Unstable oil prices, climate change, and safety concerns about nuclear energy have emphasized the need to develop sustainable and eco-friendly alternative energy sources, and countries around the world are focusing on developing eco-friendly alternative energy sources such as wind power, hydrogen/fuel cells, and solar cells. Among them, solar cells using sunlight, an inexhaustible clean energy source, are attracting attention as a next-generation energy production device in that they are largely free of regional dependence unlike other energy sources that can only be used in specific regions, and they are simple to maintain and can be applied from portable power sources to large-scale power plants.Accordingly, various types of solar cells are being researched, and they are classified into inorganic solar cells, which are made up of inorganic materials such as silicon and compound semiconductors, and organic solar cells, which contain organic materials. Currently, commercially available and highly efficient inorganic solar devices have limited applications due to economic, stability, or toxicity issues and are generally inflexible. Organic solar cells, on the other hand, are lightweight, flexible, and can be mass-produced in a roll-to-roll continuous process, requiring less energy to produce and a shorter production time, thus dramatically reducing manufacturing costs.1,2 However, organic solar cells have lower efficiencies than commercially available inorganic solar cells, in part because of the relatively low external quantum efficiency of organic solar cells. In contrast, the external quantum efficiency of silicon solar cells, for example, is close to 100% up to the near-infrared region.