Solar panels utilize a physical phenomenon called the photovoltaic effect to produce an electric current.
In simple terms, there are certain materials that produce electricity when they are exposed to light. This occurs when the light knocks some of the electrons loose from their host atoms. When we attach conductors to the positive and negative sides of the material to form a circuit, we can channel this electrical energy.
When multiple solar cells are linked together, they become a solar panel.
To understand how solar panels generate electricity, we need to first provide a bit of background on a group of materials called semiconductors.
What are Semiconductors?
A semiconductor is a material that can conduct electricity under some conditions but not others. All solar cells are made from semiconductors. In semiconducting materials, there is something called a conduction band and a valance band.
The electrons generally like to go into the lowest energy levels they can. As a result, the valence band will usually be filled with electrons, and the conduction band is usually empty.
In each band, there are many different energy levels that electrons can go into. Between the two bands is called the band gap. There are no energy levels that electrons can go into.
A high band gap means that the solar cell will produce a high voltage and low current. A low band gap means that the solar cell will product a low voltage and high current.
Since Power = Voltage x Current, there is an optimal band gap value for maximizing power.
How do Solar Panels Produce Electricity?
When light is absorbed by a semiconductor, it excites electrons from the valence band into the conduction band (eg. an electron absorbs the light energy and jumps across the band gap). After an electron makes it into the conduction band, it will sit there and essentially store the energy. If we pull the electron out of the conduction band, we can make it do work for us (eg. power a light bulb).
If this is all very confusing, it might be useful to consider an an analogy. A good analogy for how solar cells work is hydropower – where we store water at an high elevation when it rains, and let the water fall towards ground level to run a generator when we need power.
In this analogy, we want the altitude of the water reservoir to be low enough to capture a lot of water, but also high enough to generate a lot of power. If the the water reservoir is at the top of a mountain (very high altitude), then not very much water will make it in. If the water reservoir is at the bottom of a mountain (very low altitude), it will collect lots of water since all of the rivers will drain into it. However, the reservoir will be so close to ground level that there’s not much gravitational potential energy left.
Similar to the water reservoir, we want the solar cell band gap to be low enough to capture a lot of light, but high enough to store a lot of power when the electrons get bumped up to the conduction band.