How Thin-film Solar Cells Function

Solar panels have long been a symbol of the global push for renewable energy. These black-paneled structures are often seen on rooftops or in expansive arrays across landscapes. However, the familiar design of the solar panel -- measuring 5.5 feet by 2.75 feet by 2 inches (1.7 m by 0.8 m by 5 cm) -- could soon become a thing of the past. This is due to the emergence of a new technology that is poised to complement traditional silicon wafer panels as a more efficient, affordable method of converting sunlight into electricity. This new innovation is the thin-film photovoltaic (PV) cell, which, by 2010, is expected to generate 3,700 megawatts of electricity globally [source: National Renewable Energy Laboratory].

After 2010, the production capacity of thin-film PV cells will continue to grow, as these cells begin to power solar installations in commercial buildings and homes around the world, from California to Kenya to China.

What makes thin-film solar cells different from traditional solar cells, beyond their flexibility? Why do they offer better cost efficiency? And could they play a significant role in making solar energy a truly viable alternative to coal and nuclear energy? Keep reading to explore the answers.

What are Thin-film Solar Cells?

You might have seen a thin-film solar cell in a solar-powered calculator. These small, compact cells measure around an inch (2.5 cm) long, a quarter-inch (0.6 cm) wide, and are wafer-thin. The defining characteristic of this technology is its thinness. In contrast to silicon wafer cells that have light-absorbing layers approximately 350 microns thick, thin-film solar cells use light-absorbing layers that are just one micron thick. For context, one micron equals one-millionth of a meter (1 µm).

The process of creating thin-film solar cells begins with the deposition of multiple layers of a semiconductor material onto a substrate, which can be made of coated glass, metal, or plastic. These semiconductors don’t require significant thickness to absorb solar energy efficiently. As a result, thin-film solar cells are lightweight, durable, and easy to handle.

Thin-film solar cells are categorized into three primary types based on the semiconductor used: amorphous silicon (a-Si), cadmium telluride (CdTe), and copper indium gallium deselenide (CIGS). Amorphous silicon is essentially a compact version of traditional silicon wafer cells. Due to its familiarity, a-Si is widely used in solar-powered gadgets, although it has some limitations.

A major challenge with a-Si solar cells is the material used for their semiconductor. Silicon can be hard to source in the market due to high demand outpacing supply. Moreover, a-Si cells are not very efficient. They experience considerable power loss when exposed to the sun. While thinner a-Si cells help to address this issue, they also absorb sunlight less effectively. As a result, these characteristics make a-Si cells suitable for small-scale uses, like in calculators, but less effective for larger applications like solar-powered buildings.

Significant progress in non-silicon thin-film photovoltaic (PV) technologies is starting to resolve the challenges faced by amorphous silicon. On the next page, we’ll explore CdTe and CIGS thin-film solar cells and compare their performance.

Understanding the Structure of Thin-film Solar Cells

Since the structure and function of solar cells are deeply interconnected, it’s important to understand their operation. The fundamental principles behind thin-film solar cells are based on the same science as traditional silicon-wafer cells.

Photovoltaic cells depend on materials known as semiconductors. In their pure form, semiconductors are insulators, but when heated or mixed with certain materials, they can conduct electricity. When a semiconductor is doped with phosphorous, it develops an excess of free electrons, becoming an n-type semiconductor. If it is doped with materials like boron, it creates an excess of "holes," or spaces that accept electrons, making it a p-type semiconductor.

A photovoltaic (PV) cell combines n-type and p-type materials, with an intermediate layer known as a junction. Even without light, a small amount of electrons naturally flow from the n-type to the p-type semiconductor, generating a small voltage. When exposed to light, photons dislodge many more electrons, which travel across the junction, producing a current. This current can then power electrical devices, ranging from light bulbs to cell phone chargers.

Traditional solar cells use silicon for both the n-type and p-type layers. However, the latest thin-film solar cells utilize thin layers of either cadmium telluride (CdTe) or copper indium gallium deselenide (CIGS). Nanosolar, a company based in San Jose, California, has developed a method to produce CIGS material as an ink, containing nanoparticles. A nanoparticle is a particle with at least one dimension smaller than 100 nanometers (one-billionth of a meter). In this form, the four elements naturally arrange themselves in a uniform distribution, ensuring the correct atomic ratio.

The two types of non-silicon thin-film solar cells each have distinct layer structures. For the CIGS-on-glass cell, a layer of molybdenum is required to create an effective electrode. This layer is not needed in the CIGS-on-foil cell, where the metal foil itself serves as the electrode. The CIGS cell also has a zinc oxide (ZnO) layer that functions as the second electrode. Between these electrodes are two additional layers: the semiconductor material and cadmium sulfide (CdS), which act as the n-type and p-type layers necessary to produce electron flow.

The structure of the CdTe solar cell is similar. One electrode consists of a carbon paste layer infused with copper, while the other electrode is made from either tin oxide (SnO₂) or cadmium stannate (Cd₂SnO₄). In this case, the semiconductor material is cadmium telluride (CdTe), which, together with cadmium sulfide (CdS), forms the n-type and p-type layers essential for the PV cell's operation.

How do thin-film solar cells stack up against traditional silicon-wafer cells in terms of efficiency? The theoretical maximum efficiency for silicon-wafer cells is around 50 percent, meaning that half of the incoming energy is converted into electricity. In practice, silicon-wafer cells achieve an efficiency of 15 to 25 percent. Thin-film solar cells are now becoming competitive, with CdTe solar cells reaching just over 15 percent efficiency, while CIGS solar cells have reached about 20 percent efficiency.

There are concerns regarding the use of cadmium in thin-film solar cells. Cadmium is a highly toxic element that, much like mercury, can accumulate in the food chain. This is a significant issue for any technology that claims to be part of the green revolution. The National Renewable Energy Laboratory, along with other research institutions and companies, is exploring cadmium-free thin-film solar cells. Many of these emerging technologies have proven to be just as efficient as those that use cadmium.

How are these next-generation solar cells produced? Keep reading to find out more.

Manufacturing Process of Thin-film Solar Cells

The cost of solar technology has been one of the major obstacles to its widespread use. Traditional silicon-wafer solar panels require a complicated and time-consuming manufacturing process, which increases the cost of electricity per watt. Non-silicon thin-film solar cells are much simpler to produce, eliminating many of these cost barriers.

The latest breakthroughs have been in the manufacturing of CIGS-on-foil solar cells. Nanosolar has developed a production method that mimics offset printing, and here's how it works:

The presses used for semiconductor printing are user-friendly and easy to maintain. Additionally, very little raw material is wasted, improving the overall efficiency of the process and lowering the cost of electricity generated by solar panels. Traditional solar panels typically cost around $3 per watt, and experts believe solar will only be competitive once it can generate electricity at a cost of $1 per watt. Nanosolar claims that its efficient manufacturing process and innovative semiconducting ink could reduce the cost of generating electricity from sunlight to just 30 cents per watt. If this proves true, solar could finally become a viable alternative to coal.

Thin-film solar technology is far from being a concept of the future. Nanosolar currently has a year’s worth of orders in progress. Their customers span corporations and municipalities globally. Other manufacturers of thin-film solar cells are equally busy. For instance, Ohio-based First Solar, in collaboration with Juwi Solar, is building a 40-megawatt thin-film CdTe solar field in Saxony, Germany, slated for completion by 2009. Additionally, Honda is actively testing the integration of thin-film CIGS at a facility in Japan.

If thin-film solar cells fully realize their potential, it's easy to envision a future where solar energy is as common as sunlight itself. These thin-film cells could cover rooftops or form the façades of buildings across cities. They could even be built into roofing shingles for seamless installation in new homes being constructed. Moreover, they could power the next generation of solar-powered cars and trucks.

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