This blog post examines the potential and future outlook of solar cell technology as a response to the problem of fossil fuel depletion.
Globally, the majority of 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, the consumption rate of fossil fuels has continued to increase since the Industrial Revolution, leading to their gradual depletion. Furthermore, the excessive use of these fossil fuels is causing serious environmental problems, such as greenhouse gas emissions. These issues act as factors threatening humanity’s sustainable development, and consequently, interest in alternative energy development is growing worldwide. Various alternative energies such as solar power, wind power, biomass, and geothermal energy are being researched. Among these, solar energy, which is not constrained by location and causes no environmental problems, is particularly gaining prominence as a replacement for fossil fuels.
A solar cell is a device that converts and stores light energy into electrical energy. The batteries we commonly use are chemical cells, distinct from solar cells. Chemical cells generate electrical energy through the chemical reactions of their internal materials. Therefore, once the stored materials are depleted, they can no longer generate power. In contrast, solar cells are physical cells that utilize the photovoltaic effect. As long as the external energy source—light—is not depleted, they can generate power indefinitely. The photovoltaic effect refers to the phenomenon where electrons are emitted from a metal when exposed to light of a certain intensity or higher. Electrons that are ejected are said to be excited. They become excited when they absorb the energy of the incident light and possess more energy than their original state. These excited electrons can either return to their original position while emitting the excess energy, or escape to another location while remaining in their excited state. Electrons choose the most stable path in each case. Solar cells provide a situation where electrons choose the latter path, allowing them to flow through a 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 where a P-type (positive) semiconductor and an N-type (negative) semiconductor, which have different electrical properties, are joined together. Since silicon is doped with small amounts of impurities (boron and phosphorus, respectively) to create these two semiconductors, they are also called silicon solar cells. Boron contains 5 electrons and phosphorus contains 15 electrons. Therefore, the phosphorus-doped N-type semiconductor has more electrons (-) than the boron-doped P-type semiconductor. For the same reason, the P-type semiconductor has more holes, called ‘holes (+)’, where electrons have been removed. When light energy is applied to the junction surface of the PN semiconductor, electrons are emitted due to the photoelectric effect, increasing the number of electrons and holes within each semiconductor. Electrons oversaturated in the N-type semiconductor attempt to move to the P-type semiconductor but cannot cross the junction due to the energy difference. Therefore, when the two types of semiconductors are connected by a wire, the oversaturated electrons in the N-type semiconductor move along the wire to the P-type semiconductor.
First-generation solar cells achieve an efficiency of 25% and are chemically stable. First-generation solar cells currently dominate over 80% of the solar cell market. However, since silicon performs both the role of absorbing light and transferring electrons, efficiency decreases as silicon purity drops, demanding high precision in the manufacturing process. Furthermore, using high-purity silicon as the primary material results in very high production costs. They also suffer from disadvantages such as low flexibility, opacity, and poor aesthetic appeal.
To address these issues, second-generation solar cells were developed with a focus on reducing production costs. Since solar cells must be installed over large areas on a massive scale, lower equipment costs directly translate to lower production expenses. These second-generation solar cells, which involve thinly coating an inorganic substrate with an organic dye that absorbs sunlight, are also called thin-film solar cells. Their operating principle is similar to first-generation solar cells, but the absorption and transport of electrons are separated and occur in different parts of the semiconductor. Silicon only acts 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. Additionally, second-generation solar cells are thin, transparent, and flexible, making them suitable for applications like building windows, greenhouses, and small electronic devices. However, their thinness results in lower efficiency compared to first-generation cells.
Third-generation solar cells, currently under active research, focus on enhancing energy efficiency while retaining the advantages of second-generation 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 identical to second-generation solar cells, the use of extremely small particles (nanoparticles and dye polymers) significantly increased the surface area per unit volume, drawing considerable attention. Since electrons can only move through the contact interface between the two particles, dye-sensitized solar cells utilizing nanoparticles achieved a substantial increase in energy efficiency. The U.S. DARPA developed a hybrid tandem solar cell combining multiple solar cells with different wavelength ranges. This increased efficiency by utilizing various wavelength regions as energy sources. Additionally, MEG solar cells, currently under active research by companies including Kolon and Samsung, enhance efficiency through a mechanism generating two or more electron-hole pairs from a single light photon. This is achieved by stacking multiple PN semiconductor layers, enabling the absorption and reabsorption of sunlight received at the surface multiple times.
Although 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 clean energy, unlike the fossil fuels we use, and can significantly contribute to global greenhouse gas reduction efforts. Consequently, solar cell technology is expected to drive innovation in the energy sector and play a vital role across various industries. Furthermore, the emergence of solar cells operating on diverse principles and the steadily increasing cell efficiency demonstrate the potential for research advancement and practical application of solar cells. Soon, we will likely see a variety of commercialized solar cells.
