SciTech

Sunlight knocks electrons loose from solar cell material

Remember those simple calculators that had a dark black rectangle above the screen and stopped working when you put your finger over it?

That dark black rectangle consists of solar cells, cells that require light to operate. But how do those cells actually convert light into the electricity used to power the calculator?

The idea of utilizing light and the sun’s energy isn’t a new concept. In the seventh century, B.C., magnifying glasses were used to concentrate the sun’s rays to make fire. Allegedly, in 212 B.C., Greek scientist Archimedes used bronze shields to focus sunlight and set fire to Roman wooden ships attacking Syracuse.

But “modern” investigations of solar energy really kicked off in 1839 when French scientist Edmond Becquerel discovered the photovoltaic effect. Using an electrolytic cell, he observed that the generation of electricity increased when the cell was exposed to sunlight. Since then, it’s been discovery after discovery, from Einstein’s photoelectric effect in 1905 to the development of the first silicon solar cell by Daryl Chapin, Calvin Fuller, and Gerald Pearson of Bell Labs in 1954.

Solar cells, also called photovoltaic cells, are made up of semiconductors, materials that have a conductivity between that of an insulator and a conductor. When light strikes the cell, the semiconductor absorbs a portion of it. As the semiconductor absorbs energy from the sun, electrons in the semiconductor are knocked loose, allowing them to flow freely and create a current. By placing metal rods on the top and bottom of the solar cell, that current can be captured and the resulting electricity can be used for power.

According to energy.gov, the most common semiconductor used for solar cells is silicon, due to its crystalline structure. The silicon atom contains four valence electrons, resulting in a half-filled outer shell. Silicon shares these electrons with four other silicon atoms, which share their four valence electrons with four other silicon atoms, et cetera, resulting in the crystalline structure necessary for a solar cell to work.

However, since silicon on its own does not conduct electricity well, other atoms are added to silicon’s structure; for example, there could be one phosphorous atom for every million silicon atoms. Because phosphorous has five valence electrons in its outer shell, it bonds with four silicon atoms and has one electron left over.

When the solar cell absorbs sunlight, those extra electrons are knocked loose and can flow freely.

The process of adding other atoms to silicon’s structure is called doping. When doped with phosphorous, like the above example, the silicon is called n-type — “n” for negative because of the free flowing electrons that have negative charge. When doped with an element like boron, which has three valence electrons in its outer shell, the silicon is called p-type — “p” for positive because instead of free electrons there are free openings that carry positive charge.

The solar cell actually begins working when the n-type and p-type silicon come into contact with each other, creating an electric field as the free flowing electrons in the n-type begin to occupy the free openings in the p-type. When light hits the cell, electrons are knocked out of the openings, again creating free flowing electrons that seek the free openings. Therefore, there is a constant flow of electrons when light hits the cell, providing the current. The cell’s electric field provides the voltage, resulting in the power needed to run your calculator.

Currently, scientists are researching ways to make solar cells more efficient and competitive with more traditional energy sources.
The main problem is cost — although sunlight is free, the electricity generated from solar cells is not; we have to factor in the cost of making the solar cells as well as the cost of installing them, remembering that solar cells don’t convert 100 percent of the light it absorbs.

Furthermore, solar cells only work when there is sunlight, so scientists are also researching ways to not only harness the energy from sunlight, but to effectively store it.

With enough time, solar cells and solar energy are a promising green technology that can positively affect our future energy consumption.