Solar-grade Multicrystalline Silicon Research Articles

Preparation and microstructural characteristics of solar-grade multicrystalline silicon by directional solidification in an axial magnetic field

Abstract

Assessment and Study on the Impact on Environment by Multi-crystalline Silicon Preparation by Metallurgical Route

The process of the production of multi-crystalline silicon is also that of incessant purification of metallurgical grade silicon, during which high energy consumption and environmental pollutants are inevitable. The paper, which is based on life cycle assessment (LCA), presents calculation and analysis on resource input, energy consumption, emissions into and comprehensive impact on environment generated from the whole process in which 1 kg of solar grade multi-crystalline silicon (SoG-Si) is produced with metallurgical route. The program is based on its research on four representative processes in metallurgical route, namely slag refining, wet purification, directional solidification and electron beam melting. Key factors determining environmental load are sought out through contribution analysis and improvement evaluation. Measures and suggestions on how to improve are proposed. For the first time, the LCA model of comparison of metallurgical route and modified Siemens process to SoG-Si environmental impact is constructed. The environmental impacts of SoG-Si production via metallurgical route is compared with purification via modified Siemens process. Results show that the comprehensive environmental impact of metallurgical route SoG-Si is only 49.57% of modified Siemens process SoG-Si. The metallurgical route enjoys obvious environmental advantage. The improvement should be focused on the reduction of electric power consumption in production processes.

Stiffness and fracture analysis of photovoltaic grade silicon plates

The rigidity and the strength of photovoltaic cells, particularly the centerpiece-embedded silicon plates, are of great importance from an economical point of view since their reliability impacts the overall cost based on production, transportation and in-service use. The present work focuses on the solar-grade multi-crystalline silicon used in PV wafers. The aim is to characterize the Young’s modulus and to analyze the fracture behavior at room temperature. The Si plates have been laser cut from two different manufacturing processes of silicon wafers, MCSi and RST. Due to the brittle behavior of Si at ambient temperature, 4-point bending tests have been performed. The beam hypothesis has been used to analyze bending tests for determining the Young’s modulus. A correction strategy has then been proposed with a numerical model in order to determine with a higher accuracy the mechanical data and the measurement uncertainty. For fracture investigation, high speed imaging technique and fractography have been used to identify the failure mode as well as the crack origin. The Young’s modulus is found to be 166 ± 5 GPa for MCSi wafers. The anisotropic stiffness of RST plates is also revealed and correlates well with the micro-structural texture. Both kinds of plates fracture in trans-granular manner from the edges, where some defects are located due to laser cutting.

Light-induced degradation in p-type gallium co-doped solar grade multicrystalline silicon wafers and solar cells

This letter focuses on the evolution under illumination of the minority carrier lifetime and conversion efficiency of p-type gallium (Ga) co-doped solar grade multicrystalline silicon wafers and solar cells. We present experimental data regarding the concentration of boron-oxygen (B-O) defects in this silicon when subjected to illumination, and the concentration was found to depend on [B]-[P] rather than [B] or the net doping p0([B] + [Ga] – [P]). This result implies that the compensated B is unable to form the B-O defect. Minority carrier lifetime and EQE measurements at different degradation states indicate that the B-O defect and Fe-acceptor pairs are the two key centers contributed to LID in this material.

Refined nano-textured surface coupled with SiNx layer on the improved photovoltaic properties of multi-crystalline silicon solar cells

Nano-porous silicon (NP-Si) and nano-inverted-pyramid silicon (NIP-Si) structures have been formed by Ag-catalyzed chemical etching without and with NaOH modification on solar-grade multi-crystalline silicon substrates, respectively. The influence of nano-structured morphology (NSM) and SiNx layer (SL) on effective reflectance (Reff) has been investigated through measurement and simulation. For typical NP-Si and NIP-Si samples, the NSM alone can suppress Reff of NP-Si sample to the lowest degree (5.87%), and the combination of NSM and SL is favorable to gain the lowest optical loss for NIP-Si sample (Reff=7.31%). Compared with NP-Si solar cell, the fabricated NIP-Si solar cells have hugely improved photovoltaic properties, resulting from reduced reflectance in visible and near-infrared wavelength, enhanced short-wavelength spectral responses and good diode parameters. Finally, an optimum design strategy of NSM and SL has been suggested to gain potentially better properties for nano-structured solar cells.

Preparation, microstructure and dislocation of solar-grade multicrystalline silicon by directional solidification from metallurgical-grade silicon

A vacuum directional solidification with high temperature gradient was performed to prepare low cost solar-grade multicrystalline silicon (mc-Si) directly from metallurgical-grade mc-Si. The microstructure characteristic, grain size, boundary, solid-liquid growth interface, and dislocation structure under different growth conditions were studied. The results show that directionally solidified multicrystalline silicon rods with high density and orientation can be obtained when the solidification rate is below 60 μm/s. The grain size gradually decreases with increasing the solidification rate. The control of obtaining planar solid-liquid interface at high temperature gradient is effective to produce well-aligned columnar grains along the solidification direction. The growth step and twin boundaries are preferred to form in the microstructure due to the faceted growth characteristic of mc-Si. The dislocation distribution is inhomogeneous within crystals and the dislocation density increases with the increase of solidification rate. Furthermore, the crystal growth behavior and dislocation formation mechanism of mc-Si were discussed.

Nanoporous black multi-crystalline silicon solar cells: realization of low reflectance and explanation of high recombination loss

Nanoporous black silicon (nb–Si) structures with/without saw damage removal (SDR) on solar-grade multi-crystalline silicon substrates have been formed by simple Ag-induced chemical etching. The nb–Si shows a unique morphology of nano-scale holes on micron-scale patterns with low reflectance (<5%). The photovoltaic properties of nb–Si solar cells show that SDR process prior to Ag-induced chemical etching is an effective way to lower recombination and ohmic losses, resulting in significant efficiency enhancement of 26.0%. To further reduce the recombination loss for the nb–Si solar cells with ∼10% efficiency, a two-layer emitter model has been introduced to explain the reduced emitter diffusion length that significantly lower spectral response. This suggests that removing residual Ag nanoparticles completely might be the key approach to enhance the spectral response of nb–Si solar cells with very low surface reflectance, thus increasing the final conversion efficiency.

Assessing the role of iron‐acceptor pairs in solar grade multicrystalline silicon wafers from the metallurgical route

AbstractThe recombination parameters of iron‐boron (FeB), iron‐aluminium (FeAl) and iron‐gallium (FeGa) pairs in Fe implanted FZ monocrystalline silicon wafers and Fe contaminated multicrystalline silicon (mc‐Si) wafers of solar grade feedstock from the metallurgical route have been studied by combining the Deep Level Transient Spectroscopy (DLTS) and Microwave Photoconductive Decay (µ‐PCD) techniques. Energy levels associated with FeB, FeAl and FeGa pairs were detected in the monocrystalline samples. The activation energy and capture cross section of these levels were determined. FeGa gave the strongest recombination effect, reducing the minority carrier lifetime of the sample from about 39 µs to 0.7 µs at a concentration of 4x1013 cm‐3. No electrically active iron‐acceptor pairs could be detected in the mc‐Si wafer. However, it was demonstrated that micrometer‐sized clusters, most likely composed of metallic oxides, collect iron from the bulk. This iron collection may reduce the available amount of iron for creating electrically active iron‐acceptor pairs below the detection limit of DLTS. The contamination did, however, degrade the lifetime from 40 µs to less than 1 µs in the wafer. This is likely a result of at least three overlapping energy levels believed to be related to iron (© 2012 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Biomimetic broadband antireflection gratings on solar-grade multicrystalline silicon wafers

We report a simple and scalable bottom-up technique for fabricating broadband antireflection gratings on solar-grade multicrystalline silicon (mc-Si) wafers. A Langmuir-Blodgett process is developed to assemble close-packed silica microspheres on rough mc-Si substrates. Subwavelength moth-eye pillars can then be patterned on mc-Si by using the silica microspheres as structural template. Hemispherical reflectance measurements show that the resulting mc-Si gratings exhibit near zero reflection for a wide range of wavelengths. Both experimental results and theoretical prediction using a rigorous coupled-wave analysis model show that close-packed moth-eye arrays exhibit better antireflection performance than non-close-packed arrays due to a smoother refractive index gradient.

Preparation Development of High-Quality Solar-Grade Multi-Crystalline Silicon by Directional Solidification

Silicon solar cell is well known as one of the cleanest and most potential renewable resources. As the major photovoltaic (PV) material in PV industry, multi-crystalline silicon (mc-Si) grown by directional solidification has recently attracted increasing attention because of its low production cost, low pollution and high throughput. Deeper understanding of the physic and optic properties, and preparation methods of the materials will lead to improved device design. This paper briefly presents basic directional solidification theory of multi-crystalline silicon, and reviews recent development of solar-grade multi-crystalline silicon. The directional solidification preparation techniques of high-quality solar-grade multi-crystalline silicon are detailed introduced and summarized. Furthermore, the existing problems and further development direction of directionally solidified multi-crystalline silicon for solar cell are discussed.

Fabrication and characterization of polycrystalline silicon nanowires with silver-assistance by electroless deposition

Single crystal silicon wafers are widely used as the precursors to prepare silicon nanowires by employing a silver-assisted chemical etching process. In this work, we prepared polycrystalline silicon nanowire arrays by using solar-grade multicrystalline silicon wafers. The chemical composition and bonding on the surface of silicon nanowire arrays were characterized by Fourier Transform Infrared spectroscope, and X-ray photoelectron spectroscope. The photoluminescence spectra of silicon nanowires show red light emissions centered around 700nm. Due to the passivation effect of Si dangling bonds by concentrated HNO3 aqueous solution, the photoluminescence intensities are improved by 2 times. The influences of surface chemical states on the wettability of silicon nanowire arrays were also studied. We obtained a superhydrophobic surface on the as-etched silicon nanowire arrays without surface modification with any organic low-surface-energy materials, and realized the evolution from superhydrophobicity to superhydrophilicity via surface modifications with HNO3 solutions.

Effect of series resistance on metal-wrap-through multi-crystalline silicon solar cells

Metal-wrap-through (MWT) is a promising technique to improve the solar cell performance cost effectively because it can be easily integrated into the current production line with only two additional processing steps. Metal filling through the via-holes is a key to obtain low series resistances and good FFs. In this study, several screen printing process conditions were examined to find out an optimal filling state of the metal contacts. Various shapes of the filled metals in the via-holes were formed with different printing conditions, and the shape of the filled metal results in different series resistance values. Optimization of the printing conditions dramatically reduced the series resistance of the MWT cells. The maximum and minimum series resistance values of the cells obtained are 8.56 and 0.114 Ω cm 2, respectively. As a result, we achieved an efficiency of 16.3% using the optimal printing condition on 156 mm×156 mm solar-grade multi-crystalline silicon wafer, which was 0.8% absolute higher than the baseline cell efficiency.

Slow down of the light-induced-degradation in compensated solar-grade multicrystalline silicon

This letter focuses on the kinetics of the light-induced-degradation in multicrystalline silicon, comparing electronic grade and strongly compensated solar-grade materials. In electronic grade material, the results fit well with the models developed for Czochralski grown single-crystals. In contrast, in solar grade material, the light-induced-degradation kinetics are much slower and cannot be described by the existing models. We discuss how the formation of boron-oxygen complexes may be altered by the effects of compensation.

Impact of Extended Defects on the Electrical Properties of Solar Grade Multicrystalline Silicon for Solar Cell Application

Aim of this work is to study the electrical properties and the minority charge carrier recombination behaviour of extended defects in multicrystalline silicon (mc-Si) ingots grown from solar grade silicon (SoG-Si) feedstock. The pure metallurgical SoG-Si feedstock has been produced directly by carbothermic reduction of very pure quartz and carbon without subsequent purification processes.This mc SoG-Si is studied by temperature-dependent Electron Beam Induced Current measurements and PhotoLuminescence spectroscopy and the potentiality of the combination of these two techniques in the identification of the defects which limit the quality of the base material is shown. The EBIC mapping technique shows the presence of electrically active grain boundaries at room temperature while dislocations result inactive. Dislocations become active only at temperatures lower than 250K, indicating a moderate level of metal decoration. The most detrimental defects in this material seem to be the grain boundaries and impurities dissolved in the matrix. Furthermore, the PL spectra reveal the presence of oxygen and carbon related complexes. In this work we show that the knowledge about the defect related recombination processes acquired by a combined application of EBIC measurements and PL-spectroscopy is of particular importance to tune the proper solar cell process step to be applied on such material.

Advances in Contactless Silicon Defect and Impurity Diagnostics Based on Lifetime Spectroscopy and Infrared Imaging

This paper gives a review of some recent developments in the field of contactless silicon wafer characterization techniques based on lifetime spectroscopy and infrared imaging. In the first part of the contribution, we outline the status of different lifetime spectroscopy approaches suitable for the identification of impurities in silicon and discuss—in more detail—the technique of temperature- and injection-dependent lifetime spectroscopy. The second part of the paper focuses on the application of infrared cameras to analyze spatial inhomogeneities in silicon wafers. By measuring the infrared signal absorbed or emitted from light-generated free excess carriers, high-resolution recombination lifetime mappings can be generated within seconds to minutes. In addition, mappings of non-recombination-active trapping centers can be deduced from injection-dependent infrared lifetime images. The trap density has been demonstrated to be an important additional parameter in the characterization and assessment of solar-grade multicrystalline silicon wafers, as areas of increased trap density tend to deteriorate during solar cell processing.

Relationship between trapping density and open circuit voltage in multicrystalline silicon solar cells

Minority carrier trapping frequently exists in solar grade multicrystalline silicon. At low illumination levels, the effect of trapping centers on open circuit voltage of multicrystalline silicon solar cells is dependent on the trap density and illumination level. In this paper, the relation between trapping density and open circuit voltage of multicrystalline silicon solar cells at different illumination levels is studied by a series of experiments. The experimental evidence suggests that the effect of trapping on open circuit voltage of multicrystalline silicon solar cells is obvious at carrier injection levels equal to and below the trap density, the trapping effect of multicrystalline silicon can be reflected by measuring open circuit voltage at low illumination levels, instead of complicated lifetime measurements, and some multicrystalline silicon solar cells with higher trap densities have higher open-circuit voltages at weak illumination levels. The measurement and analysis of the trapping effect is a relative tool to diagnose the quality of multicrystalline silicon, so a new method is presented to analyze relative quality of multicrystalline silicon by measuring open circuit voltage at weak illumination levels.

Electronically stimulated degradation of silicon solar cells

Carrier lifetime degradation in crystalline silicon solar cells under illumination with white light is a frequently observed phenomenon. Two main causes of such degradation effects have been identified in the past, both of them being electronically driven and both related to the most common acceptor element, boron, in silicon: (i) the dissociation of iron-boron pairs and (ii) the formation of recombination-active boron-oxygen complexes. While the first mechanism is particularly relevant in metal-contaminated solar-grade multicrystalline silicon materials, the latter process is important in monocrystalline Czochralski-grown silicon, rich in oxygen. This paper starts with a short review of the characteristic features of the two processes. We then briefly address the effect of iron-boron dissociation on solar cell parameters. Regarding the boron-oxygen-related degradation, the current status of the physical understanding of the defect formation process and the defect structure are presented. Finally, we discuss different strategies for effectively avoiding the degradation.

Electronically Stimulated Degradation of Crystalline Silicon Solar Cells

AbstractCarrier lifetime degradation in crystalline silicon solar cells under illumination with white light is a frequently observed phenomenon. Two main causes of such degradation effects have been identified in the past, both of them being electronically driven and both related to the most common acceptor element, boron, in silicon: (i) the dissociation of iron-boron pairs and (ii) the formation of recombination-active boron-oxygen complexes. While the first mechanism is particularly relevant in metal-contaminated solar-grade multicrystalline silicon materials, the latter process is important in monocrystalline Czochralski-grown silicon, rich in oxygen. This paper starts with a short review of the characteristic features of the two processes. We then briefly address the effect of iron-boron dissociation on solar cell parameters. Regarding the boron-oxygen-related degradation, the current status of the physical understanding of the defect formation process and the defect structure are presented. Finally, we discuss different strategies for effectively avoiding the degradation.

Weak light effect in multicrystalline silicon solar cells

Minority carrier trapping centers frequently exist in solar grade multicrystalline silicon, such trapping centers cause a drastic increase in photoconductance at carrier injection levels equal to and below the trap density, this phenomenon leads to higher open circuit voltage for multicrystalline silicon solar cells at illumination levels below about 0.2 suns compared to high performance crystalline silicon solar cells. In this paper, the open circuit voltage of multicrystalline silicon solar cells are investigated at low illumination levels, the experiments prove that some multicrystalline silicon solar cells which have higher trap density have higher open circuit voltage at weak illumination levels, and have lower efficiency, so a new method is presented to analyze quality of multicrystalline silicon by measuring open circuit voltage at weak illumination levels in-line, this makes cells manufacturers gain insight into the quality of multicrystalline silicon wafer from different multicrystalline silicon manufacturers easily with the same cell process before screenprinting and firing.

On the use of a bias-light correction for trapping effects in photoconductance-based lifetime measurements of silicon

The effectiveness of a method for analytically reducing the effect of trapping centers on photoconductance-based recombination lifetime measurements in silicon is examined. The correction method involves the use of a “bias-light” term to subtract out the underlying photoconductance due to the traps. The technique extends, by approximately an order of magnitude, the range of carrier densities over which reasonably accurate (within 30%) measurements of the recombination lifetime can be made. Guidelines for determining which bias-light intensity will produce the best correction for solar grade multicrystalline silicon wafers, and the range over which it is valid, are developed for several practical cases.