Polycrystalline Silicon Wafers Research Articles

Integrating machine learning and the finite element method for assessing stiffness degradation in photovoltaic modules

This study introduces a novel machine learning (ML) method utilizing a stacked auto-encoder network to predict stiffness degradation in photovoltaic (PV) modules with pre-existing cracks. The input data for the training process was derived from numerical simulations, ensuring a comprehensive representation of module behavior under various conditions. The findings highlight the robust predictive capability of the model, as evidenced by its impressive R2 value of 0.961 and notably low root mean square error (RMSE) of 4.02%. These metrics significantly outperform those of other conventional methods, including the artificial neural network with R2 of 0.905 and RMSE of 9.43%, the space vector machine with R2 of 0.827 and RMSE of 17.93%, and the random forest (RF) with R2 of 0.899 and RMSE of 11.02%. Moreover, the findings suggest that the predictive dynamics of degradation are affected by the varying weight functions of different input parameters, such as climate temperature (CT), grain size (GS), material effort, and pre-crack size, as the degradation level changes. Furthermore, a geometric analysis reveals model deficiencies where significant overestimations correlate with thicker glass components, while pronounced underestimations are predominantly associated with thinner layers of polycrystalline silicon wafer and Ethylene Vinyl Acetate in the module. As a case study, it demonstrated that to maintain a constant degradation level between 1.30 and 1.32 in a PV module with components featuring consistent geometric attributes, the input parameters must be kept within specific ranges: CT ranging from 33 °C to 57 °C, GS ranging from 36 to 81 μm, material effort ranging from 0.74 to 0.81, and pre-crack size ranging from 24 to 32 μm. Therefore, this underscores that the ML model not only predicts degradation but also delineates the parameter space required to achieve a consistent output value.

Numerical homogenization of poly-crystalline silicon wafer based photovoltaic modules including pre-cracks

The photovoltaic (PV) modules containing multiple polycrystalline silicon solar cells (PSSCs) are one of the most common devices for solar energy production. PSSCs are finding different applications such as on buildings and on solar-driven airplanes, hence, their mechanical analyses are required. During the lifespan of PSSCs, the performance degrades, which is predominantly associated with the cracking of the crystalline silicon wafer (PSW). Moreover, because of the multiple materials involved and the complicated microstructure of the energy-producing component, polycrystalline silicon wafer (PSW), modeling becomes computationally expensive. Therefore, model reduction techniques, such as homogenization, are needed. The mechanical response of polycrystalline silicon solar cells (PSSCs) is investigated in this work. The crystalline patterns of the PSW were generated using a Voronoi-tessellation scheme. A mean-field homogenization scheme using the finite element (FE) method was employed to predict the homogenized response of the PSSCs. The response of the PSSC with heterogeneous and homogeneous modeling was compared. The stiffness degradation due to the existing microcracks of the PSW was investigated. It is evident from these investigations that the homogenized FE solution provides an accurate and computationally efficient representation of the progressive failure in solar cells. The approach presented here offers a basis to further enhance the knowledge of failure analysis in PV modules and quantify the subsequent degradation of their energy producing capacity.

Experimental study on micro-crack initiation in photovoltaic polycrystalline silicon wafer

Brittle fracture of polycrystalline silicon wafer is critical to long-term reliability of electronic devices and solar cells for its wide use as components or substrates in semiconductor industry. In order to observe its initiation under loads, classical tensile test was modified by introducing the poly-methyl methacrylate (PMMA) layer to determine critical stress of the micro-crack initiation in the wafer. The method is verified by the finite element computation and then the fracture criterion of the maximum principal stress criterion for the wafer material is deduced from a series tests on the material strength. It was found that the existence of dislocations and impurities in the wafer significantly influence the threshold stress levels which is modeled by the Weibull distribution. Especially, micro-cracks are more likely to initiate in the dislocation position but not in the impurity position. The proposed method can be further extended for studying the tensile fracture and related issues on such brittle materials as ceramics, quartz, and ice.

Life Cycle Assessment of Crystalline Silicon Wafers for Photovoltaic Power Generation

A life cycle assessment(LCA) was conducted over the modified Siemens method polycrystalline silicon(S-P-Si) wafer, the modified Siemens method single crystal silicon(S-S-Si) wafer, the metallurgical route polycrystalline silicon(M-P-Si) wafer and the metallurgical route single crystal silicon(M-S-Si) wafer from quartzite mining to wafer slicing in China. A large amount of data was investigated from relevant literature and factories in this study. Based on the contribution analysis and sensitivity analysis, the key points for improvement were found. The result included primary energy demand (PED), chemical oxygen demand (COD), SO2, NH3-N, NOX, CO2, and industrial water use (IWU). The above seven indexes were weighted and then added to get China’s thirteenth five-year plan for energy conservation and emission reduction total environmental impact indexes(ECER-135) of S-P-Si wafer, S-S-Si wafer, M-P-Si wafer and M-S-Si wafer were 1.47 × 10−9, 2.12 × 10−9, 3.30 × 10−10 and 1.22 × 10−9 respectively. The ECER-135 of silicon wafers purified with modified Siemens method was higher than that purified with metallurgical route by 3.1 times on average; the ECER-135 of single crystal silicon wafers production was larger than that of polysilicon wafers production by 2.3 times on average. When the four kinds of silicon wafers were used to generate the same amount of electricity for photovoltaic modules, the ECER-135 of S-P-Si wafer, S-S-Si wafer and M-S-Si wafer were 3.3, 4.5 and 2.8 times of that of M-P-Si wafer respectively. During the whole production process, the electricity consumption was of the highest sensitivity for ECER-135. PED had the highest contribution to the ECER-135 for four kinds of silicon wafers, mainly due to the use of thermal power. If hydropower were used instead of thermal power, the ECER-135 could be reduced by 46% to 62%.

Experiment and theoretical prediction for surface roughness of PV polycrystalline silicon wafer in electroplated diamond wire sawing

The electroplated diamond wire sawing technology is the mainstream processing method of cutting PV polycrystalline silicon ingots. Surface roughness is one of the most significant evaluation indexes for wafers surface quality, and has an important influence on subsequent processes such as surface texturization, screen printing, subsurface damage layer prediction and so on. In this paper, a mathematical model, considering the random distribution characteristic of abrasive parameters and its location on saw wire surface, was developed. The influence of brittle and ductile material removal modes on the wafers surface morphology was taken into account, the model can obtain the cutting depths of abrasives by discretizing the cutting groove and wire profile during the sawing process, and then the material removal modes of abrasives are judged by cutting depth. According to the calculation nodes closest to the sawn surface, the wafer surface profile and roughness along the feed speed direction were calculated. The correctness and reasonability of the model were verified by experiments. Then, the model was used to predict the variation trend of surface roughness under different process parameters. The results showed that the relationships between surface roughness and feed speed Vf, wire speed Vs and their ratio Vf /Vs satisfied power functions. Finally, the calculation result of surface roughness was used to generate a 3D surface morphology of wafer.

Laser Application in Photovoltaics for Surface Texturization of Silicon and Front Electrode Deposition

Abstract The laser technology in the manufacturing of solar cells has become an indispensable element of modern photovoltaic technology. The main reason for taking up such research is because conventional methods used for monocrystalline silicon texturization are ineffective when applied to polycrystalline silicon texturing. This is related to random distribution of grains of different crystalographic orientations on the surface of polycrystalline silicon. Texturing a polycrystalline silicon surface using an Nd:YAG laser makes it possible to increase absorption of the incident solar radiation. The additional technological operation of etching in potassium hydroxide solution introduced into the technology of the photovoltaic cells manufactured from laser-textured wafers allows for improvement in electrical performance compared with cells produced from nontextured wafers. Moreover, this article presents the influence of selective laser sintering of front metallization that was manufactured using paste based on silver nanopowder on contact resistant. The electrodes were formed on monocrystalline silicon solar cells. The microstructures of laser-textured silicon and sintered front metallization were investigated by scanning electron microscope and confocal laser scanning microscope. The reflectance of produced textures was measured by spectrophotometer with an integrating sphere. Current-voltage characteristics of solar cells manufactured from laser-textured polycrystalline silicon wafer were measured under standard AM 1.5 radiation. The contact resistance between the metal and semiconductor was investigated using the transmission line model.

Polycrystalline silicon wafer defect segmentation based on deep convolutional neural networks

Defect segmentation is an important way for defect detection in machine vision. For polycrystalline silicon wafer production, it is difficult to automatically segment defects due to its inhomogeneous background and unpredictable defect shapes. The conventional methods based on handcrafted models or features not only heavily rely on the expertise, but also are not flexible from one application case to another. In this paper, we propose a defect segmentation method for polycrystalline silicon wafer based on the deep convolutional networks. Firstly, we apply Region Proposal Network (RPN) to generate underlying defect regions. Then, these generated image patches are processed to suitable sizes for feeding into a improved segmentation network which is modified based on U-net and a dilation convolution. Real defect images are used to test the proposed method and experimental results show that proposed method achieves better performance compared with the state-of-the-art methods.

The effect of cut depth and distribution for abrasives on wafer surface morphology in diamond wire sawing of PV polycrystalline silicon

Due to the existence of an acid etch resistant thin amorphous silicon layer over the smooth grooves of the diamond wire sawing polycrystalline silicon wafer surface, the anti-reflection effect is usually not ideal using the mature acidic texturization. The amorphous silicon layer will be produced on the machined surface by material ductile removal. Therefore, during the process of cutting photovoltaic polycrystalline silicon wafers, the material removed in the brittle way is expected and the surface topography of the wafers formed with the brittle fracture is better for the texture fabricating. In this paper, a mathematical model considering the influences of process parameters and wire saw parameters was developed based on indentation fracture mechanics. The variations of cutting groove profile formed by different material removal modes were also included. The effect of abrasives distributed on the wire saw on material removal and surface formation of polysilicon was analyzed. The results showed that most of abrasives removed material with ductile removal mode, however, the volume of the material removed by abrasive in ductile mode is less than 10% of the total removal volume. Brittle fracture removal mode was still the major way of material removal in diamond wire sawing. With the same ratio of the feed rate and wire speed, the faster feed rate and wire speed will not only improve the cutting efficiency, but also is easier to obtain a brittle fracture surface. There is a critical angle θc for the distribution of abrasives on the wire saw surface. Only when the position angle of the abrasive removing material in brittle mode is less than θc, the brittle fracture can be formed on the wafers surface.

Modelling, simulation, optimization of Si/ZnO and Si/ZnMgO heterojunction solar cells

Currently, crystalline and polycrystalline silicon wafer based solar cells dominate the photovoltaic market, which employs expensive manufacturing processes. Zinc oxide is one among the inexpensive alternatives for acting as emitter/anti-reflection coating layer which can be used in combination with widely available p-type silicon. Hence it becomes essential to gain in-depth knowledge about the charge transport mechanisms involved in the n-type ZnO/p-type silicon (Si) heterojunction devices. Therefore, Si/ZnO solar cell is modeled and the carrier transport mechanisms are carefully examined by analyzing the band edge discontinuities, electric field distributions at the Si/ZnO interface, carrier generation-recombination profile for varying ZnO thickness, carrier density and affinity values. The photo- generation rate degrades with the increase in ZnO emitter thickness owing to increased absorption in the blue region and is independent of ZnO affinity and donor concentration. The cliff like configuration with high conduction band discontinuity at the Si/ZnO interface for higher values of ZnO affinity has resulted in the formation of poor electric field and the lattice mismatch resulting in high defects formation (recombination centers) at Si/ZnO interface are the predominant causes for deficit in open circuit voltage and fill factor. Hence the conduction band offset alignment is further engineered by designing Si/MgZnO solar cell model and Mg doped ZnO emitter has significantly improved the band offset alignment at the p-n heterojunction when compared to ZnO emitter. Moreover, the simulation results revealed that the stronger electric field strength developed at the heterojunction is suggested to favor the carrier separation process by drift motion thereby improving the open circuit voltage and fill factor of the device. As a next step, both the models are optimized for varying emitter parameters like thickness, carrier density etc using Silvaco ATLAS simulator. Based on this investigation, the optimized Si/ZnO and Si/MgZnO designs anticipate improved conversion efficiencies of 11.57% and 14.46% respectively.

Comparative study on multiple transmission–reflection infrared spectroscopy and infrared methods for the measurement of substitutional carbon and interstitial oxygen contents of polycrystalline silicon

Multiple transmission–reflection (MTR) infrared (IR) spectroscopy was established as a new IR method to measure the interstitial oxygen and substitutional carbon contents of polycrystalline silicon wafers at ambient temperature. The superiority of MTR–IR to conventional IR was verified through practical measurements of polycrystalline silicon samples. With single incidence, the following characteristics of the former were superior to those of the latter. (1) The areas of the absorption peaks of interstitial oxygen at 1107 cm−1 and substitutional carbon at 605 cm−1 can be amplified linearly with transmission times N between 10 and 12, thereby extending the limit of detection of oxygen and carbon to approximately one order of magnitude lower. (2) The strength of the interference fringes of the baseline can be decreased to 5 times that of the single incidence for a 1 mm polycrystalline silicon slice. (3) In contrast to conventional IR method in which data are gathered from one sampling point at each measurement, MTR–IR collects information from many sampling points in a long sample for one-time measurement. In conclusion, MTR–IR is more sensitive, reliable, and reproducible than the conventional IR method in terms of measuring the interstitial oxygen and substitutional carbon contents of polycrystalline silicon.

Study on nano-pores enlargement during Ag-assisted electroless etching of diamond wire sawn polycrystalline silicon wafers

In this paper, diamond wire sawn (DWS) mc-Si wafers were textured using Ag-assisted electroless etching (AgNO3+HF+H2O2) combined with an auxiliary etching (HF+HNO3). The evolution mechanism of the surface texture structure on the wafers was investigated in detail. It was noted that during the AgNO3+HF+H2O2 etching, there existed the difference in the axial direction and the depth of nano-pores among different grains. This difference made the different grains form different texture structures during the following HF+HNO3 etching. Since the etching of poly-silicon is isotropic in HF+HNO3 solution, it was the AgNO3+HF+H2O2 etching rather than the auxiliary etching (HF+HNO3) that resulted in the different texture structure among different grains. During the HF+HNO3 etching, the diameter of the etched pits on the wafer surface was enlarged quickly through the adjacent pits combination. The depth of the etched pits hardly increased because the top and bottom of pits were etched at the same time. As a result, the ratio of depth to diameter of the pits decreased. The texture difference among the grains and decrease of the ratio of depth to diameter of the pits limited the further enhancement of the PV efficiency of mc-Si solar cell. Therefore, the improving the uniformity of AgNO3+HF+H2O2 etching for the different crystal orientation grains and obtaining a large ratio of depth to diameter of the pits during the HF+HNO3 etching are two critical issues to enhance the PV efficiency of mc-Si solar cell.

Nano silver-catalyzed chemical etching of polycrystalline silicon wafer for solar cell application

Silver nanoparticles were deposited on the surface of polycrystalline silicon wafer via vacuum thermal evaporation and metal-catalyzed chemical etching (MCCE) was conducted in a HF-H2O2 etching system. Treatment of the etched silicon wafer with HF transformed the textured structure on the surface from nanorods into nanocones. An etching time of 30 s and treatment with HF resulted in nanocones with uniform size distribution and a reflectivity as low as 1.98% across a spectral range from 300 to 1000 nm.

Influence of laser texturization surface and atomic layer deposition on optical properties of polycrystalline silicon

Reducing the reflectance of light as a result of texturing the front surface of the silicon and depositing an antireflection coating allows to increase the efficiency of the solar cell by up to several percent. In the present paper, the surface texturing of polycrystalline silicon based on the laser treatment and Al2O3 antireflection coating (ARC) deposited by atomic layer deposition ALD is discussed. Polycrystalline silicon wafers are characterized by their different grain sizes and orientation (grains are randomly oriented). Much effort has recently been devoted to the development of techniques for creating more isotropic textures further reducing the reflectance of polycrystalline wafers. Therefore in the present work polycrystalline silicon surface was shaped with a laser beam. In order to further reduce the reflectance of light R(λ) to the surface of the solar cell Al2O3 antireflection coating was deposited by ALD method. Al2O3 thin film in the structure of finished solar cell functions as both an antireflection and the passivation coating, which simplifies the process. A unique advantage of ALD method is the ability to uniformly deposition on geometrically complex surfaces. For this reason, atomic layer deposition method may be used for deposition of the optical thin film on the silicon solar cells as antireflection coating. There are many methods of optical thin films, for example, sol–gel, CVD, PVD but with the advantage of ALD, earlier mentioned, seems to be the most promising.

The optical properties of solar cells before and after encapsulation

In this work we investigate how the optical properties of monocrystalline silicon and polycrystalline silicon wafers are affected by texturing techniques and encapsulation. For monocrystalline wafers, the KOH etching is better than acid etching while reactive ion etching (RIE) is proven to be preferred compared to acid etching for polycrystalline wafers. The differences in reflectance (R) between two textures are apparent before encapsulation, but when the textured wafers are encapsulated with glass especially antireflectance coated (ARC) glass, the difference can be reduced from about five percentage points to below a percentage point. More important, the optical losses caused by reflectance (R) losses and parasitic absorption losses (A) for four types of commercial monocrystalline and polycrystalline silicon module are quantified and compared. Analyses are carried out for eight configurations using a stratified model consisting of solar cell wafers as well as cover glass and ethylene vinyl acetate (EVA). The model is to first obtain the external reflectance (R) and transmittance (T) in each layer, and the spectrally parasitic absorption loss associated with the cover glass and EVA is calculated with the aid of R and T measurements, which allows us to do a complete optical loss analysis for the solar module. The results show that the KOH texture with ARC glass encapsulation may be better choice for monocrystalline wafers, which gives a 94.04% of effective light collected by silicon. The RIE technique with ARC glass is suitable for polycrystalline substrates with excellent light trapping of 94.14%. After all, reflectance from glass surface and silicon surface accounts for over 70% of total loss, while the absorption of glass and EVA accounts for the rest and less loss.

Investigation of diffusion length distribution on polycrystalline silicon wafers via photoluminescence methods.

Characterization of the diffusion length of solar cells in space has been widely studied using various methods, but few studies have focused on a fast, simple way to obtain the quantified diffusion length distribution on a silicon wafer. In this work, we present two different facile methods of doing this by fitting photoluminescence images taken in two different wavelength ranges or from different sides. These methods, which are based on measuring the ratio of two photoluminescence images, yield absolute values of the diffusion length and are less sensitive to the inhomogeneity of the incident laser beam. A theoretical simulation and experimental demonstration of this method are presented. The diffusion length distributions on a polycrystalline silicon wafer obtained by the two methods show good agreement.

Scratch Fracture of Polycrystalline Silicon Wafers

Fracture of silicon wafers is responsible for lower than desirable manufacturing yields in the photovoltaic industry. This study investigates the fracture response of polycrystalline silicon wafers under sliding contacts at different length scales, by means of macro and microscratch tests which simulate cutting processes. The dominant fracture modes were found to be partial cone cracking (macro) and radial cracking (micro). Statistical analysis of the critical loads for crack initiation showed that polycrystalline wafers are weaker than their single‐crystal counterparts, that is, they crack at lower applied loads under comparable conditions. Moreover, the Weibull modulus of polycrystalline silicon was found to be the average of the relevant single‐crystal directions. Subsequent microscopic observations and flexure tests reveal that the lower resistance of polycrystalline silicon to scratch fracture is due mainly to the presence of relatively large polishing defects, and not to the weakness of its grain boundaries. Alternatives are proposed to minimize damage during ingot cutting, with a view to minimizing wafer breakages during wafer handling and machining.

Cu(In,Ga)Se$_{\bf 2}$ Thin-Film Solar Cells and Modules—A Boost in Efficiency Due to Potassium

Thin-film solar cells based on the chalcopyrite Cu(In,Ga)Se2 (CIGS) absorber material show high potential for further cost reduction in photovoltaics. Compared with polycrystalline silicon (p-Si) wafer technology, thin-film technology has inherent advantages due to lower energy and material consumption during production but has typically shown lower conversion efficiency. However, in the past two years, new scientific insights have enabled the processing of CIGS solar cells with efficiencies up to 21%, surpassing the p-Si wafer value of 20.4% efficiency for the first time. Now several research groups report record cell efficiency values above 20% using different deposition processes and buffer layers. The presence of potassium was observed in many CIGS devices over the years, but it is only very recently that differences with Na have started being taken into full consideration for device processing and that K was added intentionally to the absorber. In this study, previous reports showing the presence of potassium are reviewed and discussed in more detail. Furthermore, on a scale-up perspective, additional progress has also taken place with CIGS minimodules achieving efficiency up to almost 19% and where further increase can be expected in the near future with the improvements induced by the use of potassium. This shows that the CIGS technology is continuously progressing not only on scientific level but on technological level as well.

Sol–gel deposited phosphorus and boron doped thin silica films for diffused n+p and n+pp+ structures

AbstractThe realization of a silicon solar cell emitter is a critical and very important step in photovoltaic technology. Indeed, the surface concentration of boron (B) or phosphorus (P) and junction depth determines the photovoltaic conversion efficiency of silicon solar cells. Several techniques are used for the realization of emitters n+p and n+pp+ among which the vapor diffusion, ionic implantation and diffusion from solid sources are the most common. This work will be devoted to the realization of emitters made by diffusion of P or B from solid doping sources developed by the sol–gel method associated to spin coating on monocrystalline and polycrystalline silicon wafers. The doping solutions were prepared by sol–gel method using methyltriethoxysilane “MTEOS” or tetramethoxysilane “TEOS” and H4PO3 or H3BO3 as precursors. The n+ junction was realized by emulsion of phosphoric acid in isopropanol while the p+ junction was obtained by emulsion of boric acid H3BO3 in isopropanol or by diffusion of aluminium at high temperatures. Our samples were annealed at temperatures ranging from 850 to 1000 °C in a classical furnace under Nitrogen atmosphere. The four point technique has been used to study the electrical properties of n+p and n+pp++ structures. The results of measurements showed values of the sheet resistance Rs comparable to those of literature and confirm the diffusion of phosphorus and boron from liquid and solid sources prepared by sol–gel method. (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Sol–gel synthesized Titania nanoparticles deposited on porous polycrystalline silicon: Improved carbon dioxide sensor properties

Titania nanoparticles (TNPs) were synthesized by a sol–gel method in our laboratory using titanium tetrachloride as the precursor and isopropanol as the solvent. The particles׳ size distribution histogram was determined using ImageJ software and the size of TNPs was obtained in the range of 7.5–10.5nm. The nanoparticle with the average size of 8.5nm was calculated using Scherrer׳s formula. Homogeneous and spherical nanoparticles were characterized by X-ray diffraction (XRD), atomic force microscopy (AFM), field emission scanning electron microscopy (FESEM) and UV–visible spectroscopy (UV–vis). The X-ray powder diffraction analysis showed that the prepared sample (TNPs) has pure anatase phase. TNPs were deposited on porous polycrystalline silicon (PPS) substrate by electron beam evaporation. The TNPs thickness was 23±2nm at 10−5mbar pressure at room temperature. Porosity was performed by an anodization method. Since polycrystalline silicon wafers consist of different grains with different orientations, the pore size distribution in porous layer is non-uniform [1]. Therefore, the average diameter of pores can be reported in PPS layer analysis. Average diameter of pores was estimated in the range of 5μm which was characterized by FESEM. The nanostructured thin films devices (Al/Si/PPS/TNPs/Al and Al/Si/PPS/Al) were fabricated in the sandwich form by aluminum (Al) electrodes which were also deposited by electron beam evaporation. Electrical measurements (I–V curves) demonstrated the semiconducting behavior of thin film devices. The gas sensitivity was studied on exposure to 10% CO2 gas. As a result, conductivity of devices increased on exposure to CO2 gas. The device with TNPs thin film (Al/Si/PPS/TNPs/Al) was more sensitive and, had better response and reversibility in comparison with the device without TNPs thin film (Al/Si/PPS/Al).

Polycrystalline Silicon Wafer Surface Color Defect Inspection Based on Machine Vision

As non-uniform color and complex texture exist on the polycrystalline silicon solar cell, manual surface inspection adopted by most domestic factories suffers from low efficiency and poor repetitive detection ability. To overcome the shortcomings of manual inspection, based on machine vision and SVM, an automatic silicon wafer surface defect detection and classification system has been developed in this paper: through feature extraction of color images and defect areas, a series of wafer classifiers are trained and used to separate the defective products from qualified ones. Experiments on samples and actual application in the enterprise show that the system has achieved high accuracy and fast run-time performance, indicating that machine vision is an effective and promising method for polycrystalline silicon solar cell inspection.