Geminate and non-geminate recombination of triplet excitons formed by singlet fission

Singlet fission is a promising route to enhance solar cells by harvesting two electron-hole pairs from high-energy photons. Through singlet fission, an optically generated singlet exciton is transformed into two spin-correlated triplet excitons, which serve as a unique signature of the process. We use optically detected magnetic resonance to identify and study triplet excitons created through singlet fission in the solution-processable small molecule TIPS-tetracene (bis(triisopropylsilylethynyl)tetracene). Through changes in photoluminescence under spin resonance, we identify geminate recombination of triplet pairs directly following singlet fission, as well as recombination from bimolecular triplet-triplet annihilation. We show that both processes can be present in spin-coated films, and correlate the two distinct annihilation pathways to film morphology.