Dielectric Constant: A Key Factor Limiting the Future Development of Organic Solar Cells

Nowadays, with the rapid development of nonfullerene acceptors, organic solar cells (OSCs) have been pushed to the level of industrialization. One of the breakthroughs is the significant reduction of voltage losses by minimizing the energy offset between donor and acceptor materials. Therefore, the local excited (LE) states would intermix with charge transfer (CT) states through hybridization and thermal population effects, and form the LE‐CT states. According to the Shockley–Queisser theory, an ideal semiconductor in a solar cell should exhibit 100% of photoluminescence quantum yield and perfect charge separation. As reported, the strongly intermixed LE‐CT states indeed enhance the photoluminescence quantum yield, but it delays the CT generation and accelerates the charge recombination due to the strong bound excitons. Hence, it is the time to focus on increasing the dielectric constant ( ε r ) of organic materials. This review systematically summarizes the influence of ε r on OSC performance, such as exciton dissociation, CT state characteristics, and voltage loss. Two major techniques of impedance spectroscopy and spectroscopic ellipsometry are introduced for ε r measurement. Then, strategies for increasing ε r by backbone engineering, side‐chain polarity modification, and additive/third‐component incorporation are discussed. Finally, we proposed the remained problems that should be concerned for the future studies of dielectric constant.