Solar cells are an important technology that can help solve humanity’s energy problems by converting the sun’s light energy into electricity. Solar cells utilizing silicon semiconductors are currently the most efficient and are emerging as an alternative to future energy depletion.
Imagine an Earth without the sun. It would be a barren planet, unable to support life at all. The sun has been sending massive amounts of light energy to Earth ever since it was born. Almost all life on Earth is powered by energy stored through photosynthesis in plants. Even petroleum, the most common source of energy used by humans, is the result of the bodies of long-ago thriving organisms that have been transformed over time in the ground. Modern civilization is over-consuming and depleting the sun’s energy reserves. If all sources of energy are depleted, where does humanity get its energy from? Can’t we use the sun’s light energy directly, even now? The answer is solar cells.
Solar cells are devices that convert the sun’s light energy into electricity, and silicon solar cells are currently the most popular due to their high efficiency, relatively low unit cost, and simplicity of manufacture. The structure of a silicon solar cell is fairly simple, consisting of two types of silicon semiconductors bonded together. So how do silicon semiconductors turn light into electricity? The secret lies in the electrons inside silicon. In a stable state, electrons are bound to the nucleus of an atom and cannot move freely. However, when electrons are energized, they can move freely. These electrons are called free electrons. Light has energy, and when the electrons in a substance bump into light and absorb energy, they become free electrons, which travel along an electrical circuit and provide energy where it is needed. So a solar cell can be thought of as a pump. The sun’s light pumps the electrons up, causing current to flow, making them do work.
This begs the question. Since electrons are present in every atom, does that mean that any material can generate electricity by simply connecting electrodes and shining sunlight on it? Unfortunately, no. The problem is that different materials have different energies of resting electrons and excited electrons, which means that the amount of light energy they can absorb is different. In other words, the height of the pump is different for different materials. Light can be categorized into different energies, with infrared and visible light accounting for a high percentage of sunlight, so solar cells need to absorb infrared and visible light well. However, the pumps in a semiconductor are too high to allow the sun to push the electrons all the way up, and the pumps in a conductor are too low to do much work, absorbing lower-energy light instead of infrared and visible light. Silicon, however, is a semiconductor, and the energy required for electrons to be lifted is somewhere between that of a conductor and a semiconductor, so it can effectively absorb infrared and visible light. Silicon is the perfect pump for sunlight’s energy, just the right height.
So, can you make a solar cell using just silicon? Unfortunately, it’s not enough to just pump up electrons. Just like water is useless if it runs off to the bottom before it can be used, electrons can absorb energy and become excited, but they are useless if they can’t move through a circuit. So you need a channel that can properly transport the pumped electrons. This is why two types of silicon semiconductors, p-type and n-type, are bonded.
A silicon atom has four electrons to participate in the bond. Two atoms each contribute one electron to form a bond, so that one atom has four valence bonds to form a crystal. But if you replace some of the silicon atoms with atoms that have five bonding electrons, such as phosphorus (P), the one remaining electron becomes a free electron and can go anywhere. Semiconductors with many free electrons are called n-type semiconductors. On the other hand, if you replace some of the silicon atoms with atoms with three bonding electrons, such as boron (B), you get holes with missing electrons called holes. These holes can move like particles, which is easy to understand if you imagine a slide puzzle. Just as a slide puzzle has a blank space, and when a puzzle piece is moved into that space, the space becomes blank again, so when an electron next to the hole moves to fill the hole, it looks like the hole has moved into the electron’s place. Semiconductors with many holes are called p-type semiconductors.
By themselves, n-type semiconductors and p-type semiconductors are electrically neutral. However, when they are joined together, the free electrons from the n-type semiconductor fill the holes in the p-type semiconductor, giving the n-type semiconductor a positive charge and the p-type semiconductor a negative charge. When the electrons at this junction absorb light and become excited, the free electrons and holes are separated, and the negatively charged free electrons move toward the n-type semiconductor and the positively charged holes toward the p-type semiconductor. The electrons travel through the n-type semiconductor electrode to do work in the external circuit, and return through the p-type semiconductor’s + electrode to combine with the holes.
The sun will continue to provide enough light energy until the day humanity ends. A solar cell that harnesses this energy to generate electricity is a dream energy source. The process of making a solar cell is actually quite simple. All you need is silicon, a pump to pump up the electrons, and a p-n junction, a channel to move the electrons through the circuit. Silicon solar cells, as well as all other solar cells, just need the right pumps and channels. With a little materials science common sense, anyone can create a new innovative solar cell and contribute to the salvation of humanity.
