CSBurton TO41 

## FINAL DRAFT



# PFC-Emissions during Photovoltaic (PV) Cell Fabrication: A Scoping Report

Draft Report

April 2006

Prepared for: ICF Consulting 1725 Eye St., N. W. Suite 1000 Washington, D. C. 20006

Prepared by: C. Shepherd Burton, Ph. D. 2047 Huckleberry Road San Rafael, CA 94903 (415) 902-5504

ICF Subcontract No. 22J00074 ICF EPA Contract: GS-10F-0124J ICF Contract No. 023015 Task Order 041This page intentionally left blank.CSBurton i ICF Consulting

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# Table of Contents

1. Introducti on...........................................................................................................1-1

2. Solar cells—how they work, their efficiency , cost and size and the prospect for growth....................................................................................................................2-1
2.1. How solar cells work.................................................................................................2-1
2.2. Power conversion efficiency.....................................................................................2-2
2.3. Cost and the economic prize of solar electricity .......................................................2-2



### 3. PV Cell Manufact ure .............................................................................................3-1
3.1. Links to semiconductor and flat panel display manufacturing..................................3-1
3.2. PV Cells fabricated with x-Si ....................................................................................3-2
3.3. PV Cells fabricated with amorphous-Si....................................................................3-3
3.4. Emerging PFC-using fabrication technologies .........................................................3-5
3.5. Potential PFC emissions from existing and emerging PFC-using technologies.......3-6



### 4. Global PV Industry: Installed Capacity, Growth, Regional Shares of Production Capacity, PFC Emissions a nd Equipment S uppliers.........................................4-1
4.1. PV production growth...............................................................................................4-1
4.2. An initial estimate of uncontrolled, global PFC emissions for 2004 and 2010..........4-3
4.3. PV-cell equipment manufacturers ............................................................................4-6



### 5. PFC Emission Contro l Technologi es..................................................................5-1

6. Referenc es ............................................................................................................6-1



## List of Figures

Figure 1. Forecast of global PV-cell production, MWp

................................................................................ 4-1 Figure 2. Country shares of installed PV capacity

.......................................................................................... 4-2 Figure 3. Trends in regional PV production: 1995-2004

................................................................................ 4-2 Figure 4. Trends in regional PV-cell production, m2, 1995-2004

.................................................................. 4-3



## List of Tables

Table 1. Reported status of PFC-use du ring fabrication of some PV cells................................................3-9 Table 2. PV-cell equipment manufac turers...................

.................. .................. ................... ............. . ......... 4-8Table of Contents

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# 1. Introduction

Similarities between the photovoltaic (PV) industry on the one hand, and the semiconductor and active matrix liquid crystal display (AMLCD) i ndustry on the other, prompt concerns about the potential for uncontrolled atmospheric releases of high GWP greenhouse gases (GHG) during PV fabrication. Those similarities include use of (1) silicon and thin-film technologies (TFT), and (2) sustained high growth rates.

As this report will reveal, however, the analogy of the PV industry to the semiconductor and LCD industries cannot be pressed too far at this time: PV-cell technology, manufacturing processes and the associated infrastructure across the PV-industry lack the semiconductor and LCD industries’ unity of technologies (e.g., CMOS 1 and TFT technologies) as well as tight integration of R&D and manufacturing technology planning (e.g., International Technology Roadmap for Semiconductors and FPD Roadmap). The result is a relatively fragmented PV industry compared to the tightly coordinated semiconductor and LCD industries. 2 All three industries are highly competitive and aggressive.

This scoping report, after first summarizing the workings, cost, growth rate and manufacture of solar cells, examines actual and potential emissions of high-GWP GHG gases during PV fabrication. The report (1) identifies the processes that use high-GWP gases, (2) identifies emission factors (utilizations and/or emission/unit ac tivity); (3) prepares preliminary (worst case) estimates of current emissions for US, Japan and Europe; (4) provides published estimates of industry growth rates; and (6) describes available emission reduction technologies.

Information for preparing this report was obtained from searching the internet. Budget constraints prevented the author from contacting experts in PV manufacturing.

1 CMOS technology refers to complimentary metal oxide semi conductors. CMOS is the most widely used technology for

fabricating the transistors in today’s integrated circuits (IC) . CMOS technology is in almost every electronic product from handheld devices to mainframe computers. CMOS’ popularity stem s from its relative power demand; CMOS transistors consumer almost no power until switched.

2 It is also worth noting that semiconductor and LCD-TFT tec hnologies support and enable a seemingly ever expanding array of products that offer buyers a wide range of value proposition and, in turn, can support very large investments. The corresponding PV-technologies, however, support one product—the so lar cell—whose value seems highly constrained by the cost of conventional means for generating electricity.CSBurton 2-1 ICF Consulting

TO 041 April 2006 

# 2. Solar cells—how they work, their efficiency, cost and size and the prospect for growth

2.1. How solar cells work

Solar cells exploit the photovoltaic effect exhibited by semiconductor materials. When light of an appropriate energy (or color) strikes an electron attached to a semiconductor atom in the crystal lattice, the electron is freed to move in the lattice, which creates a “hole” (a region with a net positive charge) that can also move. These light-generated negative (n) and positive (p) charges are the constituents of electricity.

This process is described as the creation of mobile electron-hole pairs (or charge carriers). These charge carriers are then separated by an internal electric field formed when semiconductors with different electrical characte ristics come into contact. The electrical field drives positive and negative charges in opposite directions. That movement of charge through an external circuit is electricity (Landsberg, 2002).

While there are several ways to form an electric field in crystalline silicon, the most common is to slightly alter the structure of an atomically pre-altered form of the silicon crystal. This technique, known as doping, introduces an atom of another element (the dopant) into the atomically pre-altered silicon crystal to modify its electrical properties. This pre-altered form of silicon is called p-type, and will be described shortly.

The most common dopant in photovoltaic cells is a phosphorous atom, which occupies the same place in the lattice that was formerly o ccupied by the silicon atom that was replaced. However, phosphorous has one more electron than silicon that is free to move. If there are many phosphorous atoms substituted for silicon there are many additional electrons that are free to move. This type of silicon is called n-ty pe silicon because it has more (or extra) negative carriers (electrons) than normal silicon. The most common method of phosphorous-substitution is to apply the top layer of crystalline silicon with phosphorous and then heat the surface to force phosphorous to diffuse into the silicon crystal lattice. Other methods include gaseous diffusion, a liquid spray-on method and ion implantation (where high velocity phosphorous atoms are targeted at the crystalline silicon surface).

N-type silicon cannot form the electric field by itself, which is why the doping process starts with an atomically pre-altered form of silicon. This pre-altered form is a p-type silicon. P-type silicon consists of silicon that has been doped with boron atoms during the manufacture of the silicon crystal. When boron takes the place of silicon in the lattice, an electron is missing creating an electron hole. In this p-type silicon, then there are many more positive charges (or holes) than free electrons, so there are extra positive charge carriers. 3

3 It should be realized that n-type and p-type s ilicon are both electrically neutral: each material contains an equal number of negatively charged electrons and positively charged protons.CSBurton 2-2 ICF Consulting

TO 041 April 2006 It is the presence of layers of n-type and p-type silicon materials that creates the electric field. The field is strongest where the p-type and n-type silicon are in contact—the p-n junction. The strength of the field (the greater the difference in semiconductor electrical properties) is governed by the amount of dopant in the silicon. At the junction, the extra charge carriers— electrons in n-type and holes in p-type silicon—respond to the electric field the greatest. Electrons are attracted to and holes are repelled by the internal electric field.

The p-n junctions in PV cells are constructed to maximize the flow of charge carriers across the junction before they can recombine, which would result in no current flow. It is the construction of p-n junctions, including doping and many other factors that determine the efficiency of a PV cell to convert solar power into electrical power. 4



## 2.2. Power conversion efficiency

The dominant consideration in PV manufacture is providing the highest solar-to-electricity conversion at the lowest manufacturing cost.

The power conversion efficiency of a solar cell—the amount of power produced per solar power input—and the cost per watt produced, determines the commercial utility of solar cells. Efficiency is a ratio of the electric power (watts, W e ) output to light power input. Near the Earth’s equator at noon on a clear day, the solar radiation is approximately 1000 W s /m 2 (W s denotes the power from the sun in watts.). A solar cell of 1 square meter with an efficiency of 10 percent can, therefore, power a 100 W e light bulb (=0.1 W e /W s x 1000 W s ).

The average conversion efficiency in popular sola r cells (using the semiconductor silicon) is 13 percent, and varies depending on the semiconductor material used and method of fabrication (Wilcox et al., 2004). (The maximum theoretical e fficiency for a semiconductor is approximately 30 percent (Landsberg, 2002). For commercial devices, efficiencies vary between 5 and 17 percent (Wilcox et al., 2004).

Decreasing the cost per W p generated in the shipped system is a driving commercial metric of the industry. W p refers to peak watts produced, which occurs at mid-day. Increasing conversion efficiency at the smallest possible increase in manufacturing cost—so that the cost per W p generated is competitive with conventional sources of electricity—is one of two major R&D initiatives of the PV industry. The second major R&D initiative seeks the greatest reduction in manufacturing cost with the smallest (or no) loss in conversion efficiency.



## 2.3. Cost and the economic prize of solar electricity

The economic prize the solar industry seeks is penetration of the grid-connected electricity generation market 5 . Tying into the grid offers decades of sustained double digit growth. But to compete, the cost of PV systems must drop substantially.

4 In PV cells that use thin-film technology, p-n junctions are r eplaced with p-i-n junctions, where i refers to the intrinsic la yer, which is located between the p and n layers of the p-n diode. Mo re about this PV cell structure will be described later in the section on fabrication of thin-film PV cells.

5 The term grid-connected refers to both consumer and utility sides of the meter.CSBurton 2-3 ICF Consulting

TO 041 April 2006 Currently, the average installed price for PV systems is $4/W p , which is heavily weighted toward cells manufactured using silicon wafers. A ty pical residential system, designed to provide 2.5kW p , would cost, on average, $10,000 and occupy approximately 19 m 2 or more of roof area depending on sun exposure assuming a power density of 130 W p /m 2 (=12 W p /ft 2 ). A typical utility central plant, estimated at 5MW p , would cost $20M and would occupy a PV area of approximately 38,500 m 2 , using the same power density for residential and commercial applications.

Numerous reports extol the virtues of and opportuni ties for PV cells for producing electricity. Since the mid-1970’s, average efficiencies of commercial PV cells have more than doubled, from 5.8 to 13 percent, although since 1990 the ra te of improvement appears to have slowed to approximately 0.07 percent/year (Wilcox et al., 2004). Between the mid-1980s and 2005, the installation price of high power band (>70 W) solar modules dropped from about $30/W p to $4/W p today (Maycock, 2005).

Associated with these improvements in efficiency and reductions in cost, is a 60-fold increase in annual, global PV-module shipments, from approximately 20 MW p shipped in 1982 to 1200 MW p projected shipments in 2005 (Maycock, 2005). Of the total projected shipments for 2005, approximately two-thirds are for the grid-connected market, with the remainder divided among consumer, communication/signal, commercial and off-grid residential and rural markets. Based principally on maintaining current trends in cost reductions (approximately 5 percent per year), global shipments for 2010 are projected to reach 4000 MW p , with approximately 75 percent of those shipments expected to serve the grid-connected market (Maycock, 2005).

Notwithstanding its “green label”, an ongoing debate among energy policy makers is whether solar electricity is a boutique industry that is suitable in certain applications or whether it can become an essential part of electric generation, like natural gas, nuclear or coal. Even at compound annual growth rates of 30 percent since the late 1990s, solar electricity is far from making a significant contribution to global or US energy supply (Maycock, 2005).

The prize for the solar industry—and the prize that attracts investors and that encourages state and federal policy makers to continue and to expand existing forms of policy and financial support for PV industry expansion— is penetration of the grid-connected electricity generation market. The market is huge, providing the solar industry and its investor’s decades of sustained high double digit growth.

For example, PV systems shipped in 2004 made-up roughly 0.1 percent of US grid-generated power, after two decades of double-digit annual growth rates. The annual value of that 0.1 percent share was roughly $700 – 800 million (Chaudhari et al, 2004). At lower installation costs, say in the range $1 to $1.25/W p (a very aggressive price reduction of 75 percent below the current $4/W p price), Chaudhari et al. show that, for only US roof-top applications, penetration rates could reach approximately 1.6 percent by 2010, valued at $13.10 billion and divided almost equally between residential and commercial installations.

The principal determinant of this increased penetration is faster payback, which accelerates as installed prices drop (Chaudhari et al. 2004). While a 75 percent price reduction is aggressive (and probably unachievable with time to appreciably affect 2010 shipments), other assumptions made by Chaudhari et al. that govern PV demand are neither conservative nor optimistic; their and similar analyses have attracted inventors and investors, raising more that $120M in the US since 2001 (SF Chronicle, 11 July 2005).CSBurton 3-1 ICF Consulting

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# 3. PV Cell Manufacture

To attain the grid-connection prize, the PV industry is pursuing reductions in production/material costs; improvements in PV-cell yields, efficienc y (higher power densities) and lifetime; increased fabrication productivity/throughput; or some combination thereof. Available reports suggest the PV industry is pursuing many tracks, with, except for status-quo—PV cells fabricated with crystalline silicon—no clear favorite yet evident.

These multiple tracks are grouped, for the organizational purposes of this report, into two more or less homogeneous categories: improving the co st-performance of crystalline silicon (x-Si) technology (currently the dominant technology ) and thin-film technologies (currently an emerging technology) (Maycock, 2005). Crystalline Si technology accounted for approximately 90 percent of global PV shipments in 2004 (Maycock, 2005).

Both x-Si and thin-film technologies possess, as often cited by their advocates, synergies with semiconductor and FPD fabrication methods (Roca and Carabe, 2003; Wohlgemuth et al., 2003; Soppe et al., 2003; Shah et al., 2004; Agostinelli et al., 2004; Agostinelli et al., 2005; Bosch et al., 2004; Janz et al., 2004; Rentsch et al., 2005a; Rentsch et al., 2005b; and Kenanoglu et al., 2004). Despite the advocacy there is little evidence that the case has been heard and the technologies adopted.



## 3.1. Links to semiconductor and flat panel display manufacturing

Commercial PV fabrication grew out of 1980s microelectronics technology (the era of 4 and 5 inch wafers) 6 , relying on technology for manufacturing highly purified (and costly) crystalline silicon wafers as well the microelectronic industry’s batch, wet chemical technology for fabricating p-n diodes and electrodes. This linkage produced and continues to produce an upward pressure on PV costs at current PV-production volumes.

The high-purity silicon required for IC fabrication is unnecessary for PV fabrication but the PV industry is still too small to support breakthrough, cost-reducing alternatives. While the cost of high-purity crystalline Si wafers used in chip manufacturing is spread across hundreds of chips, just one PV cell can be produced for each wafer, assuming 100 percent yield; the result is that 40 – 50 percent of each solar cell is the wafer cost (Wilcox et al. 2004). In addition, the cost of modern semiconductor-device fabrication equipment also seems beyond the economic reach of PV manufacturers at current PV-production volumes. Nevertheless, the link to the microelectronics industry continues, with t he PV industry seemingly considering wide adoption of plasma etching and plasma enhanced chemic al vapor deposition (PECVD) technology.

That wafer cost would limit reductions in PV fabrication cost led the PV industry to seek alternatives, including moving to thinner wafers, to lower quality wafers as well as to using alternative methods for manufacturing crystalline silicon (e.g., as-grown sheets of silicon or

6 Four inch (100 mm) wafers were introduced in 1975; 5-in ch (125 mm) wafers were introduced in 1979. PFC-use in semiconductor manufacturing was commercially introduced toward t he end of the 6-inch (150 mm) wafer era and the first third of the 8-inch (200 mm) wafer era in 1988 or so.CSBurton 3-2 ICF Consulting

TO 041 April 2006 ribbons of silicon). 7 In addition, some manufacturers seem to be moving away from what are mostly batch processes to more in-line, integrated methods for PV-cell fabrication on x-Si as a way to lower overall cell cost (Phylipsen and Alsema, 1995; Wholgemuth et al., 2003; Soppe et al., 2004; Shah et al., 2004; Wolke et al., 2005).

Researchers, since the 1980s, have also studied thin-film solar module concepts, i.e., solar panels using thin-films (such as amorphous-Si (a-Si), CdTe and Cu (In,Ga) Se 2 (CIGS)) deposited on large, low-cost substrates (glass, stainless steel or aluminum). These researches extol the manufacturing synergies between thin-film solar panels and AMLCD panels, which utilize a-Si thin-films deposited on glass (Shah et al., 2004). Others are pursuing not only new kinds of (proprietary) thin-films but also new substrates (plastic) and new methods for applying (roll-on printing and painting) the thin-films to those substrates (Nanosolar, 2005).

Still other researchers tout the advantages of the PV industry adopting the dry processing, highthroughput processes of microelectronic manufacturing (Agostinelli et al., 2004; Agostinelli et al., 2005; Rentsch et al., 2005a, 2005b and 2005c), which has only been adopted in limited ways in PV cell manufacture using x-Si (Phylipsen and Alsema, 1995).

What is evident is that while increasing current global production at annual rates of 30 percent and while reducing average panel costs at 5 percent per year, the PV industry is pursuing many alternatives in search of breakthrough cost-reductions to attain the prize of grid-connection. That pursuit appears relatively complex and capital-starved despite evidence of relatively large government support.

The remainder of this section describes the essential steps in PV cell fabrication, first using x-Si technology and then a-Si technology. In each case emphasis is given to the actual or potential use of high-GWP gases during PV cell manufacture.



## 3.2. PV Cells fabricated with x-Si

Fabricating PV cells using x-Si technology starts with p-type (boron-doped) Si wafers. To make the n-p junction, phosphorus is diffused into the wafer. Then the back and front aluminum contacts are usually deposited by screen-printing and then fired. At the end of the process, or just after phosphorous diffusion, a single passivation and anti-reflection coating is deposited on the front (incoming sunlight) side of the ce ll (Phylipsen and Alsema, 1995; Lossen et al., 2005).

A typical x-Si production line follows 7 major steps:

7 For example, the PV industry is a large and growing user of pol ycrystalline silicon (consisting of multiple small crystals rat her a single cystral) from which monocrystalli ne silicon wafers are made for fabricati on of semiconductor devices. The demand for polysilicon by solar cell manufacturers has increased to the point that shortages in 200 mm and 300 mm wafers for semiconductor manufacturing are projected for 2006 and possibl y 2007 (LaPedus, EETimes, 2005 “Polysilicon shortages to cause wafer price increases”). Recent r eports describe a constrained supply of pol ycrystalline silicon such that PV production in 2006 will be but 5 percent above 2005 production levels, compared to recent annual growth rates of 30 – 40 percent (Pichel and Yan, 2006).CSBurton 3-3 ICF Consulting

TO 041 April 2006 (1) alkaline damage etch repair and texturing 8 —damage due to wafer cutting is repaired and texturing accomplished using a solution of NaOH (or KOH) and H 2 SO 4 ;

(2) emitter (n-type) formation by phosphorous diffusion;

(3) edge isolation to reduce current leakage (using either plasma [CF 4 ] or wet etching)

(4) phosphorous silicate glass (PSG, formed during n-p junction formation) etch using a solution of HF

(5) silicon nitride (SiNx) deposition using PECVD (SiNx film provides passivation and antireflection properties to cell)

(6) screen printing or metallization contacts/electrodes, and

(7) firing of metal contacts.

Two reports describe PFC use during x-Si PV-cell fabrication (Phylipsen and Alsema, 1995; Alsema et al., 1997). Production-line CF 4 -usage occurs during step 3. However, CF 4 -use is not universal because of reported plasma damage that lowers panel yield (Pylipsen and Alsema, 1995). Alsema et al. (1997) reports SF 6 -use in cleaning PECVD chambers used to deposit SiNx, which provides cell passivation and improved light capture through anti-reflection. (Similarly formed anti-reflection films are used in the photoresist steps during IC and AMLCD fabrication.)

It should also be noted that despite many repor ts describing the increased performance of PVcells that incorporate SiNx films technology, only Alsema et al. (1997) reports SF 6 use in chamber cleaning. Among all the other reports identified and reviewed in preparing this report, a method of chamber cleaning was not described (e.g., Liehr, 2001; Wohgemuth et al, 2003; Winderbaum et al., 2003; Shah et al, 2004; and Kumar et al., 2005). However, Fthanakis (2005) reported that PECVD chambers are usually cleaned manually 9 . Shah et al. does report that yield losses due to particle contamination during PV fabrication are not important unlike it is during AMLCD and IC fabrication (Small particles c an short TFTs in AMLCDs, for example and the particle problem during IC manufacture is a major failure mechanism for semiconductor devices. However, the presence of particles in PV fabrication would seem to be limited to reducing optical properties, which on average might be acceptably small and, if so, not a cause of PV-cell failure. Thus, unless in-situ chamber cleaning offers productivity increases, it may be simply a cost-ineffective cleaning method.)



## 3.3. PV Cells fabricated with amorphous-Si

Amorphous-Si solar cells, first developed in 1976, offer the advantage of manufacturing entire arrays of cells—instead of connecting individual solar cells from silicon wafers onto panels and wiring them together to form systems—with a si ngle series of steps on sheets of a low-cost

8 Texturing refers to surface texturization of x-Si to attain improved conversion efficiencies. Su rface textures diminish reflec tions losses and cause an oblique coupling of light into the silicon lattice. More net charge-carriers result and cell efficiency improves.

9 A confidential report to the author from a PV manufacture (that uses a-Si technology) boasts of PECVD chamber designs with shields around the substrate that captures 99.9 percent of the f ugitive film material before it reaches the chamber walls. In addition, the report notes that “walls (are designed) to be co ld and biased so that very little (fugitive) material deposits...an d cleaning (when needed) can be accomplished with HCl and hydrogen.”CSBurton 3-4 ICF Consulting

TO 041 April 2006 substrate, such as glass, metal or plastic. Vapor deposition methods are used to fabricate photodiodes (p-i-n diode) from several micro-thin layers of a-Si alloys on a transparent film of tin oxide (called the transparent conducting electrode, TCO), which has been previously deposited on the substrate (e.g., glass). The TCO acts both as a conductor and a means for increasing thepanels light absorption in the red portion of the spectrum. a-Si is one of several thin-film PVfabrication technologies, also called second-generation technologies, and the only one with the prospect of using PFCs because the other films do not react with F-atoms.

The disadvantage of a-Si is reduced and unstable efficiency. Light-induced degradation of aSi:H, known as the Staebler-Wronski effect, degrades performance over time unless more complex film structures are used to enhance stabilized efficiency (Wilcox, et al., 2004). In a-Si PV cells, the p-i-n junction is employed, with a-Si:H being the i-layer, where light absorption occurs. Numerous stacked-cell designs of varying a-Si alloys are used that are intended to enhance and stabilize efficiency with light-trapping. 10

The roughly 20-year old Chronar process (or some variation of it) seems in wide use for producing a-Si solar cells (EPV, 2005; France: National Status Report 2002, available at www.oja-services.nl/iea-pvps/nsr02/fra.htm ) . Which a-Si films (alloys) are used and how they are deposited account for most of the manufacturing process variability. EPV (formerly known as Chronar), uses “standard deposition techniques to build up” two vertically-stacked paired layers of p-i-n diodes (six a-Si layers total); EPV does not describe its standard deposition technique. Free Energy Europe and many other a-Si panel as well as equipment manufacturers use a plasma enhanced chemical vapor deposition (PECVD) monochamber to fabricate a single p-i-n a-Si diode (BP Solar, 200x, United Solar Syst ems Corp., 200x,, Plasma Quest Ltd, 200x, and Applied Film, 2005). However, they do not describe their methods of chamber cleaning.

Free Energy describes a 7-step process:

(1) Low pressure chemical vapor deposition of tin oxide (SnO 2 ) on glass (2) Laser scribing the tin oxide multi-cell interconnect layer (3) a-Si alloy deposition using PECVD to produce the p-i-n junction diode (4) Laser scribing of the a-Si alloy layers (5) Rear electrode fabrication; aluminum sputtering deposition (6) Laser scribing the aluminum layer to form the cell’s other interconnect traces (alternatively the aluminum sputtering might be performed using a mask instead of making a blanket film and then laser scribing) (7) Testing and aging to stabilize efficiency; encapsulation

What is evident in this process, compared to PV cells fabricated from x-Si, is the absence of etching. Just one step might use PFCs: the cleaning of the PECVD mono-chambers. However, chamber cleaning is not mentioned, as noted earlier.

10

The term a-Si alloys refers to silicon-carbon (a-Si:C:H), silicon-nitrogen (a-Si:N:H), silicon- germanium ( a-Si:Ge:H) and sili contin (a-Si:Sn:H) alloy systems. Which alloy is chosen depends on the desired band gap (which governs the wave length for generating charge carriers), among other things. For example, large band-gap materials serve as a front window in p-i-n junctions that prevents absorption of UV light while a low band- gap material permits absorption of red- and infra-red light in multilayer (2 to 3-stacked) p-i-n junctions.CSBurton 3-5 ICF Consulting

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## 3.4. Emerging PFC-using fabrication technologies

Recently researchers have expressed the need to move away from current practices of lowthroughput batch, wet processing towards automated in-line, high-throughput, dry processing for PV fabrication using x-Si (Wohlgemuth et al., 2003; Bosch et al., 2004; Shah et al. 2004). Dry processing is a generic term indicating little or no use of water during cell fabrication and usually refers to plasma technologies for etching and film deposition (Wohlgemuth et al., 2003; Shah et al., 2004, Agostinelli et al., 2004; Agostinelli et al., 2005; Rentsch et al., 2005a; Rentsch et al.,2005b; and Rentsch et al., 2005 c). Dry-processing also entails less physical handling of the xSi wafers, which, as wafers are made thinner to reduce cost and improve efficiency, reduces breakage and, therefore, increases yields.

However, it is important to note that dry processing may not always entail fluorine chemistries produced from PFCs or even the use of plasmas. For example in texturing x-Si PVs, reactive ion etching (RIE, a plasma etching method) may use either chlorine or SF 6 /O 2 chemistry. 11 Researchers seem to emphasize the use of SF 6 /O 2 chemistry, apparently because of the hazards of using chlorine (e.g., Cl 2 or BCl 3 ). Reports indicate, however, that the only fully dry etching process that is in full production is Cl-based RIE chemistry at Kyocera, the second largest global manufacturer of PV-cells (Inomata et al., 1997).

Laser ablation is another method for texturing x-Si under development by researchers, which may or may not include plasma technology (Rentsch et al., 2005c, Abbott et al., 2004). Texturization is accomplished by stepping and focusing the laser at regular intervals across the wafer, ablating Si at each step to the desired depth. While the ablation step has high throughput and good yield, the wafer surface must then be cleaned of residual debris (molten and recrystallized silicon). The surface may be cleaned via wet chemical- or plasma-etching (Rentsch et al., 2005c). For plasma-etching, which the Rentsch et al. preferred to wet-chemical, SF 6 /O 2 chemisty was used with microwave plasma. SF 6 utilzation rates were not reported (Rentsch et al., 2005c).

When researchers examine fluorine-based plasma chemistry etching, they unanimously acknowledge the concomitant emissions of PFCs and point to work of the semiconductor industry and the PFC-reduction partnership to identify alternative technologies for abating PFC emissions. As was noted at the start of this secti on, the aim of these researchers is to improve the productivity of PV cells fabricated from x-Si, the dominant (roughly 90 percent) solar panel fabrication technology.

Researchers also optimistically express that “it is reasonable to predict that dry processing will progressively make its way to production lines as the PV market size grows ... and that, as a side effect, this [growth] will contribute to provid[ing] momentum for PV thin-film technologies (emphasis added), which are largely based on large-area PECVD processes, in that the larger market volume for deposition systems will reduce the initial [higher?] investment costs [of the PECVD systems].” (Agostinelli et al., 2005). However, the reports identified and reviewed for

11 Chlorine atoms produced in a plasma react with surface atoms of Si to eventually form SiCl 3 , a gas under process conditions that can be pumped away. Similarly, fluorine atoms produced fr om PFCs react with surface Si atoms to produce SiF 4 , which is also a gas under process conditions (see Flamm, D. L., “Mechani sms of silicon etching in fluorine- and chlorine-containing plasmas”, Pure & Appl. Chem., 62 1709 (1990)). See also Kuo, Okajima and Takeichi, “Plasma processing in the fabrication of a-Si thin-film transistor arrays”, IBM J. Res. Develop. 43 73 (1999)who also describe the relative virtues of Cl- and PFCchemistries during Si-etching.CSBurton 3-6 ICF Consulting

TO 041 April 2006 this report would not support, at least for thin-film technologies, such across-the-board optimism for, at least, a-Si PV cells (e.g., BP Solar, 2002; NREL, 2002; Photon International, 2003).

BP Solar, a global leader in solar cell production, discontinued a-Si and cadmium telluride PVcell fabrication at plants in Virginia and California in 2002. In the summer of 2003, Dunasolar Photovoltaics Inc. of Hungary closed its a-Si PV production line which was preceded by CalSolar shutting down its California plant in late 2002.

However, CSG Solar AG, a new German company born out of a joint venture between CSG Solar Pty Limited and Pacific Solar Pty Ltd of Australia, is giving new hope to the commercial viability of second-generation a-Si thin-film sili con technology. CSG (which stands for crystal silicon on glass) Solar uses a single-c hamber PECVD system supplied by UNAXIS 12 to deposit SiNx- and a-Si-alloy-films onto textured glass. The a-Si is converted to its crystalline state by high-temperature annealing. A remote plasma sy stem, which generates H-atoms, is then used to hydrogenate the crystalline silicon, which removes numerous defects in silicon. The result, according to CSG Solar publications, provides commercial-quantity panels with an efficiency that exceeds 8 percent (Keevers et al., 2005; Bascore, 2004). CSG Solar does not describe its method of cleaning the single PECVD chamber. In January 2005, CSG Solar AG raised 24 million euros to finance construction and launch its first production facility designed with an annual output of 25 MW p (approximately = 312,500 m 2 assuming an efficiency of 8 percent, or 67 percent of the annual production capacity of one average generation 5 TFT-LCD panel manufacturing plant of which there are approxim ately 13 plants globally in production in 2005, (Burton, 2005)). Initial PV-panel production is expected in 2006 (CSG Solar AG, 2005).



## 3.5. Potential PFC emissions from existing and emerging PFC-using technologies

Phylipsen and Alsema appear to be the first to report PFC use during PV-panel fabrication (Phylipsen and Alsema, 1995). As part of a life-cycle assessment (LCA) of PVs, Alsema et al. report a CF 4 usage rate of 0.006 – 0.007 kg/m 2 PV-cell area (or 0.01 MTCE/m 2 ) during the edge-isolation step (see Section 3.2, step 3). Since no CF 4 emissions were reported, it appears the authors assumed that all of the CF 4 is consumed during cell fabrication, which is not the case in semiconductor etch process with CF 4 . A typical semiconductor etch process during semiconductor fabrication utilizes roughly 25 percent of the CF 4 , with 75 percent vented to the atmosphere unless abated.

Alsema et al. (1997), as part of additional LCA work, reported SF 6 use during PECVD chamber cleaning after PECVD deposition of a-Si films. A detailed report included an emission factor based on a single pilot line from PST in Putzbru nn, Germany (now known as RWE Schott Solar of Germany that markets ASI thin-film as well as x-Si solar panels globally) (Nieuwlaar and Alsema, 1997). The figure from Nieuwlaar and Alsema is 0.5 g SF 6 emitted/W p of manufactured panels, or 12 kg CO 2 equivalent/W p (= 0.5 g SF 6 x 23,900 g CO 2 -equivalent/g SF 6 /W p ). For a-Si PV panels with an efficiency of 0.05 (i.e., a 1 m 2 panel will generate 50 W p compared to 100 W p

12

UNAXIS is a supplier of PECVD equipment to the flat panel displa y industry. UNAXIS, however, is currently divesting itself of its FPD business (UNAXIS, 2005) and focusing that capability on PV-manufacture. UNAXIS’ PECVD systems are cleaned with an in-situ SF 6 plasma with a relatively low utilization that leads to large quantities of atmospheric emissions of SF 6 . Recent product descriptions of its KAI-systems fo r PV-cell production mention use of either SF6 or NF3 for chamber cleaning (see Sec. 4.3 of this report).CSBurton 3-7 ICF Consulting

TO 041 April 2006 if the efficiency were 0.1), Nieuwlaar and Alsema’s figure becomes 600 kg CO 2 equivalent emitted/m 2 of panels produced (=12 kg CO 2 equivalent/W p x 50 W p /m 2 of panels produced), 164 kg CE emitted /m 2 of panels produced (=600 x 12/44), 0.164 MTCE/m 2 of panels produced or 0.000164 MMTCE/m 2 of panels produced.

As noted in the previous section, recent reports describe the benefits of dry-process (plasma) technologies for manufacturing x-Si solar panels (Agostinelli et al, 2004; Rentsch et al., 2005a, b; Shah et al,. 2004). Some of the reports provide PFC emission factors, albeit in somewhat unfamiliar units.

Agostinelli et al. (2004) consider PFC-plasma dry- etching and texturing processes for x-Si solar cells. They assume only SF 6 usage and provide a factor of 11.9 MMTCO 2 emitted/GW p of peak power production. In a footnote, the authors indica te that emissions would be 40 – 80 percent of this factor, suggesting that 11.9 MTCO 2 /GW p is the PFC gas usage, not emissions. Taking the high end of this range for SF 6 emissions, gives an estimate of 9.52 MMTCO 2 emitted/GW p , or 0.338 MTCE/m 2 , where an average efficiency of 13 percent (130 W p /m 2 ) has been assumed (= 9.52 MMTCO 2 /GW p x 10 -9 GW p /W p x 130 W p /m 2 x [12/44] x 10 6 MTCE/MMTCE).

Rentsch et al. (2005a) describe the benefits of plasma technologies in three process steps during x-Si manufacture: texturing, dry-edge isolation and PSG etch. Sulfur hexafluoride is used in the texturing and dry-edge isolation steps, with oxygen included in the former. Perfluoromethane (with hydrogen) is used in the PSG etch step because the carbon provides the needed selectivity between PSG and Si (see also Rentsch et al., 2005b). The authors provide an estimate of the aggregate PFC emissions rate (both SF6 and CF4) for the three steps, assuming 80 percent utilization for each step. The figure, extracted from Fig. 7, is 4.6 g CO 2 E of emissions/kWh of electricity generated. Using the authors’ assumption of 850 kWh/W p , the aggregate emission rate can be restated as 3.9 kg CO 2 /W p , or 1.07 kgCE/W p . Further, using the authors’ assumption of 15 percent cell efficiency, that aggregate emission rate becomes 161 kgCE/m 2 , which includes the influence of PFC abatement. Using the authors’ unsupported and questionable assumption that SF 6 and CF 4 abatement systems can achieve 98 and 95 percent destruction efficiencies, and using their graph to factor the aggregate process emissions rate into its SF 6 and CF 4 components, the abated emissions factor (161 kgCE/m 2 ) becomes 7.48 MTCE/m 2 (an average abatement efficiency of 97.5 percent).

A recent comprehensive survey of European and U. S. PV manufacturers by Fthankis, Alsema and Agostinelli (2005) found that 40 – 50 percent of PV manufacturers use relatively small quantities of PFCs during etching crystalline silicon wafers and C 2 F 6 during chamber cleaning after deposition of SiNx films. 13 Their survey revealed no SF 6 usage by European or U. S. PV manufacturers. Survey results showed that for 2004, 30 MT CF 4 usage for European and U. S. manufacturers and that CF 4 accounted for 99 percent of all PFC usage in 2004, with C 2 F 6 accounting for the remaining one percent. Fthenakis et al. (2005) report that no SF 6 usage by European or U. S. manufacturers. Fthenakis et al (2005) also provide an aggregate emission factor of 0.008 MTCE/m 2 of substrate (which includes CF 4 and C 2 F 6 usage and can be disaggregated into CF 4 - and C 2 F 6 -specific factors using the mass distribution and global warming potentials from the IPCC Second Assessment Report).

13

Fthenakis, Alsema and Agostinelli did not have information from J apanese manufacturers in their survey. As will be shown in Section 4 of this report (see Fig. 3), Japanese manufacturers accounted for approximately 50 percent of global PV production in 2004 compared to 40 percent for Europe and Japan.CSBurton 3-8 ICF Consulting

TO 041 April 2006 Table 1 summarizes PFC usage during PV manufacturing as identified through this research. It is immediately evident from Table 1 that PFC use is limited to Si-based technologies, specifically etching and, in some instances, chamber cleaning processes.. Solar cells made from silicon accounts for more than 90 percent of 2004 production, however, furthermore, evidence was found for only SF 6 -, CF 4 - and C 2 F 6 -use. Although not depicted in Table 1, the recent survey of Fthenakis et al. suggests the CF 4 dominated PFC-usage in 2004. As previously noted, by excluding Japanese production, the Fthenakis et al. (2005) survey included only 40 percent of the world’s production capacity. Readers are cautioned not to assume that Fthenakis’ et al. survey results are representative of those in Japan. As described in section 3.4, Japanese manufacturers may be using Cl-based etchants rather than F-based etchants.

That PFC-use (i.e., F-containing etchants) is associated with PV cells fabricated from Si stems from the chemistry of Si and fluorine. Si material can be removed via its reaction with F-atoms to form gaseous SiF 4 , which is pumped away. Importantly and not unexpectedly, PFC-use was not a process gas used in fabrication of CdTe and CIGS thin-film PV panels. Fluorine does not react with these materials. CdTe and CIGS panels, over the long-term, are considered strong (if not favored) competitors of a-Si thin-film technol ogy because they provide intrinsically higher efficiencies than a-Si solar systems (5 percent vs. 11-14 percent for solar modules) (Wilcox et al., 2004). Despite the attractiveness of thin-film technologies (principally because of the lower material cost and the still-unrealized potential for producing large mass-connected collections of panels) they account for just 10 percent of global production capacity without any strong trend that this share is growing (Maycock, 2005).

Table 1 also summarizes the reported emission factors, for the processes indicated, as identified from this research. For purposes of completeness and convenience to the reader, emission factors associated with emerging PV fabrication technologies are included. However, because both the emission factors and technologies are speculative, they will not be discussed further. Instead, attention is given to the “In-production” category at the top of Table 1. For this category, the disparateness of the emission factors is striking: two orders of magnitude, with the lower factors corresponding to the recent survey of Fthenakis et al. (2005) and the larger factors from the earlier work (vintage 1995-1997) of Alsema and colleagues. Because the work reported by Alsema and colleagues considered but a few European manufacturers that included data from pilot production rather than full production it is considered less relevant to PFC-usage in today’s PV production. Thus, only the emission fa ctors from the survey of Fthenakis et al. (2005) will be considered when estimating global emissions.CSBurton 3-9 ICF Consulting

TO 041 April 2006 Table 1. Reported status of PFC-u se during fabrication of some PV cells

PV-cell fabrication technology Process step Gas Emission rate, MTCE/m 2 Source

In-production

x-Si Edge isolation CF 4 0.01 a Phylipsen and Alsema,
1995

x-Si Etching CF 4 0.007 Fthanakis et al (2005)

x-Si SiNx deposition, chamber cleaning C 2 F 6 0.001 Fthanakis et al. (2005)

a-Si SiNx deposition, chamber cleaning SF 6 0.164 Alsema et al., 1997

a-Si a-Si alloy deposition, chamber cleaning ? NA NA

Limited/prospective production

x-Si Texturing SF 6 0.338 Agostinelli et al. 2004

“ Saw damage removal SF 6 5.4 Rentsch et al. 2005

“ Edge isolation SF 6 1.4 Rentsch et al. 2005

“ PSG etch CF 4 0.7 Rentsch et al. 2005

a-Si a:Si alloy deposition, chamber cleaning SF 6 NA Shah et al., 2004

a-Si to x-Si film a-Si alloy deposition, chamber cleaning SF 6 ? NA Keevers et al., 2005

Bascore, 2004

a Represents a chemical usage rate. As noted in the te xt, the authors appear to assume that all of the SF 6 is utilized, which is unlikely. A r ough estimate of an emission rate is 75 percent of this value, or 0.008 MTCE/m 2 , which is based on an analogy between SF 6 etch rates during semiconductor device fabrication.CSBurton 4-1 ICF Consulting

TO 041 April 2006 

# 4. Global PV Industry: Installed Capacity, Growth, Regional Shares of Production Capacity, PFC Emissions and Equipment Suppliers

The PV industry and its advocates seek a significant place in the world’s portfolio of long-term sources of energy. That the industry can deliver and is delivering clean, reliable, on-demand power in current worldwide markets is undeniable. For some time it has been a “power choice” for those in remote areas. Contemporary circum stances position PV-production, therefore, fordouble-digit growth with the concomitant potential for increasing atmospheric releases of highGWP gases used during PV-manufacture.



## 4.1. PV production growth

PV production can point to two decades of 20-plus percent compound average growth rates in PV power shipments, exceeding 30 percent per year since 1996. Cumulative installed PV power grew at a compound average growth rate of 30 percent over the period 1992 to 2003 (IEA, 2004). Further, since 1996 actual shipments have exceeded forecasts by an average of 6 percent. A recent global forecast projects shipments will reach 4,000 MW p in 2010, up 4.2-fold over actual shipments in 2004 (Maycock, 2005). This forecast is shown in Figure 1. 14

At the end of 2003, approximately 1800 MW of PV power was installed globally (IEA, 2004) 15 . Over 85 percent of this capacity is installed in three countries: Japan (859.6 MW), Germany (410.3 MW) and the U. S. (275.2 MW) (IEA, 2004). Six countries account for another 10 percent of the cumulative installed capacity in 2003 (The Netherlands, Australia, Spain, Italy, France and Switzerland, in decreasing rank order) (IEA, 2004). Figure 2 shows the trends in global shares of cumulative capacity for the top nine countries over the period 1992 - 2003.

14

Maycock’s forecast will be used despite recent reports (see f ootnote 7 of this report) that PV production will be constrained by shortages of polysilicon. The projected gr owth by 2010 can still be achieved provi ded the additional supply of polysilicon does return during 2007 and demand for solar power remains,

15

IEA tracks installed capacity from reporting countries of whic h the total is 20: Australia, Austria, Canada, Denmark, Finland, France, Germany, Israel, Italy, Japan, Korea, Mexico, Nether lands, Norway, Portugal, Spain, Sweden, Switzerland, United Kingdom and USA.. Forecast of Global PV Cell Shipments, MWp

0 500 1000 1500 2000 2500 3000 3500 4000 4500

1 9 95 19 9 6 1997 1998 1999 2 0 0 0 2001 2002 2003 2 0 04 2

0 0 5 2006 2007 2008 2 0 09 2

0 1 0Annual shipment, MWp

Fig. 1 Forecast of global PV-cell production, MWpCSBurton 4-2 ICF Consulting

TO 041 April 2006 Of most interest to estimates of PFC use and emissions are trends in regional production capacity. Figure 3 presents trends in production (shipped cells, not capacity, which, while available, could not be obtained and analyzed within the task resources) for the top regions and the rest-of-the-world (ROW) for the 10-year period 1995 – 2004 (Maycock, 2005). Over this period, global production increased fifteen-fold. Interestingly, however, Japan dramatically increased her share from approximately 20 to 50 percent, with Europe maintaining a share of approximately 25 percent and the US loosing shar e, from over 40 percent to approximately 10 percent in 2004.

Country Shares of Installed Global PV Capacity

0.0% 20.0% 40.0% 60.0% 80.0% 100.0%

199

2 199 3 1994 19 9 5 19 9 6

1 99

7

1 99

8 199 9 200 0 2001 20 0 2 20 0 3Share of global total, % NLD

ITA

FRA

ESP

CHE

AUS

USA

JPN

DEU

Trend in Regional PV Production: 1995 2004

0% 20% 40% 60% 80% 100%

19

95 199 6 19 97 19 98 19 99 2 000 20 01 20 02 2 003 20 04Share of production, % of MWp

ROW
Europe
Japan

U.S. Fig. 2. Country shares of installed PV capacity

Fig. 3.Trends in regional PV production: 1995 – 2004

.CSBurton 4-3 ICF Consulting

TO 041 April 2006 

## 4.2. An initial estimate of uncontrolled, global PFC emissions for 2004 and 2010

Figure 4 shows total production figures expressed in units of panel area shipped for the period 1996 to 2004 (Maycock, 2005). In converting MWp to panel area, the global average efficiency for each year was used, assuming that average is applicable across each region. The global average efficiencies were taken from Wilcox et al. (2004). That the growth in panel area over the 1995 – 2004 period increased 14-fold rather than the 15-fold increase in MWp (mentioned above) is attributable to the increase in efficiency of the 1995 – 2004 period (more efficiency means more power
per m 2 of cell area).

The results in Figure 4 (for example for 2004) together with a suitable emissions factor may be used to estimate the uncontrolled PFC emissions provided emissions are directly proportional to area of panel produced. The findings reported in Chapter 3 support this proportionality, and the aggregate emission factor provided by Fthenakis et al. (2005) (in Chapter 3) will be taken as the best-available emission factor of uncontrolled PFC emissions.

Using the estimate of 0.008 MTCE/m 2 from the survey of Fthenakis et al. (2005), provides a means for estimating global PFC emissions fr om PV-manufacture. This emission factor is, however, applicable to PV-cell manufactured via crystaline-Si technology, which accounts for 95 percent of the 2004 total production (Maycock, 2005). Using the area of production information in Figure 4, together with the fraction of production that employs x-Si, gives 0.07 MMTCE (=0.95
x 9.125 x 10 6 m 2 x 0.0.008 MTCE/m 2 x 10 -6 MMTCE/MTCE).

This global estimate for 2004 of 0.07 MMTCE, however, neglects any contribution to PFC emissions from PFC-use a-Si technology, which accounts for 4 percent of total PV production in 2004 (Maycock, 2005). While, as was mentioned in Section 3.4), no recent evidence was identified in this review of PFC-use to remove a-Si films present on the walls of PECVD chambers and Fthenakis et al. report that C 2 F 6 is not used by a major user of a-Si technology, an estimate is nonetheless developed for purposes of completeness.

As a starting point for estimating global emissions from the a-Si sector of the PV industry, the emission factor for C 2 F 6 provided by Fthenakis et al. for cleaning PECVD-produced films formed during crystalline-Si PV-manufacture seems appropriate (see Table 1, where factor is 0.001 MTCE/m 2 ). If this emission factor is assumed applicable to chamber cleaning of PECVD-films during a-Si PV manufacture, then the corresponding PFC emissions for 2004 is 0.0004 MMTCE (=0.04 x 9.125 x 10 6 m 2 x 0.001 MTCE/m 2 x 10 -6 MMTCE/MTCE). The emission factor 0.001 Trend in Regional PV-Cell Production (m2): 1995 - 2004

- 1,000,000 2,000,000 3,000,000 4,000,000 5,000,000 6,000,000 7,000,000 8,000,000 9,000,000 10,000,000

1995 1996 1997 1998 1999 2000 2001 2002 2003 2004Area of panels shipped, m2

ROW

Europe

Japan

U.S.

Fig. 4.Trends in regional PV-cell production, m 2 ,1995 – 2004

.CSBurton 4-4 ICF Consulting

TO 041 April 2006 MTCE/m 2 from Fthenakis et al. may, however, im plicitly reflect the proportion of C 2 F 6 usage among those crystalline-Si PV manufacturers that use PFCs, which would tend to understate the actual emissions factor (per unit area of silicon consumed during PV manufacture that uses C 2 F 6 ). If it is assumed that only one percent of x-Si PV manufacturers use CF 4 and C 2 F 6 , the 0.001 figure of Fthenakis et al. should be multipli ed by 100, to give an emission factor of 0.1 MTCE/m2 of x-Si consumed during chamber cleaning.

This upwardly adjusted figure, however, needs further adjustment to account for the preliminary findings of this review that a relatively sma ll fraction of PV manufacturers seem to use a-Si technology to clean PECVD-produced Si-films with C 2 F 6 . In fact, it should be recalled that no reports were found indicating that C 2 F 6 was used in PECVD-chamber cleaning when a-Si technology was employed. If, however, it is assumed that as much as 20 percent of 2004 a-Si production capacity employed chamber cleaning with C 2 F 6 (which this non-exhaustive review could not identify), the upwardly adjusted emission factor Fthenakis et al. would drop to 0.02 MTCE/m 2 . Using this value and the information in Figure 4, an estimate of C 2 F 6 emissions from chamber cleaning within the a-Si PV production sector is 0.007 MTCE (=0.04 x 9.125 x 10 6 x 0.02 MTCE/m 2 x 10 -6 MMTCE/MTCE), however, it is recognized that this survey is not exhaustive.

An estimate of the global uncontrolled aggregate emissions from PV manufacturing in 2004 is 0.077 MMTCE (= 0.07 + 0.007 MMTCE), where 90 percent is estimated to originate from the use of x-Si technology to manufacture PV cells and 10 percent from the use of a-Si technology..

This estimate of global PFC emissions from PV production (for 2004 levels of production) is small compared to the corresponding uncontroll ed emissions from semiconductor and TFT-FPD manufacture. The estimate for PV manufacturing is but 0.5 percent of the corresponding global emissions from semiconductor and 5 percent of global emissions from TFT-FPD manufacture (Burton, 2005a, b).

Using these emission factors for x-Si and a-Si PV-manufacturing technologies, an estimate of uncontrolled, global emissions for 2010 can be completed. Maycok (2005) forecasts that by 2010, the global production PV capacity will have grown to 4,000 MWp. Wilcox (2004) projects that by 2010, the average efficiency of PV- cells will have increased from 13 in 2004 to 15 percent (i.e. 150 Wp/m2 of panel area), which represents a weighted average over all x-Si and thin-film technologies. Forecasts of PV-production by manufacturing technol ogy (x-Si, thin-film, etc) were not publicly available. It can be expec ted that PV-manufacturers that use thin-film technologies will endeavor to increase market share during the period of constrained supply of polysilicon. If it is assumed that Maycock’s 2010 forecast will be achieved and that thin-film technologies (specifically the a-Si technology) will approximately triple its share of production capacity by 2010 at the expense of x-Si technology, the share of a-Si capacity will grow from 4 to 13 percent with the corresponding share of x-Si capacity dropping to 86 percent (from 95 percent). The remaining 1 percent of production capacity is assumed to be employing thin-film technologies that do not use PFCs (which compared to x-Si and a-Si manufacturingtechnologies are considerably more costly albeit more efficient. Assumptions about technologyspecific improvements in efficiency are also r equired. Information from Wilcox (2004) suggestsCSBurton 4-5 ICF Consulting

TO 041 April 2006 that x-Si will increase to 160 MWp/m 2 while the efficiencies of cells made using a-Si will improve to 100 MWp/m 2

. 16

These figures together with the previously developed emissions factors, yields 0.3 MMTCE foran estimate of the global, aggregate 2010 uncontrolled PFC-emissions during PVmanufacture. 17

This estimate, which may be characterized as busi ness-as-usual amid an industry that wishes to be anything but usual, may be compared with the corresponding 2010 figures for semiconductor and TFT-LCD manufacturing. The corresponding figures for semiconductor and TFT-FPD manufacturing are 28 MMTCE and 8.7 MMT CE, respectively. One can only conclude that at current PFC-usage rates, uncontrolled PFC emissions from PV-manufacturing by 2010 will be small compared to the corresponding estimates for semiconductor and FPD display manufacturing. However, it should be noted that both semiconductor and FPD manufacturers have committed to achieving 2010 emissions by as much as 90-95 percent below their projected uncontrolled emissions, or approximately 2 to 3 MMTCE. Assuming those targets are achieved, the uncontrolled, aggregate emissions from PV-manufacture might then become 10 to 15 percent of the emissions from semiconductor and FPD manufacturing.

In concluding this section it seems worth mentioning that the PV equipment manufacturers do not yet seem to offer production-line dry-processing tools—the tools that would use PFCs and if used might emit uncontrolled PFC emissions—at sustainable business levels (e.g., UNAXIS recently announced it’s aim to divest itself of its PV-equipment division). If, however, PV-cell manufacturers concluded that dry processing could deliver on the promised lower PV-cell manufacturing cost and quality, it would take several years to ramp-up equipment production at levels to affect cell production. For example, both the semiconductor and LCD fabrication industries have coordinated infrastructures and experience in quickly ramping up production using new manufacturing processes. Using the ITRS for semiconductors as a guide, it is evident that it takes 5-years to go from a decision to use a new technology to putting it into production (and that production at that time accounts for approximately 50 percent of maximum production for that technology) (ITRS, 2004).

If this technology development and adoption rate could be achieved in the PV industry, which seems doubtful in view of its seemingly fragm ented uncoordinated infrastructure, then by 2010the incremental, uncontrolled, aggregate PFC emissions associated with the use of this dryprocessing technology could approach and exc eed the corresponding projected emissions from semiconductor and FPD manufacture if the emissions factors in the lower-half of Table 1 are assumed. Such an estimate is not provided because of its highly speculative nature.

16

Note that using these projected estimates of technology-s pecific PV-module efficiencies, the 2010 weighted average module efficiency is 150 MWp/m 2 (= 0.86 x 160 MWp/m 2 + 0.13 x 100 MWp/m 2 ), the average value forecasted (fortuitously?) given by Wilcox (2004) for 2010.

17

This estimate equals [0.008 MTCE/m2 x((0.86 x 4000 MWp x 10 6 Wp/MWp)/160 Wp/m 2 ) + 0.02 MTCE/m 2 x ((0.13 x 4000 MWp x 10 6 Wp/MWp)/100 Wp/m 2 ] x 10 -6

MMTCE/MTCE where the figures are taken from the discussion in the previous paragraph.CSBurton 4-6 ICF Consulting

TO 041 April 2006 

## 4.3. PV-cell equipment manufacturers

This chapter concludes with the findings of an initial review of equipment manufacturers that offer PV-manufacturers plasma-etch (reactive ion etch or dry-etch) systems as well as PECVD systems. This review seeks to identify companies that offer equipment to PV-manufacturers that employ x-Si and a-Si technologies and to asce rtain whether those manufacturers recommend or require PFC-use. It should be noted, however, that, like the early history of semiconductor manufacturing, the PV supply chain is fragmented. Some PV-manufacturers such as BP Solar, Kyocera, Sharp and Energy Photovoltaics (a mong others) appear to design and manufacture most if not all of the required PV-fabrication equipment internally, others, such as the ErSol Group and SCHOTT Solar, produce and design manufacturing equipment as well as purchase equipment provided by others.

As described in Chapter 3, while wet-etching is currently the predominant process for etching xSi substrates during PV-manufacture, the increased plant throughput promised by dry-etching appears to be attracting interest. As also noted in Chapter 3, however, RIE may beaccomplished with either F- or Cl-based chemistries for PV-manufacture, with, in the case of Fbased chemistry, the fluorine provided by PFCs (SF 6 or CF 4 , for example). So, one cannot infer that increased use of dry-etching technologies during PV-manufacture necessarily results in proportional increase in PFC-use.

PECVD systems, it appears, are already widely us ed to deposit SiNx (passivation) films in PVmanufacture (using x-Si or a-Si technologies) and to deposit a-Si (and crystalline silicon on glass, CGS) thin-films during fabrication of the photodiode. However, PFCs are not used todeposit either SiNx or a-Si films. Instead, PFCs may be used to occasionally clean depositionreactor/chamber walls (and other reactor components) of either of the fugitively deposited Sifilms. While cleaning these chambers with PFCs is the standard practice in both semiconductor device and FPD manufacture, it has not been, appar ently, the case during PV-manufacture.

Two European equipment manufacturers appear to be leaders in offering PFC-using equipment: Roth & Rau, based in Germany, and UNAXIS, based in Switzerland. Roth & Rau equipment uses PFCs (specifically SF 6 and CF 4 ) in x-Si etching processes to achieve reported throughputs of 600 to 800 wafers per hour for PV-cell manufacture (http:www.roth-gau.de/). Roth & Rau also offer PECVD equipment, however no information was identified indicating how the chambers are cleaned. Roth & Rau appears often at techni cal conferences but evidence of equipment sales was not found. UNAXIS, as a European supplier of thin-film deposition equipment to the semiconductor and flat panel display industries, has restructured and repositioned itself to service the solar industry, at the same leaving the flat panel display business. UNAXIS provides PECVD equipment for depositing a-Si and SiNx films, which uses SF 6 and NF 3 for in-situ cleaning of the deposition chambers. UNAXIS’ equipment (particularly the KAI-1200) is a key component of CGS Solar’s novel PV-cells (see Basore, 2003). Recently the ErSol Group and SCHOTT GmbH have also placed large equipment orders for UNAXIS’ KAI-1200. (see www.unaxis.com ).

BP Solar (the UK) and Kyocera (of Japan) are two of the largest global PV-manufcturers using wet etching. BP Solar, while an early leader in the adoption of PECVD SiNx films for improving panel efficiency and longevity, published information about reactor cleaning was not found.

The PV-equipment manufacturing industry in the U. S. appears to have, compared to its European-based competitors, a fragmented customer base. GT Solar, based in New Hampshire, while offering, among a large array of PV-equipment, wet and dry etch systems andCSBurton 4-7 ICF Consulting

TO 041 April 2006 PECVD systems, it does not indicate the use of PFCs. Plasma Etch, based in Nevada, and Kurdex Thinfilm Equipment, based in California, while offering dry etch and PECVD equipment to the PV-industry, appear to be suppliers to small, specialty manufacturers in the aerospace, electronics and semiconductor industries. Energy Photovoltaics, however, while offering equipment, seems to provide full-service, turn-k ey PV-manufacturing facilities that recently emphasizes a-Si technology that does not use PFCs.

Table 2 summarizes the companies identified in this brief review. Table 2 also indicates whether the companies specialize in x-Si or a-Si technologies.CSBurton4-8ICF ConsultingTO 041April 2006Table 2. PV-cell equipment manufacturers*Any boxes left blank indicate that the answer is unknownCompany Location WebsiteCrystalline (x-Si)or Amorphous(a-Si) Silicon?PFC useforEtching?PFC useforCleaningof PECVDchambers?BOC Edwards The United Kingdom http://www.bocedwards-cme.co.uk/ x-Si NoBP Solar The United Kingdom http://www.bpsolar.co.uk a-SiCentrotherm Germany http://www.centrotherm.de/index.htm x-SiEnergy Photovoltaics New Jersey (U.S.) http://www.epv.net/EPVhome.htm a-Si No NoGT Solar Technologies New Hampshire (U.S.) http://www.gtsolar.com/index.php x-Si No NoKurdex Thin Film Equipment California (U.S.) http://www.kurdex.com/index.html a-Si No NoKyocera Japan http://www.kyocerasolar.com/ UnknownOTB Solar The Netherlands http://www.otb-solar.com/index.html a-Si NoPlasma Etch Nevada (U.S.) http://www.plasmaetch.com/index.html x-Si YesRoth & Rau Germany http://www.roth-rau.de/ x-Si YesUNAXIS Switzerland http://www.unaxis.com a-Si No YesCSBurton 5-1 ICF Consulting

TO 041 April 2006 

# 5. PFC Emission Control Technologies

In the course of preparing this scoping report many studies were identified concerning the environmental impacts of PV-cell fabrication. The industry appears concerned that, in relying on its “clean” image to advance PV technology (no CO 2 emissions during the generation of electricity and just 2-years of equivalent CO 2 emissions during PV-cell manufacture), it is obligated to minimize the environmental impact during PV-cell fabrication. Some authors mention the use or potential use of PFC emissions, while others are silent on their use.

The seminal work of Phylipsen and Alsema is an interesting example. As discussed in Chapter 3, these authors present SF 6 -usage rates, but they tacitly appear to assume that all of the SF 6 is consumed—none is emitted. In more recent work, Alsema et al., who seem to track and to rank carefully the environmental impacts of alte rnative PV-manufacturing technologies for the industry, fail to even mention a potential PFC use in their recent reports. Fthenakis, who also publishes frequently on the health, environmental and safety concerns of PV-manufacturing alsofails to mention PFC use. A recent comprehensive study of the environmental impacts of PVfabrication, sponsored by EPRI and California Energy Commission, also fails to make mention of PFC use (EPRI, 2003).

Still others bring to the front and center PFC-use during PV fabrication as well as available technologies for reducing those emissions. The work of Agostinelli et al. (2004, 2005) and Rentsch et al. (2005a, 2005b) are examples. These authors, as previously mentioned in Chapter 3, rely heavily on published work by the PFC Reduction/Climate Partnership for the Semiconductor Industry. It seems, however, these authors may not fully appreciate that, because of the high flows that might be associated with PV-cell fabrication, the abatement technologies available to semiconductor manufacturing may not directly apply to PV-cell fabrication. Of importance here is the assumption by Agostinelli et al. and Rentsch et al. that the DREs reported by the PFC Reduction/Climate Partnership and its vendors are directly applicable to PV-cell fabrication. Nevertheless, these authors conclude that both plasma and combustion abatement are applicable.CSBurton 6-1 ICF Consulting

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# 6. References

Abbott et al (2004). M. Abbott, I. Mai and J. Cotter, “Laser Texturing of Multicrytsalline Silicon Solar Cells”, presented at 14 th International Photovoltaic Science and Engineering Conference , Bangkok, Thailand, 2004.

Agostinelli et al (2004). G. Agostinelli, H. F, W. Dekkers, S. De Wolf, G. Beaucarne, ”Dry Etching and Texturing Processes for Crystalline silicon Solar Cells: Sustainability for Mass Productions”, presented at 19 th European Union Photovoltaic Solar Energy Conference

, Paris, 2004.

Agostinelli et al (2005). G. Agostinelli, P.Choulat, H. F, W, Dekkers, S. De Wolf and G, Beaucame. “Advanced Dry Processes for Cystalline Silicon Solar Cells”, presented at 31 st IEEE Photovoltaic Solar Conference , Orlando, Florida, January 3 – 7, 2005.

Alsema et al. (1997). E. A. Alsema, A. E. Bauman, P. Hill and M. H. Patterson, “Health, Saftey and Environmental Issues for Thin-film Manufacturing”, 14 th European Photovoltaic Solar Energy Conference , Barcelona, Spain, 1997.

Applied Films (2005). “Applied Films Sells Firs t Sputtering System for Solar Cell Production”, February 10. 2005. available at www.appliedfilms.com.de.

Bosch et al (2004). R. C. M Bosch, W. T. W. Stals, A. J. M. C. Van Erven, R. L. J. R. Pennings and M. D. Bijker, “High-throughput PECVD Production Tool for In-line Silicon-nitride Deposition on Silicon Solar Cells”, presented at 19 th European Photovoltaic Solar Energy Conference

, Paris, France (2004).

Basore (2004). Basore, P. A. ”Simplified Processing and Improved Efficiency of Cyrstalline Silicon on Glass Modules”, presented at 19 th European Photovoltaic Solar Energy Conference , Paris, France (2004).

BP Solar (2002). See Reuters, ”BP Solar drops thin film solar cells, up to 260 US jobs”, November, 27, 2002.

Burton (2005a). “PFC Emissions Vintage Model for Flat Panel Manufacture: Projected Uncontrolled and Controlled Emissions 1995-2010”, prepared under Task Order 041, Subcontract 22DJ00074, December 20, 2005.

Burton (2005b). “PEVM Update”, completed under Task Order 041, Subcontract 22DJ00074, October 10, 2005

EPV (2005). Energy Photovoltaics, Inc. See www.epv.net/products/products.htm

EPRI (2003). ”Potential Helath and Environmental Impacts Associated with the Manufacture and Use of Photovoltaic Cells”, Technical Report pub lished by Electric Power Research Institute and California Energy Commission as part of Public Interest Energy Research Program (PIER), November, 2003.

Fthenakis, Alsema and Agostinelli (2006). Private communication to Scott Bartos, US EPA, February 2005.

IEA (2004). See www.oja-services -nl/iea-pvps/isr/index.htm

ITRS (2004). International Technology Roadmap for Semiconductors ...CSBurton 6-2 ICF Consulting

TO 041 April 2006 Inomata et al (2004). Y. Inomata, K. Fukui, K. Shirasawa, « Texturizing silicon surface using Chlorine”, Solar Energy Materials and Solar Cells , ( 48 ), p. 237, 1997.

Janz et al (2004). S. Janz, S. Bau, F. Lutz, C. Schetter and S. Reber, „PECVD-SI as Intermediate Layer for Cystalline Silicon Thin- Film solar Cells on Ceramic Substrates“, presented at 19 th European Photovoltaic Solar Energy Conference , Paris, France, June 7 – 11, 2004.

Keevers et al. (2005). Keevers, M. J., A. Turner, U. Schubert, P. A Basore and M. A. Green, “Remarkably Effective Hydrogentation of Crystalline Silicon on Glass”, Proceedings of 20 th European Solar Energy Conference Barcelona, Spain (2005)

Kenanoglu et al. (2004). A. Kenanoglu, D. Borchert, M. Rinio, M. Abusnina, and S. Diez, “Deposition of a-SiNx:H solar Cells at 13.56 MHz”, presented at the 19 th European Photovoltaic Solar Energy Conference , Paris, France, June 7 – 11, 2004.

Kumar et al. (2005). B. Kumar, T. K. Pandia, E. Streekiran, S. Narayanan, “Benefit of DualLayer Silicon Nitride Anti-reflectin Coating”, presented at 31 st IEEE Photovoltaic Solar Conference , Orlando, Florida, January 2005.

Landsberg (2002). P. T. Landsberg, “Theoretical Limits of Photovoltaic Solar Energy Production”, presented at World Climate & Energy Event , Rio de Janeiro, Brazil, January 6- 11, 2002,

Liehr (2001). M. Lier, “Keeping Pace with the photovoltaic market: Applied Films delivers the firt microwave PECVD system to Japan”, available at www.appliedfilm.com.de

Lossen et al (2005). J. Lossen, L. Mittelstadt, S. Dauwe, K. Lauer and C. Beneking, “Making Use of Silicdon Wafers with Low Lifetimes by Adequate POCl 3 Diffusion”, presented at 20 th European Photovoltaic Solar Energy Conference , Paris, France, June 7 – 11, 2004.

Maycock (2005). Paul D. Maycock, “PV Market Update:2004”, PV News, Vol 23, No. 3, March 2004. Information purchased via www.pvenergy.com

Nanosolar (2005). See www.nanosolar.com for descriptions of its roll-on and painting processes for PV cell manufacture.

Nieulwaar and Alsema (1997). E. Nieulwaar and E. A. Alsema, “Environmental Aspects of PV Power Systems, report on the IEA PV Task 1 Workshop , Utretch, The Netherlands, pp 25-27, June 1997.

NREL (2002). ”Nationial Center for Photovoltaic Statement on BP Solar Announcement”, National Renewable Energy Laboratory, Golden, Colorado, November 27., 2002.

Pacific Solar (2005) See www.pacificsolar.com

Phylipsen and Alsema, (1995). “Environmental life-c ycle assessment of multicyrstalline silicon solar cell modules” report prepared for Netherlands Agency for Energy and the Environment, Report No. 95057. September, 1995.

Pichel and Yan (2006). J. W. Pichel and M. Yang, “2005 Solar Year-end Review and 2006 Solar Industry Forecast”, Piper Jaffray, January 11, 2006. See also “Polysilicon Shortage Becoming Critical?”, Electronic News , July 11, 2005.CSBurton 6-3 ICF Consulting

TO 041 April 2006 Photon International (2003).; “Putting on a brave thin-film face”, Photon International: The Photovoltaic Magazine , August, 2003.

Rentsch et al (2005a). J. Rentsch, J. Jaus, Kristin Roth and Raif Preu, “Economical And Ecological Aspets of Plasma Processing for Industrial Cell Fabrication”, presented at Proceedings of 20 th European Solar Energy Conference Barcelona, Spain June 6-10, 2005.

Rentsch et al (2005b). J. Rentsch, C. Schetter, H. Schlemm, K. Roth and R. Preu, “Industrialization of Dry Phosophorous Glass Etching and Edge Isolation for Crystalline Silicon Solar Cells”, presented at 20 th European Solar Energy Conference Barcelona, Spain June 6- 10, 2005.

Rentsch et al (2005c). J. Rentsch, F, Bamberg, E.l Schniederlochner and R. Preu., „Texturing of Multicrystalline Silicon by Laser Ablation“, presented at 20 th European Solar Energy Conference Barcelona, Spain June 6-10, 2005.

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Shah et al (2004). A. Shah, J. Meir, a. Buechel, U, Kroll, J. Steinhauser, F. Meillaud and H. Schade, „Towards Very Low-Cost Mass Production of Thin-film silicon Photovoltaic (PV) Solar Modules on Glass” presented at ICCG5 Conference in Saarkbrucker, Germany. July, 2004.

Soppe et al (2004). W. J. Sopper, B. G. Duijvelaar, S. E. A Schiermeir, A. W. Weebar, A. Steiner and F. M. Schuurmans, “A High Throughput PECVD Reactor for Deposition of Passivating SiN Layers”, presented at 19 th European Photovoltaic Solar Energy Conference

, Paris, France, June 7 – 11, 2004

United Solar Systems (2004) “Amorphouse Silicon (a-Si) Solar Technology” from United Solar Ovonic Corp. (2004). See www.uni-solar.com..

Wholgemuth et al (2003). J. Wohlgemuth, M. Narayanan, R. Clark, T. Koval, s. Roncin, M. Bennett, D. Dunningham, D. Amin and J. Creager, Large-scale PV Module Manufacturing using Ultra-thin Polycyrstalline Silicon Solar Cells”, presented at WCPEC-3, Osaka, Japan, May 2003.

Wilcox et al (2004). A. Baumann, Y. Bhargava, Z. X. Liu, G. Nemet and J. Wilcox, “PhotovoltaicTechnology Review”’ available at http://ist-socrates.berkeley.edu/~kammen/C226/BerkeleyC226-PVTechComp.pdf

Winderbaum et al (2003). S. Winderbaum, A. J. Leo, S. P. Shea, T. D., Koval and B. Kumar, “Characterization jof Industrial-scale Remote PECVD SiN Depositions”, presented at WCPEC-3, Osaka, Japan, May 2003.

Wolke et al (2005). W. Wolke, J. Catoi, G. Emanuel, J. Liu, M. Ruske and R. Preu, “Surface Passivation for Solar Cells by Large Scale Inline Sputtering of Silicon Nitride”, presented at Proceedings of 20 th European Solar Energy Conference Barcelona, Spain June 6-10, 2005.CSBurton 6-4 ICF Consulting

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