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Primer on Solar Energy

photovoltaic process
The photovoltaic process

How do solar cells (photovoltaics) work?

Sunlight is comprised of photons.  When a photon hits a solar cell, it can pass straight through, be reflected back out, or be absorbed. When photons are absorbed, negatively charged electrons are knocked loose from their atoms, freeing them to flow through the material to produce electricity. Complementary positive charges, called holes, are left behind and flow in the opposite direction. It takes a certain minimum amount of energy to free an electron from the atom, known as the “bandgap” of the material. The value of the bandgap will vary by material.  The photon must have at least this much energy to be absorbed by the solar cell and produce electricity.  If the photon has more energy, the excess energy is typically lost as heat. Photons coming from the Sun have a wide range of energies, which corresponds to the different colors that make up the solar spectrum (as well as invisible photons such as ultraviolet and infrared).

Depending on the type of solar cell, the electron and hole may separate from one another freely or may require additional steps to be separated. Once separated, the movement of the electron and hole to their respective electrodes can take place either by drift or diffusion. Drift is driven by an electrostatic field across the cell whereas diffusion results in electrons or holes moving from areas of high concentration to areas of lower concentration. Traditional solar cells rely on drift while next-generation technologies such as dye-sensitized solar cells and organic solar cells generally rely on diffusion.

One of the most common metrics for the performance of a solar cell is the power conversion efficiency, or how much of the incident energy from sunlight is turned into useful electrical energy. Current photovoltaic technologies such as crystalline silicon, cadmium telluride (CdTe), and copper indium gallium selenide (CIGS) have commercial solar cell module efficiencies ranging from about 10-20%. These numbers are relevant for standard conditions, but efficiencies can be enhanced by collecting sunlight from a large area and concentrating it onto the solar cell. A more telling metric than efficiency, however, is the cost. Levelized cost of energy (LCOE) is a measure of the lifetime costs compared to the lifetime energy output. This metric will depend on the geographic location, financing, device efficiency, and many other factors.

Next » How does concentrated solar power (CSP) work?

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