Yu. V. Kryuchenko

Quantum-size resonance tunneling in the field emission phenomenon

Theoretical analyses have been performed of the quantum-size (QS) resonance tunneling in the field-emission (FE) phenomenon for different models of the emitting structures. Such experimentally observed peculiarities have been considered as the enhancement of the FE current, the deviation from the Fowler-Nordheim law, the appearance of sharp current peaks, and a negative resistance. Different types of FE cathodes with QS structures (quantized layers, wires, or dots) have been studied experimentally. Resonance current peaks have been observed, from which the values of the energy-level splitting can be estimated.

Quantum efficiency of exciton luminescence in low-dimensional structures with indirect energy gap

A theoretical model is proposed, which allows to evaluate the efficiency of the exciton luminescence in semiconductor nanostructures (NSs). Calculations have been made for Si–SiOx quantum structures with account of image potentials and finite barriers for electrons and holes, the barrier heights being functions of the content parameter x. Within effective mass and parabolic dispersion law approximations a relationship between concentrations of electron–hole pairs and excitons in quantum layers (QL) and quantum wires (QW) has been evaluated. The relationship depends on the intensity of pumping, temperature and exciton binding energy Ex which in turn is a function of NS width d. It is shown, that in the case of high barriers the internal quantum efficiency of the exciton luminescence in extremely narrow NSs (d≈1nm) can achieve almost 100% even at room temperatures. In NSs with lower barriers the exciton luminescence achieves maximal intensities at larger widths, but absolute values of the intensities are lower than those in the case of high barriers.

Temperature dependence of photoconversion efficiency in silicon heterojunction solar cells: Theory vs experiment

Silicon heterojunction solar cells (HJSC) with the efficiency of about 20% are manufactured. Their short-circuit current, open-circuit voltage, photoconversion efficiency, and fill factor of the current–voltage curve are measured in a broad temperature range from 80 to 420 K. It is established that the open-circuit voltage, the fill factor, and the photoconversion efficiency are non-monotonic functions of temperature, having a maximum in the vicinity of 200 K. A new approach to modeling of HJSCs is proposed, which allows one to obtain quantitative agreement with the experimental results at temperatures above 200 K, as well as to describe the results published in the literature on the solar cells under AM1.5 conditions. The temperature coefficient of photoconversion efficiency in HJSCs is discussed, and its low value is shown to be related to the low surface and volume recombination rates. Finally, a theoretical expression for the SCs temperature under natural working conditions is derived.

Observation of stimulated emission in an ultrashort-period nonsymmetric GaAs/AlAs superlattice

Nonsymmetric short-period GaAs/AlAs superlattices, for which the well thickness is at least a factor of 2 larger than the barrier thickness, have been shown to exhibit a direct band gap for any well thickness. These superlattices are characterized by an enhanced intensity of the luminescence as compared to their symmetric indirect-gap counterparts with the same well width, and, thus, may be used as light-emitting devices, in particular, as low-threshold lasers in the red visible spectrum. This conjecture is supported by the observation of stimulated emission at T=80 K for a GaAs/AlAs superlattice with six monolayers well and three monolayers barrier width.

Exciton Characteristics and Exciton Luminescence of Silicon Quantum Dot Structures

The exciton binding energy, the energies of the basic radiative exciton transition, and the zerophonon radiative lifetime of excitons in silicon quantum dots embedded in the SiOx matrix are calculated in effective mass approximation with quadratic dispersion relation. In addition, the spectra of steady-state photoluminescence and of time-resolved photoluminescence of excitons in the silicon quantum dots are calculated, and the kinetics of the photoluminescence relaxation is considered. The theory is compared with the experiment. It is shown that, for nanostructures involving silicon quantum dots with diameters smaller than 4 nm, the governing factor in the broadening of the spectral photoluminescence bands is the effect of mesoscopic quantum fluctuations. In this case, either an even one dangling bond at the interface, or one intrinsic point defect, or one foreign atom located inside the small-sized nanocrystallite or in its close surroundings produces a pronounced effect on the energy of the exciton transition.

Analysis of the possibility of high-efficiency photovoltaic conversion in tandem heterojunction thin-layer solar cells

The possibility of creating tandem heterojunction-with-intrinsic-thin-layer (HIT) solar cells possessing photovoltaic conversion efficiency greater than that of the best existing single-junction HIT structures is analyzed. It is established that, because of small carrier lifetimes and high degrees of compensation, the use of amorphous silicon in tandem HIT cells cannot provide for record high conversion efficiency. Key parameters of the material for a widegap p-n junction on the frontal side of tandem solar cells are determined that will allow a photovoltaic conversion efficiency above 25% under AM1.5 conditions to be reached.

Simulation of daytime variations in the characteristics of a-Si:H solar cells

The time dependences of the key characteristic of a-Si:H solar cells over daylight hours are theoretically simulated. The model is used to calculate the time dependences for an arbitrary geographic latitude in the interval 30°–60° and arbitrary day of the year. The calculated results are illustrated for a geographic latitude of 45° and equinox. The relative variations in the characteristics of the a-Si:H solar cells are valid with a relatively high accuracy for the solar cells based on alternative semiconductors provided that their efficiency ranges form 7 to 20%.

Annual dependences of generated power and electrical energy for a-Si:H-based solar cells

The annual dependences of the powers and energies generated by the unit area of a solar cell (SC) are calculated for a-Si:H-based SCs operating at latitudes of 45°N, 50°N, 55°N, and 60°N and in some geographical localities of Russia. Normalization of these dependences gives an idea about the corresponding annual dependences for SCs based on other semiconductors. Combined with the data on the average number of sunny days in a year (or the total duration of sunshine per year) for a specific region in Russia, this information makes it possible, in particular, to judge about the prospects for constructing solar power plants in these regions. As a result, the regions in Russia for which the excess over the average values of electrical energy generated by solar power plants may reach 24% are determined.

Exciton states and photoluminescence of silicon and germanium nanocrystals in an Al2O3 matrix

This paper deals with the theoretical and experimental study of radiative processes in zero-dimensional Si and Ge nanostructures consisting of a system of Si or Ge nanocrystals embedded in an Al2O3 matrix. The Al2O3 films containing Si or Ge quantum dots were produced by pulsed laser-assisted deposition. The timeresolved photoluminescence spectra of the films were recorded in the energy range from 1.4 to 3.2 eV in the range of photoluminescence relaxation times between 50 ns and 20 μs. The exciton binding energy and the energy of radiative excitonic transitions are calculated, taking into account the finite barrier height and the polarization of heterointerfaces. In addition, the excitonic photoluminescence spectra are calculated, taking into account the effect of quantum mesoscopic fluctuations and the possible nonmonotonically varying dependence of the radiative zero-phonon lifetime of excitons on the dimensions of the quantum dots. The observed agreement between the calculated and recorded photoluminescence spectra confirms the excitonic nature of photoluminescence and provides a means for the determination of the model parameters of photoluminescence in the nanostructures.

Simulation of the natural characteristics of vertical a-Si:H/μc-Si:H tandem solar cells. 1. General relations

An approach to calculating the characteristics of vertical (series) a-Si:H/μc-Si:H tandem solar cells (SCs) at arbitrary angles of incidence of sunlight is developed. The multiple reflection and refraction of electromagnetic waves at the internal interfaces of a tandem SC, in particular, at the a-Si:H/μc-Si:H interface is taken into account. In the calculation of the ideality factor and saturation current density of the diode component of the I–V characteristic of a tandem SC, as well as in the calculation of all its photovoltaic characteristics on the basis of these parameters at arbitrary sunlight incidence angles, general relations are used that take into account the recombination of excess carriers in both the quasi-neutral regions and in the space charge regions of the investigated structure. Expressions are obtained for determining all the main parameters of the component parts of a tandem SC, which are necessary for calculating the photovoltaic characteristics of the entire tandem SC. The results of calculation for standard AM1.5G (1000 W/m2) illumination conditions are reported.