In this blog post, we will look at whether solar cells can be an alternative to solve the problems of excessive resource consumption and energy depletion.
Imagine a world without the sun. The Earth would be a barren planet where no life could exist. The sun has been continuously sending a tremendous amount of light energy to the Earth since its birth. Almost all living things on Earth live off the energy stored through photosynthesis in plants. Petroleum, the most commonly used energy source by humans, is also the result of the transformation of the remains of long-dead organisms over a long period of time in the ground. Modern civilization is consuming and depleting the resources of accumulated solar energy. If all energy sources are exhausted, where will humanity get energy from? Is it possible to directly use the light energy from the sun even at this very moment? The answer lies in solar cells.
Solar cells are devices that convert the light energy of the sun into electricity. Among them, silicon solar cells are currently the most widely used due to their high efficiency, relatively low cost, and ease of manufacture. The structure of a silicon solar cell is quite simple because it is just two types of silicon semiconductors joined together. So how do silicon semiconductors convert light into electricity? The secret lies in the electrons inside the silicon. In their stable state, electrons are bound to the atomic nucleus and cannot move freely. However, when electrons are excited by energy, they become free to move. The excited electrons are called free electrons. Light has energy, and electrons in a material absorb energy by colliding with light and become free electrons, which move along an electric circuit and provide energy where it is needed. Therefore, a solar cell can be thought of as a kind of pump. Just as the light of the sun pumps electrons up, causing an electric current to flow and do work.
Then, the question arises: “If electrons are present in all atoms, can any material produce electricity just by connecting electrodes and shining sunlight on it?” Unfortunately, the answer is no. The problem is that the difference between the energy of stable electrons and the energy of excited electrons varies from material to material, and the amount of light energy that can be absorbed varies accordingly. In other words, the height of the pump is different for each material. Light can be classified into various types according to its energy, and among them, infrared rays and visible rays account for a high proportion of sunlight. Therefore, solar cells must absorb infrared rays and visible rays well. However, the pump of a non-conductor is too high, so the sun cannot pull the electrons all the way up, and the pump of a conductor is too low, so it absorbs lower-energy light instead of infrared and visible light and does not do much. However, the energy required for electrons to move in a semiconductor, silicon, is about halfway between that of a conductor and a non-conductor, so it can effectively absorb infrared and visible light. Silicon is a pump of just the right height, perfectly suited to the energy of sunlight.
So, can solar cells be made simply from silicon? Unfortunately, pumping electrons up is not enough. Just as it is useless to pump water and have it flow back down to the bottom before it is used where you want it, it is useless for electrons to absorb energy and become excited if they cannot move to a circuit. Therefore, a waterway is needed to properly transport the electrons that have been pumped up. This is why p-type and n-type silicon semiconductors are joined together.
A silicon atom has four electrons participating in bonding. Two atoms each provide one electron to form a bond, and four such bonds form a crystal. However, if a part of a silicon atom is replaced with an atom with five electrons participating in bonding like phosphorus (P), the remaining electron becomes a free electron and can move anywhere. Semiconductors with many free electrons like this are called n-type semiconductors. On the other hand, if you replace some of the silicon atoms with atoms with three electrons that participate in the bond like boron (B), you will have a hole with an electron called a hole. This hole can move like a particle, which is easy to understand if you imagine a slide puzzle. There is a blank space in the slide puzzle. When a piece of the puzzle moves into this blank space, it looks as if the space where the piece was is now blank again, and when the electrons next to the hole move to fill the hole, it looks as if the hole has moved into the space where the electrons were. A semiconductor with many holes like this is called a p-type semiconductor.
N-type and P-type semiconductors are electrically neutral on their own. However, when the two are connected, the free electrons of the N-type semiconductor fill the holes of the P-type semiconductor, resulting in a positive charge on the N-type semiconductor and a negative charge on the P-type semiconductor. At this junction, when electrons absorb light and become excited, free electrons and holes are separated, and the negatively charged free electrons move toward the n-type semiconductor and the positively charged holes move toward the p-type semiconductor. Electrons that have moved through the n-type semiconductor electrode will move to the external circuit to do work, and then return through the p-type semiconductor + electrode to combine with the holes.
The sun will continue to provide sufficient light energy even on the day that humanity perishes. Solar cells that use this to generate electricity are truly a dream energy source. The method of making solar cells is simpler than you might think. All you need is silicon, a pump that draws electrons out, and pn junctions, which are channels that transfer electrons to circuits. Not only silicon solar cells, but all other solar cells require only the right pumps and channels. With a little knowledge of materials engineering, anyone can create new, innovative solar cells and contribute to the salvation of humanity.
