In this blog post, we’ll explore the background, operating principles, and recent research trends of organic solar cells—which are evolving into various forms beyond traditional silicon solar cells.
Limitations of Conventional Solar Cells and the Emergence of Organic Solar Cells
When visiting government offices or schools, don’t just look at the entrance—take a moment to glance up at the roof, and you’ll easily spot gleaming solar panels. Solar cells are now a common power-generation technology in everyday life and are so widely known that their basic principles are even covered in South Korea’s school curriculum. However, a review of recently published papers and research trends reveals that the field of solar cells continues to evolve rapidly. This is because new types of solar cells—beyond conventional inorganic solar cells—are being continuously developed. Representative examples include organic solar cells and dye-sensitized solar cells. In this article, we will focus on organic solar cells, which use organic materials, to examine their origins, operating principles, and current research trends.
There is a reason why solar cells are often referred to as an “old” technology. The photoelectric effect, which forms the basis of solar cells, was discovered in the 19th century, and silicon solar cells have been in practical use since the mid-20th century, finding applications in various fields such as the space industry and power generation facilities. Subsequently, with the development of the energy industry, commercialization gained full momentum, and silicon-based inorganic solar cells, which offer high efficiency and stability, remain the most widely used to this day. However, as existing silicon solar cell technology has reached a highly mature stage, the potential for further efficiency improvements is becoming increasingly limited. Consequently, research is actively underway not only to improve existing materials but also to develop next-generation solar cells with broader performance capabilities and applications using new materials; organic solar cells are gaining attention as a prime example of this trend. While most conventional inorganic solar cells are manufactured in the form of rigid, thick plates, recent trends in electronic devices are moving toward thinner, lighter designs in a variety of shapes. In line with this trend, there is a growing demand for solar cell technology that is thin, capable of producing a variety of colors, and even flexible. Organic solar cells are attracting significant attention as next-generation solar cells capable of meeting these demands.
How do organic solar cells generate electricity?
While inorganic and organic solar cells differ not only in appearance but also in internal structure, they share the same basic principle of converting light into electricity. Since electric current is generated by the movement of electrons, the process of using sunlight to separate and move electrons plays a key role in both types of solar cells. While inorganic solar cells use the junction between a p-type semiconductor and an n-type semiconductor to separate electrons and holes, in organic solar cells, electron donors and electron acceptors perform this role.
When sunlight is absorbed, electron-hole pairs are generated in the donor material. The generated electron-hole pairs move in various directions and, upon reaching the interface with the acceptor, separate into electrons and holes. The electrons then move along the acceptor, and the holes move along the donor, thereby generating an electric current. Therefore, the donor material must effectively absorb light to generate a large number of electron-hole pairs, while the acceptor material must effectively separate the electrons and holes to facilitate the smooth movement of electrons.
Ultimately, the efficiency of an organic solar cell is determined by how many electron-hole pairs are generated and how effectively those pairs are separated. Current research on organic solar cells is focused on achieving high power conversion efficiency and stability, as well as reducing production costs; various studies are underway to improve these two factors by developing new materials or modifying the structure.
How far has research on organic solar cells progressed?
Electrons and holes do not naturally exist in a separated state; rather, holes are formed when electrons absorb energy and transition to a higher energy state. Therefore, electrons and holes can easily recombine. In organic solar cells, electron-hole pairs exist for only a very short time, and the distance they can travel is also very short. Consequently, if the donor layer is too thick, the likelihood of light absorption and the generation of electron-hole pairs increases; however, the probability that the generated electrons and holes will recombine before they can separate also rises.
A representative structure developed to address this issue is the blend structure. The blend structure involves uniformly mixing the donor and acceptor on a very small scale, which significantly increases the surface area at the interface where the two materials meet and reduces the distance that electron-hole pairs must travel. As a result, the probability of electron-hole separation increases, and the power conversion efficiency of the solar cell improves.
Organic solar cells are classified into polymer solar cells and small-molecule solar cells depending on the materials used, and are often named directly after the materials themselves. This indicates that active research is underway on a wide variety of materials and structures, but it also means that no single technology has yet been established that can completely replace conventional silicon solar cells in every respect. However, thanks to their advantages—such as being thin, lightweight, and capable of being produced in various colors and shapes—organic solar cells are expanding their applications into areas where conventional solar cells were difficult to implement, such as building facades, wearable devices, vehicles, and portable electronic devices. While it remains to be seen which of the various next-generation solar cell technologies will become mainstream, organic solar cells are being actively researched as one of the leading candidates for future solar power technology.
