Thin-Film Photovoltaics Capture More of the Spectrum

Silicon-based photovoltaic (PV) cells have been used for decades in calculators, and have come into wide use for garden lighting, but the industry is just beginning to ramp up for increased demand on a power-grid level. Crystalline silicon (c-Si) technology — comprising both multicrystalline and monocrystalline — makes up some 90% of the PV market. But thin-film solar cells are on the rise for several reasons, not the least of which is a polysilicon shortage that has kept an already rapidly growing solar industry from realizing more of its potential.

Around 2004, the amount of silicon being used to make solar cells equaled that being used to make ICs. Solar has since surpassed ICs, and has been facing stilted growth because of the difficult polysilicon supply situation. Although polysilicon suppliers are ramping up production and the materials are becoming more readily available,¹ thin-film solar technology offers a way to bring polysilicon use to a minimum. Thin-film silicon panels use just a thin layer of amorphous silicon (a-Si) on a glass substrate (Fig. 1) rather than the bulk silicon substrate of c-Si.

1. Thin-film silicon panels, like those shown here, use just a thin layer of amorphous silicon (a-Si) on a glass sub-strate rather than the bulk silicon substrate of c-Si. (Source: Oerlikon Solar)

Other thin-film technologies use no silicon at all, relying instead on materials such as cadmium telluride (CdTe) or copper indium gallium selenide (CIGS). Although CIGS has shown promising results in the lab, it has not yet reached commercialization on a significant scale. Meanwhile, First Solar (Tempe, Ariz.) is having great success with thin-film CdTe modules, reporting its ability to lower the cost per watt to <$1.25.

Lowering solar module cost to $1/W has been the brass ring for the industry because it's approximately at this point that solar energy will reach grid parity — the ability to economically compete with traditional fossil fuel energies. In some places, grid parity is coming soon or has already arrived, according to a recent talk by Mark Pinto, senior vice president and general manager of Applied Materials' Energy and Environmental Solutions (Santa Clara, Calif).²

Government incentives throughout the world contribute in a way to lowering the total cost, but the goal is to be able to reach grid parity even without such help. What the incentives have done is speed up the timetable, according to Charlie Gay, corporate vice president and general manager of Applied Materials' Solar Business Group. He noted that this is particularly true in Germany, where feed-in tariffs essentially guarantee a price for the electricity to be paid out over 20 years. "The amount that will be paid declines at about 8% per year so that the solar business has to continue to reduce its prices because the economics for an individual to put solar on their roof are dictated by that long-term power purchase agreement, how it's structured," Gay said. "So there's a direct correlation between the scaling of the industry, the lowering of the size of those incentives year by year, so that we can basically be freestanding, directly competitive with bulk power in the not very distant future."

In the near term, it's very important to have policies that support the transition to the next phase of market growth, where we are beyond grid parity, argued Chris O'Brien, head of market development for North America for Oerlikon Solar (Trübbach, Switzerland). "We're higher than grid electricity today in most markets. And so, if the policies stop today...there would be a significant stop in new investment to drive down the cost of solar to the point where that grid crossover happens sometime over the next several years."

Working in solar energy's favor, however, is the fact that the cost of fossil fuels will continue to rise. "The price of coal had been assumed to be quite stable because we've got very large reserves of coal," O'Brien noted. "Well, in fact, the price of coal has increased substantially because of the increase of other energy fuels." Also, in Washington, there's widespread confidence that climate legislation will be enacted to put a cap and trade restriction on carbon emissions, O'Brien added. "That could add 2–4 cents per kilowatt hour to the price of generating from conventional fossil fuels."

Much of the ability to get to grid parity comes with the help of veterans of the semiconductor and flat panel display (FPD) industries — bringing to the solar industry proven technologies to improve manufacturing efficiencies and thereby lower the total cost of the modules. Two of the semiconductor industry's biggest equipment makers — Applied Materials and Oerlikon — are putting their focus on large-panel thin-film silicon solar technology. Oerlikon Solar has a promising micromorph tandem cell that adds a microcrystalline absorber to the a-Si layer. Applied's SunFab line produces 5.7 m² panels to reduce the cost of manufacturing and installation.

Thin-film solar, currently accounting for ~10% of the solar market, will likely grow to 15–20% of the total solar PV market within the next 4–5 years, according to Jeannine Sargent, CEO of Oerlikon Solar. "Solar PV is growing well over 50% compounded annual growth rates, but thin-film solar is actually growing probably closer to 80–100% compounded annual growth rate over the next five years," she said.

Although thin-film silicon does not achieve as high efficiencies as c-Si (typically 6–8% vs. 13–19%), it has other benefits that make it appealing. "Fundamentally, we believe the cost performance of thin film has been desired almost over 10–15 years, but there were not technologies and products available to make it production-worthy for high volume," Sargent said, adding that more recent innovations have enabled volume production levels. "And when you look at the cost performance of thin film against crystalline silicon, even today, with what I'd call early market entry of thin-film silicon, it already outperforms crystalline silicon in total cost of ownership."

In addition to thin-film panels being cheaper to produce (largely because of much lower substrate costs), they perform better in some locations. Although module manufacturers often rate performance based on watts, or power output, the end market is more interested in the kilowatt hours of energy the module can provide, and that is significantly influenced by the panel's location on the planet. "One of the advantages of thin-film technologies is that it performs better even in diffuse light," Sargent said. "So that even in cloudy areas or less sun-filled areas like Germany, for example, where you see a lot of the thin film and a lot of the solar market, you get more effective kilowatt hours produced using thin-film technologies than you do with crystalline."

2. The roof of the Stillwell Avenue station in Brooklyn, N.Y., consists of thin-film solar modules that supply about two-thirds of the station’s electricity. (Source: Schott Solar)

The temperature coefficient of the cell is also better for thin film, Sargent explained, meaning that, as the temperature goes up, the effective efficiency of crystalline goes down. "So if you think about these large solar parks in deserts, whether it be in the Mojave Desert in the United States, or in Abu Dhabi or Dubai, actually crystalline silicon has a less effective performance in high-temperature regions."

Making the thin-film panels large makes further economic sense, particularly for the gigawatt-level solar farms for which they are best suited. Larger panels save money on both ends of the equation — manufacturing and installation. Much the same way the semiconductor industry has improved cost and productivity by moving to progressively larger wafer sizes, making a larger solar panel has cost benefits. "Each time a robot moves a part, the cost to move is the same, whether it's moving a big part or a little one," Gay said. "If you're going to have to move something, you may as well move a big thing as a small thing. You need to spend time aligning it; you need basically the same mass flow controllers associated with a big chamber as a small chamber."

By the same token, larger solar panels save time and cost during installation. Roughly speaking, half the cost of a solar system is lumped into what's called the "balance of systems" — cabling, brackets, concrete, steel, copper connectors, inverters, DC and AC switch gear, and maybe a transformer, Gay explained. Installers can save a lot of hassle by essentially having most of the wiring done in the factory, he said, and they can save labor and hardware costs because there are fewer parts to be bolted together.

All of these elements contribute to the total cost per watt, and drive solar closer to grid parity.

Although the solar industry has been around for decades, only recently is it finally reaching a scale that can match the scale of tools provided by the IC and flat-panel industries, Gay explained, which is part of what is encouraging involvement from those other sectors. "The throughput of one tool to make a big-screen TV, making a single junction at the amorphous silicon, is a 50 MW per year tool. And it's only been in the last three years or so that any solar manufacturer needed a tool that had 50 MW per year of throughput," Gay said. "And what's now occurring is that some factories are being built that have 1000 MW per year total throughput, so that they're looking at installing maybe upwards of 15–20 parallel manufacturing lines to meet that market scale. And, at that scale, tools from other businesses other than solar are entering into the solar manufacturing space, making it possible to piggyback on the learning curve of toolmaking from other industries and accelerating the cost reduction in solar manufacturing."

The 1980s saw companies like BP Solar, Shell Energy and other energy companies try to develop thin-film technologies, but then had difficulty scaling them, Sargent noted. But equipment companies from the semiconductor and FPD industries have been able to apply the production-level technologies they've developed over the past 20 years to solar cell manufacturing to make it viable for high-volume production.

"This is like the cusp of one giant tsunami of opportunity here," Gay said. "This is a period in time when innovation is incredibly vibrant and active — it's the most I've seen in this space since 1981." At the scale that grid parity will bring, he added, the market is enormous.

"I think what's compelling for those of us who have spent almost 25 years in semiconductor-related industries, this is a fairly elastic market in its demand because, obviously, the energy market, even outside of solar PV or renewable, is quite large and growing," Sargent said. "The demand and the ability to consume the output is dramatic, as compared to, for example, flat panel or semiconductor, where you needed a killer application such as a Playstation or an iPod or a large-screen TV. In this case, energy is energy; the demand is by far tremendous — infinite almost."

Right now, solar PV represents perhaps 0.1% of the total electricity generated, according to O'Brien. Looking at overall energy needs, electricity demand in a country will continue to grow by, on average, 1–2% per year, he added. "So what that says is, every year, we've got incremental energy requirements that dwarf even the current installation of solar energy installations."

The whole solar industry, including solar PV and solar thermal, is forecasted to be almost $150B by 2015, Sargent said. "And we're sitting today at single-digit billions. So it's a ride, I think, not the likes of which we've seen since the late '70s in semiconductor."