Multicrystalline Silicon Wafers Research Articles

Damage and residual layer analysis of reactive ion etching textured multi-crystalline silicon wafer for application to solar cells

As part of the surface texturing process of multi-crystalline silicon solar cells, isotropic etching is performed by using an acidic solution. Furthermore, metal catalyst chemical etching (MCCE), reactive ion etching (RIE), and laser etching are used to further decrease surface reflectance. This study aimed to increase the power conversion efficiency of the solar cell by improving the short-circuit current density (Jsc) using MCCE and RIE. During RIE, a byproduct and a plasma damage layer are formed on the silicon surface, which decrease the efficiency of the solar cell and therefore need to be identified and effectively removed. Transmission electron spectroscopy, energy dispersive X-ray spectroscopy and X-ray photoelectron spectroscopy analyses were performed to identify the byproducts formed during RIE to confirm that it was amorphous silicon oxide. Furthermore, an etching process using acidic and alkaline base solutions was used to remove the plasma damage layer and the Jsc loss was calculated using the reflectance. As a results, confirm a difference of up to ∼ 1.6 mA/cm2, and an improvement of approximately 0.6% was observed in the solar cell efficiency. These results show a method of minimizing Jsc loss and removing surface damage in a silicon solar cells fabricated using the RIE method.

Wet sandblasting pretreatment of diamond wire sawn multi-crystalline silicon wafer for surface acid texturization in photovoltaics

Wet sandblasting pretreatment of diamond wire sawn multi-crystalline silicon wafer for surface acid texturization in photovoltaics

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.

Anti-reflection effect of large-area ZnO nano-needle array on multi-crystalline silicon solar cells

The current diamond-wire-sawed multi-crystalline silicon wafer lacks an effective light trapping structure, which results in high reflectivity and hinders the improvement of efficiency. In this work, we proposed an enhanced light trapping structure outside the junction of multi-crystalline silicon solar cells, that is, constructing a ZnO nano-needle array on the surface of the textured multi-crystalline silicon to minimize the reflection in the visible-near infrared band to achieve light trapping effect. Solar cells with ZnO nano-needle structure exhibit excellent light trapping performance over a wide band (350–1100 nm). The low-cost and simple preparation method makes this enhanced light trapping structure has broad application prospects.

Understanding Light- and Elevated Temperature-Induced Degradation in Silicon Wafers Using Hydrogen Effusion Mass Spectroscopy

Hydrogen has been long known for its ability to passivate defects in silicon devices. However, multiple recent studies on understanding the mechanism behind light- and elevated temperature-induced degradation (LeTID) have proposed that hydrogen plays an important role in this degradation mechanism. Despite its important role in photovoltaic applications, the quantitative assessment of hydrogen is difficult and seldom reported. In this work, we applied hydrogen effusion mass spectroscopy to quantify the hydrogen released from hydrogenated silicon nitride (SiN <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> :H) and atomic layer deposited (ALD) aluminum oxide (AlO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> ) dielectric films at elevated temperatures. We demonstrate that the amount of hydrogen effused from these layers strongly correlates with the extent of LeTID observed in the multicrystalline silicon wafers passivated with these monolayers and their stacks. It is shown that the hydrogen effusion scales linearly with the SiN <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> :H thickness, similar as the extent of LeTID. The effusion measurements on the AlO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> /SiN <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> :H stack revealed that the presence of the AlO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> film modifies the total amount of hydrogen that is effused, whereas it was found to slow the hydrogen in-diffusion. This result is consistent with the LeTID extent determined after contact firing where ALD AlO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> layers were found to act as a hydrogen diffusion barrier, strongly reducing LeTID when placed in between c-Si and SiN <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> :H and increasing LeTID when placed on top of SiN <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> :H.

Role of metal impurities in multicrystalline silicon solar cell degradation

Controlling light- and elevated temperature-induced degradation (LeTID) is one of the great challenges in silicon photovoltaics industry. Here, we performed systematic high-resolution elemental mapping and spectroscopic electrical measurements in multi-crystalline silicon wafers that underwent standard solar cell processing during LeTID. The prominent correlations between the metal precipitate formation/dissociation steps and the appearance of band-like or localized electronic states during LeTID evolution were observed. Of note, the re-precipitation of iron was found to co-localize with copper atom agglomerations. Finally, we discuss the potential mechanisms that enhance metal redistributions in mc-Si during LeTID in the context of the existing literature.

The Influence of Minority Carrier Density on Degradation and Regeneration Kinetics in Multicrystalline Silicon Wafers

The rate at which silicon solar cells and wafers degrade and regenerate when subjected to light and elevated temperatures (LeTID) is known to depend on the minority carrier concentration. In this article, we utilize spatial differences in the rate of degradation and regeneration in multicrystalline silicon wafers to estimate the dependence of these rates on minority carrier concentration. We apply the relation obtained from spatial investigations in a model describing temporal LeTID defect evolution. With the updated model in which the evolving minority charge carrier density during degradation and regeneration is considered, we obtain a drastically improved fit between measured and modeled data. The activation energy for the regeneration process determined from our proposed model (0.56±0.12eV at 1 sun) is lower than most values reported in the literature but correspond perfectly to the activation energy reported by Graf et al. by whom evolving carrier injection is accounted for experimentally. Hence, our results underpin the importance of taking the evolution of charge carrier injection into account in the evaluation of LeTID kinetics. Finally, we use our updated model to obtain spatial maps of degradation and regeneration parameters. Clear correlations are shown between LeTID kinetics parameters and local processing conditions.

(Invited) Application of Machine Learning for High-Performance Multicrystalline Materials

Many practical materials are multicrystalline to contain complex microstructures and crystal defects. Owing to its complexity, universal guideline regarding ideal microstructures to maximize macroscopic performance has not been fully established. Therefore, we attempt to pioneer “multicrystalline informatics” through collaboration of experiments, theory, computation, and machine learning to show how we can obtain high-performance multicrystalline materials. We employ silicon as a model material, and prepare various useful machine learning tools for efficient materials development.One example is to predict distribution of crystal orientations in a large sample (e.g. whole 6-inch wafer) from optical images. We started with surface treatment of multicrystalline silicon wafers so that the reflection become sensitive to the crystal orientations. Multiple optical images were captured from each wafer with white-light illumination from different orientations. In addition, orientations of each crystal grain for a couple of wafers were measured by X-ray Laue scanner. The obtained data sets of reflection patterns and crystal orientations were used for training, validation, and test of a long short-term memory recurrent neural network. The trained model was applied to a test wafer to contain ~1,000 crystal grains, and the median of the estimated errors was ~3 degree.Another example is to predict probability of generation of dislocation clusters. For this purpose, a large quantity of photoluminescence (PL) images was collected from wafers obtained from a same ingot. Three-dimensional reconstruction of PL images after extracting dislocation clusters by image processing successfully visualized distribution of dislocation clusters. Therefore, we can collect many small PL images as positive samples to act as the source of dislocations as well as negative samples. The obtained data sets were used for transfer learning of pre-trained convolutional neural network for image classification. The final output layer was changed to contain two outputs if the input image acts as the source of dislocations or not, and weights for the last edges were determined using the data sets. The network was applied to test wafers so that we can visualize probability distribution of generation of dislocation clusters. A part of sources was used for multiscale structural characterizations to clarify microscopic origins.In addition, we are working on various machine learning models and statistical analysis for multicrystalline materials. Integration of such models is believed to be beneficial to show how we can maximize macroscopic performance of multicrystalline materials.This work was supported by JST/CREST, Grant No. JPMJCR17J1 (2017-2023).

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.

Metal-catalyzed chemical etching using DIO3 as a hole injection agent for efficient submicron-textured multicrystalline silicon solar cells

The introduction of ozonated deionized water (DIO3 water) to the silicon wafer wet cleaning process has attracted interest from the photovoltaic industry. In this work, a simple and cost-effective approach to fabricate submicron-texture multicrystalline silicon (mc-Si) wafer is presented using Ag metal-catalyzed chemical etching (MCCE) in DIO3/HF solution, where DIO3 was used as a hole injecting agent. The submicron-texture was successfully applied to fabricate efficient mc-Si PERC solar cells with an average efficiency of 20.31%, which is 0.58%abs higher than those with an acid micron-texture and exceeds those with conventional MCCE submicron-textures using H2O2 as the hole injecting agent. We demonstrated that using DIO3 as a hole injection agent in metal-catalyzed chemical etching is feasible. This novel MCCE approach is simple, low-cost, and ecofriendly, making it an attractive alternative to conventional MCCE methods for use in industrial-scale production.

The Kinetics and Stoichiometry of Metal Cation Reduction on Multi-Crystalline Silicon in a Dilute Hydrofluoric Acid Matrix.

In this study, the process of metal cation reduction on multi-crystalline silicon in a dilute hydrofluoric acid (HF) matrix is described using Ag(I), Cu(II), Au(III) and Pt(IV). The experimental basis utilized batch tests with various solutions of different metal cation and HF concentrations and multi-crystalline silicon wafers. The metal deposition kinetics and the stoichiometry of metal deposition and silicon dissolution were calculated by means of consecutive sampling and analysis of the solutions. Several reaction mechanisms and reaction steps of the process were discussed by overlaying the results with theoretical considerations. It was deduced that the metal deposition was fastest if the holes formed during metal ion reduction could be transferred to the valence bands of the bulk and surface silicon with hydrogen termination. By contrast, the kinetics were lowest when the redox levels of the metal ion/metal half-cells were weak and the equilibrium potential of the H3O+/H2 half-cells was high. Further minima were identified at the thresholds where H3O+ reduction was inhibited, the valence transfer via valence band mechanism was limited by a Schottky barrier and the dissolution of oxidized silicon was restricted by the activity of the HF species F−, HF2− and H2F3−. The findings of the stoichiometric conditions provided further indications of the involvement of H3O+ and H2O as oxidizing agents in addition to metal ions, and the hydrogen of the surface silicon termination as a reducing agent in addition to the silicon. The H3O+ reduction is the predominant process in dilute metal ion solutions unless it is disabled due to the metal-dependent equilibrium potential of the H3O+/H2 half-cell and the energetic level of the valence bands of the silicon. As silicon is not oxidized up to the oxidation state +IV by the reduction of the metal ions and H3O+, water is suspected of acting as a secondary oxidant. The stoichiometric ratios increased up to a maximum with higher molalities of the metal ions, in the manner of a sigmoidal function. If, owing to the redox level of the metal half-cells and the energetic level of the valence band at the metal–silicon contact, the surface silicon can be oxidized, the hydrogen of the termination is the further reducing agent.

19.2%-Efficient Multicrystalline Silicon Solar Cells via Additive-Free Mechanical Grinding Surface Pretreatment for Diamond-Wire-Sawn Wafers

The introduction of diamond wire sawing (DWS) technology has resulted in significant cost reduction in the fabrication of crystalline silicon wafers. However, the DWS process results in parallel wheel marks, saw damage, and formation of an amorphous silicon layer on the surface, which causes difficultly in effectively forming the desired surface texture using conventional acidic etching (also known isotropic etching) techniques for multicrystalline silicon (mc-Si) wafers. In this study, we propose a novel pretreatment grinding (NPTG) technique as a method to address such issues. This is a relatively simple and inexpensive method that does not utilize processes that require the use of expensive equipment, such as vacuum equipment. Additionally, it makes use of environment-friendly procedures that do not require materials such as metal catalysts and additives that cause environmental pollution. The proposed NPTG method provides a good surface topology for effective texturing using a conventional acidic etching solution, and as a result, a uniform texture can be applied to DWS mc-Si wafers. Under the optimized experimental conditions in this study, a weighted average reflectance of 22.63% was achieved after the NPTG was applied. This reflected 6.67% less light than the 29.3% after conventional acidic texturing without the NPTG using DWS mc-Si wafers. Further, a solar cell fabricated using the DWS mc-Si wafers treated with the NPTG method achieved a cell efficiency of approximately 19.2%.

Occurrence Prediction of Dislocation Regions in Photoluminescence Image of Multicrystalline Silicon Wafers Using Transfer Learning of Convolutional Neural Network

Occurrence Prediction of Dislocation Regions in Photoluminescence Image of Multicrystalline Silicon Wafers Using Transfer Learning of Convolutional Neural Network

Effective Surface Texturing of Diamond-Wire-Sawn Multicrystalline Silicon Wafers Via Crystallization of the Native Surface Amorphous Layer

To increase the market shares of multicrystalline silicon (mc-Si) wafers obtained using diamond-wire sawing (DWS), it is necessary to develop efficient and cost-effective texturing methods. Surface texturing processes that increase light absorption are essential for manufacturing high-efficiency solar cells. The smooth surface, phase transformations, and amorphous silicon (a-Si) layer on mc-Si wafers manufactured via DWS hinder the effective texturing under conventional acidic conditions, which are typically used for multiwire slurry sawing (MWSS). To address this issue, this article focused on an efficient texturing process of DWS wafers by adapting a pre-thermal treatment (TT) process. We found that DWS mc-Si wafers could be effectively textured after the TT process by altering the surface roughness through the crystallization of surface amorphous silicon (a-Si) and phase transformation layers. For the DWS mc-Si wafers textured under conventional acidic etching conditions without TT, Rw was found to be 31.7%. The altered surface conditions achieved via the proposed TT method facilitate effective texturing on the DWS mc-Si wafer surface. Consequently, a weighted average reflectance (Rw) of 24.5% was obtained, which was comparable to that for MWSS mc-Si wafers subjected to conventional acidic etching (Rw = 24.4%). As a result of depositing a SiN x for surface passivation and antireflection coating in solar cell applications, Rw values of 8.1% and 8.3% were obtained for the thermal treated DWS mc-Si wafer and MWSS mc-Si wafer, respectively. The proposed texturing process for DWS mc-Si wafers can easily be applied to industrial inline processes using cost-effective existing acidic etching solutions.

HF/HCl/H2O/MnO2 System for High-Performance Texturization on Multi-Crystalline Silicon

We proposed a HF/HCl/H2O system with manganese dioxide (MnO2) particle as the oxidant to obtain high-performance texturization on multi-crystalline silicon (mc-Si) wafers. The texturization obtained by the HF/HCl/H2O/MnO2 system had a more uniform distribution and a lower surface reflectance than that obtained by the conventional HF/HNO3/H2O system, especially on the surface of diamond wire sawn (DWS) mc-Si wafers. The etching mechanism was revealed by studying the effects of the usage variation of HF, HCl and MnO2. The etching only occurs at the locations where MnO2 particles are deposited on the silicon surface and will self-terminate with the full consumption of MnO2. Thus the texture morphology can be easily controlled by adjusting the usage of MnO2 particles with sufficient HCl and HF. Furthermore, such method is environmentally friendly not to use HNO3.

Variations in Minority Carrier-Trapping Effects Caused by Hydrogen Passivation in Multicrystalline Silicon Wafer

In a multicrystalline silicon (mc-Si) wafer, trapping effects frequently occur in the carrier lifetime measurement based on the quasi-steady-state photoconductance (QSSPC) technique. This affects the accurate measurement of the carrier lifetime of an mc-Si solar cell by causing distortions at a low injection level close to the Pmax point. Therefore, it is necessary to understand this effect and effectively minimize the trapping-center density. In this study, the variations in the minority carrier-trapping effect of hydrogen at different annealing temperatures in an mc-Si were observed using QSSPC, time-of-flight secondary ion mass spectroscopy, and atom probe tomography. A trapping effect was confirmed and occurred in the grain boundary area, and the effect was reduced by hydrogen. Thus, in an mc-Si wafer, effective hydrogen passivation on the grain area and grain boundary is crucial and was experimentally proven to minimize the distortion of the carrier lifetime.

Excellent passivation with implied open-circuit voltage of 710 mV for p-type multi-crystalline black silicon using PECVD grown a-Si:H passivation layer

Hydrogenated amorphous silicon (a-Si:H) deposited using plasma-enhanced chemical vapor deposition (PECVD) is used as the passivation layer for p-type multi-crystalline black silicon (p-mc-b-Si) wafers. The effects of deposition conditions on passivation quality are investigated. The optimized a-Si:H passivation layer enables the best surface passivation with an implied open-circuit voltage (iVoc) of 710 mV, which is better than other conventional passivation materials. The excellent passivation quality possibly resulted from that the activated hydrogen from a-Si:H at ~200 °C was able to passivate both surface defect states and bulk trap states of p-mc-b-Si effectively. Moreover, an additional SiNx capping layer helps to reduce reflectance significantly and to keep passivation quality. This work suggests that b-Si passivated by a-Si:H/SiNx stack film shows potential for various optoelectronic devices that requires both excellent optical anti-reflectance and surface passivation.

Temperature Effect of Nano-Structure Rebuilding on Removal of DWS mc-Si Marks by Ag/Cu MACE Process and Solar Cell

The absence of an effective texturing technique for diamond-wire sawn multi-crystalline silicon (DWS mc-Si) solar cells has hindered commercial upgrading from traditional multi-wire slurry sawn silicon (MWSS mc-Si) solar cells. In this work, we present a novel method for the removal of diamond-wire-sawn marks in a multi-crystalline silicon wafer based on metal assisted chemical etching process with Cu/Ag dual elements and nano-structure rebuilding (NSR) treatment to make a uniform inverted pyramid textured structure. The temperature effect of NSR solution was systematically analyzed. It was found that the size of the inverted pyramid structure and the reflectance became larger with the increase of the NSR treatment temperature. Furthermore, the prepared unique inverted pyramid structure not only benefited light trapping, but also effectively removed the saw-marks of the wafer at the same time. The highest efficiency of 19.77% was obtained in solar cells with an inverted pyramid structure (edge length of 600 nm) fabricated by NSR treatment at 50 °C for 360 s, while its average reflectance was 16.50% at a 400–900 nm wavelength range.

Evolution of LeTID defects in industrial multi-crystalline silicon wafers under laser illumination - Dependency of wafer position in brick and temperature

In the past few years, a hot topic in both research and industrialization of p-type multi-crystalline silicon (mc-Si) solar cells is to investigate the mechanism, measurement and mitigation of the light- and elevated temperature-induced degradation (LeTID), which has been found to be a bulk-sensitive degradation behavior and is dependent on the degradation condition. In this paper, we study the influence of silicon bulk property on LeTID from five representative positions along a relative low resistivity (0.82–1.33 Ω cm) p-type mc-Si brick. The evolution of degradation and regeneration under different laser illumination conditions are investigated. Identical defect capture cross section ratio k values of ~35 at mid-gap along the brick height are found. For the first time, the activation energy for degradation (Ea,deg) and regeneration (Ea,reg) along the brick are studied, which shows Ea,deg = Ea,reg and a tendency of larger values towards the brick bottom. These results indicate that the LeTID in the whole brick might be induced by a single defect and thus mitigated in a single manner. Besides, a laser illumination of 45 kW/m2 at 142 °C for 100 s is able to induce over 90% degradation for all wafers, which could be used as a universal and fast LeTID test condition.