An energetics perspective on why there are so few triplet-triplet annihilation emitters

The efficiency of organic solar cells may be increased by utilizing photons with energies below the band gap of the absorber. This may be enabled by upconversion of low energy photons into high energy photons via triplet-triplet annihilation (TTA). The quantum yield of TTA is often low due to competing processes. The singlet pathway where a high-energy photon is emitted has a 1/9 probability according to the Clebsch-Gordon rules. The quintet pathway is typically too high in energy to be accessible, leaving the triplet pathway as the main competing process. Using many-body perturbation theory in the GW approximation and the Bethe–Salpeter equation, we calculate the energy release in both the singlet and triplet pathways in 61 chromophores of different chemical families. We find that in most cases the triplet pathway is open and has a larger energy release than the singlet pathway. Thus, the energetics perspective explains why there are so few TTA emitters and their low quantum yields.