An efficient control of Curie temperature TCT_C in Ni-Mn-Ga alloys

We have studied the influence of alloying with a fourth element on the temperature of ferromagnetic ordering $T_C$ in Ni-Mn-Ga Heusler alloys. It is found that $T_C$ increases or decreases, depending on the substitution. The increase of $T_C$ is observed when Ni is substituted by either Fe or Co. On the contrary, the substitution of Mn for V or Ga for In strongly reduces $T_C$.Comment: presented at ICM-200

Peculiarities of the influence of high- and low-energy proton and electron irradiations on the characteristics of silicon solar cells

Experimental data on degradation of photovoltaic and photoenergetic characteristics of silicon solar cells exposed by high-energy electrons and protons as well as low-energy protons have been obtained. The previously proposed theoretical model that can describe degradation of the solar cell characteristics under the influence of irradiation, including that creating spatially inhomogeneous defect distribution over the structure thickness, has been experimentally confirmed. It was ascertained that in the cases of 1 MeV energy electron and 20 MeV energy proton irradiations, when there is a relatively homogeneous defect distribution over the silicon solar cell thickness, its shortcircuit current degrades faster than the open-circuit voltage. On the contrary, in the case of low-energy 0.1 MeV proton irradiation, when the distribution of defects is spatially inhomogeneous, the open-circuit voltage degrades faster than the short-circuit current

Training of the Ni-Mn-Fe-Ga ferromagnetic shape-memory alloys by cycling in a high magnetic field

The temperature and magnetic field dependencies of Ni-Mn-Ga polycrystals deformation are investigated. Ingots were prepared by arc-melting in argon atmosphere and further annealing. A training procedure (cycling across the martensitic transition point) for the two-way shape-memory effect was performed with Ni$_{2.16}$Fe$_{0.04}$Mn$_{0.80}$Ga samples. Changes in sample deformations were noticed with changing the magnetic field at a constant temperature. The first cycle deformation increment as compared with the initial value (in the austenitic state at zero field) in the course of the martensitic transition was 0.29%, and 0.41% and 0.48% for the second and third cycles, respectively.Comment: Presented at the Second Moscow International Symposium on Magnetism (Moscow-2002

Catalytic oxidation of acetone and ethanol on a platinum wire

The current-voltage characteristics of a thin long platinum wire in air with acetone or ethanol small admixtures were obtained. Using the quadratic dependence of platinum resistance on temperature, the temperature-current dependences for the wire were calculated. At concentrations of combustible gas vapors above a certain value, these dependences show a hysteresis character. Using the assumption of acetone and ethanol complete oxidation on platinum, as a catalyst, and the first order of the oxidation reaction, temperature-current dependences was analysised. The temperature difference between the high and low-temperature stationary regimes of catalytic oxidation on the wire makes it possible to estimate the admixture concentration. As a result, the experimental dependence of the critical current value by catalytic ignition and extinction gas mixture on platinum on the admixture concentration was constructed. With its analytical description, it is possible to fairly accurately estimate the apparent values ​​of the activation energy and the pre-exponential multiplier for the oxidation reaction in a wide temperature range. A method of determining the kinetic parameters for the oxidation reaction based on the experimentally found parameters of degeneracy critical conditions is proposed

Exciton effects in band-edge electroluminescence of silicon barrier structures

A theoretical analysis of the band-edge electroluminescence efficiency in silicon diodes and p-i-n-structures has been made. We have shown that maximal possible efficiency can achieve 10 % both at room and liquid nitrogen temperatures. Maximal values of the efficiency are restricted by the interband Auger recombination process. It is found that electroluminescence efficiency decreases rapidly with the decrease of characteristic Shockley- Reed-Hall nonradiative lifetime for minority carriers. It is shown that even at room temperatures the main contribution into the edge electroluminescence in silicon barrier structures is given by excitonic effects. Dark I-V characteristics of directly biased silicon diodes measured both at room and nitrogen temperatures are used to explain anomalous temperature dependencies of silicon diode electroluminescence

The study of solar cells with back side contacts at low illumination

Theoretical analysis and experimental research of Si solar cells (SC) with interdigitated back side contacts (BSC) photovoltaic parameters and photoconversion efficiency at low light level have been done in presence of floating p⁺-n junctions and isotype n⁺-n junctions on frontal (illuminated) surface. It has been found that in case of floating junction the magnitudes of short-circuit current, open-circuit voltage and efficiency, as well as of internal quantum efficiency of photocurrent can decrease significantly due to recombination in the space charge region (SCR) rather than to surface recombination. In case of isotype junction, this decrease is absent. These results allow to conclude that the floating p⁺-n junctions at the front surface of the silicon BSC SC would be appropriate for use only in case of an illumination intensity ≥ 1000 W/m²

Analysis of features of recombination mechanisms in silicon solar cells

Investigated in this paper are theoretical and experimental spectral dependences of the short-circuit current as well as small-signal photo-e.m.f. in silicon solar cells. The authors have considered two constructions of solar cells. The first construction is a solar cell with contacts on the front and back surfaces, and the second – solar cells with back barriers and contact metallization. Analyzed in the work are spectral dependences of the internal quantum efficiency for the short-circuit current and smallsignal photo-e.m.f. It has been shown that the short-wave drop of the short-circuit current is related with recombination on deep centers at the front surface as well as inter-band Auger recombination in the heavily doped emitter. At the same time, availability of the shortwave drop in the small-signal photo-e.m.f. is related with limitation of the efficient rate of surface recombination Seff(l) due to diffusion inflow. The latter takes place when a layer with the thickness dp and increased recombination is available near illuminated surface. In this case, the mechanism providing decrease in the small-signal photo-e.m.f. in the area of strong light absorption is related with increasing the efficient rate of surface recombination near the front surface, when the dominant amount of electro-hole pairs is generated in the layer with the increased recombination rate. The same mechanism is responsible for the short-circuit current drop in solar cells with back barriers and contact metallization. Juxtaposition of theoretical and experimental results enabled to determine parameters that characterize sub-surface properties of solar cells, namely: the thickness of the surface layer with increased recombination, lifetime of carriers in it, and dependences Seff(l)

Influence of nanostructured ITO films on surface recombination processes in silicon solar cells

This paper describes the results of comparative studies of illumination currentvoltage characteristics and spectral characteristics of silicon solar cells with rear location of the collector p-n-junction for the cases of non-passivated and passivated front illuminated surface. Passivation was performed by silicon dioxide layer or ITO layer. It was found that ITO layer surface passivation with formation of ITO/silicon heterojunction, unlike silicon dioxide layer passivation, leads to a significant reduction of the effective surface recombination velocity. It significantly increases the value of the internal quantum efficiency in the wavelength range from 550 to 1050 nm and, as a result, significantly increases the value of short-circuit current of solar cells

Analysis of the silicon solar cells efficiency. Type of doping and level optimization

The theoretical analysis of photovoltaic conversion efficiency of highly effective silicon solar cells (SC) has been performed for n-type and p-type bases. Considered here is the case when the Shockley–Read–Hall recombination in the silicon bulk is determined by the deep level of Fe. It has shown that, due to asymmetry of recombination parameters inherent to this level, the photovoltaic conversion efficiency is increased in SC with the n-type base and decreased in SC with the p-type base with the increase in doping. Two approximations for the band-to-band Auger recombination lifetime dependence on the base doping level are considered when performing the analysis. The experimental results are presented for the key characteristics of SC based on a-Si:H–n-Si heterojunctions with intrinsic thin layer (HIT). A comparison between the experimental and calculated values of the HIT cell characteristics has been made. The surface recombination velocity and series resistance are determined from it with a complete coincidence of the experimental and calculated SC parameters’ values. Apart from the key characteristics of SC, surface recombination rate and series resistance were determined from the results of this comparison, in full agreement with the experimental findings

Coexistence of ferro- and antiferromagnetic order in Mn-doped Ni2_2MnGa

Ni-Mn-Ga is interesting as a prototype of a magnetic shape-memory alloy showing large magnetic field induced strains. We present here results for the magnetic ordering of Mn-rich Ni-Mn-Ga alloys based on both experiments and theory. Experimental trends for the composition dependence of the magnetization are measured by a vibrating sample magnetometer (VSM) in magnetic fields of up to several tesla and at low temperatures. The saturation magnetization has a maximum near the stoichiometric composition and it decreases with increasing Mn content. This unexpected behaviour is interpreted via first-principles calculations within the density-functional theory. We show that extra Mn atoms are antiferromagnetically aligned to the other moments, which explains the dependence of the magnetization on composition. In addition, the effect of Mn doping on the stabilization of the structural phases and on the magnetic anisotropy energy is demonstrated.Comment: 4 pages, 3 figure