Microstructure and Mechanical Aspects of Multicrystalline Silicon Solar Cells

Abstract

Due to pressure from the photovoltaic industry to decrease the cost of solar cell production, there is a tendency to reduce the thickness of silicon wafers. Unfortunately, wafers contain defects created by the various processing steps involved in solar cell production, which significantly reduce the strength of the wafers and cells. Therefore, a higher breakage rate is to be expected if thinner wafers are produced with identical fracture strength in combination with the same forces applied during processing. It should be noted that if identical displacements are applied to thinner wafers, the breakage rate will be decreased. Wafer breakage has become an important issue in the photovoltaic silicon industry, since it limits production yield and results in a further price reduction. Micro-flaws generated during wafer sawing, as well as impurity precipitations, structural defects, and residual stresses are the leading sources of crack initiation/propagation and mechanical strength degradation of silicon wafers and solar cells. In this work aspects related to microstructure, defects and stress state of crystalline silicon solar wafers and cells were studied. The aim of this work is to determine which stage during the manufacturing process, from wafer to a complete cell, is critical with respect to the introduction of stresses or cell damage, both potentially resulting in premature failure. Emphasis is placed on fundamental understanding of the microstructure and of defect and stress development and the resulting fracture strength at all stages during the production process from wafer to solar cell. The results presented in this thesis may be applied to enhance production yields, improve solar cell reliability and help to establish mechanical criteria, which all lead to a reduction in cell production costs. Microstructure and Mechanical Properties of Aluminium and Silver Contacts The research presented in this thesis provides a deeper understanding of the microstructure and mechanical properties of screen-printed and fired aluminium and silver contacts of solar cells. Furthermore, new models are proposed for the Al and Ag contact formation. It is shown that the Al layer has a porous outer part with a complex composite-like microstructure, consisting of three main components: 1) spherical (3 - 5 microns) hypereutectic Al-Si particles, surrounded by a thin aluminium oxide layer (150-200 nm); 2) a bismuth-silicate glass matrix (3.3 vol.%) 3) and pores (14 vol.%). As a result of a reaction between the porous Al and the Si wafer, a eutectic layer develops. The thickness of this eutectic layer depends on the Al particle size, the amount of Al paste and the surface roughness of the textured silicon. Smaller Al particles preferentially fill the bottom of the textured surface, while interdiffusion and alloying are facilitated by a lower melting temperature of the finer particles, resulting in a thicker and more uniform eutectic layer. Larger Al particles sinter more slowly and require higher sintering temperatures and/or longer sintering times, resulting in a wavy eutectic layer. It is also found that the drying process of the aluminium paste layer needs special attention, otherwise volatilizing solvent can cause cavities to develop in the fired layer, which affect mechanical stability and most likely electrical performance of the entire solar cell. Drying aluminium paste at a temperature of 250 °C gives small cavities and a dense Al layer structure; hence it can be recommended as an appropriate drying temperature. The Young’s modulus of the Al back contact layer, measured by nanoindentation, is found to be 44 GPa, which is in good agreement with the Young’s modulus, calculated on the basis of bowing data and a bilayer strip model. In this work the formation of fired Ag front contact layers was studied and an alternative “ionic reduction” mechanism is proposed. It is suggested, that under an oxidizing environment (atmosphere) Ag dissolves as Ag+ ions into the molten glass and there is a redox reaction between diffused Ag+ ions and the silicon substrate, which creates inverted pyramidal pits on the Si surface. The Ag atoms reduced by the reaction with the Si substrate can precipitate as Ag particles in the molten glass during firing or as Ag crystals in the inverted pyramidal pits during the subsequent cooling process. There are two main processing parameters affecting the uniformity 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 wafer by the glass layer, resulting in a good contact and a lower incidence of large voids, compared to the case of highly textured surfaces. In the case of such a textured surface, non-uniformity of the glass layer and large voids at the Ag/Si interface have a negative effect on the mechanical strength of the solar cell. The Young’s modulus of the bulk layer of Ag agglomerates was measured by nanoindentation and found to be ~54 GPa. Mechanical Stability of Crystalline Silicon Solar Wafers and Solar Cells Special fracture strength tests suitable for thin specimens, to be used in combination with Weibull statistics, were developed within this study in order to investigate the influence of the industrial processing steps on the mechanical stability of silicon wafers and solar cells. It is concluded that a combination of a 4-point bending and a ring-on-ring test method allows a more accurate evaluation of the effects of different processing conditions on the fracture strength of silicon wafers and solar cells than each test individually. In the analysis of the stresses developing during 4-point bending tests, silicon solar cell samples were treated as composite beams, consisting of two or three layers, namely a wafer and either a silver layer or aluminium porous and eutectic layers. In the ring-on-ring test stresses were analysed with a finite-element (FE) model, which was validated using digital image correlation. The combination of ring-on-ring test and FE modelling provides a new biaxial fracture strength test method for thin solar cell samples. The use of this method can be recommended for those applications where surface properties of solar cells have to be investigated, such as the effects of crystallinity and impurity concentrations on fracture strength. The results of both types of fracture tests (ring-on-ring and 4-point bending) are in good agreement. The fracture strength of crystalline silicon wafers was measured by means of both 4-point bending and ring-on-ring tests. The results show that removal of the layer containing saw damage through etching significantly increases the strength of both multicrystalline (mc) and single crystalline silicon wafers. Furthermore, the effect of mc-silicon crystallinity on fracture strength shows similar trends for both types of mechanical testing, indicating that weak grain boundaries are more detrimental than edge defects that possibly affect results from 4-point bending tests. It is found that this crystallinity has a significant effect on the strength of polished wafers, i.e. a lower strength if more grain boundaries are present, for samples taken from the middle of an mc-Si cast ingot where the impurity concentrations are low. The location where the wafer is extracted from the mc-Si cast ingot also has an effect on mechanical strength, namely samples taken from the bottom of the ingot are 30% stronger than those taken from the top. This observation was most significant for samples with many grain boundaries. This could be related to a higher carbon concentration at the bottom of the ingot. The study shows that there is a significant decrease in fracture strength when an anti-reflective coating is applied. It is thought that this is caused by high thermal stresses in this SiNx layer, which result from the high application temperature (375 °C). These high stresses probably cause fracture in the SiNx layer (before and/or during wafer loading), which consequently results in early failure of the complete wafer. The composition of the aluminium rear side contact paste has an effect on the mechanical strength of a solar cell through the total thickness of the Al layer, the thickness of the eutectic layer, the amount of porosity and the bismuth glass concentration. It was found that the larger the Al particle size, the more porous the aluminium layer is and consequently the less uniform the resulting eutectic layer is. This leads to a reduction of fracture strength, due to a non-uniform stress distribution (stress concentrations within the thinner areas of the ‘wavy’ eutectic layer). The Al-Si eutectic layer appears to show some plasticity and possibly serves to shield critical microcracks at the silicon wafer surface, thus improving the strength. Furthermore, both eutectic layer uniformity and microcrack removal contribute to the improvement of mechanical strength of Si wafers. Both the ring-on-ring and the 4-point bending test results indicate that an aluminium paste with a fine particle size can be considered the most optimal from a mechanical point of view. A strong correlation is found between the maximum firing temperature of the Al rear contact and the amount of bowing and the fracture strength of solar cells. The higher the firing temperature, the higher the bowing and the stronger the cell, effects that are related to the thickness of Al-Si eutectic layer. Aluminium contact firing temperatures between 800 °C to 850 °C are the most optimal with respect to the amount of bow and the fracture strength. Conversely, the silver paste type showed no significant influence on the fracture strength of solar cells. Samples where the Si-wafer surface is polished prior to applying a Ag layer show higher strengths, because of the stronger Ag-Si contact interface resulting from a good glass wetting on the silicon surface. Non-uniformity of the glass layer and large voids at the Ag/Si interface, observed for as-cut and textured wafer-surface conditions, have a negative effect on the mechanical strength of the solar cell and result in a lower Weibull modulus. Stress Characterization in Silicon Solar Cells Stress measurements through X-ray diffraction, in combination with bow measurements and bending tests, proved to be a powerful non-destructive qualitative and quantitative experimental technique that provides information about the stress state in the metal contact layers of silicon solar cells. Results reveal the relationship between silicon microstructure, processing conditions, defects and residual stress. The study shows that it is necessary to combine conventional X-ray diffraction, synchrotron diffraction and bow measurements in order to obtain a complete picture of the residual stress distribution in Al and Ag contacts. There is a strong correlation between maximum firing temperature, amount of bowing and the residual stress level in a solar cell, i.e. the higher the firing temperature the higher the residual stresses and the amount of bowing. Furthermore, synchrotron diffraction analysis revealed that there is a stress gradient along the thickness direction in both the Ag and Al layers. Laboratory and synchrotron X-ray diffraction methods are not appropriate for a complete stress analysis of the coarse-grained mc-silicon substrates studied in this thesis. Therefore, residual and bending stresses in the silicon substrate were investigated using Raman spectroscopy in combination with 4-point bending loading. This Raman study shows that residual stresses at the grain boundaries are higher than within the grains. The presence of grain boundaries is therefore considered the most probable reason for the lower mechanical strength of mc-Si wafers relative to sc-Si wafers. An amorphous Si phase was found in the layer damaged by the wafer-cutting process and it is thought that the presence of this transformed amorphous Si also affects the mechanical stability of as-cut wafers, caused by a transformation-induced volume change resulting in high stresses. The studies reported in this thesis provide the photovoltaic industrial and academic audience with a more fundamental understanding of the microstructure and mechanical property development during industrial multicrystalline silicon solar cell processing. On the basis of this work it can be concluded that wire-sawing, texturing, applying a SiNx antireflection coating and firing of metallic contacts are the most critical solar cell processing steps. Recommendations for the most suitable processing parameters are proposed in this thesis.