Cast Multicrystalline Silicon Research Articles

Interfacial characterization of non-metal precipitates at grain boundaries in cast multicrystalline silicon crystals

Interfacial characterization of non-metal precipitates at grain boundaries in cast multicrystalline silicon crystals

Growth and Defects in Cast‐Mono Silicon for Solar Cells: A Review

Cast‐mono silicon (CM‐Si) is a promising substrate to fabricate cost‐effective photovoltaic (PV) wafers used for solar cells with higher efficiency compared with conventional cast multicrystalline silicon (mc‐Si) due to its high throughput and good ingot quality. However, the unavoidable appearance of defects like dislocation, subgrain boundaries, impurities, multicrystallization, and twins, seriously limits its mass‐scale applications in the PV industry. In this review, the growth characteristics of the CM‐Si method are introduced first. Then, the attention is focused on the defects in CM‐Si. Other issues related to production costs that restrict the development of CM‐Si are also addressed in detail, such as seed cost and recycled seeds, n‐type CM‐Si, and large‐size CM‐Si. Finally, a conclusion is made and the prospects are given about the CM‐Si technique.

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

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

Growth of multicrystalline silicon assisted by electrophoretic deposited quartz

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

Rapid‐Thermal‐Annealing‐Induced Passivation Degradation and Recovery of Polysilicon Passivated Contact with Czochralski and Cast Multicrystalline Silicon Substrates

Metallization of high‐efficiency n‐type solar cells with tunnel oxide passivated contact (TOPCon) structures often requires a high‐temperature rapid‐thermal‐annealing (RTA) process, which potentially induces a notable degradation of device performance. Herein, the RTA‐induced passivation degradation mechanisms and potential recovery processes are clarified by postthermal annealing or hydrogenation based on both Czochralski (Cz) silicon and cast‐multicrystalline (cast‐mc) silicon wafers. It is found that the RTA process induces at least two kinds of defects, i.e., thermal‐quenching‐induced defects and hydrogen‐release‐induced defects. The passivation quality can be partially recovered under a high‐temperature annealing at 820 °C, which, however, can be fully restored via a postmoisture/nitrogen hydrogenation treatment at 450 °C. In general, the two mentioned wafers, i.e., n‐type Cz c‐Si and cast‐mc silicon substrates, demonstrate similar passivation behaviors in most cases before and after RTA treatment. Finally, it is inferred that the inferior passivation properties of the n‐type cast‐mc silicon samples are related to the relatively low bulk quality. This can be confirmed by the fact that samples with n‐type cast‐mc Si substrates show an abnormal passivation behavior with highly effective lifetimes in the low‐density carrier region, suggesting the existence of electron trapping states.

Industrial TOPCon solar cells on n-type quasi-mono Si wafers with efficiencies above 23%

Since 2017, the efficiency record for cast multi-crystalline silicon (mc-Si) solar cells has been 22.3% using a TOPCon design on a 2 × 2 cm2 area of an n-type wafer processed in the laboratory. As the efficiency of industrial PERC and TOPCon cells on monocrystalline wafers has been higher than that, there is widespread expectation that multi-crystalline materials will be used less and less. Recently, we applied our industrial TOPCon (i-TOPCon) technology to 157.4 mm × 157.4 mm quasi-mono n-type wafers and achieved an efficiency of 23.22% with Voc of 711.9 mV with illumination for the front side, independently confirmed by ISFH CalTeC in Germany. To the best of our knowledge, this is the first solar cell based on cast-mono material that exceed 23% efficiency. This gives scope that cast materials may still be used in future cell fabrication because such high efficiencies and lower cost may offset the lower production yield compared to Czochralski (Cz) wafers. We report on details about our cell structure and processing, we analyze the bulk lifetime before and after cell fabrication, and give details about the optimization of the rear passivating contact.

Defect control based on constitutional supercooling effect in cast multicrystalline silicon: Boron-indium co-doping

The impact of constitutional supercooling (CS) on the grain structure of cast multicrystalline silicon (mc-Si) materials was investigated. Indium impurity was chosen as the CS dopant because of its low equilibrium segregation coefficient so that CS effect could be obtained at relatively low doping concentration. Industrial indium (In) doped G2 and boron-indium (B–In) co-doped G5 seed-assisted multicrystalline silicon (mc-Si) ingots were fabricated and characterized. Self-nucleated grains and epitaxial grains could be obtained, respectively, as the result of tuning the doping amount of indium. Constitutional-supercooling-related defects (CSD) were found in the co-doped ingot and could suppress the growth and propagation of dislocation-type defects. As a result, the overall deleterious defect density of B–In co-doped mc-Si wafers was well reduced compared to that of referential B-doped wafers, and the average solar cell performance of B–In co-doped ingots was obviously enhanced.

Collaborative R&D between multicrystalline silicon ingots and battery efficiency improvement—effect of shadow area in multicrystalline silicon ingots on cell efficiency* * Project supported by the National Natural Science Foundation of China (No. 51532007) and the Fundamental Research Funds for the Central Universities.

We characterized strip-like shadows in cast multicrystalline silicon (mc-Si) ingots. Blocks and wafers were analyzed using scanning infrared microscopy, photoluminescence spectroscopy, laser scanning confocal microscopy, field-emission scanning electron microscopy, X-ray energy-dispersive spectrometry, and microwave photoconductivity decay technique. The effect on solar cell performance is discussed. The results show that the non-microcrystalline shadow region in Si ingots consists of precipitates of Fe, O, and C. The size of these Fe–O–C precipitates found at the shadow region is ~25 μm. Fe–O–C impurities can slightly reduce the minority carrier lifetime of the wafers while severely decrease in shunt resistance, leading to the increase in reverse current of the solar cells and degradation in cell efficiency.

Residual stress analysis and bow simulation of crystalline silicon solar cells

The pressure to reduce solar energy costs encourages efforts to reduce the thickness of silicon wafers. Thus, the cell bowing problem associated with the use of thin wafers has become increasingly important, as it can lead to the cracking of cells and thus to high yield losses. In this paper, a systematic approach for simulating the cell bowing induced by the firing process is presented. This approach consists of three processes: (1) the material properties are determined using a nanoidentation test; (2) the thicknesses of aluminum (Al) paste and silver (Ag) busbars and fingers are measured using scanning electron microscopy; (3) non-linear finite element analysis (FEA) is used for simulating the cell bowing induced by the firing process. As a result, the bowing obtained using FEA simulation agrees better with the experimental data than that using the bowing calculations suggested in literature. In addition, the total in-plane residual stress state in the wafer/cell due to the firing process can be determined using the FEA simulation. A detailed analysis of the firing-induced stress state in single crystalline silicon (sc-Si), cast, and edge-defined film-fed growth (EFG) multi-crystalline silicon wafers of different thicknesses is presented. Based on this analysis, a simple residual stress calculation is developed to estimate the maximum in-plane principal stress in the wafers. It is also proposed that the metallization pattern, Ag busbars and fingers screen printed on the front of a solar cell, can be designed using this approach. A practical case of a 3-busbar Si solar cell is presented.

The Effect of Phosphorus Diffusion Gettering on Recombination at Grain Boundaries in HPMC-Silicon Wafers

The influence of phosphorus diffusion gettering on recombination at grain boundaries has been studied in a commercially cast high performance multicrystalline silicon block. Wafers from four different heights have been studied with high resolution photoluminescence-imaging. The recombination at grain boundaries was studied from linescans perpendicular to the grain bounadary of interest. The change in recombination activity at grain boundaries after gettering has been correlated with grain orientation measured by electron backscatter diffraction and classified according to Brandon criterion. The relative change in carrier lifetime after gettering depends on the height in the cast, and is very sensitive to the injection level. Iron concentrations were also found from photoluminescence-imaging of iron in Fei and FeB states, respectively. After gettering recombination grain boundaries in the middle section of the ingot has become strongly recombination active. Fei, has been efficiently removed and no longer constitute the main recombination path at any height.

Grain growth of cast-multicrystalline silicon grown from small randomly oriented seed crystal

Multicrystalline silicon was grown from seeds with small grains of random orientation and the growth mechanism was studied with respect to grain size, shape, boundary character and orientation. The average grain size perpendicular to growth direction increased steadily initially, became constant and then increased steadily again. Grain size parallel to growth direction increased rapidly with growth due to grain elongation in the growth direction. Grain shape with respect to growth direction changed from spherical to columnar with growth. Initially non-CSL grain boundary fraction was very high but decreased with growth as the Σ3 grain boundary fraction increased. A simple model was proposed to explain the results.

Effect of crystal defects on mechanical properties relevant to cutting of multicrystalline solar silicon

The effect of crystalline defects such as dislocations and grain boundary on the cutting behavior of multicrystalline solar silicon is investigated. Diamond scribing experiments reveal significant intra-granular variations in the critical depth of cut for ductile-to-brittle transition in the material. This is explained by characterizing the local dislocation density variations in (100) and (311) grains of a cast multicrystalline silicon wafer and measuring the corresponding elastic modulus, nanoindentation hardness, and fracture toughness. Measured elastic moduli are shown to be higher than theoretical values for defect-free single crystal silicon of the same crystallographic plane. For a given grain orientation, a higher dislocation density is shown to be correlated with higher fracture toughness and a larger critical depth of cut.

Effect of Subgrains on the Performance of Mono-Like Crystalline Silicon Solar Cells

The application of Czochralski (Cz) monocrystalline silicon material in solar cells is limited by its high cost and serious light-induced degradation. The use of cast multicrystalline silicon is also hindered by its high dislocation densities and high surface reflectance after texturing. Mono-like crystalline silicon is a promising material because it has the advantages of both mono- and multicrystalline silicon. However, when mono-like wafers are made into cells, the efficiencies of a batch of wafers often fluctuate within a wide range of >1% (absolute). In this work, mono-like wafers are classified by a simple process and fabricated into laser doping selective emitter cells. The effect and mechanism of subgrains on the performance of mono-like crystalline silicon solar cells are studied. The results show that the efficiency of mono-like crystalline silicon solar cells significantly depends on material defects that appear as subgrains on an alkaline textured surface. These subgrains have an almost negligible effect on the optical performance, shunt resistance, and junction recombination but significantly affect the minority carrier diffusion length and quantum efficiency within a long wavelength range. Finally, an average efficiency of 18.2% is achieved on wafers with hardly any subgrain but with a small-grain band.

Transmission electron microscopy analysis of extended defects in multicrystalline silicon using in‐situ EBIC/FIB sample preparation

AbstractThis paper reports results of microstructural investigations of block cast multicrystalline silicon materials deliberately contaminated with iron and copper impurities from the melt. Electron microscopy techniques have been used to study interactions of grain boundaries, light element impurities such as nitrogen and oxygen, and metal impurities. The key to get access to extended defects typically present in a small density is the recently developed in‐situ EBIC/FIB approach to sample preparation.Special attention is drawn to oxygen and nitrogen containing precipitates at grain boundaries and resulting secondary defects. Strong accumulation of copper impurities is observed which is related to reduced excess carrier lifetimes in these regions. It is shown that copper silicide precipitation mainly accounts for local dislocation networks with a high recombination activity. These observations provide evidence that decoration of grain boundaries with light element contaminants leads to efficient nucleation of metal impurity precipitates. (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

On the role of stacking faults on dislocation generation and dislocation cluster formation in multicrystalline silicon

The microstructure of highly dislocated stacking fault regions (dislocation density >106 cm−2) in industrial cast multicrystalline silicon has been investigated by light microscopy, scanning electron microscopy, and transmission electron microscopy. Our observations indicate that stacking faults form strong barriers to lattice dislocation movement and to the formation of sub grain boundaries. Stepped and curved stacking fault edges appear to generate dislocations. The observations suggest that stacking faults play an important role in the plasticity as well as in the formation of the microstructure of dislocations in multicrystalline silicon.

Simulation of Iron Distribution after Crystallization of mc Silicon

Interstitial iron (Fei) and iron-boron pairs influence or even limit the recombination lifetime in industrial block cast multicrystalline (mc) silicon, though the proportions in the total iron concentration are generally small. Most of the iron in mc silicon is precipitated and less recombination active. This work aims for a better understanding of the distribution of iron in its different states (precipitated or dissolved) over the block height, as well as in regions of different crystal quality. In experimental studies several features of iron in mc silicon were observed, which occur due to the high extended defect density. In our 2-dimensional model for mc silicon, trapping of interstitial Fe at extended defects and precipitation at the extended defects are taken into account. The results are compared with NAA-data and spatial resolved measurements of the Fei concentration.

As-grown iron precipitates and gettering in multicrystalline silicon

We report here the results of a theoretical study concerning the iron precipitation in multicrystalline silicon during crystal growth and its implications on phosphorus gettering. In our model the average size and density of iron precipitates in the final structure depends on the growth method, initial iron concentration and the density of possible heterogeneous precipitation sites. With the same model we can simulate phosphorus diffusion gettering (PDG) of iron in cast multicrystalline silicon by using the iron precipitate size distribution obtained from crystal growth simulations as initial condition for gettering.

Metallic Impurities in Mono and Multi-crystalline Silicon and Their Gettering by Phosphorus Diffusion

The concentrations of 42 metal impurities in mono-crystalline microelectronic grade (EG) and mono-crystalline solar cell grade (SOG) and cast multi-crystalline SOG silicon have been measured by ICP-MS. In mono-crystalline EG material, all 42 elements were found to be below the detection limit. In mono and multi SOG material we could measure Al, Ti, Cr, Mn, Fe, Ni, Co, Cu, Ga, Mo and Sn. No significant difference was observed between the metal content of the top, middle and bottom of cast multi-crystalline Si. In contrast, a significant difference was observed in top, middle and bottom of solar grade mono-crystalline Si. The outer part of cast multi-crystalline silicon contains higher metal impurities than the center part, likely caused by impurities from crucibles. In cast wafers, phosphorus diffusion gettering (PDG) at 900 C leads to complete gettering of Cu and Ni. Fe can be partially removed, effectively by ramp-down annealing to 700 C. In this case we found that the Fe segregation coefficient increases by a factor of 36 for lightly doped Si. In some cast material PDG could reduce metal impurities significantly without a strong improvement of lifetime. Depth profiling of metal impurities in PDG areas shows no Cr because it evaporates during PDG annealing. Also Fe is found to evaporate to some extent.

Bulk lifetime and efficiency enhancement due to gettering and hydrogenation of defects during cast multicrystalline silicon solar cell fabrication

Cast multicrystalline silicon (mc-Si) shows a significant variation in quality depending on the location of the brick in the ingot and the location of the wafer in the brick. Variation also occurs in ingots from different suppliers, which is attributed to the difference in the cleanliness of the crucible used for growth and the quality of the silicon feedstock used. Process-induced lifetime investigation conducted in this paper showed that wafers from the top region of mc-Si ingot grown by Heat Exchanger Method (HEM) benefited most from the gettering step during phosphorus diffusion to form the n + junction. Wafers from the bottom of the ingot, however, benefited most from the hydrogenation taking place from the SiN x film during the co-firing cycle used to form simultaneous front and back contacts and aluminum back surface field. Wafers from the middle region benefited from both, the diffusion-gettering, and the SiN x -hydrogenation. Un-textured, 4 cm 2, screen-printed, best solar cell efficiencies of 15.9% and above were achieved on wafers from top, middle, and bottom regions of most of the ingots used in this study because the bulk lifetime exceeded 100 μs after gettering and hydrogenation. Lifetimes in excess of 300 μs were achieved from the middle region of some mc-Si ingots. Solar cell efficiencies of 16.7% were attained from the middle regions of two out of the three ingots investigated in this study. Device modeling was performed to provide guidelines to reduce the efficiency variation across different regions of the ingots and to obtain the highest possible efficiency with a given bulk lifetime and device structure.

Bulk multicrystalline silicon growth for photovoltaic (PV) application

Cast multicrystalline silicon ingots are widely used in photovoltaic manufacturing. A key issue to achieving high solar cell efficiencies is attaining an optimized temperature profile during ingot fabrication in order to control the formation of extended defects such as grain boundaries, sub-grain boundaries and dislocations. Numerical simulation is a powerful tool to help understand and depict the temperature distribution inside the growth furnace and to allow tailoring of the growth conditions to achieve high quality with a minimum of defects. Two major types of casting furnaces, direct solidification system and heat exchange method system, will be compared from the aspects of thermal field, crystal quality and related numerical simulations.