Will Copper Make Solar Power Competitive? Thin-Film CIS Photovoltaics Reduce The Cost Of Solar Cell Manufacture

Copper Applications in Innovative Technology

By Konrad J. A. Kundig

• Introduction
• But Solar Costs Too Much
• What Does All This Have to do with Copper?
• Fabrication Process Commercialized
• Market Implications
• Global Markets Beckon
• What Does All This Mean for Copper?

Introduction

U.S. electric power consumption is rising at the rate of about two percent per year. That's good for copper, because about one-half of all new copper is made into wire and cable products. But recent experience in California and elsewhere has taught us that growing power needs also have a dark side: they lead to power shortages when the infrastructure -sufficient generating capacity and the means to get the power where it's needed - hasn't kept pace with demand, as now seems to be the case.

Additional power plants, conventional or nuclear, will take years, and risks increasing the emission of greenhouse gases an/or running into stiff public opposition. Conservation is certainly a prudent option and reduces our dependence on imported fossil fuels, but long-term implications are problematic. Copper, with its superior electrical conductivity, helps conserve energy by improving efficiency, allowing us to do more work with available resources. Interested readers can learn more about this topic at CDA's Electrical Energy Efficiency section.

Where else might we turn? Today, combustion of fossil fuels accounts for around 70% of U.S. power generation; nuclear energy, 14.9%; hydropower, 14.6%; and so-called "renewable" sources like windmills, biomass fuels and solar energy, less than 1%.

Figure 1. U.S. Sources of Electrical Energy, 1999. Energy Information Administration, Department of Energy.

Find additional information in DOE publication: "Inventory of Electric Utility Power Plants in the United States 1999".

It may be time to take a closer look at those alternative energy sources, especially solar power. Why solar? Because solar offers a number of advantages, some of which surpass other alternative energy sources:

• Solar is a proven (although continually improving) technology. It is already in use worldwide.
• It emits no pollutants, and the technology needed to create solar facilities is likewise generally clean;
• Solar is visually and aurally unobtrusive, and installations can easily be integrated with existing structures, such as in rooftop arrays;
• It uses no moving parts and therefore requires little or no maintenance. Indeed, tests by the U.S. Department of Energy's (DOE's) National Renewable Energy Laboratory (NREL) at Golden, Colorado show that one type of solar cell - based on copper - has exhibited several years of service life without deterioration in properties, and
• It provides an acceptably predictable output.

Equally important is the fact that solar-based power generation facilities can be dispersed and located near existing power lines, thereby reinforcing the distribution infrastructure. Solar could, conceivably, complement a recently proposed alternative (in California) that would emplace small gas-fired power plants along the existing grid, thereby distributing the source of power, maximizing the efficient use of the grid and minimizing the risk of disruptions due to natural or man-made catastrophes. The big "if" is whether solar power can be made cost-effective.

But Solar Costs Too Much

Economics has always been solar's main obstacle. One important element here is the cost of the photovoltaic (PV) cells themselves, along with the cost of the fabricated modules, and the batteries, inverters, switchgear and other "balance of system" hardware that makes up a solar facility. Even though both manufacturing costs and consumer prices for solar gear have fallen dramatically over the past quarter-century (Figures 1 and 2), U.S. applications for multi-watt-sized solar devices remain primarily in remote locations, where they provide power for relay antennas, road signs, motorist call boxes, vacation cabins and marine buoys.

The reason that situation will largely continue—and the principal impediment to the growth of solar power—is that conventional power is cheap in the U.S., averaging around $0.085/kWh for residential customers and a few cents less for industrial and commercial users. Quite understandably, solar power has gained acceptance more readily in, for example, Japan ($0.207/kWh) and Germany ($0.161/kWh). It is also gaining acceptance in some developing countries where conventional power sources are scarce or nonexistent. Several extensive reviews of the technology and global economics of solar power are available on DOE Web sites.

Figure 2. The price of solar power modules has fallen dramatically since 1975, but costs are still too high to gain widespread acceptance for large-scale deployment in the U.S., where electricity is relatively inexpensive.
Source: U.S. DOE

Figure 3. Improving technology and increasing manufacturing capacity have driven down the cost of manufacturing solar modules.

Costs are now decreasing at an industry -average rate of approximately 5% per year, although Siemens' CIS thin-film technology promises to accelerate this rate significantly. Source: Witt, C.E., R.L. Mitchell, H.P. Thomas, M. I. Symko, R. King, Manufacturing Improvements in the Photovoltaic Manufacturing Technology (PVMaT, PDF-49KB) Project, July 1998, NREL/CP-520-24923. See the NREL site for more information. Back to Top

What Does All This Have to do with Copper?

The situation may change, however, and once again copper will play an essential high-tech role, thanks to an innovative development by Siemens Solar Industries (SSI), Camarillo, CA, the world's largest supplier of "conventional" silicon-based solar power devices and a unit of the multinational electrical and electronic equipment manufacturer based in Germany.

First, we should note that silicon, the stuff of transistors and other solid-state electronic devices, remains the most widely used PV material by far. But silicon has some disadvantages, including long-term stability problems and high cost. Constructing large-scale silicon solar collectors involves a number of labor-intensive batch-type processing steps, all of which must be performed nearly flawlessly if large-scale arrays are to function reliably and at expected performance levels.

Working both independently and in partnership with the DOE's Photovoltaic Manufacturing Technology ( PVMaT) Project, SSI has become a leader in the development of an advanced type of solar cells known as thin-film photovoltaics. There are a number of thin-film PVs currently in use, including several varieties under development at private and government laboratories, but Siemens has concentrated its efforts on a complex copper-indium-gallium-selenium intermetallic compound, Cu(In,Ga)Se₂, commonly known as CIS, or more recently, CIGS. (Technically, the latest and most efficient CIS is an alloy of indium- and gallium-containing copper diselenides, hence the "G" in the current acronym.)

Like other PV materials, CIS/CIGS is a semiconductor. That is, it acts like an electrical insulator until it is induced by various means to conduct electricity very much like an ordinary metal. One of those means is expose it to electromagnetic radiation - sunlight. CIS is an example of what are known as p-type semiconductors in which electrical conduction occurs by the movement of positively charged "electron holes" in the crystal lattice. Contacting the material with an n-type semiconductor (where conduction occurs by negatively charged electrons) creates a p-n junction, the basis for a successful solar cell. The junction ensures that current flows in only one direction, much as it does in a battery, with positive and negative poles at opposite surfaces of the composite film. Siemens' thin-film cell utilizes cadmium sulfide, CdS, as the n-type semiconductor.

Fabrication Process Commercialized

Siemens creates its CIS/CIGS film through a combination of electrochemical (plating), vapor deposition and annealing steps that produce a material with just the right composition and electronic properties. As it forms, the CIS/CIGS is deposited onto a polymeric substrate coated with thin layers of molybdenum (which serves as the positive (+) electrode) and copper (which improves adhesion to the CIS/CIGS).

Once the CIS/CIGS has been formed and annealed, the n-type CdS layer is deposited on it, again using an electrochemical process. Two layers of zinc oxide, ZnO, are then laid down. The first of these layers is very pure; the second contains a dispersion of aluminum oxide, Al₂O₃. The ZnO layers prevent short-circuiting, provide efficient current collection and enhance cell performance. At this stage, the composite film is only a few micrometers (1µm=0.000039 in) thick, or about 1/25th the thickness of a human hair.

A metallic grid through which sunlight can pass is then deposited over the ZnO layers. The grid acts as the negative (-) electrode. Finally, the assembly is covered with a tempered glass sheet that has been treated with an anti-reflecting coating akin to that found on thermal-barrier windowpanes. Electrical leads are attached to the electrodes, a mounting frame is provided, and the high-tech solar sandwich is complete. SSI's process development took place over many years, during which both the collector material (CIS/CIGS) and the deposition process itself underwent numerous improvements. SSI's crowning accomplishment has been to reduce this extremely complex combination of technical and economic factors to commercial practice. As of the publication of this article (2001), one production line is already in service, and several more will be added as demand for the new cells grows.

Market Implications

Bringing the new cells to widespread use will involve a number of additional factors, some of which are already in place, according to Arthur Rubin, SSI's marketing director. "Cost is the major impediment to growth in the U.S.," says Rubin. "At this stage, we've been able to reduce the cost of the completed module by about 10 percent, and we should be able to double that figure in the future. Efficiency - the amount of electrical energy produced for a given solar energy input - is another part of the equation since the user is ultimately interested in how much power the cell produces for a given capital investment. While our new cells exhibit nearly the same efficiency as silicon in finished modules, around 12%, small, laboratory-built CIS/CIGS cells have demonstrated nearly 18% efficiency in tests conducted at the NREL in Golden. That's currently the world record, but it will take further process improvement to approach such a high level of efficiency in production-scale units."

To explain those numbers, it is necessary to understand that there are basically three types of solar installations. The first and most viable type for home/business applications is one that is connected to the utility's power grid. In daylight, the solar panels generate 48VDC power, which is fed a grid-synched inverter and transformer, which feed power to the utility's distribution system. At night, power is returned to the consumer, who gets a credit for the power he/she has put into the grid. The second type of system is used for backup power for essential equipment. Here, the solar cells are used to charge batteries that energize the equipment. If AC power is called for, the system will incorporate an inverter. The third type of installation is for stand-alone applications such as roadside call boxes or DC motor-driven equipment. No inverter is needed in this case.

According to SSI's Rudin, "A typical home requires between two and four kW of peak power, which can be produced by between 50 and 100 of our 40-W modules. Current modules produce power for $0.25/kWh, which, for the average house and average installed costs translates to a payout of about 15 years. That isn't necessarily prohibitive because the cost of commercial power will continue to rise while the cost of solar comes down. We don't know when the crossover will occur, but studies show that a growing percentage of consumers are willing to pay more for "green" power - certainly here in California. Over 25 years, each kilowatt of peak solar power installed corresponds to avoiding the release of 49,500 lbs of CO₂, 125 lbs of NOX and 400 lbs of sulfur oxides to the atmosphere. Siemens Solar is promoting that paradigm with its Earthsafe™ systems for residential, office, retail and industrial users."

Global Markets Beckon

Rubin maintains that the generally higher commercial power costs seen in countries outside the U.S. will help drive the market for solar generation there. He notes that the U.S. currently maintains the lead in solar-power technology, but other countries are far ahead in deployment. For example, a government-subsidized program in Japan seeks to increase PV demand by 400 MW per year through 2010, and Germany wants to emplace 100 MW per year through 2005. SSI is currently supplying the solar modules for what will be the largest solar installation in the world: the 19,000 modules with a cumulative peak power of 2.3 MW for the Floriade 2002 international horticultural exhibition in Amsterdam. Worldwide, there was 175 MW worth of solar power generation equipment sold in 1999, and Siemens Solar sold 200 MW of cumulative power by 2000. Overall, solar power use will continue to increase at between 15 and 20% per year, according to government studies and Siemens' own estimates.

Here in the U.S., programs like the DOE's Million Solar Roofs program (PDF - 577MB), which seeks to reach its namesake goal by 2010, will help build awareness and demand. In addition, the Energy Policy Act of 1992 (EPAct, the law that also promotes the use of more efficient electric motors) provides financial incentives for utilities to subsidize installation of solar and other alternative energy sources through a program called the Renewable Energy Production Incentive (REPI). Such programs are in effect in several states. Other sections of EPAct provide for tax incentives to certain private-sector entities that install solar power facilities.

What Does All This Mean for Copper?

Obviously, CIS/CIGS solar collectors that are only a fraction of a hairbreadth thick won't require much copper. On the other hand, where there's electricity, copper will surely find new uses: for connectors, cables, inverters, transformers and perhaps even batteries. Wherever the efficient and environmentally friendly production, distribution and use of electrical energy are important, copper will play a role. As Arthur Rudin puts it: "Copper is a key technological element in all that we deal in." We couldn't have said it better.

Also in this Issue:

• Will Copper Make Solar Power Competitive? Thin-Film CIS Photovoltaics Reduce The Cost Of Solar Cell Manufacture
• A Case for the Role of Copper Deficiency in "Mad-Cow" Disease and Human Creutzfeldt-Jakob Disease

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