Materials Science Forum Vol. 725

Abstract: We evaluated the properties of crystalline defects in silicon substrate, and clarified the origin of small-angle grain boundaries. In order to eliminate the effects of grain boundaries, the ingot was fabricated by unidirectional solidification technique with seed crystal. In single-crystalline region, Σ3 twin boundaries and SiC precipitates were observed near the seed crystal. No obvious relationship between twin boundaries and precipitates was observed. These defect decreased once and the precipitations appeared again. The density of precipitates increased through the crystal growth procedure. These precipitates were consisted of Si, C, and N. After the precipitation density increased, the small-angle grain boundaries appeared and some precipitates were observed at the boundaries. We considered the precipitations consisted of light element impurities such as C and N were one of the major origins of the small-angle grain boundary generation.

Abstract: In this paper we combine LBIC and EL measurements of commercially multi-crystalline silicon solar cells, in order to obtain detailed information about the electrical activity around defect areas. This integrated analysis is suitable for the study of different crystal defects at both micrometric and full wafer scale. In particular, the electrical activity of some defect areas is studied in detail by means of highly spatially-resolved LBIC maps, showing important differences in their behaviours. A discussion about the origin of these differences is presented.

Abstract: Light-induced defect generation seriously reduces the minority-carrier lifetime of crystalline silicon (c-Si) wafers which causes a decrease in solar cell efficiency. In this paper we investigate the impact of boron-oxygen complexes and iron impurities on the light induced minority-carrier lifetime degradation in c-Si, comparing electronic grade and upgraded metallurgical grade materials. For the later, the characteristic of the decay process is shown to be composed of a fast initial decay and a subsequent slow asymptotic decay. We conclude that the dissociation of iron-boron pairs must be taken into account to explain the light-induced lifetime reduction.

Abstract: In this work laboratory scale multicrystalline silicon ingots were grown which have been intentionally contaminated with iron in the range between 10 to 400 ppmw by adding FeSi2 to the silicon feedstock. It is shown that an iron contamination at these high levels does not result in a structural breakdown of the columnar grain growth regime because constitutional supercooling could be avoided by strong mixing of the melt in the present crystal growth experiments. The minority carrier lifetime mappings are dominated by the iron contamination and show the distribution of the impurity over the ingot height. The measured values of the specific electrical resistivity show a significant drop from 40 to below 20 Ωcm for a contamination level of 10 ppmw Fe probably due to interactions of iron with thermal donors. At higher contamination levels the specific resistivity increases significantly with increasing iron concentration compared to the 10 ppmw ingot. Above 400 ppmw iron the specific resistivity drops below the initial value for nominally iron free material. These results indicate that interstitial iron shows a donor-like behavior in multicrystalline silicon and precipitated iron decreases the specific resistivity.

Abstract: We demonstrated high-speed imaging of photoluminescence (PL) and electroluminescence (EL) for not only band-to-band but also multiple deep-level emissions in a multicrystalline Si solar cell. We used a cooled InGaAs camera with a photosensitive range of 900 - 1700 nm equipped with band-pass filters for the selective detection of various deep-level emissions. The exposure time for imaging was only 1 - 10 seconds. Comparisons of the present PL images with the microscopic PL mappings confirmed for us that essentially the same luminescence patterns were obtained.

Abstract: A synchrotron white x-ray microbeam diffraction method was employed to investigate lattice distortion in multicrystalline silicon for photovoltaic cells. The measurements were carried out by scanning the sample, and transmission Laue patterns were observed at each position on the sample. Intensity and position maps of the Laue spots showed the distribution of the crystalline quality of the grains and the bending of the lattice planes. Strain and bending distributions were extracted from an analysis of Laue spots at diagonal positions, and these were compared with those obtained by other techniques.

Abstract: Small angle grain boundaries (SA-GBs) are known as the most electrically active defects in multicrystalline silicon. These SA-GBs are classified in “general” and “special” by the normal and strong electrical activity at 300K, respectively. In this study, the origins of these electrical activities of SA-GBs were elucidated by using electron beam induced current (EBIC) and transmission electron microscopy (TEM). It was found that both general and special SA-GBs were composed of edge-type and 60 deg / screw dislocations. The fraction of edge dislocation in special SA-GB was higher than that of general one, which suggests that strong electrical activity is mainly originated in edge dislocations.

Abstract: We investigated the new materials applicable for the field effect passivation layer in crystalline Si solar cells, ZrO2-Al2O3 and ZrO2-Y2O3 binary systems, by using combinatorial synthesis method. As-deposited samples indicated hysteresis curves and flat band-voltage (VFB) shifts at capacitance-voltage (C-V) measurements. After oxygen gas annealing (OGA) at 700 oC for 5min, an improvement of the hysteresis and a positive shift of VFB were observed. OGA process influenced defects density related to decreasing oxygen vacancy. OGA processed ZrO2 incorporated with 20 % Al2O3 and 15 % Y2O3 structures showed the maximized negative fixed charge of -5.8 × 1012 cm-2 and -7.8 × 1012 cm-2 in each system, respectively, suggesting that the ZrO2 based alloy systems were revealed to be the promising material for the passivation in the solar cell application.

Abstract: In the CuInSe₂(CIS)-based thin-film PV technology, various characterization techniques have been applied to measure the composition, crystal structure, depth profile and defect chemistry and so on, since Boeing Aerospace, for the first time, has come to the 10 % milestone in a thin-film form in 1980 by fabricating a very small single cell with top grids. More advanced and comprehensive characterization techniques are being applied after over 18 % total-area efficiency was consistently achieved employing the “three stage method”, which was developed by National Renewable Energy Laboratory (NREL). Comparing to the CIS-based absorber, there are not so many researches to investigate the absorber/buffer interface because the buffer is too thin to analyse separately and precisely and there are quite limited information on reaction pathways and composition of the buffer layer. However, in order to achieve the aperture-area efficiency of over 18 % on over 800cm²-sized large-area integrated circuits, it is remarkably important how to enhance the quality of absorber/buffer interface. Therefore, analytical works to understand how to improve the FF should tend to be more and more important.

Abstract: CuGaSe₂ single crystals were grown by a traveling heater method, which is one of the solution growth techniques. The temperature dependence of the X-ray diffraction of CuGaSe₂ was determined between 10 and 300 K. The room temeprature XRD pattern of CuGaSe₂ corresponds to the ICDD data. No secondary phases are observed in the spectrum. The lattice constant of the a-axis increases and that of the c-axis decreases with increasing temperature. The linear thermal expansion of the c-axis calculated from the lattice constants indicates a negative value from 10 to 100 K.