Harvesting the Sun With Silicon: Photovoltaic Cells

2024-01-12 | By Antonio Velasco

After talking about PIN junctions and photodiodes, it's important to give a nod to the most popular and revolutionary technology coming from it: Photovoltaic Cells-- most commonly known as the material in solar panels! In 1905, Albert Einstein talked about how when light (or photons) strikes certain materials, electrons are released, forming the basis of electricity generation within any solar cell. We've come a long way since then, so let's dive into this!

What's Behind Those Blue Panels?

Solar Panels are a collection of solar cells--basically a ton of silicon (semiconductor) material. We talked about how this creates energy in my PIN junction blog here. Essentially, the special properties of the semiconductor allow photons to excite the electrons and create an electrical current. In order to get this special property though, the silicon needs to be doped extensively with impurities.

The P-layer is doped with boron to create positively charged holes, whereas the N-layer is doped with phosphorus to bring in negatively charged electrons. The I-layer (intrinsic) between P and N is lightly doped with impurities to allow for the electron flow between the two upon certain conditions.

The energy from the light (photons) is transferred into the semiconductor material, exciting the electrons, and allowing them to move. This leaves "holes" as the electrons become excited--essentially the absorption of the photon allows for the holes and electrons to freely move. The electric field induced as a result of the PN junction pushes the electrons toward the N-layer and the "holes" to the P-layer, thus creating a voltage.

Basically, the light (photons) hit the intrinsic layer and separate the electron-hole pairs, thus generating current.

Harvesting the Sun With Silicon: Photovoltaic Cells

Above is a symbol of how a cell would look in a circuit, depicting the light going toward a junction.

Further Look into the Science

Another concept to look at is the idea of band gap--essentially the difference between the area where electrons are tightly bound and the conduction band, where electrons move more freely as there is more space. This is typically measured in electronvolts, or eV, as it can describe the extremely small energy difference. You see, the "gap" here isn't necessarily a physical distance, but rather an energy state. There lies a big difference between the two due to their energy; with a higher energy, there is more mobility, and vice versa. You can think of it like a kid with caffeine--if you give them more soda and thus energy, they'll be all over the place.

The difference in energy (band gap) between the two determines what wavelengths the solar cell can absorb. Photons need to have a higher energy than the band gap in order to excite the electrons, as it needs to compensate for that difference, essentially.

In the case of silicon, its bandgap is around 1.1 electronvolts (eV), making it suitable for absorbing visible light photons with energies ranging from 1.1 eV (red) to 3.1 eV (ultraviolet). Photons with energies lower than the bandgap are not absorbed, while those with energies higher generate excess energy as heat.

Efficiency is a crucial factor in solar cell technology, and engineers constantly strive to improve it. One approach involves creating heterostructures—combining different semiconductor materials with varying bandgaps to capture a broader spectrum of sunlight.

Harvesting the Sun With Silicon: Photovoltaic Cells

For instance, gallium arsenide (GaAs) has a narrower band gap than silicon, making it suitable for capturing higher-energy photons like those from the blue and green spectrum. By combining GaAs with silicon, tandem solar cells can efficiently convert a wider range of sunlight into electricity.

Other advanced materials, such as perovskite solar cells and organic photovoltaics, offer exciting possibilities for boosting efficiency and reducing manufacturing costs.

Wrap-Up

You'll see solar cells in a ton of applications--whether on your roof or your calculator.

Harvesting the Sun With Silicon: Photovoltaic Cells

They're extremely versatile and can be used in multiple form factors. Popularly, they're always on satellites or space vehicles as the sun's limitless energy is something that is extremely useful to take advantage of in space. Silicon's abundance has made them easy to access and the established manufacturing processes make it cost-effective.

You can also incorporate them into your projects to create your own electricity! I've linked a couple of examples of components that you can put into your projects below with a range of sizes. You could make a weather station that's powered entirely by the sun, or simply make your own power so you don't have to rely on batteries.

If you're interested in learning more about solar energy, see the articles below!

Photodiodes: Light Meets Semiconductors

PIN Junctions

Photoresistors: Opening a Path with Light!