This blog post explores whether solar cells can serve as an alternative solution to the problems of excessive resource consumption and energy depletion.
Imagine an Earth without the sun. Earth would become a barren planet where no life could possibly exist. Since Earth’s birth, the sun has continuously sent immense amounts of light energy to our planet. Nearly all life on Earth thrives on energy stored through plant photosynthesis. Even petroleum, humanity’s most widely used energy source, is the result of long-dead organisms transformed deep within the earth over eons. Modern civilization is excessively consuming and depleting resources that are accumulated solar energy. If all energy sources were to be depleted, where would humanity obtain energy? Could we not directly utilize the light energy pouring from the Sun even at this very moment? The answer lies in solar cells.
Solar cells are devices that convert the Sun’s light energy into electricity. Among these, silicon solar cells are currently the most widely used due to their high efficiency, relatively low cost, and simplicity of manufacture. The structure of a silicon solar cell is quite simple, consisting only of two types of silicon semiconductors joined together. So how does a silicon semiconductor convert light into electricity? The secret lies in the electrons within the silicon. In a stable state, electrons are bound to the atomic nucleus and cannot move freely. However, when an electron absorbs energy and becomes excited, it gains the ability to move freely. These energized electrons are called free electrons. Light carries energy. When light collides with electrons within a material, the electrons absorb this energy and become free electrons. These free electrons then travel along an electrical circuit, delivering energy where it’s needed. Therefore, a solar cell can be thought of as a kind of pump. Sunlight acts like a pump, lifting electrons to create a current that performs work.
This raises a question: since electrons exist in all atoms, can any material produce electricity simply by connecting electrodes and exposing it to sunlight? Unfortunately, no. The problem is that the difference in energy between stable electrons and excited electrons varies by material, meaning the light energy they can absorb differs. In other words, the pump height varies by material. Light can be classified into various types based on its energy. Among these, infrared and visible light constitute a high proportion of sunlight. Therefore, solar cells must efficiently absorb infrared and visible light. However, the pump height in insulators is too high, preventing the sun from fully lifting electrons to the top. Conversely, the pump height in conductors is too low, causing them to absorb lower-energy light instead of infrared and visible light, rendering them largely ineffective. Silicon, however, being a semiconductor, has an energy requirement for electron pumping that lies between conductors and insulators. This allows it to effectively absorb both infrared and visible light. Silicon can be thought of as a pump with just the right height, perfectly matched to the energy of sunlight.
So, can solar cells be made simply from silicon alone? Unfortunately, merely pumping electrons upward is not sufficient. Just as water pumped up is useless if it leaks back down before reaching its intended use, electrons that absorb energy and become excited are useless if they cannot move into the circuit. Therefore, a proper pathway is needed to transport the electrons pumped up. This is precisely why two types of silicon semiconductors—p-type and n-type—are joined together.
Silicon atoms have four electrons participating in bonding. Two atoms each contribute one electron to form a bond, and thus one atom forms four bonds to create a crystal. However, if some silicon atoms are replaced with atoms like phosphorus (P), which have five electrons participating in bonding, the remaining single electron becomes a free electron that can move anywhere. A semiconductor with many such free electrons is called an n-type semiconductor. On the other hand, if some silicon atoms are replaced with atoms like boron (B), which contribute three electrons to the bond, a hole is created where an electron is missing. This hole can move like a particle; imagining a sliding puzzle makes it easier to understand. A slide puzzle has one empty slot. When a puzzle piece moves into this slot, the space it left behind becomes empty again. Similarly, when an electron adjacent to a hole moves to fill it, the hole appears to move into the space the electron occupied. A semiconductor with many such holes is called a p-type semiconductor.
Both n-type and p-type semiconductors are electrically neutral on their own. However, when joined at a junction, the free electrons from the n-type semiconductor fill the holes in the p-type semiconductor. This causes the n-type semiconductor side to carry a positive charge (+), and the p-type semiconductor side to carry a negative charge (-). At this junction, when electrons absorb light and become excited, free electrons and holes separate. The negatively charged free electrons move toward the n-type semiconductor, while the positively charged holes move toward the p-type semiconductor. The electrons that move through the n-type semiconductor electrode travel to the external circuit to perform work, and then return through the p-type semiconductor’s positive electrode to recombine with the holes.
The sun will provide ample light energy without fail until the day humanity perishes. Solar cells that harness this energy to produce electricity are truly a dream energy source. The method for making solar cells is simpler than one might think. All that’s needed is silicon, which acts as a pump to draw up electrons, and a pn junction, which serves as the channel to move electrons into the circuit. Not only silicon solar cells, but all other solar cells require only a suitable pump and channel. With just a little knowledge of materials engineering, anyone could create a new, innovative solar cell and contribute to humanity’s salvation.
