I. J. Bennett

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.

Non-destructive testing of crystalline silicon photovoltaic back-contact modules

One-step module encapsulation and interconnection using conductive back sheet foils and conductive adhesives has advantages including i) fast module assembly, ii) limiting cell handling to a one time pick-and-place, and iii) low temperature (<160°C) processing. Drawback of the integrated module production, however, is that interconnected cells can only be inspected after the module has been laminated. Furthermore, because all electrical interconnections are located between the cells and the conductive foil, non-destructive test methods are required for the inspection of photovoltaic (PV) modules produced with this method. In this contribution complimentary non-destructive test methods, including lock-in thermography using a forward bias in the dark (power is dissipated) are compared as methods for testing back-contact modules allowing i) the accurate discrimination of failed and functioning interconnections between cells and the conductive foils, and ii) the detection of delamination of the back side foil. Included in the comparison are electroluminescence, infrared thermography, X-ray scanning, and ultrasonic inspection. Drawbacks and benefits of each test method are summarized and this shows that lock-in thermography is a fast, accurate, and economical non-destructive test method that can be applied for back-contact modules.

World first 17% efficient multi-crystalline silicon module

The overall demand in reducing the cost of electricity from solar energy has driven the cost of solar modules down. ECN developed an innovative module technology to enable this cost reduction requirement. The back-contact solar module concept caters specifically for high-efficiency solar cell types. It combines advanced module manufacturing design and process with emphasis on manufacturability and performance to provide an effective route toward the reduction of cost-performance ratio of crystalline silicon module. This paper discusses the recent achievement in the delivery of the worlds first 17% efficient multi-crystalline silicon module by utilizing the back-contact module concept, and the corresponding optimizations that resulted in this number.

Towards a better understanding of the thermo-mechanical behavior of H-pattern cells during metallization

In this paper, we present a fully adaptable three-dimensional finite element (FE) model of a solar cell with silver H-pattern on the front side and a full aluminum rear side. The model is used to simulate the metallization process followed by a four-point bend test. The model was verified by measuring bowing after metallization and force-displacement curves during four-point bending. The four-point bending model allows the ultimate breaking stress for different cell thicknesses to be determined. In combination with the model for the residual stresses generated during the metallization process, the limits of this process with respect to cell thickness can be predicted. Furthermore the robustness and limits of the model are tested using Monto Carlo analysis giving insight into the variables determining stress and bowing.

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.

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.