In this blog post, we will explore the potential and future prospects of solar cell technology as a solution to the problem of fossil fuel depletion.
Globally, most energy demand relies on fossil fuels such as oil, coal, and natural gas. Fossil fuels are energy resources formed when the remains of organisms that lived on Earth millions of years ago decomposed and were deposited under specific environmental conditions. Due to the nature of their formation process, which takes millions of years, they are classified as non-renewable resources. However, due to the continuously increasing consumption of fossil fuels since the Industrial Revolution, these resources are gradually being depleted. Furthermore, the excessive use of these fossil fuels is causing serious environmental problems, such as greenhouse gas emissions. These issues pose a threat to humanity’s sustainable development, and consequently, interest in the development of alternative energy sources is growing worldwide. Various alternative energy sources, such as solar, wind, biomass, and geothermal energy, are being researched. In particular, solar energy—which is not constrained by location and does not cause any environmental problems—is gaining prominence as an alternative to fossil fuels.
A solar cell is a device that converts and stores light energy into electrical energy. The dry-cell batteries and rechargeable batteries we commonly use are chemical cells, which differ from solar cells. Chemical cells generate electrical energy through the chemical reactions of their internal materials. Therefore, once the pre-stored material is depleted, they can no longer generate power. In contrast, solar cells are physical cells that utilize the photoelectric effect, allowing them to generate power indefinitely as long as the external energy source—light—is not depleted. The photoelectric effect refers to the phenomenon where electrons are emitted from a metal when it is exposed to light of a certain intensity or higher. When an electron is ejected, it is said to be “excited.” If it absorbs the energy of the incident light and gains more energy than its original state, it becomes excited. Such an excited electron can either return to its original position by emitting the excess energy or escape to another location while remaining in an excited state. Electrons choose the most stable path in each case; solar cells create conditions where electrons choose the latter option, allowing them to flow through the circuit.
Solar cells were first developed in the United States in 1945 and are referred to as first-generation solar cells. First-generation solar cells have a structure in which P-type (positive) and N-type (negative) semiconductors, which have different electrical properties, are joined together. Since a small amount of impurities (boron and phosphorus, respectively) are mixed into silicon to create these two semiconductors, they are also called silicon solar cells. Because boron contains 5 electrons and phosphorus contains 15 electrons, the phosphorus-doped N-type semiconductor has more electrons (-) than the boron-doped P-type semiconductor. For the same reason, the P-type semiconductor contains more holes—vacancies where electrons are missing—referred to as “holes (+).” When light energy strikes the junction of the PN semiconductor, electrons are emitted due to the photoelectric effect, increasing the number of electrons and holes within each semiconductor. The excess electrons in the n-type semiconductor attempt to move toward the p-type semiconductor, but they cannot cross the junction due to the energy difference. Therefore, when the two types of semiconductors are connected by a wire, the excess electrons from the n-type semiconductor flow along the wire toward the p-type semiconductor.
First-generation solar cells achieve an efficiency of up to 25% and are chemically stable. First-generation solar cells currently account for over 80% of the solar cell market. However, since silicon performs both the role of absorbing light and conducting electrons, efficiency decreases as silicon purity drops, requiring a high degree of precision in the manufacturing process. Additionally, because high-purity silicon is used as the primary raw material, production costs are very high. They also have the disadvantage of being inflexible and opaque, resulting in poor aesthetic appeal.
Second-generation solar cells, developed to address these issues, focused on reducing production costs. Since solar cells must be installed on a large scale over extensive areas, lower equipment costs directly translate to lower production costs. Second-generation solar cells, created by applying a thin layer of light-absorbing organic dye onto an inorganic substrate, are also known as thin-film solar cells. Although their operating principle is similar to that of first-generation solar cells, the absorption and transport of electrons do not occur simultaneously within the semiconductor but are separated. Silicon acts solely as a carrier, while the thin, widely spread organic dye absorbs solar energy. Consequently, the efficiency of the solar cell does not depend on the purity of the silicon, eliminating the need for expensive 100% pure silicon. Furthermore, since second-generation solar cells are thin, transparent, and flexible, they can be utilized in building windows, greenhouses, and small electronic devices. However, because they are thin, their efficiency is lower than that of first-generation solar cells.
Third-generation solar cells, which are currently the subject of active research, focus on increasing energy efficiency while retaining the advantages of second-generation solar cells. Dye-sensitized solar cells (DSSC), developed in 1991 by Professor Gratzel’s team at the Swiss Federal Institute of Technology, utilize extremely small nanoparticles and even smaller dye polymers. While the separation of solar energy absorption and charge transport is the same as in second-generation solar cells, the use of extremely small particles (nanoparticles and dye polymers) to increase the surface area per unit volume attracted significant attention. Since electrons can only move through the contact surface between the two particles, dye-sensitized solar cells utilizing nanoparticles were able to achieve very high energy efficiency. The U.S. Defense Advanced Research Projects Agency (DARPA) developed a hybrid tandem solar cell by combining multiple solar cells with different wavelength ranges. By utilizing a wide range of wavelengths as energy sources, they increased efficiency. Additionally, MEG solar cells—currently under active research by companies including Kolon and Samsung—have improved efficiency through a mechanism that generates two or more electron-hole pairs from a single light particle. By stacking multiple PN junctions, sunlight absorbed at the surface can be absorbed and reabsorbed multiple times.
Although their efficiency is not yet high enough to replace fossil fuels, unlike depleting chemical fuels, the energy source for solar cells is infinite. Solar energy is considered a clean energy source, unlike the fossil fuels we use, and can contribute significantly to global efforts to reduce greenhouse gas emissions. Consequently, solar cell technology is driving innovation in the energy sector and is expected to play a vital role across various industries. Furthermore, the emergence of solar cells operating on diverse principles and the ever-increasing efficiency of these cells demonstrate the potential for further research, development, and practical application. It won’t be long before we see a variety of commercialized solar cells.
