Multicrystalline Silicon Wafers Research Articles

Innovative temperature-based texturization process for reducing reflectivity in boron-doped as-cut multi-crystalline silicon wafers

Innovative temperature-based texturization process for reducing reflectivity in boron-doped as-cut multi-crystalline silicon wafers

Multicrystalline Informatics Applied to Multicrystalline Silicon for Unraveling the Microscopic Root Cause of Dislocation Generation.

A comprehensive analysis of optical and photoluminescence images obtained from practical multicrystalline silicon wafers is conducted, utilizing various machine learning models for dislocation cluster region extraction, grain segmentation, and crystal orientation prediction. As a result, a realistic 3D model that includes the generation point of dislocation clusters is built. Finite element stress analysis on the 3D model coupled with crystal growth simulation reveals inhomogeneous and complex stress distribution and that dislocation clusters are frequently formed along the slip plane with the highest shear stress among twelve equivalents, concentrated along bending grain boundaries (GBs). Multiscale analysis of the extracted GBs near the generation point of dislocation clusters combined with ab initio calculations has shown that the dislocation generation due to the concentration of shear stress is caused by the nanofacet formation associated with GB bending. This mechanism cannot be captured by the Haasen-Alexander-Sumino model. Thus, this research method reveals the existence of a dislocation generation mechanism unique to the multicrystalline structure. Multicrystalline informatics linking experimental, theoretical, computational, and data science on multicrystalline materials at multiple scales is expected to contribute to the advancement of materials science by unraveling complex phenomena in various multicrystalline materials.

Improvement of charge carrier lifetime in heat exchange method multicrystalline silicon wafers by extended phosphorous gettering process

External phosphorous diffusion gettering were applied using homogenous and extended schemes on multicrystalline silicon (Mc-Si) wafers obtained from solar grade (SOG) silicon feedstock by the heat exchange method (HEM) growth technique. X-ray fluorescence (XRF) characterization shows the presence of transition metals like Chromium (Cr), manganese (Mn) and iron (Fe) in the analyzed as-cut Mc-Si substrates with a predominance of Mn and Fe elements. Before and after gettering process, we have observed that the Cr atoms concentration drop from 4.1015 at.cm-3 in the as-cut Mc-Si samples to 2.5 1015 at.cm-3 with homogeneous gettering and to 3.1014 at.cm-3 in the wafer undergoes extended gettering. The quasi-steady-state photo conductance (QSSPC) technique is used to characterize the minority charge carrier’s lifetime tn before and after gettering process. It was found that the response of Mc-Si wafers is better (form point of view of lifetime improvement) to the extended process but in some regions it is less effective and the homogenous process give the best amelioration due probably to the spatial distribution of crystalline defects and metallic precipitates densities. With tn initial values before gettering in the range of 2.5-8 usec we have measured lifetime values in the range of 15 to 37 usec with extended process and from 10 to 30 µsec with the homogenous one which ensure cell efficiencies in the range of 15 to 16%.

Adequate method to study the surface passivation effectiveness in HEM multicristallin silicon wafers

In this work we have examined the effectiveness of surface passivation on ascut multicrystalline silicon (mc-Si) wafers using different techniques. The study is based on minority carrier lifetime measurements with quasi steady state photo-conductance, ‘QSSPC’. Effective minority carrier lifetime ()measured values of 12.4, 8.9, 4.9 and 3.1 are obtained respectively with four silicon surface passivation techniques: 1- Shallow phosphorous diffusion emitter (n+p), 2- Iodine-Ethanol (I-E), 3-Hydrofluoric acid (HF) emersion and 4-SiNx layer deposition. These results suggest that the shallow n+p emitter gives the eff close to the bulk lifetime due to the better surface passivation quality. Simulations made with Hornbeck-Haynes model indicate that the improvement can be correlated with the decrease of the surface recombination velocity (SRV) and the increment of bulk lifetime.

Hierarchical Structuring of Black Silicon Wafers by Ion-Flow-Stimulated Roughening Transition: Fundamentals and Applications for Photovoltaics.

Ion-flow-stimulated roughening transition is a phenomenon that may prove useful in the hierarchical structuring of nanostructures. In this work, we have investigated theoretically and experimentally the surface texturing of single-crystal and multi-crystalline silicon wafers irradiated using ion-beam flows. In contrast to previous studies, ions had relatively low energies, whereas flow densities were high enough to induce a quasi-liquid state in the upper silicon layers. The resulting surface modifications reduced the wafer light reflectance to values characteristic of black silicon, widely used in solar energetics. Features of nanostructures on different faces of silicon single crystals were studied numerically based on the mesoscopic Monte Carlo model. We established that the formation of nano-pyramids, ridges, and twisting dune-like structures is due to the stimulated roughening transition effect. The aforementioned variety of modified surface morphologies arises due to the fact that the effects of stimulated surface diffusion of atoms and re-deposition of free atoms on the wafer surface from the near-surface region are manifested to different degrees on different Si faces. It is these two factors that determine the selection of the allowable "trajectories" (evolution paths) of the thermodynamic system along which its Helmholtz free energy, F, decreases, concomitant with an increase in the surface area of the wafer and the corresponding changes in its internal energy, U (dU>0), and entropy, S (dS>0), so that dF=dU - TdS<0, where T is the absolute temperature. The basic theoretical concepts developed were confirmed in experimental studies, the results of which showed that our method could produce, abundantly, black silicon wafers in an environmentally friendly manner compared to traditional chemical etching.

Enhancing multi-crystalline silicon wafer performance through surface modification and AZO treatment

ABSTRACT In the current work, we studied the influence of surface modification on reflectivity and minority carrier lifetime of mc-Si wafer taken from ingot grown by DS process. Two etchants combination namely, KOH/IPA/DI water, and NaOCl/KOH/IPA/DI water are used for different time durations. Reflectance and minority carrier lifetime vary significantly for both etchants used in this work. Optimised micro pyramidal structure formation will aid in enhancing the total internal reflection thereby increasing light absorption. Increase in minority carrier lifetime will be helpful in increasing the efficiency of solar cell. In order to establish the effect of etching on reflectivity of wafers, Al-doped ZnO was sputtered on etched surface and reflectivity was found to decrease further as observed from the reflectivity spectrum. I-V measurements were carried out on Ag/Ti/AZO/mc-Si/Ag and effect of etching on contact behaviour was observed. AZO-coated Si surface with optimised surface micro pyramids served as a good antireflection layer.

Influence of Temperature on Acid‐Based Wet Chemical Texturization for Ultralow Optical Reflectance (&lt;2%) of mc‐Si Wafer for PV Applications

AbstractThis study proposes a method to enhance the efficiency and optical properties of silicon solar cells made from as‐cut boron‐doped p‐type multicrystalline silicon (mi‐Si) wafers through acid‐based chemical texturization. Achieving lower reflectance is crucial for improving solar cell efficiency, and this can be attained through optimized chemical etching. The research demonstrates achieving a reflectance below 2% on the wafer surface after optimal chemical etching. Wet chemical etching is employed as the surface treatment method, using the same chemical etchant with varying conditions for the texturing process. The study compares temperature‐maintained chemical etching and without temperature maintenance. The chemicals used include HF + HNO3 + CH3COOH at temperatures of 50 °C and room temperature, with etching durations of 2, 5, 10, and 15 minutes, respectively. The as‐cut boron‐doped p‐type mc‐Si wafer grains are analyzed using UV‐vis spectroscopy, optical microscopy, Fourier transform infrared spectroscopy, thickness profilometry, scanning electron microscopy, and X‐ray diffraction spectroscopy, both before and after etching. Among the different conditions tested, the chemical etchant HF + HNO3 + CH3COOH at 50 °C for a 5‐minute etching duration yields favorable results, with reflectivity below 2%. The orientation of mc‐silicon wafer grains is determined using X‐ray diffraction spectroscopy.

Influence of pulsed Nd:YAG laser oscillation energy on silicon wafer texturing for enhanced absorption in photovoltaic cells

The influence of the Nd:YAG laser's oscillation energy on creating textured surfaces on silicon wafers is investigated in this study. The silicon wafer surfaces were directly ablated by a pulsed Nd:YAG laser beam to create the texturing, which was then examined using UV–Vis spectroscopy, AFM andFESEM to determine its characteristics. The reflectance was reduced up to 15% after laser texturing of multicrystalline silicon wafers. The silicon surface suffers from structural defects and a laser damage layer as a result of the laser texturing process, which in turn has an effect on the lifespan of the photo-generated carriers. To compensate for the laser damage layer, the silicon surface was cleaned with diluted KOH (15%) to remove undesirable particles and the oxide layer. Surface roughness and reflectance in silicon solar cells were changed depending on the laser oscillation energy utilized for texturing. The roughness and reflectance were measured using AFM and UV–Vis, respectively. The laser oscillation energy of 84 J/p had the highest average roughness of 0.2104 m and the lowest reflectance of 5%. As a result, standardsilicon solar cell devices revealed photovoltaic conversion efficiencies of roughly 7.5% and 5.0% for laser-textured grid and one-dimensional line patterns, respectively.

A Grain Orientation-Independent Single-Step Saw Damage Gettering/Wet texturing Process for Efficient Silicon Solar Cells.

Improving electrical and optical properties is important in manufacturing high-efficiency solar cells. Previous studies focused on individual gettering and texturing methods to improve solar cell material quality and reduce reflection loss, respectively. This study presents a novel method called saw damage gettering with texturing that effectively combines both methods for multicrystalline silicon (mc-Si) wafers manufactured using the diamond wire sawing (DWS) method. Although mc-Si is not the Si material currently used in photovoltaic products, the applicability of this method using the mc-Si wafers as it contains all grain orientations is demonstrated. It utilizes saw damage sites on the wafer surfaces for gettering metal impurities during annealing. Additionally, it can crystallize amorphous silicon on wafer surfaces generated during the sawing process to allow conventional acid-based wet texturing. This texturing method and annealing for 10min allow for the removal of metal impurities and effectively forms a textured DWS Si wafer. The results show that the open-circuit voltage (ΔVoc =+29mV), short-circuit current density (ΔJsc =+2.5mA cm-2 ), and efficiency (Δη=+2.1%) improved in the p-type passivated emitter and rear cells (p-PERC) manufactured using this novel method, as compared to those in the reference solar cells.

Novel technique to study the wet chemical etching response of multi-crystalline silicon wafers

The current work aimed to demonstrate the application of a technique where white light interferometry (WLI) and Laue X-ray crystallography scanner characterisation were combined to study the chemical etching response of diamond cut multi-crystalline Si (mc-Si) wafers. Using this technique, the effect of different texturing additives (isopropyl alcohol, natrium hypochlorite) was evaluated by examining the topography of the mc-Si surfaces before and after etching. The etching responses of monocrystalline Si wafers of (100), (110) and (111) orientations were used as reference for comparison with the multi-crystalline wafers investigated. The texturing results illustrated the influence of different crystal-orientations on the etching rate. It was revealed that for the mc-Si wafers, the etching speed of the different crystal grain-planes is increasing with their crystallographic similarity with the main (hkl) planes (100, 110,111). The comparison of isopropyl alcohol (IPA) and sodium hypochlorite (NaOCl) additives to KOH solutions showed that NaOCl additive is favourable for the polishing of mc-Si wafers, while IPA can be used as polishing only for crystal grains close to the (111) orientation.

Phase transformation pre-treatment of diamond wire-sawn multi-crystalline silicon wafers for metal-assisted chemical etching of solar cells

Phase transformation by the heat treatment, prior to metal-assisted chemical etching (MACE) of solar cells, has been proposed and studied, as a pre-treatment approach for diamond wire sawn (DWS) Si wafers. The results of the study show that the phase transformation pre-treatment can modify the microstructure of amorphous Si surface layers of DWS multi-crystalline (mc) Si wafers. The modified surface layer proved to be more reactive to the chemical etching solution, which was the primary factor for the improvement in the treated solar cells. A significant improvement on the surface uniformity and morphology of the textured Si wafer was achieved, subsequent to the pre-treatment, during the MACE texturization process. These solar cells based on the treated DWS mc-Si wafers displayed a gain of about 0.18% (absolute value), in the overall photoelectric conversion efficiency (η), when compared to the solar cells of the same batch, for the untreated wafers, on the same production line. It is hence, proven that this pre-treatment method, when used as a supplementary technique to the wet-chemical texturization process, has great potential as an effective application technology for solar cells based on DWS mc-Si wafers.

Novel chemical texturizing process in Boron–doped As-cut multi-crystalline silicon wafer for increasing the optical properties

The aim of this paper, the improvement of optical properties of as-cut boron doped p-type multi-crystalline silicon (mc-Silicon) wafer grains obtained by chemical texturization is presented. mc-Silicon wafer gives lower reflectance is one of the important parameters for improving solar cell efficiency and it can be obtained by the optimized texturizing conditions. In this work, we have carried out different types of chemical etchants in the same ratios for the texturizing process. Here, we have carried out HF + HNO3 + KMnO4 = 2.5:1:0.2M (set-1), HF + HNO3 + K2Cr2O7 = 2.5:1:0.2 M (set-2) and HF + HNO3 + IPA =2.5:1:0.2M (set-3) for the individual etchant etching time duration is 2 minutes, 5 minutes and 7 minutes. The mc-Silicon wafer grains were analyzed by the optical microscope, UV- visible, X-ray diffraction (XRD), Scanning Electron Microscope (SEM), Thickness profilometer, and Fourier transforms infrared (FTIR) spectroscopy for before and after texturizing. The set-1 with a 2-minutes etched wafer gives better results as the lower reflectivity value is below 12.5%, lower surface roughness, lower oxidation, and better surface contained.The same orientation of the grains were used as indicated by XRD results. The sheet resistance of the mc-Silicon wafer grains were calculated by the four-probe method.

Comprehensive cell to module optical loss analysis of metal assisted chemically etched inverted pyramid textured multi-crystalline silicon solar cells and modules by ray-tracing method

Most of the recent articles addressing the texturing issues of diamond wire sawn multi-crystalline silicon (mc-Si) wafers propose metal-assisted chemical etching (MACE) inverted pyramid texturing scheme as the best suited industrial process for the fabrication of low-cost, high efficiency solar cells. However, a detailed generation loss analysis of such textured solar cells are not widely investigated yet, especially after module conversion. We present a comprehensive analysis of various optical losses in MACE inverted pyramid textured mc-Si solar cells and modules using the SunSolve module ray-tracer simulator from PV Lighthouse. The simulation model accurately predicted the optical properties of MACE inverted pyramid textured samples and the simulated reflectance values were nearly identical to experimentally measured values at different stages of solar cell processing (before and after ARC). This study quantifies and benchmarks the optical generation losses of not-encapsulated and encapsulated inverted pyramid textured cells with well established models of random pyramid textured and iso-textured solar cells for aluminum back surface field (Al-BSF) architecture. Moderate cell to module (CTM) generation current loss was noticed for inverted pyramid textured solar cells, which was nearly 0.71 mA-cm−2 lower than random pyramid textured and 1.05 mA-cm−2 higher than iso-textured solar cells. We find that adoption of MACE inverted pyramid texturing over acid texturing enhances the photo-generation in mc-Si modules by ∼0.5% and that the total photo-generation current density in mc-Si solar modules approach 99.6% of that in random pyramid textured c-Si modules.

Solar Cells Produced by Diamond Wire Sawn Multicrystalline Silicon Wafer by Using Additive-Assisted Acidic Texturization

A novel additive-assisted acidic etching method is proposed to improve the etched morphology of the diamond wire sawn (DWS)-processed multicrystalline silicon (mc-Si) wafers. The proposed etching technique is a cost-effective method for surface texturization of DWS-processed mc-Si wafers, which can be used for large-scale production of Si-based solar cells. Moreover, the mechanism of additive-assisted etching is explained by decoupling the roles of surfactants and etching inhibitors. The additive-assisted etching of DWS-processed mc-Si wafers resulted in different morphology to the slurry wire sawn (SWS)-processed mc-Si wafers under optimized etching conditions. It has been observed that the etched morphology and reflectivity of DWS-processed mc-Si wafers are significantly influenced by the ratio of hydrofluoric acid (HF): nitric acid (HNO3) solution. High-quality etching morphologies have been obtained. Therefore, high-power conversion efficiency of 19.0% and open-circuit voltage (Voc) of 0.6386 V have been demonstrated by additive-textured DWS-processed Si-based solar cells. The improved power conversion efficiency and Voc can be ascribed to the reduced defect area of the wafer surface. In summary, the proposed additive-assisted acidic etching is an effective strategy to obtain the desired surface texturization of DWS-processed Si wafers for high-performance solar cell applications.

Quality improvement of multi-crystalline silicon ingot by the Hot-Zone modification

Directional solidification process is the dominant method for the multi-crystalline silicon wafer production due to the lower production cost, simple operating process and mass production. Numerical simulation is the best tool to understand and enhance the multi-crystalline silicon ingot quality by using the directional solidification process for photovoltaic application. We improved the multi-crystalline silicon ingot quality by adding an additional graphite layer on the hot-zone of directional solidification furnace. We have simulated both modified and conventional directional solidification process and compared the temperature distribution, growth rate and stress distribution of both systems. The modified directional solidification system significantly increases the multi-crystalline silicon ingot quality compared to the conventional directional solidification process grown multi-crystalline silicon ingot. With the modified directional solidification system we have prevented the wall growth during the casting process.

Etching methods for texturing industrial multi-crystalline silicon wafers: A comprehensive review

Surface texturing for suppressing the reflection losses is the first and foremost step in the solar cell fabrication process. Over the years, multi-crystalline silicon (mc-Si) wafer solar cells dominated the PV market due to their cost-effectiveness. This article reviews various etching methods reported for texturing mc-Si wafers under the light of basic reaction mechanism, general composition of chemicals used, merits, drawbacks, and the progress made towards their commercialization. The article starts with introducing the alkaline etching process used in initial days of mc-Si solar cell production along with its process limitations. Subsequently, evolution of acid texturing process to address the challenges associated with alkaline etching and its journey as an established industrial process are discussed. Thereafter, progressive but expensive texturing methods such as reactive ion etching, laser etching, mechanical grooving and colloidal lithography are also reviewed. Later, the challenges imposed by introduction of diamond wire sawn (DWS) mc-Si wafers on established acid texturing process are presented. Further, remarkable approaches reported towards the effective texturing of DWS mc-Si wafers are discussed. Finally, a summary of reaction chemistry, recent process advances and future roadmap of proposed next-generation texturing scheme known as metal assisted chemical etching is being compiled and presented.

Investigation of veryintenseD3-band emission in multi-crystalline silicon wafers using electron microscopy and hyperspectral photoluminescence imaging

Defects in high performance multi-crystalline silicon wafers can be detrimental to the lifetime of the solar cell. It is, therefore, important to study and understand the underlying structure and chemical elements present at these defective areas in order to suppress them. The underlying cause of the D-band emission line “veryintenseD3” (VID3) has not yet been understood, although many theories have been proposed. In this paper, we have investigated the underlying causes of the d-band emission peak VID3 by hyperspectral photoluminescence imaging, scanning electron microscopy, electron backscatter diffraction, scanning transmission electron microscopy, and density functional theory (DFT) to understand the defect structure in areas of a VID3 emission peak in more detail. We found a high VID3 peak intensity at sub-grain and Σ3 twin boundaries bordering to grains with a small misorientation, which suggests higher stress in these regions. Defects close to the twin boundary indicate a light element dopant in the area, such as oxygen. DFT calculations show that oxygen is prone to segregate to this boundary.

Experiment and theoretical prediction for subsurface microcracks and damage depth of multi-crystalline silicon wafer in diamond wire sawing

Diamond wire saw slicing is an important process of multi-crystalline silicon (mc-Si) solar cell substrate processing. In wire sawing of silicon crystal, the brittleness removal of materials will lead to subsurface microcrack damage, reduce the fracture strength, increase the breakage probability of the as-sawn wafer and affect the subsequent surface etching texture of wafer. In this paper, a mathematical model calculating subsurface damage depth (SSD) of mc-Si is established based on the analysis of subsurface crack system in diamond wire sawing process. The correctness and rationality of the model are verified by experiments. Then, the model is used to predict the variation trend of the microcracks number (CN) and subsurface damage depth under different process parameters and saw wire parameters. The results show that the SSD decreases and CN increases with the increase of wire speed and decrease of feed speed; With the increase of abrasive density on wire surface, the SSD decreases and CN increase. The change of abrasive average size has little effect on the SSD and CN; a smaller abrasive size range is beneficial to reduce the SSD and increase CN.

Investigation of orientation, surface morphology, impurity concentration and reflectivity of the multi-crystalline silicon wafers

Multi-crystalline silicon (mc-silicon) ingot was grown using the directional solidification (DS) method. The grown ingot was sliced, and the grain orientations, grain size, surface morphology, and impurities concentration in the wafers were studied. The XRD pattern of as-cut wafers exhibited the formation of a more number of (111) oriented plane in the grown ingot. The scanning electron microscopy images showed the formation of worm-like trenches after the chemical etching of the sliced wafers. FTIR spectra showed the higher carbon concentration on the top wafers and higher oxygen concentration at the bottom wafers. At the central wafer of the middle brick, the calculated carbon and interstitial oxygen concentration result exhibited 4.740 × 1016 and 1.369 × 1017 atoms/cm3, respectively. The impurity concentration of carbon and oxygen is validated with simulation results. The reflectivity spectra revealed low reflectivity for the wafers sliced from the central brick compared to the peripheral brick. The presence of very less impurity concentration and low reflectivity of the middle wafer in the central portion of the ingot enhances the light absorption, which is beneficial for photovoltaic energy generation.

Surface properties of diamond wire sawn photovoltaic mc-Si wafers by stirred slurry pretreated and additive-free acid textured

Diamond wire saw cutting multi-crystalline silicon (mc-Si) wafers has the advantage of high cutting rate. However, it is difficult to follow the current additive-free wet acid etching process to prepare the anti-reflective textured surface due to the obvious saw marks on the surface of the as-sawn wafer. In response to this problem, this paper proposes a turbine-type stirred slurry pretreatment process to modify the surface of mc-Si wafers cut by diamond wire to make it suitable for the subsequent conventional wet acid texturing process. The experimental results show that the consistency of the surface structure of the mc-Si wafers can be effectively improved after turbine-type stirred slurry pretreatment by choosing suitable process parameters. The reflectivity of the pretreated mc-Si wafers after acid texturing is greatly reduced compared with that before processing, which can meet the production requirements of solar photovoltaic cells. Furthermore, based on ANSYS Workbench and Fluent, numerical simulation of the surface pretreatment process of large-size silicon wafers in practical industrial applications was carried out, and the uniformity of the surface modification treatment was analyzed. The research results provide ideas for promoting the adaptation of the diamond wire sawing of mc-Si silicon wafers to the acid wet texturing technology.