How Thin-film Solar Cells Work

If you've used a solar-powered calculator, you've seen a solar cell based on thin-film technology. Clearly, the small cell in a calculator is not big and bulky. Most are about an inch (2.5 cm) long, a quarter-inch (0.6 cm) wide and wafer-thin. The thinness of the cell is the defining characteristic of the technology. Unlike silicon-wafer cells, which have light-absorbing layers that are traditionally 350 microns thick, thin-film solar cells have light-absorbing layers that are just one micron thick. A micron, for reference, is one-millionth of a meter (1/1,000,000 m or 1 µm).

Thin-film solar cell manufacturers begin building their solar cells by depositing several layers of a light-absorbing material, a semiconductor onto a substrate -- coated glass, metal or plastic. The materials used as semiconductors don't have to be thick because they absorb energy from the sun very efficiently. As a result, thin-film solar cells are lightweight, durable and easy to use.

There are three main types of thin-film solar cells, depending on the type of semiconductor used: amorphous silicon (a-Si), cadmium telluride (CdTe) and copper indium gallium deselenide (CIGS). Amorphous silicon is basically a trimmed-down version of the traditional silicon-wafer cell. As such, a-Si is well understood and is commonly used in solar-powered electronics. It does, however, have some drawbacks.

One of the biggest problems with a-Si solar cells is the material used for its semiconductor. Silicon is not always easy to find on the market, where demand often exceeds supply. But the a-Si cells themselves are not particularly efficient. They suffer significant degradation in power output when they're exposed to the sun. Thinner a-Si cells overcome this problem, but thinner layers also absorb sunlight less efficiently. Taken together, these qualities make a-Si cells great for smaller-scale applications, such as calculators, but less than ideal for larger-scale applications, such as solar-powered buildings.

Promising advances in non-silicon thin-film PV technologies are beginning to overcome the issues associated with amorphous silicon. On the next page, we'll take a look at CdTe and CIGS thin-film solar cells to see how they compare.

Because structure and function are so closely linked with solar cells, let's take a moment to review how they work. The basic science behind thin-film solar cells is the same as traditional silicon-wafer cells.

Photovoltaic cells rely on substances known as semiconductors. Semiconductors are insulators in their pure form, but are able to conduct electricity when heated or combined with other materials. A semiconductor mixed, or "doped," with phosphorous develops an excess of free electrons. This is known as an n-type semiconductor. A semiconductor doped with other materials, such as boron, develops an excess of "holes," spaces that accept electrons. This is known as a p-type semiconductor.

A PV cell joins n-type and p-type materials, with a layer in between known as a junction. Even in the absence of light, a small number of electrons move across the junction from the n-type to the p-type semiconductor, producing a small voltage. In the presence of light, photons dislodge a large number of electrons, which flow across the junction to create a current. This current can be used to power electrical devices, from light bulbs to cell phone chargers.

Traditional solar cells use silicon in the n-type and p-type layers. The newest generation of thin-film solar cells uses thin layers of either cadmium telluride (CdTe) or copper indium gallium deselenide (CIGS) instead. One company, Nanosolar, based in San Jose, Calif., has developed a way to make the CIGS material as an ink containing nanoparticles. A nanoparticle is a particle with at least one dimension less than 100 nanometers (one-billionth of a meter, or 1/1,000,000,000 m). Existing as nanoparticles, the four elements self-assemble in a uniform distribution, ensuring that the atomic ratio of the elements is always correct.

The layers that make up the two non-silicon thin film solar cells are shown below. Notice that there are two basic configurations of the CIGS solar cell. The CIGS-on-glass cell requires a layer of molybdenum to create an effective electrode. This extra layer isn't necessary in the CIGS-on-foil cell because the metal foil acts as the electrode. A layer of zinc oxide (ZnO) plays the role of the other electrode in the CIGS cell. Sandwiched in between are two more layers -- the semiconductor material and cadmium sulfide (CdS). These two layers act as the n-type and p-type materials, which are necessary to create a current of electrons.

The CdTe solar cell has a similar structure. One electrode is made from a layer of carbon paste infused with copper, the other from tin oxide (SnO₂) or cadmium stannate (Cd₂SnO₄). The semiconductor in this case is cadmium telluride (CdTe), which, along with cadmium sulfide (CdS), creates the n-type and p-type layers required for the PV cell to function.

But how does the efficiency of thin-film solar cells compare to traditional cells? The theoretical maximum for silicon-wafer cells is about 50 percent efficiency, meaning that half of the energy striking the cell gets converted into electricity. In reality, silicon-wafer cells achieve, on average, 15 to 25 percent efficiency. Thin-film solar cells are finally becoming competitive. The efficiency of CdTe solar cells has reached just more than 15 percent, and CIGS solar cells have reached 20 percent efficiency.

There are health concerns with the use of cadmium in thin-film solar cells. Cadmium is a highly toxic substance that, like mercury, can accumulate in food chains. This is a blemish on any technology that fancies itself part of the green revolution. The National Renewable Energy Laboratory and several other agencies and companies are currently investigating cadmium-free thin-film solar cells. Many of these technologies are proving themselves to be just as efficient as those that require cadmium.

So how are these next-generation solar cells manufactured? Read on and find out. ­

Cost has been the biggest barrier to widespread adoption of solar technology. Traditional silicon-wafer solar panels require a complex, time-consuming manufacturing process that drives up the per-watt cost of electricity. Non-silicon thin-film solar cells are much easier to manufacture and therefore remove these barriers.

The biggest recent breakthroughs recently have come with CIGS-on-foil manufacturing. Nanosolar makes its solar cells using a process that resembles offset printing. Here's how it works:

The presses used in semiconductor printing are easy to use and maintain. Not only that, very little raw material is wasted. This contributes to the overall efficiency of the process and drives down the cost of the electricity generated by the solar panels. Electricity from traditional solar panels costs about $3 per watt. Conventional wisdom suggests that solar will not be competitive until it can produce electricity at $1 per watt. Nanosolar claims that its super-efficient manufacturing process and revolutionary semiconducting ink can reduce the cost of making electricity from sunlight to a mere 30 cents per watt. If that holds true, solar may finally be competitive with coal.

Thin-film solar technology is not science fiction. Nanosolar currently has a 12-month supply of orders it's trying to fulfill. Customers include corporations and municipalities all over the world. Other thin-film solar cell manufacturers are just as busy. Ohio-based First Solar is working with Juwi Solar to construct a 40-megawatt thin-film CdTe solar field in Saxony, Germany, that will be completed in 2009. And Honda is actively experimenting with building-integrated thin-film CIGS on a facility in Japan.

If thin-film solar cells achieve their full potential, however, it's easy to imagine a future where solar power is as ubiquitous as, well, sunlight. Thin-film cells could blanket the roofs or form façades of buildings across cities. They could be integrated into roofing shingles for easy installation in every new house being built. And they could help power a new generation of solar cars and trucks.

For more information on solar power, electricity and related topics, see the next page.