Solar cells, also called photovoltaic cells, have become much more common in recent years. These cells power everything from handheld calculators to household electrical systems to the most high-tech space vehicles. Solar cells were used to power an experimental airplane, as well as prototypes for sunlight-powered automobiles.
Solar energy has tremendous potential as an alternative fuel source, but several technological problems will have to be addressed before solar power can be used on a wider scale. Understanding how a solar cell works can help illuminate the current limitations of solar power as well as its potential.
A solar cell consists of two panels sandwiched together. These panels are made from silicon, the same element which made the computer revolution possible. Finding silicon in its natural state is not difficult, but processing it for purity can be expensive and technologically challenging.
To understand why silicon is an ideal material for solar cells requires a quick trip back to high school chemistry. Imagine a silicon atom as an eight-point snowflake. The points of the snowflake have room for eight particles called electrons. But each silicon atom only contains four electrons, which means it will continue to search for four more electrons to fill the remaining spaces.
If two silicon atoms meet, the bond will give each one exactly the four electrons it is missing. This process continues until no more silicon atoms are available. Pure silicon has no electrical charge of its own because it has no need for additional electrons and the bond between atoms is very stable.
In order to create a solar cell, the silicon base must be altered at the atomic level. Remember that an individual silicon atom has four electrons on its outer ring and needs four more to be complete. An element such as phosphorus contains FIVE electrons, so a bond between silicon and phosphorus would result in a total of nine electrons.
Since silicon only needs eight electrons to be complete, the extra electron would be held in a state of limbo- it’s not needed by the new silicon/phosphorus molecule, but it cannot break away on its own. The new silicon/phosphorus molecule now has an N-type (negative) charge because of this extra electron. Solar cell manufacturers must create a supply of this impure silicon/phosphorus compound in order to make their final product.
In order to create a proper environment for electrical production, another impure silicon compound must be created. The element boron only contains THREE electrons in its outer atomic band. When combined with the four electrons of silicon, the result is a P-type (positively-charged) compound with only seven electrons. If any free electrons were available, they would be drawn to the last available space in the silicon/boron molecule. This need for electrons to fill available spaces is the basic science behind a solar cell.
When a plate containing the N-type silicon/phosphorus compound is sandwiched with a plate containing the P-type silicon/boron compound, the result is a battery of sorts with potential electrical energy. The only thing missing is an outside force which will free the extra electrons and send them to the available spaces on the other plate. This is where the sun’s energy comes into play.
Raw sunlight contains a number of different particles, but the most important particle for solar cells is called a photon. When photons from the sun’s light strike the negatively-charged plate, the extra electron is sent flying from the impact. Because the positively-charged plate attracts free electrons, all of the freed electrons from the negative plate rush over to the positive plate.
The result is a usable but tiny amount of electrical current. Wires placed between the plates direct this current away from the solar cell array and back again in a circuit. Any electrical load along the way (electric motor, light filament, heating element, etc.) will be powered by this current and the remaining electrons will flow back to the solar cell. Photons continue to strike the negative plate and the current will continue to flow. In theory, this process could continue indefinitely, as long as photons strike the negative plate and set more electrons free.
This is where the problems with solar energy become more apparent. Sunlight, and to a lesser degree ANY light source, contains other elements besides photons. Some of these other particles, such as ultraviolet and infrared rays, eventually cause solar cells to degrade physically. The panels must also be aligned at a specific angle in order to absorb as many photons as possible.
If even one solar cell in an array is not receiving direct sunlight, the efficiency of the entire system can be reduced by 50 percent or more. It is possible to store some of the excess energy from solar cells in standard chemical batteries, but there usually isn’t much excess energy created. Exposure to the elements may also damage solar cells, rendering them useless as electrons escape.
At present, solar cells tend to work best with devices that require little direct current to operate. Small electronic devices and lighting systems would not require many cells to generate their electricity, but larger projects such as automobiles or home power generators require large arrays of solar panels.
These panels must be pointed directly at the sun for maximum efficiency, and they would be limited by clouds and nightfall. In the short term, most power generated by solar cells will continue to be directed towards smaller applications. Production costs of quality solar cells are substantial and their efficiency is negligible.