Temperature-Dependent Recombination of Triplet Biexcitons in Singlet Fission of Hexacene

Abstract

Singlet fission is a spin-conserving process for the multiplication conversion of one singlet exciton into two individual triplet excitons by absorbing one photon. Such a multiplication is believed to circumvent the Shockley–Queisser thermodynamic limit for improving efficiency of solar energy conversion. A mechanistic understanding of generation and yields of triplet excitons from singlet fission materials is essential for efficient exploitation of solar energy. Here we employ temperature-dependent transient absorption spectroscopy to examine the dynamical nature of singlet fission and triplet excitons in hexacene. The generation and dissociation rates of the intermediate correlated biexciton, 1(TT), are independent of temperature from 77 K to the room temperature. On the other hand, the triplet excitons in spatially separated biexcitons, 1(T···T), relax via geminate and nongeminate recombination. The former was found to be temperature-dependent, whereas the latter is temperature-independent. Quantitative analyses of the temperate-dependent rates for the two recombination processes yield the energy difference between the 1(T···T) and 1(TT), which were further confirmed by our density functional theory (DFT) calculations.