 nanotubes fabricated by rapid breakdown anodization

Photo-assisted hydrogen generation studies of platinum loaded titanium (IV) oxide nanotubes suspended in ethanol–water mixture were carried out at room temperature. The TiO₂ nanotubes synthesized by rapid breakdown anodization technique were loaded with Pt nanoparticles by chemical reduction of aqueous chloroplatinic acid solution using sodium borohydride. The chemisorption (active) surface area of the synthesized nanocomposites for hydrogen was measured by pulse chemisorption method using temperature programmed desorption reduction oxidation equipment and found to decrease with increase in platinum loading in the range 1–10 wt%. The platinum supported nanotube composites were characterized for phase and morphology by XRD, TEM and SEM. The hydrogen generated by the photocatalytic reduction of water from water–ethanol mixture at different wavelengths of incident light, using the Pt-TiO₂ nanocomposite photocatalyst, was determined by using a proton exchange membrane based hydrogen meter. The highest hydrogen generation efficiency was observed at 1–2.5 wt% of Pt loading. The maximum photocatalytic hydrogen generation of 0.03 mol/h/g of Pt-TiO₂ was observed with a 64 W UV light source (λ = 254 nm). The photoluminescence property of the Pt loaded TiO₂ has been correlated with the hydrogen generation efficiency and the reaction mechanism briefly discussed.     

Performance of micro-PEM fuel cells with different flow fields

Four designs of flow fields were applied to micro-proton exchange membrane fuel cells (μ-PEMFCs) using microelectromechanical system (MEMS) technology. The flow fields and membrane electrolyte assembly (MEA) of 2.25 cm² active area were assembled to μ-PEMFCs. Electrochemical behaviors of these μ-PEMFCs were investigated by polarization method at reactants flow rates of 15 ml min⁻¹, 30 ml min⁻¹ and 50 ml min⁻¹, respectively. This study emphasized the effects of different topologies of flow fields on performance of μ-PEMFCs. Results demonstrated that μ-PEMFCs with different flow fields have similar behavior at reactants flow rates of 50 ml min⁻¹. However, at reactants flow rates of 15 ml min⁻¹ and 30 ml min⁻¹, performance of the μ-PEMFC with long and narrow micro-channels rapidly deteriorated due to the flooding in micro-channels. The mixed serpentine design had a good ability to resist the flooding, but it displayed a low maximum power density because of its short effective length of micro-channels. The results in this study suggested that the μ-PEMFC with a mixed multichannel design flow field and long micro-channels yielded the best performance.