With shorter spring and fall seasons and more frequent weather extremes, South Korea’s four seasons are likely to become increasingly subtropical. Solar energy is gaining attention as one of the ways to reduce carbon dioxide emissions, a major contributor to global warming. Solar cells, which use the photoelectric effect of silicon semiconductors to generate electricity, are currently 10 to 20 percent efficient, and research is ongoing to improve their efficiency. Together, our small actions and eco-friendly policies can help slow global warming and create a sustainable future.
Spring and fall are getting shorter. The cherry blossoms are blooming earlier and earlier every year, and the fall foliage is fading away before we know it. Every year, the National Weather Service reports record highs and record lows. We’re suffering through increasingly intense heat and cold. It’s not just a matter of hot and cold. At this rate, Korea will one day enter a subtropical climate. For future generations, “Korea, a country with four distinct seasons” will be a distant memory. Global warming, the main cause of such extreme weather, must be delayed as soon as possible.
One of the ways to reduce carbon dioxide emissions, which is the cause of global warming, is photovoltaic energy. We can generate electricity from the sun, the light bulb that circles the earth and emits light continuously. So how do we generate electricity from sunlight? First, let’s look at some basic concepts to understand solar energy.
‘Photovoltaic’ is the Greek word for light, and ‘Volta’ is the Italian inventor of batteries. We often use the term solar cell interchangeably with solar energy. However, solar energy is a bigger concept than solar cells. Solar energy simply means harnessing sunlight and converting it into other forms of energy for use. Solar cells are one of the technologies that utilize solar energy. The converted energy is electricity. The process of generating electricity in a solar cell is explained by the principles of semiconductors and the photoelectric effect. Let’s take a quick look at them below.
Oxygen is the most abundant of the elements that make up our earth’s crust, followed by silicon (Si). Silicon is the main component of the soil, sand, and rocks we see around us. Because of its inexhaustible nature, silicon is often used as the main ingredient in semiconductors. All elements, including silicon, have an atomic nucleus made up of neutrons and protons and electrons orbiting around it. The electrons build their own apartments around the nucleus. The apartment is called an electron shell. The first floor of the apartment is very small and can only hold two electrons, while the second floor can hold up to eight electrons. The electrons that live on the top floor of the apartment are the outermost electrons. With the exception of hydrogen and helium, all elements like to have a full house of eight outermost electrons. This is because electrons that have lost their family members will easily drift apart. Silicon, which has 14 electrons, is satisfied with two in the first floor shell and eight in the second floor shell, leaving four outermost electrons. A silicon that is unstable because it lacks four electrons will join hands with its neighbors to solve the problem. It bonds by sharing its four outermost electrons with each other. This bond is called a covalent bond, and the silicones form a covalent crystal, acting as if they have eight outermost electrons.
What happens if you force phosphorus (P), which has five outermost electrons, into a group of silicones that have gotten along well with each other? Phosphorus gives up one of its outermost electrons and forms a covalent bond with the silicones. Because of that one wasted electron, the silicon crystal becomes an N-type (negative type) semiconductor. Conversely, if you force boron (B), which has three valence electrons, into a silicon crystal, it will behave as if it has seven valence electrons. A silicon crystal with a relative shortage of one electron becomes a positive-type semiconductor. When an N-type semiconductor and a P-type semiconductor are junctioned, the electrons move across the junction to generate electricity. If you connect the electron movement between the two semiconductors with a metal of high conductivity and allow it to move through the wire, you can generate a flow of electrons, or current.
However, the one remaining electron in an n-type semiconductor cannot easily find a home in a p-type semiconductor. This is because it takes a minimal force for an electron to jump out of the n-type semiconductor apartment to get to the p-type semiconductor. This minimal force can be obtained from light energy with a finite frequency. The energy of light increases proportionally to its frequency, and the minimum frequency of light that can cause an electron to jump out of a metal is called the limit frequency. In a cell made of an N-type semiconductor and a P-type semiconductor, it is sunlight that provides the force for electrons to move, which is why it is called a solar cell. The photoelectric effect is a phenomenon in which electrons that have absorbed energy jump out due to collision with photons, particles of light with energy in the light. The photon is the helper that helps the electron jump out of the apartment. In a solar cell, the photons in the sunlight give the electrons in the n-type semiconductor the power to jump out of the apartment. In the case of sunlight, however, it’s a mixture of different types of light, consisting of infrared, visible, and ultraviolet light. Among these, the visible and ultraviolet regions, which have a frequency above a certain value called the limit frequency, are the components of light that cause the photoelectric effect in solar cells.
To summarize the principle of solar cells mentioned above, when an N-type semiconductor receives sunlight, photons in the sunlight region with a frequency above a certain value move the remaining electrons in the N-type semiconductor to the P-type semiconductor, generating a current. The efficiency of solar cells developed to date is between 10 and 20 percent. That doesn’t make them cheap. If we want to slow down global warming, it’s worth further researching this bright spot. Global warming is a problem we all need to solve.
But in addition to solar energy, we can also make small changes in our daily lives to reduce carbon dioxide emissions. For example, taking public transportation, using energy-efficient appliances, and recycling more thoroughly are all small actions that can make a big difference. It’s also important for governments and businesses to work together to actively promote green policies and increase investment in renewable energy. Hopefully, our small efforts will add up and we can leave a beautiful planet for our children and grandchildren.
Global warming is a complex and multidimensional problem, but the key to solving it lies in the hands of all of us. It’s time to take action now to ensure a sustainable future.
