Experimental study on micro-crack initiation in photovoltaic polycrystalline silicon wafer
Introduction
Polycrystalline silicon has been widely used as most commonly used photovoltaic modules in the photovoltaic industry for its low cost and high photoelectric conversion efficiency [1,2]. The silicon (Si) wafer contributes about 40% to the cost of a silicon solar cell which has a large reduction by thinning in silicon solar cell wafer thickness was required to decrease the cost of solar cells and hence, of PV modules. However, thinner wafers led to lower robustness of the solar cells against mechanical loads resulting in cell cracking. The power out and lifetime of photovoltaic modules would be significantly decreased by micro-cracking in polycrystalline silicon wafers by windy or snowy loads together with the assembly stress. It has been shown in studies that the mechanical property of polycrystalline silicon plays an important role on the performance and reliability of polycrystalline silicon system [3,4]. Therefore, researchers have done many works for the evaluation of mechanical properties in polycrystalline silicon. The stress field distribution of photovoltaic silicon cells with different structures has been studied under complex loading conditions [[5], [6], [7], [8], [9]]. Among them, fracture characteristic of polycrystalline silicon is realized as the most important factor due to its significant influence on reliability and life of photovoltaic module [[10], [11], [12], [13], [14]].
In recent years, in order to explore the fracture mechanism of polycrystalline silicon, damage evolution such as crack initiation and propagation are investigated by scholars in macro and micro scale. For instance, the crack behavior in photovoltaic modules has been analyzed through a statistic analysis with ultraviolet fluorescent method [15,16]. The cracking in polycrystalline silicon wafer has been observed by a four-points bending test for mini solar cells [17]. Furthermore, under a micro scale, micro-crack propagation in photovoltaic silicon is analyzed experimentally by Nano-indentation method [18]. The effect of micro-crack in polycrystalline silicon is also studied by using a multi-resolution approach without neglecting the role of the grain boundaries considering a polycrystalline silicon solar panel under coupled thermal mechanical load [19].
In addition, dislocations and impurities will be introduced into the production process of the polysilicon wafer, which will have a certain impact on micro-crack initiation. This will lead to the reduction of its power generation efficiency and service life. Boubaker [20] and Kieliba et al. [21] investigated the effects of dislocation in the polycrystalline silicon material. The results show that the existence of dislocations can drastically alter the relevant properties of polycrystalline silicon solar cells such as mechanical and photoelectric performance. On the other hand, the influence of impurities on properties of polycrystalline silicon material [22,23] is also taken into account for the purpose of improving the out power stability and life of photovoltaic module.
Due to the influence of processing technology, the introduced dislocation and impurity distribution appear to be quite random. Therefore, it is necessary to analyze the results with the help of the Weibull distribution. For the fracture assessment of polycrystalline silicon, the Weibull distribution [[24], [25], [26]] has been introduced as a basic and reliable theoretical method. Integrating analytical and numerical method, the result of Weibull distribution provides a proper approach to solve the scattered problem in polycrystalline silicon material. Weibull distribution has been employed in scratch tests by Borrero et al. [27,28] to determine the critical fracture stress in polycrystalline silicon wafer. Furthermore, it has also been assessed for the silicon wafer fracture strength with results of three-point bending test [29]. In addition, Weibull distribution method is firstly applied in polycrystalline silicon samples with pre-notch to analyze the stress concentration results by Bagdahn [30]. However, till now, it is still limited in the researches related to the aspects of evaluating critical stress of micro-cracks in polycrystalline silicon wafer.
In the past thirty years, in-situ experimental method [35,36] has received much attention due to its advantages on direct observation of deformation and micro-crack initiation process during the experiment. A number of experimental methods have been applied to explore the fracture mechanism of material with complicated structure such as polycrystalline silicon [[31], [32], [33], [34]]. For instance, the crack initiation and growth in the wood-plastic composites is observed directly with a portable tensile test setup [37]. Their results shows that wood particle orientation has a significant influence of on fracture mechanism. For polycrystalline silicon, Chasiotis et al. [38] have studied the fracture toughness of material and the phenomenon of subcritical crack growth under micro scale by using in-situ experiment method. Propagation process of micro-crack is directly observed under scanning electron microscope and critical stress intensity factor is measured.
Although many explorations have been carried out based on in-situ experiment, the application in micro-crack initiation of polycrystalline silicon wafer is still lacked. In particular, it is important to assess life and out power stability of PV modules especially in the polycrystalline silicon wafer under a product level, which is regarded as being qualified by industry without apparent defects. Further study on the evaluation of fracture mechanism of micro-crack initiation is needed.
The aim of this paper is to investigate the initiation behavior of micro-cracks in polycrystalline silicon wafer by using in-situ experiment method. In order to obtain the critical failure stress under tension, a novel design on specimen of polycrystalline silicon wafer slice was glued on poly-methyl methacrylate (PMMA) backboard for observing micro-crack initiation. Subsequently, numerical simulation was used to determine the dominant stress factor and assess the critical stress level of micro-crack initiation. The result was then explained with Weibull distribution to describe the fracture behavior which can provide a failure stress probability curve for the critical stress level of the micro-crack initiation. It was found that the material original characteristics such as dislocation and impurity significantly influence the micro-crack initiation.
Section snippets
Sample preparation
The identical microstructural characteristics of polycrystalline silicon wafer samples is shown by photoluminescence method in Fig. 1. The polycrystalline silicon wafer samples provided by Canadian Solar Inc were prepared from a thin sheet in dimensions of 156 × 156 × 0.19 mm3 which is used for producing the component of photovoltaic devices. As is seen in Fig. 1(a), the sheet was cut from the bulk polycrystalline silicon with randomly distributed grains, samples was obtained by cutting the
Numerical implementation
In this section, in order to investigate the complicated influences of high stress zone, a finite element method is implemented to get full stress field distribution on the polycrystalline silicon wafer. The simulation process is based on the fracture load of each sample obtained from experiment shown in Fig. 8. Comparing the micro-crack initiation location of each sample in experiment with simulation results, the dominant stress factor for micro-crack initiation can be determined.
Weibull distribution of critical crack initiation stress
For the failure of brittle materials, it is hard to determine a very precise stress value as the critical stress. A criterion based on probability distribution l is used to describe such brittle failure problems [24]. It has been indicated that the Weibull distribution is a useful tool to give a description of fracture probability under a certain stress value.
In the present work, the failure probability function based on Weibull distribution is defined as:where m is a shape
Discussion of dislocations and impurities on micro-crack initiation
The results of the critical principal stress in different samples show an obvious discreteness. Based on the critical maximum principal stress and Weibull distribution mentioned in Sections 3&4, the influence of dislocation and impurity on micro-crack initiation in polycrystalline silicon wafer is discussed in this section. PL (Photoluminescence) method is used to check the integrality of the polycrystalline silicon wafer to provide the impurity and dislocation distribution information as shown
Conclusion
In this paper, the threshold stress in polycrystalline silicon wafer has been studied. The main conclusions are as follows:
- 1.
The in-situ experiment scheme with sample combining PMMA backboard and polycrystalline silicon wafer is feasible to explore the mechanism of micro-crack initiation.
- 2.
Based on the results of experiment and FEM simulation, maximum principal stress is found as the dominant factor for micro-crack initiation.
- 3.
Existence of dislocation and impurity will all decrease the threshold
Author statement
I have made substantial contributions to the conception or design of the work; or the acquisition, analysis, or interpretation of data for the work; AND
I have drafted the work or revised it critically for important intellectual content; AND
I have approved the final version to be published; AND
I agree to be accountable for all aspects of the work in ensuring that questions related to the accuracy or integrity of any part of the work are appropriately investigated and resolved.
All persons who
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgement
This work was mainly supported by Program Mechanical Study on Initiation and Propagation of Micro-cracks in Photovoltaic Polycrystalline Silicon Wafers (Program No. 201511109) which is financially supported by EDF (Electricite De France) & CSI (Canadian Solar Inc).
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