Abstract
Singlet fission is the photoinduced conversion of a singlet exciton into two triplet states of half-energy. This multiplication mechanism has been successfully applied to improve the efficiency of single-junction solar cells in the visible spectral range. Here we show that singlet fission may also occur via a sequential mechanism, where the two triplet states are generated consecutively by exploiting oxygen as a catalyst. This sequential formation of carriers is demonstrated for two acene-like molecules in solution. First, energy transfer from the excited acene to triplet oxygen yields one triplet acene and singlet oxygen. In the second stage, singlet oxygen combines with a ground-state acene to complete singlet fission. This yields a second triplet molecule. The sequential mechanism accounts for approximately 40% of the triplet quantum yield in the studied molecules; this process occurs in dilute solutions and under atmospheric conditions, where the single-step SF mechanism is inactive.
Highlights
Singlet fission is the photoinduced conversion of a singlet exciton into two triplet states of half-energy
In favourable cases, singlet fission (SF) outcompetes singlet-state deactivation by fluorescence and internal conversion, with intersystem crossing being of no concern for such a large EðS1Þ À EðT1Þ energy bias: photon-to-electron conversion efficiencies close to 200% are achievable in the visible spectral range if charges can be efficiently extracted from the long-living triplet states
In an initial energy transfer stage, the chromophore is excited to the first excited singlet-state S1 by an incoming photon. It crosses to its T1 state in a spin-allowed singlettriplet annihilation involving the excited chromophore and 3O2
Summary
Singlet fission is the photoinduced conversion of a singlet exciton into two triplet states of half-energy This multiplication mechanism has been successfully applied to improve the efficiency of single-junction solar cells in the visible spectral range. We show that singlet fission may occur via a sequential mechanism, where the two triplet states are generated consecutively by exploiting oxygen as a catalyst This sequential formation of carriers is demonstrated for two acene-like molecules in solution. This work uses transient absorption and time-resolved photoluminescence measurements to demonstrate a sequential SF mechanism with ground-state triplet molecular oxygen 3O2 acting as a catalyst (Fig. 1b) In this process, two triplet states are produced in a step-by-step manner in solution. The sequential mechanism accounts for ~40% of the triplet quantum yield in TIPS-pentacene, as well as tetrachlorphenazinothiadiazole, and is transferrable to all SF molecules with suitable S1 and T1 energies
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