The critical role of gas and chemical suppliers in PV cell manufacture

Transcription

1 The critical role of gas and chemical suppliers in PV cell manufacture Gases and chemicals contribute a significant part of the bill of materials for PV cells, and as the industry grows, material suppliers will face a number of key challenges to meet demand. Linde Electronics offers PES an illuminating view of the sector. The production of photovoltaic (PV) electricity has doubled every two years since 2002, with a CAGR of 48 percent, making it the world s fastest-growing power generation technology. At the end of 2008, according to industry data, cumulative global solar module production capacity was pegged at approximately 8000 megawatts. While actual production has been hampered by lack of poly silicon, this shortage is expected to ease from 2009 onwards. Demand through 2008 has been strong driven by financial incentives, such as preferential feed-in tariffs for solargenerated electricity and net metering, in many countries including Germany, Spain, Italy, Japan and parts of the United States. There has been a transition from mostly residential rooftop installations to more commercial and utility scale projects. While the recent financial conditions have tempered the growth, cell production capacity is still expected to grow to over 30GW by 2012 according to several market analysts. New technologies based on thin film silicon, Cadmium Telluride and CIGS are expected to take a growing share of the market due to their lower cost basis. Out of these, thin-film silicon is particularly promising due to its scalability, good performance in diffuse light and well known material properties. All of the above mentioned technologies for solar cell manufacturing rely on the same principle the creation of an 48 PES: Europe optical semiconductor device. This device can be made by processing wafers of crystalline silicon, or depositing a film on glass, metal foil or ceramics. There are three key technologies: crystalline silicon (c-si), advanced thin films, and amorphous silicon-based thin films (thin film Si). Each of these technologies has different conversion efficiency and cost structure, measured in terms of cost per square meter and watt per square meter. Combined, these two measures determine the cost per Watt. This article addresses those technologies adopted on an industrial scale: c-si & thin film Si. In both cases, as shown in Figure 1, gases and chemicals play a significant part in the manufacturing process, and hence can influence the cost per watt. Fig.1 - Gases & chemicals as a proportion of overall production costs for crystalline & thin film cells Crystalline cell manufacture Crystalline silicon wafer solar cells represent more than 85% of the total worldwide shipments for solar installations today and are expected to grow 20-30% annually for the next several years. C-Si cells offer high efficiency (15-20%) in terms of transforming light into electricity. With the present high cost of silicon wafers, representing 66% of the overall c-si cell manufacturing cost, the average solar cell cost of US$4/Watt is expensive. However, this is anticipated to reduce this year as supplies of silicon wafers become more abundant with additional global silicon capacity and given the current slow down and/or delays in cell manufacturing expansion capacity from the impact of global economic conditions. Innovative technology advances in c-si manufacturing techniques are emerging to produce higher efficiency, lower cost cells that are predicted to reach grid parity within the next 4-5 years. Process

2 materials, both gases and chemicals, play a key function in cell manufacturing, and represent 7% of the overall c-si cell manufacturing cost. Fig.2 Typical crystalline cell manufacturing processes showing gases & chemicals used Many new c-si companies are implementing turnkey cell manufacturing processes provided by OEM equipment and solutions suppliers. The OEM s typically specify process material purity specifications that are tied to cell efficiency yield guarantees. C-Si cell companies are then adding their own developed solutions to the turnkey line installations to optimize such critical processing steps as etch/texturization, doping, PSG removal, edge isolation, antireflective coating (ARC) deposition, and various cleaning steps that are essential to cell performance. Process material know-how and expertise in applications are routinely considered for cell efficiency impact. The value of process materials purity in each manufacturing step is constantly being evaluated for the effects on cell performance and for a path to further reduce manufacturing cost. Table 1 lists the principal gases and chemicals used, along with typical purity requirement and container sizes Table 1 - Principal gases and chemicals used in crystalline cell manufacture Product Typical Grade Typical Package (<500MW) Typical Package (>500 MW) Silane (SiH 4 ) Electronic Cylinder / Cylinder bundle ISO module (6000kg) Ammonia (NH 3 ) Electronic Tonne tank ISO module (6000kg) Tetrafluoromethane (CF 4 ) Electronic Cylinder / Cylinder bundle MCP (330 kg) Nitrous Oxide (N 2 0) Electronic Tonne tank Tonne tank Nitrogen (N 2 ) Electronic Liquid tank On-site generator Oxygen (O 2 ) Electronic Cylinder bundle / Liquid tank s Liquid tank Argon (Ar) Electronic Liquid tank Liquid tank Hydrogen (H 2 ) Electronic Cylinder bundle Liquid tank Acetic Acid (CH 3 COOH) <100 ppb * 200L drum / 1m3 tote 200L drum / 1m3 tote Ammonium Hydroxide (NH 4 OH) <50 ppb* 200L drum / 1m3 tote 1m3 tote / ISO tank ** Buffered Oxide Etchants (NH 4 F:HF) <50 ppb* 200L drum / 1m3 tote 200L drum / 1m3 tote Ethanol (C 2 H 5 OH) <10 ppb* 200L drum / 1m3 tote 1m3 tote / ISO tank** Hydrochloric acid (HC l ) <10 ppb * 200L drum / 1m3 tote 1m3 tote / ISO tank** Hydrogen Peroxide (H ) <10 ppb * 200L drum / 1m3 tote 1m3 tote / ISO tank ** Hydrofluoric acid (HF) <10 ppb* 200L drum / 1m3 tote 1m3 tote / ISO tank ** Isopropyl alcohol (C 3 H 8 O) <10 ppb * 200L drum / 1m3 tote 1m3 tote / ISO tank ** Potassium Hydroxide (KOH) < 5 ppm * 200L drum / 1m3 tote / ISO tank** 1m3 tote / ISO tank ** Nitric acid (HNO 3 ) <50 ppb * 200L drum / 1m3 tote 1m3 tote / ISO tank ** Phosphoric acid (H 3 PO 4 ) <100 ppb * 200L drum / 1m3 tote 1m3 tote / ISO tank ** Sulfuric acid (H 2 SO 4 ) <50 ppb* 200L drum / 1m3 tote 1m3 tote / ISO tank ** Sodium Hydroxide (NaOH) <50 ppb* 200L drum / 1m3 tote / ISO tank** 11m3 tote / ISO tank ** * Specification - max. impurity level per element analysed. **ISO tank contains between 6,000-17,000 litres of chemical PES: Europe 49

3 Thin film Si cell manufacture Thin film Si solar cells use amorphous silicon in single junction devices, or a combination of amorphous silicon and microcrystalline silicon for tandem junction devices on a substrate, usually glass. They offer a significant advantage in manufacturing large size panels up to 1.4 or 5.7 square meters, depending on the technology. A further significant advantage that this technology enjoys is the availability of large scale turnkey production equipment developed and proven in the flat panel display industry. In the past year many projects have been initiated in both Europe and elsewhere, while several companies have recently announced very large scale projects ranging from 500MW to 1000MW production capacity. Such gigafabs are poised to exploit the benefits of economy of scale to drive down module cost - currently in the range of US$2.50/watt, to nearer US$1.50/watt mark by Thin film silicon accounts for more than 50% of most new solar cell production capacity planned between 2008 and Large scale manufacturing of thin-film silicon cells requires very large quantities of gases and chemicals. Figure 1 highlights that gases can be between 15% & 20% of the bill of material, although as figure 3 shows, differences in production technology can also affect the mix of gases needed. Table 2 lists the range of gases needed for typical thin film operations. The critical process step in all thin-film silicon technologies is deposition of doped silicon film from a silane (SiH 4 ) precursor in a Chemical Vapor Deposition (CVD) system. The result is a thin film of silicon on the glass. Typically hydrogen (H 2 ) is also introduced to control the kinetics of the film growth. Dopants are incorporated through precursors such as Fig. 3 Key gases for 3 typical thin film cell processes Table 2 - Principal gases and chemicals used in thin film cell manufacture Product Typical Grade Typical Package (<200MW) Typical Package (>200 MW) Silane (SiH 4 ) Electronic Tonne tank / ISO module (6000kg) ISO module (6000kg) Nitrogen trifluoride (NF 3 ) * Technical Tonne tank / ISO module (8000kg) ISO module (8000kg) Fluorine (F 2 ) * Electronic On-site plant On-site plant Sulphur hexafluoride (SF 6 ) * Technical Cylinder N/A Nitrogen (N 2 ) Electronic Liquid tank / on-site plant On-site plant Hydrogen (H 2 ) Electronic Tube trailer**/ Liquid tank / On-site On-site plant plant Helium (He) Technical Tube trailer** Tuber trailer** Trimethlyboron (TMB) mixtures Electronic Cylinder Cylinder Phosphine (PH 3 ) mixtures Electronic Cylinder Cylinder bundle Diborane (B 2 H 6 ) mixtures Electronic Cylinder Cylinder Methane (CH 4 ) Technical Cylinder Cylinder Argon (Ar) Electronic Cylinder bundle / Liquid tank Liquid tank Oxygen (O 2 ) Technical Cylinder Cylinder /Liquid tank Diethylzinc (DEZ) Electronic Drum /Tank Drum /Tank Disilane (Si 2 H 6 ) Electronic Cylinder Cylinder * PECVD chamber clean gas choice depends on process, scale and environmental considerations ** Tube trailers can contain up to 3,000 m3 of H 2 and 5,000 m3 of He 50 PES: Europe

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5 trimethyl boron (TMB), diborane (B 2 H 6 ) and phosphine (PH 3 ). This process also results in amorphous silicon deposition on other surfaces in the process chamber, such as the showerhead and chamber walls, which must be periodically cleaned. Typically a fluorine based etch process using NF 3, SF 6 or F 2 can be used for this purpose. Finally, nitrogen (N 2 ) must be used to dilute the pump lines. Another critical step is the deposition of a transparent conductive oxide (TCO) film on the top glass. This is typically tin oxide or zinc oxide deposited via sputtering or using an organometallic precursor such as Diethyl Zinc (DEZ). With gases playing such an important role, reducing cost of these critical materials is essential for the success of this technology. Such cost reductions can be achieved in various ways. On site generation of bulk gases: Due to the large scale of the gigafabs, the economics favor on site generation of major bulk gases such as hydrogen and nitrogen. This eliminates the transportation and delivery cost and enhances security of supply. Hydrogen is provided through Steam Methane Reformers or electrolytic cells. On-site hydrogen is the preferred delivery method for flows exceeding 150 Nm 3 /hr. Nitrogen can also be generated on-site via packaged N 2 -generators such as the one shown in Figure 4. Depending on location, this may be a cost effective solution for requirements as low as 500 Nm 3 /hr. Lowering cleaning costs: Up to 70% of the capital cost, and over 40% of the direct materials cost are related to the CVD process that deposits the active silicon layers. These CVD tools require frequent chamber cleans to remove silicon residue built up during the deposition cycles. Replacing the gases currently used (NF 3 or SF 6 ) by fluorine (F 2 ) can cut cleaning costs by up to 40%. Fluorine can be generated on-site using packaged fluorine generators such as those shown in Fig. 5. Increasing throughput: By utilizing F 2 based cleaning, the throughput of the CVD process may be increased by up to 10% at no additional cost. Additionally, additives in the silane gas may increase deposition rates and thereby increase throughput as well. Improving cell efficiency: The cell efficiency is strongly affected by the composition of the active p-i-n layers in the amorphous and microcrystalline steps. The cell efficiency may be improved by controlling critical impurities or through additives such as disilane. Silane cost reduction: Silane is the largest contributor to the cost of gases, and one in potentially short supply. Silane cost reduction can be achieved through recycling, reducing film thickness and improving film quality & device efficiency by the control of critical impurities. Since these changes impact critical process steps, the impact must be evaluated and the benefits weighed against potential risks. Dopant management: While dopants (PH 3 & TMB) are used in relatively small quantities, they are very expensive and are typically shipped as 0.5% mixtures in H 2. Since on-site H 2 generators are part of the gas supply solution, costs may be reduced by shipping pure dopants and making the blends on-site. Conclusion Clearly gases and chemicals contribute a significant part of the bill of materials for PV cells. As the industry grows rapidly material suppliers will face a number of key challenges supplying ever larger amounts of critical materials, managing safety and environmental issues, and developing materials technology that will both reduce costs and increase cell efficiency. To meet these challenges, material companies must therefore evolve from being a traditional supplier to become an integral part of the manufacturing industry. Authors Anish Tolia is PV Market Development Manager for Linde, specializing in thin film Si applications. His background includes roles with Applied Materials & Photon Dynamics in the semiconductor and TFT-LCD industries. Greg Bauer is Business Development Manager for Linde, specializing in wet chemical applications for c-si cell manufacture. Prior to joining Linde he worked for Ashland, Dow Chemical and BOC Edwards. For more information, visit Fig. 4 Typical on-site nitrogen generator Fig. 5 Typical on-site fluorine generator 52 PES: Europe