High Performance Multi-crystalline Silicon Ingots Research Articles

Effects of Heat Extraction Methods on the Quality of High Performance Multi-Crystalline Silicon Ingot

Effects of Heat Extraction Methods on the Quality of High Performance Multi-Crystalline Silicon Ingot

Improve solar cell performance of high-performance multicrystalline silicon seeded with low-cost compact nucleation layer

In order to further improve the quality of high efficiency multicrystalline silicon and the performance of multicrystalline silicon solar cells, we designed a compact nucleation layer on the crucible bottom for casting high performance multicrystalline silicon ingots. The morphology of nucleation grains, the minority carrier lifetime mappings and the defects were analyzed and compared with that of the conventional nucleation layer. The results show that the initial nucleation grains of the compact nucleation layer were finer and more uniform, and there are fewer grains with high defect density at the ingot bottom，which is beneficial to reduce the defect ratio of the whole ingot. Moreover, the solar cell efficiency and electrical performance parameters, such as Rsh was also discussed. It was found that the Rsh has a great impact on the efficiency of multi-busbars solar cells, and the average solar cell conversion efficiency of the wafers grown with the compact nucleation layer is 0.11% higher than that of the wafers grown with the conventional nucleation layer.

Origin of recombination activity of non-coherent Σ3{111} grain boundaries with a positive deviation in the tilt angle in cast-grown silicon ingots

Non-coherent Σ3{111} grain boundaries (GBs) with a positive deviation in the tilt angle (θ 〈110〉 > 70.5°) exhibit a high recombination activity in high-performance multicrystalline silicon ingots. Most of the GB segments are composed of edge-type dislocations with the Burgers vector b of a/3〈111〉, unlike Lomer dislocations with b = a/2〈110〉 observed for negative deviations, arranged on coherent Σ3{111} GB segments. Stretched 〈110〉 reconstructed bonds along the tilt axis are introduced so as not to form dangling bonds, and large strains are generated around the dislocation cores. Oxygen and carbon atoms segregating due to the strains would induce the recombination activity.

Generation of dislocation clusters at triple junctions of random angle grain boundaries during cast growth of silicon ingots

Three-dimensional distribution of grain boundaries (GBs) and generation sources of dislocation clusters is examined in a cast-grown high-performance multicrystalline silicon ingot for commercial solar cells. A significant number of dislocations are generated nearby some triple junctions of random angle GBs, although it is believed that such non-coherent GBs would not induce large strain during the cast growth. This explosive generation of dislocations would take place when the triple junctions are interacted with multiple Σ3{111} GBs. A segment of the random angle GB connected with a pair of Σ3{111} GBs nearby the triple junction would act as a dislocation source.

High-performance Multicrystalline Silicon Ingots Assisted by Recycled Multicrystalline Silicon Block Tailing as Seeds

High-performance Multicrystalline Silicon Ingots Assisted by Recycled Multicrystalline Silicon Block Tailing as Seeds

Production of high performance multi-crystalline silicon ingot by using composite nucleant

A composite nucleant with Si powder as the main component was introduced to prepare high-performance multicrystalline (HPM) silicon ingot in this work. The effect of different sizes of Si powder on the ingot performance was investigated and compared with the conventional HPM silicon ingot which seeded from poly-Si particles. It is proved that the composite nucleant with Si powder of large particle size (D50 = 355 μm) can induce small and uniform grains, showing uniform minority carrier lifetime distribution and low dislocation cluster area ratio. Meanwhile, the length of red zone at the ingot bottom is significantly shorter than conventional HPM silicon ingot, which significantly increases the yield of the ingot. The ingot grown with suitable composite nucleant can achieve high cell efficiency and the solar cells have low light-induced degradation (LID) value due to low interstitial oxygen concentration which are comparable to the conventional HPM silicon ingot.

Numerical and experimental investigation of sectional heater for improving multi-crystalline silicon ingot quality for solar cells

The sectional heater of industrial-scale directional solidification furnace was designed for improving the quality of the high performance multi-crystalline silicon ingots used for solar cell fabrication. A global numerical model was adopted to investigate the effect of the modified furnace on the thermal field distribution, the shape of the crystal-melt interface and the thermal stress distribution in the solidified silicon ingot during the directional solidification process. Simulation results indicate that the crystal-melt interface shape in the modified furnace with the sectional heater changes from convex at the earlier stage to slightly flat at the later stage, and the thermal stress level at the bottom corner of the solidified ingots is clearly lower than that in the conventional furnace. Furthermore, the designed furnace with the sectional heater was built and experiments corresponding to the numerical simulations were carried out. It was found that a more uniform minority carrier lifetime distribution, lower dislocation cluster density, and lower oxygen concentration were obtained for the silicon ingot grown using the modified furnace. Additionally, the average conversion efficiency of the solar cells fabricated using the grown ingots was evaluated, and it was found that a higher efficiency was obtained for the cells fabricated using the ingots obtained with the modified furnace (18.64%) compared to that for the cells fabricated using the ingots obtained with the conventional furnace (18.54%).

Influence of modified bottom insulation on the seeded directional solidification process for high-performance multi-crystalline silicon

This study proposes two kinds of bottom insulation with a moveable insulation block, with either a narrower or large horizontal width, for use in an industrial-size seeded directional solidification (DS) furnace to produce high-performance multi-crystalline silicon ingot. A transient global model was established to study the effects of the modified configurations on the thermal field in the solidified silicon ingot during the DS process. The numerical results indicate that the shapes of the s-m and m-c interfaces remained relatively flat or slightly convex throughout the entire DS process when the narrower bottom insulation block design was used, and the highest thermal stress in the bottom corner areas of the ingots was obviously lower. To further verify the simulation results, ingot casting experiments and testing were carried out with the use of photoluminescence and minority lifetime mappings. The results show that the dislocation clusters exhibited a lower density in the modified designs. This phenomenon could be attributed to the uniform impurity and lower thermal stress distributions induced by the flat or slightly convex interface. Additionally, the corresponding average conversion efficiency is discussed.

Production of high performance multi-crystalline silicon ingots for PV application by using contamination-free SixNy seed particles

In this work a new type of contamination-free SixNy seed particles for the directional solidification of HPM silicon ingots is introduced and evaluated. Several G1 crystallization experiments have been carried out to examine the influence of the seed particle size, the introduced mass and the influence of the composition of the underlying Si3N4 coating at the crucible bottom. Finally, multi-PERC cells over the ingot height from one of the most promising SixNy seeded ingot have been prepared.The results show that an oxygen contamination is not occurring by the new particles. A minimum seed particle density of around 120/cm2 at the crucible bottom is sufficient to generate a HPM like grain structure with random grain boundary fractions R of 65%±5%, which is very close to typical values of classical silicon seeded HPM. A further increase of the particle density doesn’t lead to a further increase of the R value due to the competition between the very close packed seeds.While lower oxygen levels of the Si3N4-coating promote significant higher R values in crucible setups without seed particles, the Si3N4-coating composition has almost no influence on the resulting grain structure properties when SixNy seed particles are used. This could be shown even for small seed densities of ~30 cm−2 and a remaining coating area fraction of 80%.The cell analysis shows same efficiencies of the ingots seeded on the SixNy particles and on the silicon feedstock layer, but a higher wafer yield can be achieved in the case of the SixNy seeding method since a full melt process can be used.

Simulated and measured temperature coefficients in compensated silicon wafers and solar cells

In this paper we perform a thorough investigation of the temperature coefficients of c-Si solar cells and wafers, based on both experimental data and device simulations. Groups of neighboring wafers were selected from different heights of four high performance multicrystalline silicon ingots cast using different dopants concentrations and Si feedstocks; Three different target resistivities of compensated silicon ingots based on Elkem Solar Silicon (ESS®), which are purified through a metallurgical route, and one non-compensated reference ingot. The wafers were processed into Al-BSF and PERCT type solar cells, as well as into lifetime samples subjected to selected solar cell processing steps before being etched down and re-passivated to assess the bulk lifetime of the final cells. Temperature-dependent photoluminescence imaging was used to study the spatial distribution of the carrier lifetime and the temperature coefficient of the lifetime. We observe a beneficial effect of higher doping levels on the temperature coefficients of the short circuit current JSC in PERCT type cells, with the most favorable temperature coefficients are found in the ingot doped to 0.5 Ω-cm. Increasingly beneficial temperature coefficients in JSC is also observed with increasing height in each ingot, which might be attributed to a combination of small variations in the dopant concentrations and to a clear increase in the temperature coefficient of the carrier lifetime with increasing ingot height. The latter point is also in agreement with the changes in the so-called gamma parameter of the open circuit voltage throughout the ingot. To build a more fundamental understanding of the experimental results the temperature dependence of the solar cells was simulated using PC1Dmod6.2. We find that the experimental temperature coefficients of the final cells can be reproduced in device simulations within 8% of and 14% for the VOC and JSC, respectively. The compensation level itself is of minor importance for the cell performance. Instead, the simulations indicate that the observed behavior in the temperature coefficients of the solar cells is mostly explained by differences in the net doping and carrier lifetime of the wafers.

Evaluation of improvement strategies of grain structure properties in high performance multi-crystalline silicon ingots

High performance multi-crystalline silicon (HPM-Si) for the use in photovoltaics is characterized by a very fine grain structure and a high content of random grain boundaries, finally resulting in a low dislocation density and consequently in a high material quality. Typically, the grain size increases and the fraction of random grain boundaries decreases over ingot height due to annihilation mechanisms, especially in the first 150 mm. One approach for further material improvement is to further increase the initial random grain boundary fraction and to maintain it as high as possible over the complete ingot height.In this work, several theoretical approaches to achieve these points were evaluated by experiments in G1 scale. Firstly, the influence of the silicon seeding material on the initial grain structure was investigated regarding the effect of extremely fine Si particles in the µm to nm range and the bulk density of the particle layer. Secondly, the effect of the initial geometrical grain boundary arrangement in the seed layer was evaluated. For that purpose, special seed alignments similar to the Quasimono approach were tested. Finally, the process parameter growth rate was varied in a wide range to investigate its influence on the evolution of the grain boundaries during growth.The results show that the optimum for the initial random grain boundary fraction is already reached by the existing methods/commonly used seed materials. Concerning the decrease of the random grain boundary fraction over ingot height, some technical aspects were identified which are able to keep the amount of random grain boundaries at a high level. However, the practical realization within an industrial setup might be difficult.

Seed Size: Influence of Seed Size on the Quality of High‐Performance Multicrystalline Silicon Ingots Grown by Fused Quartz Particle Seeding (Crystal Research and Technology 3/2019)

Crystal Research and TechnologyVolume 54, Issue 3 1970014 Cover PictureFree Access Seed Size: Influence of Seed Size on the Quality of High-Performance Multicrystalline Silicon Ingots Grown by Fused Quartz Particle Seeding (Crystal Research and Technology 3/2019) First published: 12 March 2019 https://doi.org/10.1002/crat.201970014AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinkedInRedditWechat Graphical Abstract Growing Silicon ingots by fused quartz particle seeding is critically dependent on seed size. In article number 1800098, Xinming Huang and co-workers report on the investigation of four different seed sizes. Experimental results show that ingots obtained in the presence of 50-70 mesh seeds have the best quality. Volume54, Issue3March 20191970014 RelatedInformation

Evolution of the light sensitive defects in high performance multicrystalline silicon wafers

Sequential degradation measurements have been performed on passivated high performance multicrystalline silicon wafers, first at room temperature under low intensity illumination followed by a higher intensity illumination at an elevated temperature. The presence of two main degradation mechanisms, affecting the lifetime under different conditions has been demonstrated, namely, the well-studied light induced degradation caused by boron-oxygen-complexes and the less understood light and elevated temperature induced degradation. Light and elevated temperature induced degradation is the main lifetime limiting the recombination path when fully activated, but the contribution from boron-oxygen complexes is not negligible. This separation of the two degradation mechanisms might, therefore, be necessary for proper evaluation of the dominant recombination mechanism. Experiments also show regeneration of the minority carrier lifetimes caused by deactivation of both the lifetime limiting defects at comparable time scales, and under similar illumination and temperature conditions. Wafers from different heights in a high performance multicrystalline silicon ingot have been evaluated to better understand the underlying causes for the different degradation mechanisms. Effects of the iron-boron-splitting on the carrier lifetime are only visible in ungettered wafers.

Crystallization of multicrystalline silicon from reusable silicon nitride crucibles: Material properties and solar cell efficiency

In this paper experimental results on High Performance multicrystalline silicon ingots (HPmc-Si) grown from reusable silicon nitride crucibles are presented. Five ingots are grown from the same crucible and the material is analyzed for carbon and oxygen, and metallic impurities, by means of Fourier transform infrared (FTIR) spectroscopy and Neutron Activation Analysis (NAA), respectively. Microwave Photoconductance Decay (μW-PCD) and Quasi-Steady-State Photoconductance (QSSPC) is applied on samples to reveal the resulting charge carrier lifetime distribution. Full-area aluminum back surface field (BSF) solar cells are processed from 156 mm × 156 mm wafers and the solar cell efficiency is analyzed. Material properties and solar cell efficiencies are comparable to reference material grown from high purity electronic grade (EG) quartz crucibles. Obtained results are discussed with respect to results published recently.

Investigation of Nanoscale Quasi Pyramid Texture for HIT Solar Cells Using n-type High Performance Multicrystalline Silicon

In this work, an n-type high performance multicrystalline silicon (nHPMC) ingot (τ >1.2 ms) was cast at NTNU. A new method to manufacture nHPMC solar cells with hetero junction solar cell architecture is presented. Texturing of the wafers is performed through silver assisted chemical etching followed by an alkaline solution treatment process. The slight differences of the reactions between the concentration of the alkali solutions and the nanopores on silicon surface has been determined and used to tailor different textures. Alkaline solutions with gradient concentration were developed to improve the surface morphology for HIT solar cells after a silver assisted chemical etching process. A homogeneous texture with quasi pyramid morphology was then obtained through balancing the reflectance and surface properties on nHPMC wafers. This kind of quasi pyramid texture is also a good starting point for the surface passivation during HIT processes. An efficiency of 19.03% has been obtained, demonstrating the potential of using n-type HPMC in the production of cost effective solar cells.

Minority carrier lifetime evaluation of periphery edge region in high-performance multicrystalline ingot produced by seed-assisted directional solidification

A high-performance multicrystalline silicon (mc-Si) ingot was produced by seed-assisted directional solidification, and the minority carrier lifetime of the periphery edge region was evaluated. The defects and impurities in the periphery edge region of the silicon wafers were systematically studied with photoluminescence (PL) imaging, minority carrier lifetime mapping, and Fourier transform infrared (FTIR) spectroscopy. Their relationships with the minority carrier lifetime were investigated. The concentration of substitutional carbon, interstitial oxygen, and dislocation clusters is not directly correlated with the low minority carrier lifetime of the edge zone of the mc-Si ingot. Inhomogeneous grain size distribution and contamination with iron impurities were demonstrated to be the main factors affecting the low minority carrier lifetime. By controlling the impurities and improving the grain size distribution, a modified furnace was designed and a higher-quality mc-Si ingot was manufactured.

Recombination sources in p-type high performance multicrystalline silicon

This paper presents a comprehensive assessment of the electronic properties of an industrially grown p-type high performance multicrystalline silicon ingot. Wafers from different positions of the ingot are analysed in terms of their material quality before and after phosphorus diffusion and hydrogenation, as well as their final cell performance. In addition to lifetime measurements, we apply a recently developed technique for imaging the recombination velocity of structural defects. Our results show that phosphorus gettering benefits the intra-grain regions but also activates the grain boundaries, resulting in a reduction in the average lifetimes. Hydrogenation can significantly improve the overall lifetimes, predominantly due to its ability to passivate grain boundaries. Dislocation clusters remain strongly recombination active after all processes. It is found that the final cell efficiency coincides with the varying material quality along the ingot. Wafers toward the ingot top are more influenced by carrier recombination at dislocation clusters, whereas wafers near the bottom are more affected by a combination of their lower intra-grain lifetimes and a greater density of recombination active grain boundaries.

N-type high-performance multicrystalline and mono-like silicon wafers with lifetimes above 2 ms

Combined with advanced crystal growth technology and reduced dislocation densities, the higher tolerance to metal contamination of n-type silicon makes n-type cast-grown silicon a potential option for low cost high quality substrates for solar cells. Using a combination of photoconductance based lifetime testing and photoluminescence imaging, we have investigated the carrier lifetime in wafers from the bottom, middle, and top parts of a n-type high-performance multicrystalline (HPM) silicon ingot, and wafers from n-type mono-like silicon ingots after each high temperature solar cell processes, including after boron diffusion, phosphorus diffusion, and hydrogenation. Although boron diffusion leads to a degradation of the sample lifetime, phosphorus diffusion and hydrogenation is effective at recovering the lifetime in the intra-grain region and at the grain boundaries respectively. Quasi-steady-state photoconductance (QSSPC) measurements show that the arithmetic average lifetime of HPM silicon wafers and mono-like silicon wafers can reach up to 1.8 and 3.3 ms respectively for a process sequence including a boron diffusion, with corresponding implied open circuit voltage of about 720 mV. If the boron diffusion can be avoided, average lifetimes up to 3.0 and 6.6 ms can be achieved respectively, highlighting the excellent potential of n-type cast-grown materials.

Evolution of grain structure and recombination active dislocations in extraordinary tall conventional and high performance multi-crystalline silicon ingots

In this work one high performance multi-crystalline silicon ingot and one conventional multi-crystalline silicon ingot, each with an extraordinary ingot height of 710mm, were replicated by the successive growth of eight G1 ingots to evaluate the potential advantage of extraordinary tall HPM ingots in industrial production. By analyzing different grain structure parameters like mean grain size, grain orientation and grain boundary type distribution as well as the recombination active dislocation area over the complete ingot height, it was observed that the material properties strongly differ in the initial state of growth for the two material types. However, at ingot heights above 350mm, the difference has vanished and the grain structure properties for both materials appear similar. It is shown that the evolution of the grain structure in both material types can be explained by the same grain selection and grain boundary generation/annihilation mechanisms whereas the current grain structure determines which mechanisms are the most dominant at a specific ingot height. Since the grain structure directly influences the dislocation content in the silicon material, also the recombination active dislocation area becomes equal in high performance and conventional multi-crystalline silicon material at ingot heights above 350mm. From these results it is concluded that the advantage of high performance silicon material is limited to the first grown 350mm of the ingot.

Different nucleation approaches for production of high-performance multi-crystalline silicon ingots and solar cells

Two different process approaches for growing high-performance multi-crystalline silicon (HPM-Si) material for photovoltaic applications are presented. Assisted nucleation was achieved by seeding on fine polycrystalline silicon chunks and on a rough silica crucible bottom, respectively. For each of the two methods industrially sized ingots of 650kg were directionally solidified. Wafers of these ingots and those from conventionally solidified multi-crystalline (mc) silicon ingots were used to produce solar cells on industrial line. Solar cell parameters were analyzed and compared.Both assisted nucleation methods are suitable for industrial growing of HPM-Si ingots. The grain size is more homogeneously distributed resulting in improved material quality. Less dislocation clustering results in higher average conversion efficiency of solar cells compared to those made of conventionally solidified mc-Si material. Moreover, it is also experimentally proven that assisted nucleation procedure is effective for both Siemens polysilicon feedstock and cheaper solar grade silicon. Further improvements of nucleation procedures, which can be used to achieve higher conversion solar cell efficiency and lower product cost, are proposed.