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The results of steady state and transient capacitance spectroscopy for ZnO/CdS/Cu(In,Ga)Se2 solar cells are presented. A minority carrier signal from the interface region of absorber has been investigated using Laplace-DLTS. Contributions belonging to three discrete electron traps with thermal emission rates distorted by electric field-assisted tunnelling have been identified and assigned to InCu antisite defect. Support for these conclusions has been also provided by admittance spectroscopy of samples in various metastable states created by prolonged exposition to light, voltage bias or elevated temperature. Two other deep electron traps have been revealed by the use of DLTS injection. One of them, tentatively assigned to the VSe defect, is involved in the metastable phenomena observed in Cu(In,Ga)Se2 – based solar cells. Judging from the high value of capture cross section for carriers of both signs we conclude that it might be a dominating recombination centre in these devices.
Keywords: solar cells, Cu(In,Ga)Se2, defect levels, DLTS, admittance spectroscopy.
This paper presents current technology used to produce 100 cm2 multicrystalline silicon solar cells of efficiency above 13% which was one of the main goals of the National Photovoltaic Project undertaken in the Institute of Metallurgy and Materials Science (IMMS). The general concept of the technology consists of maximum seven steps. The process sequence is based on diffusion from POCl3 and screen printed contacts fired through a PECVD SixNy or TiOx antireflection coating (ARC). Co-metallisation annealing was performed in an IR furnace. The multicrystalline wafers are described using four-point probe, scanning electron microscopy (SEM), secondary ion mass spectrometer, and spectrophotometer with an integrating sphere. The completed solar cells are characterized with internal spectral response and a current-voltage characteristic. All aspects playing a role in a suitable manufacturing process are discussed. At present, multicrystalline silicon solar cells capture around a 45% share of the world photovoltaics market with a total of 540 MW being produced in 2002. Taking into account the decreasing cost of multicrystalline substrates, this relation will rise systematically.
Keywords: solar cells, multicrystalline silicon, industrial technology.
Photoluminescence (PL) of the absorber layer of ZnO/CdS/Cu(In,Ga)Se2 solar cells has been studied. Baseline process solar cells as well as structures subject to a damp heat treatment and sodium-free structures have been investigated. The excitation intensity and temperature dependence of the photoluminescence spectra have been measured. A large blue shift of the photoluminescence bands for increasing excitation intensity has been observed with a per decade shift value ranging from 10 meV for baseline cells to 35 meV for Na free cells. This is characteristic behaviour for spectral bands due to transitions involving random potential fluctuations in highly compensated In-rich near-interface layer of the Cu(In,Ga)Se2 film. The temperature evolution of the spectra indicates two types of PL transitions: the tail-impurity and band-impurity transitions. The change of the PL spectra upon the damp heat treatment is discussed.
Keywords: thin film solar cells, photoluminescence, defect states.
This paper presents the improved method of obtaining the stable hydrogenated amorphous silicon (a-Si:H) due to the protocrystalline nature of the Si:H materials. The latest experiments and results on silicon thin film solar cells are presented in this publication. The relation of microstructure and the efficiency of amorphous and microcrystalline silicon single photovoltaic structures as well as tandem on glass and triple on steel foil solar cell are also discussed.
Keywords: silicon thin film solar cells, hydrogenated amorphous silicon (a-Si:H), microcrystalline silicon, single-, tandem- and triple-junction solar cells, RF PECVD.
The results of investigation of monocrystalline silicon solar cells with screen-printed metallisation are presented. The efficiency of typical cell is about 15%. The participation of all responsible factors for reducing the efficiency was determined based on PC-1D program. It was shown that the profile of the donor dopant in the emitter n+ is the most critical factor which reduces the efficiency by about 9% in comparison with the maximal theoretical value. The second important factor is the area of the top grid contact which reduces the efficiency by about 7%. The other factors are reflectance from TiOx/SiO2/Si, series resistance and high saturation current which is caused by recombination in the space charge region of cell. To achieve higher efficiency and to approach 20% efficiency, cell design needs to evolve significant and further improvement in the fine-line screen-printing. Moreover, elaboration of selective emitter technique suitable for mass production is necessary.
Keywords: solar cells, crystalline silicon, manufacturing and processing.
Manufacturing processes and investigation of properties
of thin film materials forming the CuInSe2 (CIS) solar cell
have been described. The cell consisted of the following layers: glass/
Mo/ p-CuInSe2/ n-CdS/ n+–ZnO/. CIS absorbers were
obtained by pulse magnetron sputtering of metallic targets in argon yielding
the multilayer precursors structures which were successively chalcogenised
in selenium vapours. Cadmium sulfide buffer layer was manufactured by chemical
bath deposition (CBD) method which offers the films with optimal properties.
Window zinc oxide layers were obtained by RF magnetron sputtering of
metallic Zn:Al target in oxygen reactive atmosphere.
Thin film CIS solar cells with the efficiencies of the order of 6% have
been produced. Further improvement in technology leading to CIS cells with
better parameters have been discussed.
Keywords: chalcopyrite, CuInSe2, thin film technology, solar cells.
The use of single mode polarization-maintaining side-hole (SMPM-SH) fibres for direct measurement of transverse force has been studied theoretically and experimentally. Based on the results of a qualitative analysis of stress distribution in such a fibre, it was predicted that SMPM-SH fibres can exhibit higher sensitivity to transverse force than a standard SMPM fibre. The modelling of two types of SMPM-SH fibres with bow-tie and round holes was conducted using finite element method and modified coupled-mode method. It showed that while there is a marked increase of the sensitivity for the SMPM-SH fibre with bow-tie holes, the sensitivity of the SMPM-SH fibre with round holes can be even lower than that of a standard SMPM fibre. Subsequently, in an experiment, which to the authors’ knowledge is the first application of SMPM-SH fibres to direct transverse force measurement, the sensitivity of an SMPM-SH fibre with bow-tie holes was measured. Measurement results agreed well with theoretical predictions. However, more systematic research is needed to determine the relationship between the sensitivity, dimensions and geometry of the holes.
Keywords: force measurement, fibre sensor, polarisation,
optical fibre, birefringence, finite element modelling, side-hole
fibre.
In this paper, a low cost velocity sensor based on the self-mixing effect in a laser diode is described. Theory of the self-mixing effect in the laser diode is shortly presented. Experimental velocity measurements are presented in order to evaluate the operation of the velocity sensor. In the design, the attention is focused to develop a budget sensor, which frequency response of the detection electronics is up to 85 MHz. This limits the maximum measurable velocity to 27.5 m/s. The total material costs of the velocity sensor were 234 euros. The experimental measurements conducted so far show that the linearity of the developed velocity sensor is at least as good as that of a mirror moved by a translation stage with velocities ranging from 1.0 to 48.5 mm/s. The velocity of the translation stage was controlled by a computer. When the mirror velocity is lower than 20 mm/s, the maximum relative precision with the mirror velocity is less than 3.5%. When the mirror velocity is higher than 20 mm/s the relative precision with the mirror velocity is below 0.5%. In an additional experiment with a vibrating loudspeaker’s membrane, it is also demonstrated that a maximum Doppler frequency is clearly detectable over the noise level at 12.5 MHz.
Keywords: velocity sensor, laser diode, self-mixing effect, Doppler frequency.
The InNxAs1–x alloy is a very promising, although so far almost completely unexplored, novel material for mid-IR emitters and detectors. InNAs/GaAs multiple quantum wells were grown by MOCVD on GaAs substrates, using dimethylhydrazine (DMHy) as nitrogen precursor. The crystalline quality and solid phase composition were evaluated by high-resolution x-ray diffraction. The nitrogen content in InNAs wells was determined to be 18%. The measurements indicate high quality of quantum-well structures. The peak photoluminescence emission wavelengths of ~6.5 µm at 30 K and ~7.2 µm at 193 K are the longest reported so far for dilute nitride semiconductors.
Keywords: InNAs, dilute nitrides, MOCVD, dimethylhydrazine, multiple quantum wells, mid-infrared emitters.
The advantages of optical wavelet transform used as a preprocessor for an artificial neural network are investigated. We show by digital simulation that this set-up can successfully identify and discriminate complex biometric images, such as fingerprints. The achieved capabilities include limited shift-, rotation-, scale- and intensity-invariance. We also show that the edges-enhancement filter, applied before the wavelet transform, significantly improves abilities of the system.
Keywords: optical fingerprint identification, biometrics, artificial neural network, optical wavelet transform.
General idea of the super-resolution method based on the signal-spectrum extrapolation beyond transmission-channel band has been presented. Main limitations of former realizations of the method have been discussed. A new realization procedure of the method, removing those limitations, has been proposed. It consists in signal fragmentation before transmission through a low-pass channel. Results of a few numerical experiments done by means of original computing tools has been presented and discussed.
Keywords: signal restoration, super-resolution, signal processing.
The Lorentzian width of the spectral line HeI 504.7 nm emitted in the negative glow of a hollow cathode discharge (HCD) was found to increase depending on the distance from the emitting volume to the cathode axis. This effect manifests itself at low buffer gas pressures pHe = (0.0–0.28) Torr. The later is typical for the experiments on SFS, radiative width and other constants. Thus, the observed radial inhomogeneity concerns the accuracy of these measurements. It is checked by determination of radiative width of the HeI 504.7 nm line emitted by two regions along the radius of HCD.
Keywords: hollow cathode discharge, shape of spectral line, convolution, radiative width.