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How does solar work?

July 28, 2021 by VCT Group

As long as we’ve been in the business of solar energy, we keep getting asked one question: How do solar cells work? 

In short, solar cells are energy converters. At the most basic level, they absorb the energy from sunlight and convert it into electrical energy. That is the simplest answer, but with just a little physics we can gain a deeper understanding. 

The photovoltaic effect 

All atoms are made of three types of sub-atomic particles: protons, neutrons and electrons. Protons and neutrons are found in the nucleus at the center of the atom, forming the bulk of its mass. Electrons are much lighter and are normally found orbiting around the nucleus. However, if an electron absorbs enough energy it can get so “excited” that it can break free from its atom, leaving behind an “electron hole”.  

In some materials the energy from light is enough to excite atoms’ outer electrons to escape. This is called the photovoltaic effect. Once the electrons are knocked loose, they are free to flow through the material. When a bunch of electrons flow together that’s electricity.

Schematic diagram of a silicon atom showing electrons, nucleus, orbitals. A photon is shown energizing an electron to ionize the atom, leaving an electron hole in the outer shell.

One material in which this happens efficiently is silicon. Silicon is commonly found in sand or quartz. It’s cheap and abundant, and most solar cells you’ve seen are made from it. (Solar cells can also be made from other materials, but for this article we’ll stick with silicon.)

Atoms will always try to find a way to balance their electric charge to become electrically neutral. If they have an electron hole, they will readily absorb any free electron to fill it. For free electrons to be useful to us and generate an electric current, we must find a way to separate them from the positively charged holes they leave behind. This “charge separation” is at the heart of how solar cells work.

Solar cell design

When you look closely at a solar cell, you can see it is like a sandwich. On the top is a fine web of metal wires, in the middle is a layer of silicon, often in crystalline form, and on the bottom is a layer of metal, or another web of wires. When the solar cell is connected, these metal components are the electrodes of the circuit, negative on the top, positive on the bottom.

If you examine the silicon crystal layer chemically, you’ll find that it isn’t pure silicon. It contains other elements, or impurities. Looking even closer, you’ll see that the impurities are not evenly spread throughout the silicon. The top part of the layer has one kind of impurity and the bottom another. This is by design and it is how the solar cell separates the charges.

Diagram of solar cell showing n-doped and p-doped layers, electrodes and current flow.

The power of doping

When impurities are added to a crystal during manufacturing, the process is called “doping”. There are two kinds of doping: n-type where arsenic or phosphorus is added in small quantities; and p-type, where boron or gallium is added. By selectively doping the silicon crystal as it is grown, n-doped and p-doped layers are created within it. Where the two layers meet the boundary between them will only let electrons flow in one direction. This property can separate the electrons and electron holes.

When sunlight strikes a solar cell its energy is absorbed, creating electron/electron-hole pairs. When this happens at the boundary the electrons are drawn into the n-doped layer on top, while the holes migrate towards the bottom p-doped layer.

Away from the boundary, high concentrations of free electrons or holes that are created will naturally diffuse from areas of high concentration to low concentration. As these diffusing electrons and holes come near the boundary, they will also be drawn to opposite sides of the boundary, effectively being sorted.

The electrons that gather on the top surface are collected by the electrodes, where they flow out of the solar cell creating an electrical current. This current is what we use to power our devices. When the current flows back through the bottom electrode of the solar cell the electrons combine with the holes there, completing the circuit.

Closeup diagram of a solar cell showing electrons and electron holes created by the absorption of energy from sunlight. A silicon atom is depicted absorbing sunlight, and the separation of electrical charges within the cell.

At-a-glance summary.

Electricity is the flow of electrons. Atoms, such as silicon, contain electrons. By absorbing enough energy electrons can break free from their atoms. In silicon crystals, sunlight provides sufficient energy for this to occur. By carefully creating crystals doped with impurities, these liberated electrons are concentrated in one layer of the solar cell where they can flow out to generate an electric current.

How solar cells work for you.

The modular nature of solar cells makes them ideal for many applications because they are scalable systems that can be designed at any size. They have reached a level of efficiency – and such a low manufacturing cost – that they are now the cheapest form of power generation. On average the sun beams 1000 Watts of energy onto every square meter of the Earth – imagine the light and heat from ten 100W lightbulbs and you get the scale of it. Solar panels can convert about 20% of this energy directly into electricity. After the panels have paid for themselves with energy savings, this is literally free power.

In Ontario, the economic model for solar is net-metering. Net-metering calculates the difference between the power you generate and the power you use from the hydro grid. If you use more power than you generate, you draw the extra power from the electrical grid and only pay for the difference. If you generate more power than you use, the surplus is fed into the grid, and you are credited for the difference against future power use. The ability to carry a credit balance means that you don’t need to build battery storage into your system for days when the sun doesn’t shine. The grid is your battery.

The benefits of solar go beyond saving on power costs. Solar power is a highly visible way to demonstrate your commitment to a sustainable future. Modern customers want to do business with companies that share their vision. Your adoption of solar power creates that connection. Green is good for business.

How solar cells work for everyone.

Climate change is here. We can already see its effects in the marketplace and in the wild. Adaptability and resilience are key benefits of solar power. By moving to solar, not only do we reduce our environmental footprint over time, we also create a network of distributed renewable power generation that is more resilient to interruption. Solar can power us cleanly today and let us adapt to the future. There are many challenges that face the solar industry in the future, without question. Our transition to new forms of energy generation is staggeringly massive. The push will take global adoption and investment in technologies that promote a healthy energy mix. Not one single solution will be the “silver-bullet”, no matter how much the advocator states it so. We are here to work as one, to fight for the future we want to see.

At VCT Group, we are passionate about building solar energy solutions that transform how the world is powered. We believe solar is as impactful to adapting to climate change as it is on your operations budget. We will work with you to incorporate renewable energy into your business. We make solar work for you, for everyone, for the world.