How Solar Cells Work

While this section will provide you with a basic understanding of how modern solar cells function, most of the detailed information is available on the World Wide Web. The web links posted at the end of this section are far more complete in terms of information and illustrations. You are encouraged to examine these links if you want to really understand how this wonderful technology evolved and how it continues to grow and expand.

The Basics

Solar cells are quiet, non-polluting energy converters. They convert one form of energy, light, to another form of energy, electricity. When light energy is reduced or stopped, as when a cloud passes in front of the sun or when the sun goes down in the evening, then the conversion process slows down or stops completely. When the sunlight returns, the conversion process immediately resumes. Solar cells do not store electricity; they just convert light to electricity when sunlight is available. To have electric power at night, a solar electric system needs some form of energy storage, usually batteries, to draw upon.

What is marvelous about solar cells is that they perform this conversion without any moving parts, noise, pollution, radiation or constant maintenance. These advantages are due to the special properties of semiconductor materials that make this conversion possible. The following discussion will guide you through the conversion process without any mathematical equations or complicated physics.

Semiconductors - The "Current" Solar Cell Material Of Choice

Today the majority of solar cells are made from silicon. While other materials are also used, the fundamental process of how solar cells work is the same as for silicon cells, like transistors. Silicon is basically a "semiconductor" or "semi-material" whose name reflects that it has properties of both a metal (a conductor) and an insulator (a nonconductor). In a conducting metal, like copper, atoms have loosely bound electrons that easily flow when a voltage is applied. Conversely, atoms in an insulator, like glass, have tightly bound electrons that cannot flow even when a strong electric voltage is applied. In between metals and insulators, atoms in a semiconductor material bind their electrons somewhat tighter than metals, but looser than insulators. Another feature of semiconductors is that introducing small amounts of impurities, called "dopants," into the semiconductor structure can vary the amount of electrons that are available to break away from their atoms and flow under electric voltage. Two dopant elements that are typically used in silicon are phosphorous and boron.

A Little History

In the 1950s, scientists tinkering with semiconductors found that by introducing small, minutely controlled amounts of dopant impurities to the semiconductor matrix, the density of free electrons could be shepherded and controlled. The dopants, similar enough in structure and valence to fit into the matrix, have one electron more or less than the semiconductor. For example, doping with phosphorus, which has five valence electrons, produces a (negative) n-type semiconductor, with an extra electron, which can be dislodged easily. Aluminum, boron, indium, and gallium have only three valence electrons, and so a semiconductor doped with them is (positive) p-type, and has "holes" where the missing electrons ought to be. These holes behave just like electrons, except that they have an opposite, positive charge. (Holes are theoretical, but so are electrons and either or both may or may not exist, but we know for sure that if one exists, they both do, because we can't create something out of nothing in the physical world!) It is important to understand that, although loosely bonded or extra carriers exist in a substance, it is still neutral electrically, because each atom's electrons are matched one for one by protons in the nucleus.

The fun begins when the two semiconductor types are intimately joined in an NP-junction, and the carriers are free to wander. Being of opposite charge, they move toward each other and may cross the junction, depleting the region they came from and transferring their charge to their new region. This produces an electric field, called a gradient, which quickly reaches equilibrium with the force of attraction of excess carriers. This field becomes a permanent part of the device, a kind of slope that makes carriers tend to slide across the junction when they get close. Figure 3-1 shows a typical solar cell.

Photovoltaic reaction causes electron flow

When light strikes a photovoltaic cell, atoms are bombarded with photons, and give up electrons. When an electron gets lopped off an atom, it leaves behind a "hole", which has an equal and opposite charge. Both the electron, with its negative charge, and the hole, with its positive charge, begin a random walk generally down the gradient. If either carrier wanders across the junction, the field and the nature of the semiconductor material discourage it from re-crossing. A proportion of carriers which cross this junction can be harvested by completing a circuit from a grid on the cell's surface to a collector on the backplane. In the cell, the light "pumps" electrons out one side of the cell, through the circuit, and back to the other side, energizing any electrical devices (like the battery in Figure 3-1) found along the way.

Solar Cell Efficiency

If asked about how much sunlight energy gets converted into electricity, the best commercial solar cells are about 25% efficient, however the typical value is around 15% or less. Why so low? The answer lies in the electromagnetic spectrum of sunlight, which is composed of many wavelengths. We can see some of these wavelengths as visible light while other wavelengths fall below (infrared) and above (ultraviolet) the visible spectrum. Some wavelengths won't have enough energy to move electrons inside the solar cell's material and other wavelengths have too much energy.

The Solar Spectrum: The vast majority of the Sun's radiation is emitted at wavelengths between 290 and 3,200 nanometers. The spectrum of visible light falls within this range. Table 3-1 shows the wavelengths for common colors along with the outer boundaries in the ultraviolet and infrared ranges.

Table 3-1: Colors and Approximate Wavelengths of the Solar Spectrum (Values in Nanometers)

Color

Wavelength

Color

Wavelength

Ultraviolet

290-390

Orange

630

Violet

400

Red

780

Blue

470

Near infrared

800-1000

Green

565

Infrared

1000-2000

Yellow

590

Far infrared

2000-3,200

In effect, the solar cell's material can react to only certain energy bombardments from sunlight in order to produce electricity. By changing the solar cell materials the efficiency may, or may not, improve. This is where a major part of solar cell research is being conducted. The ideal solar cell material, or materials, would absorb more of the wavelengths of sunlight thus producing that much more electrical energy.

Other losses also enter into the efficiency figure of solar cells. One conventional loss has to do with the internal resistance of the material itself. Most silicon-based solar cells have a high internal resistance, which serves to reduce the amount of usable power that is produced by the cell. Another related electrical loss involves where to put the wires that connect to the cell's energy producing material. If the wires are placed at the edge of the cell, then electrons on the opposite edge have further to travel in the high resistance material thus creating additional energy loss. Most modern solar cells use a grid of conductors on the surface to more efficiently route the photovoltaic energy to the wires. However, this grid on the surface of the solar cell also blocks the sunlight from entering the cell's material. So as you can see, creating an efficient solar cell is really a matter of selective "trade offs" or compromises.

Future Solar Cells

The most futuristic approach to new solar cell technology involves arranging nanosize semiconductors in a matrix of plastic-like materials that are expected to be much less expensive to produce. Companies like Nanosys, Inc. are working on "nanorods" that are just 7 nanometers by 60 nanometers in a polymer. Because of their size (a nanometer is about 10,000 times narrower than a human hair) nanorods are arranged by chemical reactions. The manufacturing of nanocomposite solar cells is more like the production of photographic film, which is done in extremely high volumes with miles of precisely engineered materials per day at extremely low costs. According to a company official, the efficiency should be on par with crystalline silicon within three years. Plus, it will be easy to simply "paint" the solar cell material onto existing surfaces like outdoor walls and rooftops, thus making the entire structure produce power when the sun is shining. Yet there are doubts over how quickly such technology might be on the market.

On the horizon: a virtually perfect solar cell! An unexpected discovery at the Department of Energy's Berkeley National Laboratory (LBNL) may push the implementation of photovoltaics (PVs) sooner than later. A serendipitous discovery by the lab about the electronic properties of indium nitride may eventually yield high efficiency solar PVs, by making use of the entire spectrum of the Sun's radiation. While researching the properties of LEDs (light emitting diodes) researchers found that the band-gap energy of indium nitride is 0.7 electron volts, much lower than the 2.0 electron volts previously expected.

„ Band-Gap Energy: The band-gap energy is the amount of energy needed to free an 1 electron from its atom; a solar cell material can only capture sunlight at energies equal to or greater than its band-gap energy.

The low band-gap energy of indium nitride means that it can capture sunlight at much lower energies than expected, so the material is able to capture low-frequency, red or infrared light, a broader spectrum of radiant energy. This discovery may lead to two-layer PVs that could reach a phenomenal 50% efficiency in converting sunlight to electricity.

Work is proceeding to capitalize on newer, less expensive and more efficient materials and techniques that can make solar cells as ubiquitous as light bulbs in our homes, schools and offices. Much more work and effort remains, so it's not too late to think about a career in this exciting field.

Credit

A portion of this information was reprinted with permission from The Independent Home by Michael Potts. His book and many other interesting titles can be obtained from the publisher Chelsea Green at www.chelseagreen.com.

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