High Performance Multicrystalline Silicon Research Articles

Twenty years crystal growth of solar silicon: My serendipity journey

The past two decades have been a transformative era for solar silicon crystal growth, especially in the competition between multi-crystalline silicon (Multi-Si) and mono-crystalline silicon (Mono-Si). As a critical sector of the solar photovoltaic (PV) industry, the demand for this crucial material has surged exponentially, expanding over a thousand-fold. This remarkable increase has led to an accumulative deployment of silicon solar panels, which now approach a striking terawatt (TW), capturing over 95 % of the global PV market share. Furthermore, the rapid advancements in crystal growth technology during this period have set an unprecedented historical benchmark. This paper reviewed our early effort in the hot-zone design of Czochralski (CZ) solar silicon and then discussed its recent advancements. In between, the small-grain high-performance multi-crystalline silicon (HPM-Si), which we proposed, and the mono-like silicon (ML-Si) were briefly introduced. Because of the much better ingot quality and growth yield, HPM-Si became the mainstream until 2018. Its market share peaked at more than 70 % in 2017 but slid down quickly and was replaced by CZ silicon due to the emergence of diamond wire sawing (DWS); the CZ silicon was easier to cut, and its kerf loss was much less. Since then, the CZ ingot size has also started to increase quickly. The wafer size increased from 156 mm × 156 mm (M0) to 217 mm × 217 mm (M12+) in a few recent years; the increase in wafer area was almost doubled. Nowadays, the market share of CZ mono-Si is over 80 %.

Higher quality composite silicon ingot with monocrystalline and multicrystalline bricks

Cast monocrystalline silicon is a high-quality photovoltaic material with low fabrication cost, the developing trend is to increase quality stability. This work focuses on solving the edge multicrystalline region and related defects. By setting the modified seeds using Siemens polycrystalline raw silicon at the ingot edges, high-quality cast monocrystalline silicon and high-performance multicrystalline silicon can be obtained simultaneously in the same ingot, reducing the use of Czochralski seed and significantly improving the crystal quality and cell conversion efficiency in the edge region.

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

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

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.

The potential of cast silicon

The interest in cast mono silicon is increasing due to its lower energy consumption and resulting smaller carbon footprint, lower oxygen content and resulting less oxygen-related defects as well as easy scalability to large wafer formats like 210 × 210 mm2 full square. As a cast silicon alternative to high performance multicrystalline (hpm) silicon, which rapidly lost market share, we analyze the cell efficiency potential of cast mono silicon in a TOPCon cell structure.We show how the absence of grain boundaries and the exceptional tolerance of the material quality towards high temperature processing enable this significant increase of the cell efficiency potential compared to hpm silicon. The very effective suppression of crystal defects by the Seed Manipulation for ARtificially controlled defect Technique (SMART) results in a very low lateral variation of the high material quality.We present certified cell efficiencies of 23.3% on n-type material crystallized in our labs, which demonstrates the high efficiency potential even for our lab-type G2 crystallization. An additional crystallization experiment for 210 × 210 mm2 wafers demonstrates that SMART mono is compatible to large wafer sizes. A significant difference of the crystallization costs for Cz and cast mono crystallization as a function of electricity costs is discussed.

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.

Influence of temperature gradient at interface on defect multiplication in seed-assisted multicrystalline silicon

The control of defect multiplication in the upper part of seed-assisted multicrystalline silicon (SAMC-Si), also called as high-performance multi-crystalline silicon, has become crucial for the further improvement of multicrystalline silicon (mc-Si) crystal quality. This work investigates the impact of the temperature gradient at solid/liquid interface on the defect multiplication and solar cell performance. To get the in-time temperature gradient at solid/liquid interface, an equation and an experiment method were developed. It is found that the multiplication rate at the upper ingot is strongly relevant to the temperature gradient at solid/liquid interface. After modifying the temperature gradient, the defect density in the upper part of ingot is reduced about 40%, and the solar cell efficiency increases averagely about 0.07%. Finally, the mechanism and control of defect multiplication were discussed.

Interfacial atomic structure and electrical activity of nano-facetted CSL grain boundaries in high-performance multi-crystalline silicon

The interfacial structure of coincidence site lattice grain boundaries in multi-crystalline silicon plays a decisive role in their electrical behavior as revealed by high-resolution (scanning) transmission electron microscopy investigations. Considering only the global misorientation of the adjacent grains can lead to a false correlation between the structural and electrical properties of a grain boundary. The grain boundary habit plane as well as local deviations in the orientation and misorientation that induce additional structural defects need to be analyzed. Indeed, a Σ9 {221} grain boundary, presenting a perfect coincidence and periodicity at the atomic scale, appears electrically non-active. However, a grain boundary also identified as Σ9 {221} at the mesoscale is found to be composed of nano-sized triangular structures involving Σ3 {111} and Σ3 {112} facets at the nano-scale. This leads to the formation of grain boundary kinks and triple junctions that induce additional structural defects and turn the overall grain boundary interface electrically active. The possible origin of such a grain boundary dissociation, as well as its impact on the electrical activity, is discussed.

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%).

Atom Probe Tomography Study of Gettering in High-Performance Multicrystalline Silicon

During the production of high-performance multicrystalline silicon (HPMC-Si) solar cells, gettering occurs inherently during the formation of an emitter. The material benefits with an increase in minority carrier lifetime from the external gettering of impurities into the diffused layer. However, depending on the thermal budget and parameters of the emitter diffusion, as well as the specific material properties, the process can also be detrimental in terms of increased recombination activity of specific crystallographic defects. Thus, it is important to understand the root causes behind the change in recombination activity of defects following gettering. Here, we present a correlative atom probe tomography study of grain boundaries in both p- and n-type HPMC-Si before and after gettering. The presence of nitrogen was found to directly correlate with the increase in recombination activity at grain boundaries. Additionally, an estimation of the atom probe tomography detection limit for transition metals in silicon is made and found to be greater than known impurity levels in commercial HPMC-Si.

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.

The electrical properties of high performance multicrystalline silicon and mono-like silicon: Material limitations and cell potential

We compare the electrical properties of p-type and n-type industrially grown High Performance (HP) multicrystalline silicon (mc-Si) and mono-like silicon. The materials are characterised in terms of their bulk lifetimes, their implied open circuit voltages, and the density and recombination velocity of recombination active grain boundaries and dislocation clusters within the materials. Quokka3 is applied to simulate the cell performance potential of the studied materials, and quantify the corresponding efficiency loss due to different recombination mechanisms occurring in the bulk. Our results show that bulk recombination causes a noticeable efficiency loss in high efficiency cast-grown silicon solar cells, varying from 0.1% to 3.1% absolute in the simulated devices, depending on the materials and also the ingot position of the wafers. The performance of p-type HP mc-Si is affected by both recombination at extended crystal defects and in the intra-grain regions. N-type HP mc-Si shows a slight advantage over p-type HP mc-Si in the intra-grain regions. The advantage of p-type mono-like Si over p-type mc-Si mainly comes from optical gains due to alkaline texturing, but the material exhibits large performance variations along the ingot of up to 2.4% absolute. N-type mono-like Si shows the most promising material properties, featuring less performance variation along the ingot than p-type mono-like Si, and the highest efficiency potential among the four studied materials.

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.

Random angle grain boundary formation and evolution dynamics during Si directional solidification

The growth of multicrystalline silicon and the formation of a random angle grain boundary, as well as the dislocation generation and expansion is observed dynamically in situ, by Synchrotron X-ray imaging techniques. The focus is kept on a random angle grain boundary since its behavior is particularly important to better understand the HP mc-Si (High Performance Multi-crystalline Silicon) photovoltaic properties. Due to the process conditions and to the grain competition that occurs during the solidification, a facetted {111}/facetted {111} groove is formed by this random angle grain boundary at the solid/liquid interface. It is shown how the shape of the solid/liquid interface allows the change of the preferential {111} growth facet and affects the grain boundary propagation direction. In one of the groove configurations, the two adjacent {111} facets do not have the same growth velocity and as a consequence the corresponding grain boundary does not follow the bisector of the angle between the two facets. Indeed, the direction of the grain boundary is determined by the growth velocities of the facets which control the grain competition. Moreover, under these experimental conditions a clear relationship is observed between the existence of random angle grain boundaries and the local generation of dislocations as well as their expansion. By comparison, dislocation emission is not observed at the level of Σ3 {111} grain boundaries.

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.