In this blog post, we’ll explain the structure and operating principles of DSSCs (dye-sensitized solar cells) in an easy-to-understand way.
Energy plays a vital role in the many benefits our civilization enjoys. As the limitations of conventional energy sources and environmental destruction become increasingly apparent due to the risk of fossil fuel depletion, climate anomalies, and global warming, renewable energy technologies are gaining attention, and solar cells are emerging as a key alternative.
Solar cells can be broadly divided into inorganic and organic types, with inorganic cells further categorized into silicon-based and compound-based types. Silicon-based cells offer high efficiency and stability but are expensive, inflexible, and opaque. Compound-based cells, on the other hand, offer not only efficiency and stability but also transparency, flexibility, and low cost; however, they present toxicity concerns. Although organic solar cells have limitations in terms of relatively low efficiency and stability, they are actively being researched due to their advantages of being inexpensive, transparent, and flexible. A representative example among organic solar cells is the DSSC (Dye-Sensitized Solar Cell), and this article explains the structure and operating principle of DSSCs.
A solar cell is a device that absorbs solar energy through the photoelectric effect and converts it into electrical energy. The photoelectric effect refers to the phenomenon in which electrons that were previously bound are released when an object absorbs a certain amount of light energy. DSSCs use dye molecules with a selective absorption band for visible light as light absorbers to effectively utilize the visible portion of the solar spectrum, and their operating principle is similar to that of photosynthesis. The general structure of a DSSC is sandwich-like, consisting of a light-absorbing dye, a semiconductor oxide (primarily TiO₂) that receives electrons from the dye and transfers them to a conductive substrate, a transparent conductive substrate (TCO) that simultaneously transmits light and facilitates electron transport, and an electrolyte that regenerates the dye and completes the electron pathway.
When sunlight strikes the cell, dye molecules absorb photons and emit electrons, which are then injected into a semiconductor oxide such as TiO2. To increase the photoelectric conversion efficiency, a dye capable of absorbing a broad wavelength range is required. To increase the dye loading, nanoparticles are used on the electrode surface; as particle size decreases, the surface area increases, allowing for the adsorption of more dye. However, if the particles become too small, although the loading increases, the increased surface area leads to a higher frequency of electron recombination with the dye, which is a drawback.
Oxidizes studied as electrodes for DSSCs include TiO₂, SnO₂, ZnO, and Nb₂O₅, among which TiO₂ is known to be the most efficient. Electrons passing through the oxide flow to the external circuit via a transparent electrode layer coated on a transparent conductive substrate. The transparent conductive substrate consists of glass coated with a transparent conductive oxide and must possess both low sheet resistance and high transmittance. While ITO was once the standard material, the use of FTO—which has relatively good compatibility with TiO₂—has expanded due to the scarcity of indium resources.
The most significant obstacle to reducing the unit cost of DSSCs is the expensive transparent electrode. Since the cost of substrates coated with transparent electrodes accounts for a significant portion of manufacturing costs, resolving this issue is urgent. When electrons are emitted from the dye, the resulting vacancy is called a hole; the electrolyte supplies electrons to this hole, thereby regenerating the dye. Once the dye is regenerated and the electrons that flowed through the external circuit return to the counter electrode, the electrical circuit is completed.
South Korea relies heavily on imports for its energy sources, which are primarily limited to fossil fuels, raising concerns about long-term depletion and price increases. Solar cells utilize the abundant resource of sunlight, offering the significant advantages of long-term reliability after installation and environmental friendliness. However, they currently have relatively low efficiency, requiring large installation areas, and are vulnerable to output fluctuations due to solar irradiance as well as weather and regional influences. To overcome these limitations, various types of solar cells—such as flexible thin-film solar cells that can be bent or curved—are being developed, and there are already examples of solar cells being applied to building exteriors and roofs. Although they have not yet reached the level of a primary energy source, they have great potential to grow into a next-generation energy source as research and technology advance.
