The Many Faces of Solar Energy: An Overview of Solar PV Technologies

You are about to embark on your solar journey, and the first thing you notice is that you need language lessons to pronounce the jargon. Terms like monocrystalline do not necessarily flow easily off the tongue. If you know the difference between a solar panel and a solar module (short answer: none) and a solar collector (short answer: different technology), then you may not need this primer, but for the rest of you, please read on. 

Not all solar energy is created equal and there are essential differences between solar electricity primarily produced by photovoltaic (much easier to say PV) panels, and solar thermal (much easier to say solar heating) primarily produced by solar collectors that heat water on your roof.  If you are interested in producing electricity, then you should look at our home solar panels, and if you are interested in heating your water, you should look at solar water heaters.

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If you are not confused by the difference between solar PV and solar thermal, then you may be ready to start learning about another technology that is used to generate electricity on a large scale. Concentrating Solar Power (CSP) uses mirrors and a heat-conducting liquid to generate electricity at a large scale. CSP, solar thermal and solar PV are the major three technologies used today to harness the power of the sun. In this learning article, we take a look at solar PV technology, which provides electricity from the sun.

Photovoltaics is the process of converting sunlight directly into electricity. There are three types of solar PV:

• Crystalline silicon (c-Si)
• Thin film
• Third generation technologies, including Concentrator PV (CPV-not to be confused with Concentrating Solar Power, CSP) and organic photovoltaic cells (OPV)

A solar energy installation is made up of solar modules and the balance of system components. A solar panel is the same thing as a solar module, which is actually a collection of several solar PV cells. Solar cells generate the electricity in the system through reactions that convert light particles, or photons, directly into electricity. This electricity (called a direct current, or DC) is unusable in your house, which runs on alternating current (AC). Do not plug anything that is running on DC into your home electrical outlet.

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Inverters are connected to the module and convert the DC electricity into AC. The AC is what you use to turn on your lights. Inverters, batteries (which you will need if your system is off-grid) and solar racking that point the panels in the direction of the sun make up the balance of system components.

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Now you are purchasing solar panels for your home. Your options for PV technologies are between crystalline silicon (c-Si) or thin film, although the vast majority of homes going solar in the U.S. are installing crystalline solar modules. And in the future you may want to look out for third generation technologies such as CPV.

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Crystalline Silicon

You may have heard of crystalline silicon, since this technology represents most (85-90%) of the PV technology in the global market today. There are three types of crystalline silicon cells:

• Monocrystalline silicon (or single-crystalline silicon) (sc-Si)
• Polycrystalline silicon (or multi-crystalline silicon) (p-Si)
• Ribbon and sheet-defined film growth (ribbon/sheet c-Si)

The manufacturing process of mono- and polycrystalline silicon solar cells begins with a block of silicon (the second-most abundant element on Earth) called an ingot. The ingot is cut into thin slices, or wafers. Each wafer is treated with anti-reflective coating and metal contacts in order to generate electricity and to increase the absorption capacity of the cells. The manufacturing of ribbon cells is accomplished by drawing flat thin films directly from molten silicon, reducing manufacturing costs but also efficiency.

At a measured efficiency of 12-19%, monocrystalline modules are the most efficient in the market. They generally contain 60 to 72 solar cells and have a nominal power ranging from 120 to 300 Watt-peaks (Wp-a measure of the power of a PV device in laboratory conditions). Monocrystalline modules are this efficient because they are made from pure silicon crystals.

You can tell poly- and monocrystalline cells apart by the speckled blue aspect of polycrystalline cells. Polycrystalline cells are made from less pure silicon, making them cheaper to manufacture and to buy. Because of the price, polycrystalline cells are becoming more popular for producers and consumers. They are, however, less efficient than monocrystalline silicon cells. In contrast to both, ribbon silicon modules represent less than 5% of the crystalline silicon cell market.

Thin Film

The name for thin film technology comes from the process by which it is manufactured: each film is deposited in very thin, consecutive layers of atoms, molecules or ions. Thin film solar panels are the cheapest to produce, and are becoming more and more popular, to the point that some companies offer only thin film technology. Thin film is less efficient than crystalline-based technology.

There are three main types of thin film cells:

• Cadmium telluride (CdTe)
• Amorphous Silicon (a-Si)
• Copper, Indium, Gallium, Selenide (CIGS/CIS)

CdTe cells are made of cadmium and tellurium, and are the most popular of all three thin film technologies. You may have seen amorphous silicon before, which was the first thin film material available commercially, in your solar-rechargeable calculator. This thin film technology is mostly used for items such as calculators. CIGS just recently became available for small commercial applications. Manufacturing is still expensive, but high efficiency in the lab has been achieved.

All currently manufactured thin film cells rely on rare earth elements such as indium or tellurium, which might limit the large-scale production of this technology.

Thin film is less efficient than crystalline silicon-based solar cells, but it is cheaper to manufacture and to buy. Cyrstalline solar cells are still the most popular solar cells, but thin film is being installed at a much higher rate. In fact, between 2004 and 2009, the growth in thin film CdTe was more than three times that of the growth of PV in general. Nevertheless, the most efficient PV cells are still crystalline-silicon cells (see table below).

Figure 1: Current efficiencies of different PV technology commercial modules 

Table adapted from Paolo Frankl and Stefan Nowak. Technology Roadmap: Solar Photovoltaic Energy. Paris: International Energy Agency, 2010.

Concentrator PV (CPV) and Experimental Technologies

You will probably not install these on your roof, but you may want to keep your eyes peeled for these developing technologies that are emerging in the marketplace today.

CPV modules use lenses to focus sunlight onto solar cells made from small amounts of highly efficient and expensive PV material so that the most sunlight possible is collected. CPV cells are generally based on silicon or III-V compounds such as gallium arsenide (GaAs). Commercial efficiencies of up to 25% have been obtained for silicon-based cells, and up to 30% with GaAs, which is incredibly high compared to other technologies.

There are other emerging technologies, such as organic photovoltaic solar cells, which include fully organic solar cells (OPV) and dye-sensitized solar cells (DSSC). These are based on organic electronics, which is basically the use of small organic molecules that absorb light and generate electricity.

In summary, there are three main technologies available on the market today: crystalline silicon solar cells, thin film and third-generation. In addition, there are three key points when considering solar PV technologies today:

• The share of thin film technology has increased considerably in the past few years;
• Crystalline silicon PV still dominates today's market;
• Crystalline silicon cells are the most efficient commercial PV cells.