Since the industrial revolution, the consumption of fossil fuels has skyrocketed, leading to resource depletion and environmental concerns. To address this, various alternative energies are being researched, and solar cells are gaining attention. Solar cell technology is gradually improving and showing great promise as a future energy source.
The majority of the world’s energy needs are met by fossil fuels such as oil, coal, and natural gas. Fossil fuels are energy resources created by the decay and deposition of the bodies of organisms that lived on Earth millions of years ago in specific environments. Due to the nature of their formation process, they take millions of years to be created and are therefore categorized as non-native resources. However, due to the ever-increasing consumption rate of fossil fuels since the industrial revolution, they are becoming increasingly depleted. In addition, the excessive use of these fossil fuels is causing serious environmental problems such as greenhouse gas emissions. These issues threaten the sustainable development of humanity, which is why there is a growing interest in developing alternative energy sources around the world. Solar, wind, biomass, geothermal, and other alternative energies are being researched, and solar energy, in particular, is gaining traction as an alternative to fossil energy because it is not limited by location and does not cause any environmental problems.
Solar cells are devices that convert light energy into electrical energy and store it. The batteries and batteries we usually use are chemical cells, which are different from solar cells. Chemical cells use the chemical reactions of the substances they contain to generate electrical energy, so they can’t generate any more power once they’re depleted. Solar cells, on the other hand, are physical cells that utilize the photoelectric effect and can generate electricity indefinitely as long as they don’t run out of light, an external source of energy. The photoelectric effect is a phenomenon in which electrons pop out of a metal when it is bombarded with a certain intensity of light. The electrons are called excited. They become excited when they absorb the light energy and have more energy than their original state. The excited electron can either expel the excess energy and return to its original position, or it can escape to another location in an excited state. In each case, the electrons choose the most stable way, and solar cells provide a situation where the electrons choose the latter, allowing them to flow through the circuit.
Solar cells were first developed in the United States in 1945, and are known as first-generation solar cells. First-generation solar cells are made up of P (positive) and N (negative) semiconductors, which have different electrical properties. They are also called silicon solar cells because the two semiconductors are made by mixing silicon with a small amount of foreign substances (boron and phosphorus, respectively). Since boron contains five electrons and phosphorus contains 15 electrons, there are more electrons (-) in the N-type semiconductor with phosphorus than in the P-type semiconductor with boron. For the same reason, P-type semiconductors have more holes with protruding electrons called “holes (+)”. When light energy is applied to the junction of a PN semiconductor, the photoelectric effect causes electrons to jump out, further increasing the number of electrons and holes in each semiconductor. The electrons that are oversaturated in the N-type semiconductor try to move to the P-type semiconductor, but they cannot pass through the junction due to the energy difference. So when the two types of semiconductors are connected by a wire, the electrons from the supersaturated N-type semiconductor migrate along the wire to the P-type semiconductor.
The efficiency of the first generation of solar cells can reach 25%, and they are chemically stable. They account for more than 80% of the solar cell market today. However, because silicon is responsible for both absorbing light and transferring electrons, the efficiency decreases as the purity of the silicon decreases, requiring a high degree of precision in the manufacturing process. In addition, the cost of production is very high because high-purity silicon is used as the main raw material. Another disadvantage is that it is inflexible and opaque, making it less aesthetically pleasing.
The second-generation solar cells developed to address these issues have focused on lowering the cost of production. This is because solar cells need to be installed on a large scale over a large area, so the lower the unit cost of the installation, the lower the production cost. Second-generation solar cells are also called thin-film solar cells because they are made of a thin layer of sunlight-absorbing organic dye on an inorganic substrate. The principle of operation is similar to that of first-generation solar cells, but the absorption and transfer of electrons are separated rather than occurring simultaneously within the semiconductor. The silicon only acts as a carrier, and a thin, widely-spread organic dye absorbs the sun’s energy. This means that the efficiency of the solar cell doesn’t depend on the purity of the silicon, eliminating the need to use expensive, 100% pure silicon. Second-generation solar cells are also thin, transparent, and bendable, allowing them to be used in building windows, greenhouses, and small electronics. However, because they are thin, they are less efficient than first-generation solar cells.
Third-generation solar cells, which have been under active research in recent years, focus on increasing energy efficiency while maintaining the advantages of second-generation solar cells. Dye-sensitized solar cells (DSSCs), developed by Prof. Gratzel’s team at the Swiss Federal Institute of Technology in 1991, use very small nanoparticles and even smaller dye polymers. The separation of solar energy absorption and charge transfer is the same as in second-generation solar cells, but the use of very small particles (nanoparticles and dye polymers) increases the surface area per unit volume. Since electrons can only move through the contact surface of two particles, dye-sensitized solar cells using nanoparticles can be very energy efficient. The US DARPA has developed a hybrid tandem solar cell that combines several solar cells with different wavelength regions. In addition, MEG solar cells, which are currently being actively researched by companies including Kolon and Samsung, increase efficiency through a mechanism that generates two or more electron-hole pairs from a single light particle. By stacking several different PN semiconductors, they are able to absorb and reabsorb sunlight received on their surface multiple times.
Although not yet efficient enough to replace fossil fuels, solar cells have an inexhaustible energy source, unlike chemical fuels, which are depleting. Solar energy is considered clean energy, unlike the fossil fuels we use, and can contribute significantly to global greenhouse gas reduction efforts. Because of this, solar cell technology is expected to revolutionize the energy sector and play an important role in various industries. In addition, the emergence of solar cells that operate on different principles and the increasing efficiency of solar cells demonstrates the potential for further research and commercialization of solar cells. We can expect to see a wide variety of commercialized solar cells in the near future.
