Magnetic dipolar interaction between correlated triplets created by singlet fission in tetracene crystals

Singlet fission can potentially break the Shockley-Queisser efficiency limit in single-junction solar cells by splitting one photoexcited singlet exciton (S₁) into two triplets (2T₁) in organic semiconductors. A dark multiexciton state has been proposed as the intermediate connecting S₁ to 2T₁. However, the exact nature of this multiexciton state, especially how the doubly excited triplets interact, remains elusive. Here we report a quantitative study on the magnetic dipolar interaction between singlet-fission-induced correlated triplets in tetracene crystals by monitoring quantum beats relevant to the multiexciton sublevels at room temperature. The resonances of multiexciton sublevels approached by tuning an external magnetic field are observed to be avoided, which agrees well with the theoretical predictions considering a magnetic dipolar interaction of ∼0.008 GHz. Our work quantifies the magnetic dipolar interaction in certain organic materials and marks an important step towards understanding the underlying physics of the multiexciton state in singlet fission.