V.A. Popovich

Application of X-ray computed tomography in silicon solar cells

The present study outlines the characterization of the internal microstructure in a multicrystalline silicon solar cell, by means of a powerful non-intrusive experimental method, namely X-ray computed tomography. The purpose of this research is to give a better understanding of the silicon solar cells metallization layers and defects related to its processing. Resulting tomographic images showed the distribution of bismuth glass and porosity in Al and Ag contact layers. At the same time, 3D tomographic images revealed the presence of process induced defects. In this work the usefulness of the CT technique for the in depth study of silicon solar cells is shown.

Effect of microstructure and processing parameters on mechanical strength of multicrystalline silicon solar cells

Silicon wafer thickness reduction without increasing the wafer strength leads to a high breakage rate during subsequent handling and processing steps. Cracking of solar cells has become one of the major sources of solar module failure and rejection. Hence, it is important to evaluate the mechanical strength of solar cells and influencing factors. The purpose of this work is to understand the fracture behavior of silicon solar cells and to provide information regarding the bending strength of the cells. Triple junctions, grain size and grain boundaries are considered, to investigate the effect of crystallinity features on silicon wafer strength. Significant changes in fracture strength are found as a result of metallization morphology and crystallinity of silicon solar cells. It is observed that the aluminum paste type influences the strength of the solar cells.

Residual and bending stress measurements by X-ray diffraction and synchrotron diffraction analysis in silicon solar cells

The presence of residual stresses in multicrystalline silicon solar cells has become a problem of growing importance, especially in view of silicon wafer thickness reduction. Without increasing the wafer strength, this leads to a high fracture rate during subsequent handling and processing steps. The most critical processing step during the manufacture of screen-printed solar cells is the firing of metallic contacts. In this work we evaluate the development of mechanical stresses in metallic contacts (Al, Ag and Al/Ag bus bars) with respect to different processing steps. For this purpose we combine X-ray diffraction (XRD) stress measurements, Synchrotron measurements, cell bowing measurements with a laser scanning device and in-situ bending tests. Synchrotron diffraction analysis showed that there is a stress gradient in both Ag and Al layers. It was found that the Al back contact layer represents a very porous/loose microstructure, which does not affect the mechanical stability of the solar cell. It was also found that the thickness and composition of the eutectic layer are the most important factors influencing the bowing of a complete solar cell. Furthermore, residual stresses and stresses developing during cell bending in Ag, Al/Ag bus bars are measured and discussed in detail in this work.

Raman spectroscopy characterization of residual stress in multicrystalline silicon solar wafers and solar cells: Relation to microstructure, defects and processing conditions

Stress in multicrystalline silicon (mc-Si) is a critical issue for the mechanical stability of the material and it has become a problem of growing importance, especially in view of silicon wafer thickness reduction. Without increasing the wafer strength, the high fracture rate during handling and subsequent processing steps leads to excessive losses. A non-uniform stress distribution could be expected in critical areas such as grain boundaries, wire-saw-damaged layer and areas near the metallization and soldered contacts. Therefore, a non-destructive method to locally determine stress in mc-Si solar cells is of technological importance. In this paper stress characterization based on a combination of Raman spectroscopy, electroluminescence imaging, cell bowing measured with a laser scanning device, confocal microscopy and ex-situ bending tests will be presented. The most critical processing steps during the manufacture of screen-printed solar cells are wafer cutting, firing of metallic contacts and the soldering process. In this work the development of mechanical stress in silicon wafers as a result of different processing steps will be evaluated. Furthermore, residual stress and stress developing during silicon cell bending are measured in relation to microstructure and defect density. It was found that there is an inhomogeneous distribution of stress along grain boundaries and metallic inclusions. It was found that at a certain load grain boundaries in mc-Si wafers experience a higher stress (∼50 MPa) than grains themselves (∼30 MPa). A significant Raman shift was observed in samples with a wire-saw-damaged layer and in the areas close to Ag fingers and Al/Ag bus bars. Raman scanning was also performed along the solar cell cross section with different metallization patterns.

Fracture Toughness of Welded Thick Section High Strength Steels and Influencing Factors

One of the most significant problems associated with the implementation of high strength steels for offshore applications lies with the guarantee of welded joint reliability. An aspect of particular interest is the through thickness variation of heat affected zone toughness of thick section (above 60 mm) high strength steels. An experimental study combining microstructural investigations with a new adjusted sub-sized CTOD test was conducted in order to assess the fracture toughness in the heat affected zone (HAZ) of welds on high strength steels. The emphasis was placed on the coarse grain HAZ. The effects of variations in alloying and inclusion content on the microstructure and toughness properties have been studied. Results show that fracture toughness decreases in the mid-section of thick plates, which could be a result of non-uniform microstructure with local centerline segregation bands, larger grain sizes and inclusions, which act as crack initiation sites.

The Use of Circumferentially Notched Tension (CNT) Specimen for Fracture Toughness Assessment of High Strength Steels

The fracture toughness of high strength steels is commonly determined by standard methods using Compact tension (CT) or Single edge notched bend (SENB) specimens. In the past the Circumferentially Notched Tension (CNT) geometry has been reported as a potential candidate for determining the fracture toughness of highly constrained cracks, theoretically approaching plane strain conditions, even for small specimen dimensions. The goal of this study is to develop a more fundamental understanding of the CNT methodology and apply it to high strength S690QT steel. An alternative prefatiguing method was developed and a straightforward relation was established between the Crack Mouth Opening Displacement (CMOD) and the Crack Tip Opening Displacement (CTOD). With the new experimental aspects, it proved feasible to determine upper-shelf CTOD values for S690QT steel, using small CNT specimens (D = 12 mm), tested at room temperature with a relative high loading rate. Furthermore, CNT low temperature values were found comparable to those of conventional SENB tests. Hence, the research demonstrates that CNT geometry allows for small scale high loading rate specimen testing, resulting in simple, rapid and cost effective fracture toughness determination.

Creep and Thermomechanical Fatigue of Functionally Graded Inconel 718 Produced by Additive Manufacturing

Inconel 718 is a nickel-based superalloy commonly used in aircraft engine and nuclear applications, where components experience severe mechanical stresses. Due to the typical high temperature applications, Thermo-Mechanical Fatigue (TMF) and creep tests are common benchmarks for such applications. Additive manufacturing offers a unique way of manufacturing Inconel 718 with high degree of design freedom. However, limited knowledge exists regarding the resulting high temperature properties. The objective of this work is to evaluate creep and TMF behaviour of Inconel 718, produced by selective laser melting (SLM). A novel microstructural design, allowing for grain size control was employed in this study. The obtained functionally graded Inconel 718, exhibiting core with coarse and outside shell with fine grained microstructure, allowed for the best trade-off between creep and fatigue performance. The post heat-treatment regimens and resulting microstructures are also evaluated and its influence on creep and TMF is discussed.

Microstructure and Mechanical Properties of a Screen‐Printed Silver Front Side Solar Cell Contact

The most critical processing step during the manufacture of screen-printed crystalline solar cells is firing aluminium and silver contacts, which generates residual stresses and solar cell bowing. In this paper, an alternative Ag contact formation mechanism is proposed and aspects related to electrical contact properties, residual stresses and layer delamination are investigated. It is found that there are two main processing parameters affecting the uniformity and delamination of the Ag/Si interface, namely the peak firing temperature and the silicon surface roughness. Silicon surface polishing gives a better wetting of the silicon surface by the glass layer, resulting in a good contact and lower incidence of large voids, compared to the case of highly textured surfaces. The non-uniformity in the glass layer and large voids at the Ag/Si interface (in the case of a textured surface) are expected to have a negative effect on the mechanical strength of the solar cell.

Mechanical Strength of Silicon Solar Wafers Characterized by Ring-on-Ring Test in Combination with Digital Image Correlation

Avoiding wafer breakage is a big challenge in the photovoltaic silicon industry, limiting production yield and further price reduction. Special fracture strength tests suitable for thin silicon solar wafers and solar cells, to be used in combination with Weibull statistics, finite-element (FE) modelling and digital image correlation have been developed in order to study the mechanical stability of solar wafers. The results show that removal of the saw damage significantly increases the strength of crystalline silicon wafers. Furthermore, it was found that silicon crystallinity and the location where the wafer is extracted from the cast Si ingot have a significant effect on the strength, namely samples taken from the bottom of the ingot are 30% stronger than those taken from the top. The study also showed that there is a decrease in fracture strength when an anti-reflective SiNx coating is applied, which is caused by high thermal stresses.

Stress measurement by X-ray diffraction in multicrystalline silicon solar cells

Residual stresses in multicrystalline silicon solar cells has become a problem of growing importance, especially in view of silicon wafer thickness reduction. Without increasing the wafer strength, this leads to a high fracture rate during subsequent handling and processing steps. The most critical processing step during the manufacture of screen-printed solar cells is the firing of metallic contacts. In this work we evaluate the development of mechanical stresses in metallic contacts (Al, Ag and Al/Ag bus bars) with respect to different processing steps. For this purpose we combine X-ray diffraction (XRD) stress measurements, cell bowing measured with a laser scanning device and in-situ bending tests. It was found that the Al back contact layer represents a very porous/loose microstructure, which does not affect the mechanical stability of the solar cell. It was also found that the thickness and uniformity of the eutectic layer are the most important factors influencing the bowing of a complete solar cell. Furthermore, residual stresses and stresses developing during cell bending in Ag, Al/Ag bus are measured and discussed in detail in this work.