What's Next for Organic Solar Cells? The Frontiers and Challenges

Abstract

The power conversion efficiency (PCE) of organic solar cells (OSCs) is improved dramatically in recent years and now approaches >19% for single-junction cells and >20% for tandem cells. Therefore, the practical use of OSCs is becoming a reality. This perspective summarizes the state of the art of OSC characteristics and discusses the challenges that remain in further improving PCE. The short-circuit current density (J SC) of the state-of-the-art OSCs almost approaches 30 mA cm−2. As the internal quantum efficiencies of these devices exceed 90%, for further improvement in J SC, it is necessary to suppress reflection at interfaces and increase the active layer thickness to absorb as many photons as possible. Further suppression of the nonradiative voltage loss is imperative, as there is still large room for improvement compared to inorganic and perovskite counterparts. Hence, increasing the exciton lifetime and photoluminescence quantum yield of nonfullerene acceptors are pivotal to improving the open-circuit voltage of OSCs. The fill factors of the latest OSCs approach 80%; however, because the optimized active layer thickness remains ≈100 nm, further suppression of bimolecular charge recombination is needed. Finally, this perspective discusses to what extent PCE can be improved and what can be done to achieve this.