Solar Grade Silicon Feedstock Research Articles

Assessing the potential of TOPCon solar cells architecture using industrial n-type cast-mono silicon material

Cast-mono silicon material is interesting for its lower carbon footprint compared to Czochralski (Cz) monocrystalline silicon. However, solar cells fabricated using cast-mono (CM) silicon show lower performances. In this work, two routes to make cast-mono silicon advantageous over Cz silicon are considered. The first route is to further reduce carbon footprint of cast-mono silicon, by using Upgraded Metallurgical Grade silicon (UMG-Si) feedstock instead of Solar Grade silicon (SoG-Si) feedstock. TOPCon solar cells are fabricated using both feedstocks, and cast-mono growth technology, using industrial-type furnaces. Laboratory studies show that UMG-Si can result in efficiencies higher than solar cells made of SoG-Si when feeding the material to a CM crystallization process. But when compared to Cz, CM-UMG-Si TOPCon solar cells conversion efficiency values are still 0.5%abs lower. The second route is to take advantage of the TOPCon passivation layer (e.g., poly-Si) ability to getter metallic impurities, and thus improve the quality of cast-mono material. Several TOPCon sequences are tested and their effect on the carrier recombination properties of the device are studied. In the end, solar cells are fabricated and again, UMG-Si solar cells show better results than SoG-Si solar cells, with efficiency up to 22.65%, independently confirmed.

Improvement of charge carrier lifetime in heat exchange method multicrystalline silicon wafers by extended phosphorous gettering process

External phosphorous diffusion gettering were applied using homogenous and extended schemes on multicrystalline silicon (Mc-Si) wafers obtained from solar grade (SOG) silicon feedstock by the heat exchange method (HEM) growth technique. X-ray fluorescence (XRF) characterization shows the presence of transition metals like Chromium (Cr), manganese (Mn) and iron (Fe) in the analyzed as-cut Mc-Si substrates with a predominance of Mn and Fe elements. Before and after gettering process, we have observed that the Cr atoms concentration drop from 4.1015 at.cm-3 in the as-cut Mc-Si samples to 2.5 1015 at.cm-3 with homogeneous gettering and to 3.1014 at.cm-3 in the wafer undergoes extended gettering. The quasi-steady-state photo conductance (QSSPC) technique is used to characterize the minority charge carrier’s lifetime tn before and after gettering process. It was found that the response of Mc-Si wafers is better (form point of view of lifetime improvement) to the extended process but in some regions it is less effective and the homogenous process give the best amelioration due probably to the spatial distribution of crystalline defects and metallic precipitates densities. With tn initial values before gettering in the range of 2.5-8 usec we have measured lifetime values in the range of 15 to 37 usec with extended process and from 10 to 30 µsec with the homogenous one which ensure cell efficiencies in the range of 15 to 16%.

Impact of silicon feedstock contamination by gas phase diffusion on material quality of cast silicon ingots

The impurity level of cast silicon ingots for photovoltaic application has become more and more crucial over the last years with respect to the competitiveness to the high quality monocrystalline Czochralski material. Typical contaminants diffusing into the silicon melt and ingot during the casting process are metals, dopants and light elements like C, O and N. They originate from the consumables like crucible, crucible coating and silicon feedstock as well as the furnace atmosphere. If high quality electronic grade or solar grade silicon feedstock is used, it exhibits several orders of magnitude lower impurities than the other consumables and therefore is mostly neglected during optimization processes in industrial environment. However, the pure feedstock is exposed to the furnace atmosphere during the heating process for long timescales at high temperatures even before melting and solidification. This can lead to a pronounced contamination of the feedstock particles, which negatively influences the resulting material quality of the silicon ingots.In this work, this phenomenon is investigated in laboratory scale by carrying out several two-step G1 experiments containing a pre-annealing step of the feedstock in different crucible setups followed by a regular crystallization step in a virgin crucible. Through lifetime, interstitial iron, resistivity and C-/O-measurements, the effect of feedstock contamination by the surrounding environment on the ingot quality was evaluated. The results show that a significant feedstock contamination by boron, iron and carbon species takes place if the vacuum phase during heating phase is too long. In contrast, no significant oxygen contamination could be observed. Further, the main sources for iron and boron contamination via gas phase diffusion were identified to be the silica crucible and the Si3N4 coating, respectively. In contrast, the contamination by the furnace itself (e.g. graphite parts) plays only a minor role concerning these elements, but is mainly responsible for carbon contamination.

Life cycle assessment on PERC solar modules

The screen-printed aluminium back surface field (Al-BSF) technology is the current industry standard process for crystalline silicon solar cells but, due to the search for higher efficiency, much attention has been paid to the passivated emitter and rear cell (PERC), which is gaining significant share in the world market. We undertake an environmental analysis comparing Al-BSF and PERC monocrystalline solar modules. Through the life cycle assessment (LCA) method we calculate the global warming, human toxicity (cancer and non-cancer effects), freshwater eutrophication, freshwater ecotoxicity, abiotic depletion potentials and energy payback time of these technologies considering solar, electronic and upgraded metallurgical grade silicon feedstock. The functional unit considered is 1 kWh of energy delivered over the modules’ lifetime. As a result of this work, we showed that PERC technology generates a slight improvement in the environmental impacts when compared with Al-BSF. The use of electronic and upgraded metallurgical grade silicon results in lower environmental impacts in most cases, compared with the other technologies analysed, based on the assumptions made in this LCA.

Effect of acid leaching conditions on impurity removal from silicon doped by magnesium

The effect of magnesium addition into a commercial silicon and its leaching refining behavior is studied for producing solar grade silicon feedstock. Two different levels of Mg is added into a commercial silicon and the leaching of the produced alloys by 10% HCl solution at 60 ℃ for different durations is performed. It is shown that the microstructure of the alloy and in particular the distribution of eutectic phases is dependent on the amount of the added Mg. Moreover, the metallic impurities in silicon such as Fe, Al, Ca and Ti are mainly forming silicide particles with different compositions. These silicides are physically more detached from the primary silicon grains and their removal through chemical and physical separation in leaching is better for higher Mg additions. It is observed that the leaching is more effective for the purification of smaller silicon particles produced from each Mg-doped silicon alloy. It is shown that acid leaching by the applied method is effective to reach more than 70% of phosphorous removal. It is also shown that the purity of silicon is dependent on the total Mg removal and effectiveness of leaching on removing the Mg2Si phase.

Boron Removal from Silicon by CaO-Na2O-SiO2 Ternary Slag

Boron removal from silicon is an important issue for solar-grade silicon feedstock production. In the present study, the removal of B from liquid silicon by a CaO-Na2O-SiO2 slag is studied and it is shown that B can be rapidly removed from silicon within short refining times. Based on mass balance and chemical analysis of the reacted silicon and slag, it is indicated that the kinetics of B removal is dependent on the Na2O in the slag, the main reactive agent for B removal from the system. The transported B into the slag is gasified at the slag-gas interface through sodium metaborate evaporation, which is a rate controlling reaction for B transport to the gas phase. It is indicated that B removal rate by CaO-Na2O-SiO2 slag is considerably higher than that by CaO-SiO2 slags. It is proposed that boron oxide (B2O3) is better embedded in the structure of CaO-SiO2 slags than Na2O-containing slags.

A low cost, low energy route to solar grade silicon from rice hull ash (RHA), a sustainable source

Polycrystalline silicon, with impurity levels lower than those of the SEMI III standard for solar grade silicon feedstock (≈99.9999% pure), was produced using rice hull ash (RHA) as a biogenic silica source.

Distribution and impact of chromium in compensated solar grade silicon

In this study we have investigated two multicrystalline silicon ingots made from a compensated solar grade silicon (SoG-Si) test feedstock. Both ingots were solidified under the same conditions in a pilot-scale directional solidification furnace. In order to study the distribution and the impact of Cr in the compensated SoG-Si feedstock, 50ppmw Cr (27ppma) was added to one of the ingots. Electrical properties, such as resistivity and minority carrier lifetime, have been compared for the two ingots. Solar cells performance is also presented. The minority carrier lifetime is significantly reduced by the addition of Cr, being up to 19μs for the undoped reference ingot, and <1μs for the Cr-doped material. The maximum solar cell conversion efficiency of the solar cells made from the Cr-doped compensated solar grade silicon in this study is 14.4%, which is similar to the cells made from the ingot with no Cr addition. This work shows that Cr contamination significantly impairs electrical properties but they can, to some degree, be improved by gettering during solar cell processing.

Investigation of open-circuit voltage in solar grade silicon solar cells from a metallurgical process route and cell’s defect

The single crystalline silicon solar cells made by solar grade silicon feedstock from a metallurgical process route have an inferior performance than that made by polycrystalline silicon from the chemical route, but the former has a higher open-circuit voltage than the latter under low illumination. In this paper, the open-circuit voltages are calculated under low illumination in solar grade silicon solar cells from a metallurgical process route, and the relationship between open-circuit voltage and trap density are obtained. The result is in excellent agreement with the recent experimental measurements and in harmony with other theoretical calculations. The scientific phenomenon in metallurgical silicon solar cells is explained by modelling and simulation for the first time. It should be emphasized that this phenomenon exists in not only single crystalline silicon solar cells from a metallurgical process route but also multicrystalline silicon solar cells. Finally, the detrimental characteristics of single crystalline silicon solar cells from a metallurgical process route are revealed.

Use of rice husk for emission neutral energy generation and synthesis of solar-grade silicon feedstock

The potential use of rice husk as a source of energy and also for producing high-purity silicon has been investigated by a combination of pilot trials, theoretical calculations and laboratory experiments. Rice husk was combusted at 850°C. Mass balance together with the pilot results indicates an electricity generation potential of 1 MWh per tonne rice husk. The process also generates 180 kg rice husk ash (RHA) that is used for production of high-purity silicon through a hybrid pyrometallurgical–hydrometallurgical process. The process consists of three major steps: purification of RHA by leaching and roasting, reduction by magnesium and recovery of silicon through two steps of leaching. The silicon generation process steps were optimized, showing that about 85% of silicon in the RHA can be recovered. The silicon product has a total impurity of <0.7%, hence it may be used as a suitable feedstock for production of solar-grade silicon.

Processes for Upgrading Metallurgical Grade Silicon to Solar Grade Silicon

The photovoltaic (PV) industry is in rapid growth and a large supply of PV feedstock materials must be provided to maintain this growth. Since silicon is still the dominant material for the fabrication of solar cells, low-cost solar-grade silicon (SoG-Si) feedstock is demanded. The most cost-effective and direct approach for producing SoG-Si is to purify and upgrade metallurgical-grade silicon. Many impurities in MG-Si can be effectively removed through directional solidification of molten silicon. However, the removal of boron (B) and phosphorus (P) by this method is difficult and expensive due to the relatively large distribution coefficients of these elements. Therefore, the elimination of B and P to the levels required for SoG-Si feedstock requires the development of new processes. In the present study, the effect of impurities on the solar cell efficiencies and the impurity contents in silicon materials are studied. The chemical and metallurgical processes that can be applied to purify metallurgical grade silicon to solar grade silicon are reviewed and evaluated. It is shown that under development silicon refining processes are applicable to produce solar grade silicon. However, there are specific challenges for each process and their benefits and risks must be considered for economic production.

Liquidus of Silicon Binary Systems

Thermodynamic knowledge about liquid silicon is crucial for the production of solar-grade silicon feedstock from molten silicon. In the current study, liquidus for silicon binary alloys is formulated using a previously developed method in which the liquidus curve is calculated using two constants. The liquidus measurements for the silicon portion of the silicon alloys with Al, Ca, Mg, Fe, Ti, Zn, Cu, Ag, Au, Pt, Sn, Pb, Bi, Sb, Ga, In, Ni, Pd, Mn, and Rh are reviewed, and the consistent data were used to determine the liquidus constants. The liquidus curves for silicon binary systems are calculated and plotted. It is indicated that the calculated liquidus curves fit well with the experimental data. A correlation between the determined liquidus constants is also observed, which can be used to gain a better understanding of the thermodynamics of the silicon binary melts.

Investigation of bulk and solar cell properties of ingots cast from compensated solar grade silicon

AbstractIn this work, the effect of the concurrent presence of B and P on bulk and solar cell properties of directionally solidified multicrystalline ingots from commercially compensated solar grade silicon (SoG‐Si) feedstock produced by Elkem Solar was investigated. The initial B and P content prior to the directional solidification experiment was 1260 and 762 ppba, respectively. Two reference ingots have been solidified in a silica crucible from 100% electronic grade silicon (EG‐Si) feedstock, with 332 ppba of boron added. All ingots have been cast under similar process parameters. The resistivity measurements by Four Point Probe (FPP) are in good agreement with the net dopant content, i.e., NA − ND for p‐type material, measured by Glow Discharge Mass Spectrometer (GDMS). Bulk lifetime measurements show a decrease in the values compared to the EG reference. Lifetime distributions show the highest values of 13 and 19 µs at approximately half ingot height, compared to 30 and 44 µs in the reference ingots. This decrease can be due to the concurrent effect of compensation and of other impurities present in the ingot. However, the content of several transition metals measured by GDMS at half ingot height was not significantly higher than that of the reference ingots. Oxygen content as measured by Fourier Transform Infra‐Red (FTIR) spectroscopy shows no significant difference compared to the references. Solar cells made from the compensated ingots and processed under standard process conditions show efficiency values up to 15.5% and fill factor values up to 78%, comparable to conventional multicrystalline silicon cells. Copyright © 2010 John Wiley &amp; Sons, Ltd.

Low-cost solar grade silicon purification process with Al–Si system using a powder metallurgy technique

Silicon solar cell is one of the cleanest and most potential renewable resources. However, the high cost of raw material is impeding the development of silicon solar cell. In this paper, we have investigated a purification process designed for low-cost solar grade silicon with Al–Si system, using a powder metallurgy technique. It is found that by modulating the external pressure and/or protection ambient, the alloying of Al–Si powder mixture is easily accomplished at a relatively lower temperature. The mechanism of Si products purified from the Al–Si melt has been discussed based on their morphological characterizations. The impurity contents of the purified Si products can be controlled at a very low level (∼3 ppmw). Meanwhile, the yield of the purified Si products using the powder metallurgy technique is clarified to be higher than that of the conventional Al–Si purification technique without external pressure or protecting ambient. The process is quite potential of providing low-cost solar grade silicon feedstock for photovoltaic industry.

Development of silicon purification by strong radiation catalysis method

Using a new type of solar furnace and a specially designed induction furnace, cost effective and highly efficient purification of metallurgical silicon into solar grade silicon can be achieved. It is realized by a new method for extracting boron from silicon with the aid of photo-chemical effect. In this article, we discussed the postulated principle of strong radiation catalysis and the recent development in practice. Starting from ordinary metallurgical silicon, we achieved a purification result of 0.12 ppmw to 0.3 ppmw of boron impurity in silicon by only single pass of a low cost and simple process, the major obstacle to make ‘cheap’ solar grade silicon feedstock in industry is thus removed.

Recovery of silicon from silica fume

Feasibility of producing high purity silicon from amorphous silica fume, using a low temperature magnesiothermic reduction was demonstrated. Commercial silica fume containing 97.5% amorphous silica was first purified by acid leaching and roasting to remove large quantities of transition metals and carbon. The product was then reduced using magnesium as the reductant. The effect of the amount of reductant, initial temperature, and dwell time were investigated on the quantity and type of the reaction products. The optimum reduction conditions were decided based on the maximum yield of the Si metal. These corresponded to Mg/SiO 2 molar ratio of 2.0, preheating temperature of 750 °C, and holding time of 2 h. High purity silicon (> 99 wt.%) containing < 3 ppmw B and 12 ppmw P was obtained after leaching purification of the reduction products, showing that the material is superior to metallurgical grade silicon, for use as solar-grade silicon feedstock.

The materials characteristic and the efficiency degradation of solar cells from solar grade silicon from a metallurgical process route

The rising conventional energy prices have opened up the market for photovoltaic, but the lack of polycrystalline silicon from the chemical route restricts the growth of crystalline silicon solar cells. Recently there is a trend that produces solar cells by using the newly developed solar grade silicon feedstock from a metallurgical process route. In this article, the chemical components of solar grade silicon feedstock are analyzed. The single crystalline silicon solar cells from 100% solar grade silicon feedstock from a metallurgical process route are investigated. The outdoor performance of solar modules encapsulated by such cells is reported. The experimental evidence suggests that such solar cells can achieve the average efficiency higher than 14% on single crystalline silicon wafers. However, the efficiency degradation of solar cells under natural sunlight is significant, and the electrical uniformity of small cells diced from the whole cell is too bad. The metal impurities, oxygen, carbon, and their complexes influence the performance stabilization. The article proves that the module made by such cells has a big cell mismatch loss than normal cells made by electronic grade silicon, even if these cells come from the same sort. And the operating temperature of the cells of the modules is 15–22 °C higher than normal modules under the same conditions. The solar grade silicon feedstock from a metallurgical process route has to be improved farther in order to be used in photovoltaic industry.

Crystalline Si solar cells based on solar grade silicon materials

A new method named Chemical Physics (CP) method was developed to produce solar grade silicon feedstock at a company in China. In this paper the characteristic of the solar grade silicon made by CP method was analysed. The results show that the purity of solar grade silicon is above 5 N and most of impurities are below 0.0001 wt.%. Crystalline silicon solar cells were prepared using solar grade silicon wafers based on CP method. Average efficiency of the solar cells is about 15.05%, and the highest efficiency is 15.60% under AM1.5 illumination conditions. The light-induced degradation of the solar cells was examined. Degradation by up to 15% of the initial efficiency of the solar cells is detected. The solar cell results and light-induced characteristic show that the solar cells based on CP methods have desired performance and thus have the potential for large scale production.

The impact of silicon feedstock on the PV module cost

The impact of the use of new (solar grade) silicon feedstock materials on the manufacturing cost of wafer-based crystalline silicon photovoltaic modules is analyzed considering effects of material cost, efficiency of utilisation, and quality. Calculations based on data provided by European industry partners are presented for a baseline manufacturing technology and for four advanced wafer silicon technologies which may be ready for industrial implementation in the near future. Iso-cost curves show the technology parameter combinations that yield a constant total module cost for varying feedstock cost, silicon utilisation, and cell efficiency. A large variation of feedstock cost for different production processes, from near semiconductor grade Si (30 €/kg) to upgraded metallurgical grade Si (10 €/kg), changes the cost of crystalline silicon modules by 11% for present module technologies or by 7% for advanced technologies, if the cell efficiency can be maintained. However, this cost advantage is completely lost if cell efficiency is reduced, due to quality degradation, by an absolute 1.7% for present module technology or by an absolute 1.3% for advanced technologies.

Silicon Purification by a New Type of Solar Furnace

We propose a new method to reveal a direct transformation from solar energy to solar electricity. Instead of using electricity in the process, we use concentrated solar rays with a crucibleless process to upgrade metallurgical silicon into solar-grade silicon feedstock.