mc-Si Wafers Research Articles

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.

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.

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.

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.

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.

Electrophoretic Deposited Quartz Powder-Assisted Growth of Multicrystalline Silicon

Ingot multicrystalline silicon (Mc-Si) needs to be improved in quality and reduced in cost compared with Czochralski monocrystalline silicon. A uniform and dense quartz nucleation layer was obtained by the electrophoretic deposition of quartz powder on the surface of the silicon wafer. The deposited silicon wafer was annealed at 600 °C for 1 h, and one side of the silicon wafer with the quartz layer was glued to the crucible. During the growth of Mc-Si crystal, the dense quartz powder can play a nucleation role. The results show that the average lifetime of the minority carriers a of quartz-assisted silicon ingot is 7.4 μs. The overall dislocation density of an electrophoretic deposition quartz-assisted silica ingot is low, and the defect density in the middle of the silica ingot is 1.5%, which is significantly lower than that of spray quartz (3.1%) and silicon particle (4.2%). Moreover, electrophoretic deposited quartz-assisted mc-Si can obtain oriented grains, which offers a potential to apply alkaline texturing on mc-Si wafers.

Structural modification of diamond-wire-cut multicrystalline Si by Cu-catalyzed chemical etching for surface structuring

During the multicrystalline Si (mc-Si) solar cell production, metal-catalyzed chemical etching (MCCE) is an effective method for constructing texture surface on diamond wire sawing (DWS) cut mc-Si wafer. A dense Cu film was formed on silicon surface by Cu-MCCE, significantly restricting the etching process. In this work, FeCl3, MnO2 and H2O2, were selected to modify the structure of texture surface by one-step Cu-MCCE method. A wedge-shaped structure was formed when the concentration of FeCl3 and etching time are 0.2 mol/L and 15 min, and its reflectivity is 22.9%. A composite structure with shallow pits and nanopores was formed when the amount of MnO2 and etching time are 50 g/L and 20 min, and its reflectivity is 23.2%. An inverted pyramid structure was formed when the concentration of H2O2 and etching time are 3 mol/L and 15 min, and its reflectivity is 18.2%. The best conversion efficiency (η) of solar cell obtained is 19.0%, and, the depth-to-width ratio of texture structure is 1.33.

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.

Effect of Cooling Rate during Thermal Processes on the Electrical Properties of Cast Multi-Crystalline Silicon

Photoluminescence (PL) imaging techniques and the minority carrier lifetime test system were employed to investigate the variation of the interstitial iron (Fei) concentration, the recombination activity of structural defects and the minority carrier lifetime of cast multicrystalline silicon (mc-Si) in response to the cooling rate after heating. The results showed that when the mc-Si wafers are heated to high-temperature (1000 °C) and then cooled to ambient temperature with different cooling rate, the Fei concentration, the number of recombination active dislocations and grain boundaries increased as the cooling rate rises while the minority carrier lifetime decreased. If cast mc-Si is heated followed by faster cooling at 30 °C/s, the Fei concentration increase by 223% and the electrical activity of grain boundaries, dislocations and intragrain increase significantly, that is to say, the whole wafer is heavily contaminated with metal impurities, and present extremely low minority carrier lifetime.

Emitter Quality Optimization Using Lightly Doped Phosphorus Diffusion and Thermal Oxide Anneal for Cell Efficiency Improvement in Multi-Crystalline Black Silicon Solar Cells

Improving solar cell performance by increasing solar cell efficiency by various process optimization had always been a simple straight-forward methodology followed in a R&D or in a solar cell manufacturing company. This is also the most cost-effective practice to improve a product performance using the same technology without the need to procure alternative or expensive raw materials or by adopting advanced solar cell processing techniques. Aluminium Back Surface Field (Al-BSF) technology using multi-crystalline wafers (mc-Si) had been a well-established and a dominant product in the solar industry for more than two decades. However, as the industry progresses, the demand for high efficiency solar cells and modules started going up and full area Aluminium BSF based cells suffers from a lot of inherent limitations on cell efficiency. This is primarily due to the intrinsic high density of crystal lattice defects or otherwise called as grain boundary defects present dominantly only in mc-Si wafers. These grain boundaries tends to accumulate several defects and become trap centres which cause high recombination for minority carriers thereby exhibiting lower conversion efficiency and higher dispersion in electrical parameters in batches of tested cells. Years of research using this material have helped to derive the maximum benefits using this mc-Si wafer in producing industrial full area BSF cells and we can say with certainty that the efficiency potential has reached the saturation point with this technology. An interesting development that happened in the area of improving the final product performance using mc-Si wafers at both cell and module level, is by replacing the conventional acid texturing process with an introduction of a nano-texturing process called Metal Catalysed Chemical Etching (MCCE) using specialized chemicals which improves the light trapping capabilities by creation of inverted pyramid texture on the silicon wafer surface and thereby enabling the wafers to absorb sunlight over a broader range of wavelength and incident angle. With this development done in mc-Si wafers in recent past, it is still a daunting task to surpass cell efficiencies beyond 19.0% using this wafer source. Hence for cell manufacturing lines which use mc-Si wafers, there is always a constant need to improve the cell manufacturing processes to reduce the impact of poor intrinsic quality of mc-Si wafers and improve the final product performance without adding any significant cost factor.

Impact of surface doping profile and passivation layers on surface-related degradation in silicon PERC solar cells

Illuminated solar cells are susceptible to various degradation mechanisms that can act to reduce the total energy yield when deployed. One potentially severe form is an increase in carrier recombination in the surface regions. This effect has been reported at both the undoped rear surface and phosphorous diffused emitter of PERC solar cells. This work investigates the influence of a range of surface conditions on the surface-related degradation (SRD) behaviour in PERC solar cells. It is shown that SRD is strongly affected by the doping profile of phosphorous emitters, the use of thin thermal oxides with SiN x :H dielectric passivation layers, the substrate material, and the configuration of the rear surface passivation. It finds that more lightly doped emitters result in more front side SRD, with its extent increasing with the introduction of the SiO 2 /SiN x :H surface passivation layers. Czochralski silicon (Cz-Si) wafers were observed to be significantly more susceptible to surface degradation than multi-crystalline silicon (mc-Si) wafers, which we attribute to less trapping of hydrogen in the bulk of those substrates. On the rear side of PERC cells, surface degradation was only observed in structures that incorporated the combination of SiO 2 /SiN x :H rear layers. No SRD was observed in the existing Al 2 O 3 /SiN x :H technology used in the industrial PERC cells studied. However, the results presented have implications for future commercial solar cell technologies, which are transitioning towards lightly doped emitters and commonly incorporate thermal oxides for surface passivation. • The introduction of more lightly doped emitters leads to severe SRD, with its extent increasing with the introduction of theSiO 2 . • Cz-Si wafers were observed to be substantially more susceptible to surface degradation than mc-Si wafers. • Rear side SRD was only observed in PERC precursors that incorporated the combination of SiO 2 /SiN x :H rear layers.

Growth of multicrystalline silicon assisted by electrophoretic deposited quartz

Cast multicrystalline silicon (mc-Si) requires high quality at low cost, while fully-melted seed-assisted growth has a reasonable utilization rate but urges better initial grain control. An electrophoretic deposited quartz nucleation layer is introduced to obtain a uniform and oriented nucleation layer. Carrier recombination characterization, including photoluminescence and carrier lifetime mapping, shows that the density of recombination-active structural defects is reduced due to better grain distribution. The average conversion efficiency values of silicon particle, sprayed quartz, and deposited quartz assisted wafers are 20.52%, 20.48%, and 20.68%, respectively Moreover, deposited quartz assisted mc-Si can obtain oriented grains, which offers a potential to apply alkaline texturing on mc-Si wafers.

Analysis of sawing characteristics of fine diamond wire slicing multicrystalline silicon

At present, the diamond wire sawing technology has been widely used in the slicing of photovoltaic multi-crystal silicon (mc-Si). The diameter of diamond saw wire becomes smaller and smaller, and the surface abrasive density and sawing process parameters affect the sawing characteristics of mc-Si wafer. In this paper, the sawing experiment of photovoltaic mc-Si with fine diameter electroplated diamond saw wire was carried out, and the effects of abrasive density and sawing process parameters on the sawing characteristics were analyzed. The results show that the cutting ability and sawing efficiency of diamond wire increase with the increase of surface abrasive density. However, too large surface abrasive density is easy to cause the accumulation of abrasives and the reduction of chip holding space. The core wire diameter is 60 μm and the surface fixed abrasives size is 8–10 μm, when the surface abrasive density is 200–300 grit/mm, the saw wire has relatively good sawing characteristics. The area formed by the material ductile removal domain accounts for a large proportion of the sawn surface, and the surface roughness and subsurface microcrack damage depth (SSD) of silicon wafer are lower. The surface quality of as-sawn wafer can be improved by decreasing the feed speed of workpiece and increasing the wire speed. The research results can provide a reference for the optimization of diamond wire sawing process of photovoltaic mc-Si cells.

In-depth analyses of p-type silicon solar cells: A comparison between commercial compact and laboratory LIBS systems

We report on the quality control of multicrystalline silicon (mc-Si) solar cells by laser-induced breakdown spectroscopy (LIBS). A commercial LIBS-6 compact system operating at 1064 nm was compared to a laboratory LIBS system operating at the same wavelength equipped with an Echelle spectrometer. A depth profiling analysis was performed to understand the distribution and diffusion of trace transition metallic impurities in the volume resulted from polysilicon and ingoting processes and mc-Si wafers fabrication. Eight elements were detected in this analysis, which are Silicon, Silver, Aluminum, Calcium, Magnesium, Sodium, Potassium and Hydrogen. Comparing to other analytical techniques, LIBS has the best advantage for a rapid depth profiling, with high layer-by-layer resolution, without significantly altering the sample.

Chemically microsurface-sphericizing on diamond wire-sawn multi-crystalline silicon wafers by metal-assisted chemical etching and rounding etching

The combination processes of metal-assisted chemical etching (AgNO3 + HF + H2O2) and rounding etching (HNO3 + HF + H2O) were used for the texturization of diamond wire sawn (DWS) multi-crystalline silicon (mc-Si) wafers. The evolution of texture structure on the DWS mc-Si wafers was investigated in detail. The chemically microsurface-sphericizing (CMS) textures, which were nearly smooth spherical structure with a size of approximately 2–3 μm, were fabricated successfully on the DWS mc-Si wafers with the combination processes. Compared with the conventional metal-assisted chemical etching (MACE) textures, the CMS textures were improved on surface recombination and the filling of electrode, thereby boosting the electrical performance of the solar cells and the cell-to-module (CTM) loss at the module end. These CMS textures resulted in an increase in the output power of the photovoltaic module by 0.047 W per cell. The fabrication of these CMS textures may be easily scaled up in the photovoltaic production line.

Study on a novel fixed and free abrasive combined wire sawing multi-crystalline silicon wafers for wet acid texturization

In preparation of solar cells, the multi-crystalline silicon (mc-Si) wafers cut by diamond wire sawing can't obtain the ideal texturization effect when using the wet acid etching process. In this paper, a novel fixed and free abrasive combined wire saw technology for slicing mc-Si wafers is proposed. Diamond saw wire is used as the main processing tool, and free SiC abrasive particles are added into the lubricating coolant to lap the surface of mc-Si wafers. The results show that there are no significant saw marks and ductile smooth area on the surface of mc-Si wafer processed by the combined wire sawing when the SiC abrasive mass fraction is appropriate. The surface roughness and fracture strength of mc-Si wafers in parallel and perpendicular the saw wire motion directions tend to be the same. And the anisotropy of mechanical properties of mc-Si wafers in different directions is significantly reduced, which is conducive to reducing the wafers breakage. Compared with the diamond wire sawn wafer, the reflectivity of the mc-Si wafer cut by the combined wire sawing is greatly reduced after wet acid etching, which can meet the needs of solar cell preparation. The research results provide references for the development of PV mc-Si slicing technology.