Unravelling Singlet Fission in Polymer Materials: Mechanisms, Challenges, and Prospects for High-Efficiency Solar Cells

Singlet fission (SF) is a process that produces two triplets after the absorption of one photon. This process can be utilized in solar cells to improve efficiency by capturing the energy generally lost as heat (thermalization loss). The SF-based solar cells utilizing conjugated polymers are promising because of some unique advantages of polymer materials, such as flexibility, easy film fabrication and solution processability, tunable chemical functionalization, and broad absorption bands. However, only a few SF-polymers are reported because SF in polymers is less understood. Therefore, it is necessary to review SF in polymer materials and determine a direction to promote their use in solar cells. This Review aims to provide an up-to-date account of the investigation of the SF process in polymers, including the preferred mechanism. The examples of SF in the donor–acceptor polymers and its process as intramolecular singlet fission have been discussed. In addition, the SF in the end-functionalized and pendant polymers are discussed. Furthermore, we provided challenges and possible pathways for the enhancement of the SF efficiency in the polymers.