New Hampshire, United States [Photovoltaics World magazine] The introduction of off-line systems will revolutionize the way that transparent conductive oxide (TCO) glass end-users can run their business.
The benefits of atmospheric pressure chemical vapor deposition (APCVD) for TCO-coated glass have been clearly demonstrated, and the opportunity to apply this technique to TCO thin film depositions has helped to improve cost and availability of glass products.
|Figure 1. Function of a Low-e window.|
The introduction of off-line systems will revolutionize the way that transparent conductive oxide (TCO) glass end-users can run their business. Presently, commercially available systems for deposition of TCO thin film coatings come in two forms:
- Physical vapor deposition (PVD) systems, specifically, sputtering systems, where an Ar-ion cloud bombards a target to strip material from the target, and allows it to deposit on the substrate;
- On-line (integrated with float glass lines) APCVD systems, which utilize heat to convert liquid or precursors into durable solid thin films. This technique is presently limited to deployment by glass manufacturers.
Offering lower cost operation, significantly lower power consumption, and superior film properties, APCVD is a compelling option for the glass industry. Any business is cost-sensitive, but consumers of TCO-coated glass are in the business of sustainability and alternative energy, meaning that in addition to cost savings, lower power consumption has a special incentive. Currently, there is no commercially available APCVD coating equipment that would allow the lower cost and off-line functionality necessary for a realistic effort on the part of TCO glass end-users to embark on a path of independence.
|Figure 2. Cost for processed glass.|
TCO-coated glass has found many applications in the last decade and has created a multibillion dollar market. The main applications are energy saving, low scattering (Low-e) coatings for architectural windows (Fig. 1), and high scattering TCO coating on glass for a wide variety of photovoltaic (PV) thin film modules and TCO-coated glass for display applications. The worldwide expected demand for TCO glass by 2012 is greater than 500 million m2 per year, which represents a market opportunity for TCO-coated glass well above $10B per year.
The two primary TCO coating technologies used to coat glass on a large volume scale are APCVD and PVD. In general, APCVD manufactures much more durable TCO coatings than PVD, consumes between 10-50% of the energy, and produces coatings at less than half the cost (Fig. 2). Traditionally, the two compelling reasons for PVD have been availability of commercial systems and easier process changeover. Moreover, the complexities of the APCVD systems meant they were out of the technical reach of TCO glass end-users. But a commercially available off-line APCVD system, from a company with decades of experience in a myriad of CVD applications, overcomes both challenges to adoption, and lets end-users enjoy all the benefits of the APCVD method.
There are several benefits from using APCVD:
No vacuum. APCVD does not require a vacuum
(low pressure) atmosphere as does PVD; thus, a lower capital cost.
- Lower operating cost. APCVD uses gas, liquid chemical sources and heat to produce a thin film, compared to expensive sputtering targets, plasma, and vacuum used by PVD, resulting in up to 50% lower operating costs.
- Film quality. APCVD-deposited TCO films have better hardness/adhesion.
TCO films deposited using PVD (AZO) are highly susceptible to physical damage and moisture due to their relatively low chemical stability against weak acids. For this reason, these TCO coatings force certain operational constraints upon end-users, including:
1. Need for hermetic sealing. PVD sputtered coatings must be hermetically sealed within a mechanical structure. A sputtered TCO film cannot be on the outside face of the final glass product. Moreover, it must be contained within the interior of the final assembly within three weeks of manufacture, or degradation of the TCO film will occur and its beneficial properties severely reduced or negated. In the case of thin film solar module applications, degradation translates directly to reduced lifetime. In both Low-e windows and solar modules, relatively common seal failures have a direct impact on energy performance because these films are not resistant to common atmospheric conditions.
TCO films made by APCVD are much harder with much better adhesion, allowing the final window, display, or solar panel to be assembled in whatever manner is technically or operationally superior—on the inside or outside of the assembly. There are no shelf life considerations before assembly, and the panels will continue to deliver energy performance even in the event of a seal failure.
2. Higher stability. APCVD TCO glass can be ordered well in advance due to its much higher stability. When ordering glass material, which is already coated, the end users of TCO glass (i.e., manufacturers of windows, displays, or thin film solar modules) can order larger volumes without the concern of a three-week shelf life, as with PVD sputtered TCO glass.
3. Less sensitivity. Less sensitivity to moisture and physical handling means that APCVD-coated glass can be bought in any size and cut to order at the panel assembly company.
Many of the aforementioned benefits apply equally well to manufacturers and end-users of sheet glass. Moreover, the benefits of APCVD TCO coatings are compelling, whether deployed as an on-line or off-line system. Indeed, such systems have been commercially available for some time (in an on-line form factor only), and glass manufacturers and their customers have been well served by this availability.
It is also recognized that the on-line APCVD system offers certain compelling benefits, including:
- Pre-heated glass. The glass is already heated to APCVD process temperatures from the float glass manufacturing process;
- Clean glass. There is no pre-deposition cleaning step required since the glass arrives clean to the deposition area;
- Footprint. There is no need of separate floor space;
- Recycling. Defective glass can be recycled and reused as raw material for glass manufacturing since it is still at the manufacturing facility when coated.
Float glass line limitations
But the potential mismatch of the line speed of a float glass line and the process window of a chosen APCVD process mitigates these advantages. Generally, this limits the APCVD film deposition choices for a given on-line APCVD system. With precursor selections for APCVD improving all the time in response to evolving market demand, the narrow line speed window of a given float glass line severely limits the ability to adjust to market demands. In fact, with a constant pressure to make glass thinner for PV markets, float glass lines (which run at a constant line speed inversely proportional to the glass thickness) will be even more likely to be limited by the coating process going forward.
At present, APCVD TCO films with the highest performance specifications (8Ω/?) are only available from glass manufacturers deploying an off-line coating strategy that allows them the freedom to deposit a thicker film than is commercially practical for an on-line system. Ultimately, this on-line dynamic will logically force a compromise in the form of thinner film depositions and/or lower performance parameters.
CVD is a process that operates under the combination of four physical attributes: temperature, pressure, deposition time and precursor flow rate. The first two variables are absolutely fixed for an APCVD on-line system; temperature depends on the location of the APCVD system on the float glass line (typically, 700-500º C), and pressure is atmospheric. The deposition time depends on the line speed, total deposition area, and the precursor flow rate(s). If an inappropriate parameter region is selected, the deposition modules must be moved off-line more often for cleaning/servicing, leading to a limited uptime. Moreover, changeover of precursors for various applications, and the need for multi-film layers for solar applications on a moving line is clearly challenging (Fig. 3).
Due to these considerations, it is clear that only an off-line APCVD system, which additionally allows tuning of the line speed to the optimum process window of a chosen APCVD precursor chemistry/system, has the power to change the way the market operates. Such equipment will be commercially available starting in 2010.
Historically, APCVD TCO glass has only been available by online APCVD deposition systems, which are mostly home-built by the major players in the glass manufacturing industry (although commercial systems are
Figure 3. TCO use in solar cells.
recently available). Indeed, the benefits of APCVD coating technology are available to those willing to build a new $100-$150M float glass line and to add an online APCVD system, either purchased or developed in-house. Therefore, end users have only had the choice of purchasing TCO-coated glass from a glass company with on-line APCVD coating technology, or to purchase an off-line sputtering TCO coating system and live with its limitations.
Because the supplier who has made the past investment is presently depositing TCO with a technology offering far superior quality and lower cost than anything available to the typical end-user, end users are left with little choice. The significant premium they pay for TCO-coated material—~$15-$35 /m2—is the result of this predicament.
Off-line turnkey solution
End users will soon have a new option—a commercially produced, off-line APCVD turnkey system, which clearly lays the groundwork for a paradigm shift in the business model for these end users. Depending on market conditions and business considerations, end users can buy their glass with TCO coating, or buy unprocessed glass and coat internally with their off-line systems. This approach is especially productive for smaller to mid-size volume users, allowing a glass end user to deposit custom-tuned TCO for Low-e architectural glass, or thin film PV modules as per their own specifications and customer demand.
Chemical vapor deposition (CVD) generally offers superior thin films compared to PVD. But bilding such a system is not trivial, and this complexity has been a significant impediment to the adoption of CVD for TCO applications, since only companies with very significant resources could successfully build, ramp, and integrate such a system.
The benefits of APCVD for TCO-coated glass have been clearly demonstrated for some time, and the opportunity to apply this technique to TCO thin film depositions has helped to improve cost and availability of glass products requiring these films. Allowing this technology to move downstream is taking it to the next level. It is only by providing a method for off-line APCVD to go downstream to the end user that we can really open up multiple opportunities in terms of operational flexibility.
Today, window and PV panel manufacturers have three choices: clear glass for ~$2/m2, Low-e glass for ~$15/m2, or PV TCO glass for ~$35/m2. Thanks to the availability of a professionally designed and built off-line APCVD system with a predictable process ramp and high tool availability, these end users—now vertically integrated—can sell their TCO-based products with a cost basis closer to one-third to one-half of the usual numbers.
Michael J. Gray received his BA degree from Fordham University, NYC, and is VP of sales and marketing at CVD Equipment Corp., 1860 Smithtown Ave., Ronkonkoma, NY 11779 USA; 631-981-7081; [email protected]
Karlheinz Strobl received his PhD through Max Planck Institute/U. of Innsbruck, and did post doctoral work at Los Alamos National Laboratory. He holds an MBA from Boston U., and is VP of business development at CVD Equipment Corp; 631-981-7081; [email protected]