Every technology has a foundation – that piece of innovation that makes it all possible. For solar power, it has to be the semiconductor. These wonderful, silicon-based doodads are spread across roofs around the world. But what in the world is a semiconductor? That is something the average solar enthusiast can’t exactly answer. Let’s not be too hard on ourselves, most of us couldn’t design a light bulb either. But, because rabid curiosity hasn’t killed me so far, I ventured to find out exactly what a semiconductor was. Here is what I found out…as best as I can articulate it.
Two things right off:
- One, semiconductors are not exclusive to solar. They are also used in electronics, a big reason for the recent silicon shortage.
- Two, they are not exclusively made from silicon, although it is still the most common. Other materials, such as cadmium telluride and copper indium diselenide, are also used but to lesser efficiencies so far.
Essentially, semiconductors are the devices in a solar cell that absorb sunlight and enable the separation of electrons from the heat waves. Only when electrons are freed can they be used to generate electrical currents. Hence you have the importance of semiconductors in solar cell manufacturing.
A semiconductor has two traits that are important to the production of electricity:
- Absorption Coefficient. This is a measure of how far light can penetrate a material before being absorbed. This will vary based on the type of semiconductor material and the specific wavelength of the light being absorbed. Semiconductors in solar cells have a very abrupt absorption coefficient so that sunlight can be readily absorbed.
- The Bandgap of a semiconductor represents the minimum amount of energy needed to free an electron from its “bound” state within the atom. A semiconductor material has two bands: the valence band, where the energy level is low, and the conduction band, the higher energy level where electrons are freed and able to move freely. The energy difference between the two is known as the material’s bandgap.
As you can see, the semiconductor is the part of the solar cell where science is at work. If this all seems a bit confusing to you, then we have that in common. Yet, while the physical science of semiconductors may be hard to comprehend, what semiconductors do is fairly simple. A light wave is absorbed by the semiconductor which, at a certain depth, is able to free those energetic electrons from their atomic bonds within the silicon or other semiconductor material. Essentially, the electrons that are bound in the valence band are excited and released into the conduction band where they are used to produce electricity.
There are two layers (or wafers) of semiconductor material. One of which is full of positive-type material (low electron concentration) and negative-type material (high electron concentration). The space where these two layers come in close contact is known as the p-n junction. As light heats these sensitive semiconductor layers, a flow of electrons is ignited, resulting in a direct current which ultimately results in electricity for your home.
That, quite basically, is the essence of a solar semiconductor. Note that different generations of solar cells may use different semiconductors and function in slightly different ways. Nonetheless, the goal of all solar cells, and their many components, is to create electricity by getting electrons flowing. So far silicon is the hands-down bread winner, but in the future, organic and polymer solar cells may have something important to say about that.