HOW DO SOLAR CELLS WORK?
The Simple Explanation
Solar cells 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 conducting material to the positive and negative sides of the material to form a circuit, we can channel this electrical energy.
The very first solar cell was built by Bell Labs over 50 years ago. The first serious use of solar cells was by the space industry as it was a very convenient way to power satellites. Today, the cost of solar panels are low enough for residential use.
The Full Explantion
To understand how solar panels generate electricity, we need to first provide a bit of background on a group of materials called semiconductors. We will then go into detail on how power is actually produced, and finally what affects a solar cell’s performance.
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.
In each band, there are many different energy levels that electrons can go into. Between the bands (band gap), there are NO energy levels that electrons can go into. 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.
Solar cell performance is strongly related to two factors. The first factor is how much light the cell can convert into electricity, and the second factor is how much energy gets lost due to efficiency loss.
When it comes to how much light the solar cell can convert into electricity, the most important thing to consider is what the band gap (or energy gap) should be. With current technology, we can pretty much make the band gap to be anything we want.
Ideally, we want the band gap to be slightly lower than the energy level of the light we are trying to capture. This means that the light will excite electrons just barely into the conduction band. If the light energy is much higher than the band gap, it will still excite the electron from the valence band into the conduction band, but the electron will quickly fall to the lower energy levels in the conduction band – resulting in wasted energy. If the light energy is lower than the band gap, then the electrons cannot be excited enough for the solar cell to produce power.
As shown in the graph above, the sun is emitting light at a wide range of energy levels (the shorter the wavelength the higher the energy of the light), and each semiconductor material can only have one band gap – which means that a single semiconductor cannot be optimized for all energy levels. One method to overcome this is to overlap semiconducting materials of different band gaps on top of each other. However, this greatly increases the cost of the solar cell.
When it comes to solar cell efficiency loss, the major factors to consider are:
- The fact that many photons (light) do not have enough energy to excite the electrons through the band gap.
- The high energy photons excite the electrons too high up into the conduction band. These electrons quickly drop to the lower energy levels in the band, and gives off waste heat in the process.
- Some electrons fall from the conduction band into the valence band before it gets out to the electrodes. This process is called recombination, and there are four main types to consider: Radiative, Auger, Shockley-Read-Hall, and Surface.
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.
HOW EFFICIENT ARE SOLAR CELLS?
The efficiency of a solar cell refers to how effectively it can convert the light that hits its surface into electricity. With current technology, the efficiency can be anywhere from 6% to 46%. Common residential solar cells have an efficiency range between 15%-20%.
When manufacturers measure the efficiency of their solar cells, the must do so under standard test conditions (STC) to ensure that everyone is playing by the same rules. The STC’s correspond to a clear sunny day with a temperature of 25 degrees Celsius.
WHAT MATERIALS ARE USED TO MAKE SOLAR CELLS?
There are a large number of materials one can use to create a solar cell. Most of the different materials involve trade-offs between cost and efficiency. The reason that there are so many choices is that the dollars per Watt gap between them is relatively close. This has resulted in the respective researchers trusting that their design has a chance to be the best in the future.
Here are the four most common solar cells materials.
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