Singlet exciton fission is the process of splitting a single spin zero (singlet) molecular excited state into two spin one (triplet) excited states of different molecules. It is notable because spin conservation disallows the usual competing loss process: thermal relaxation of the high-energy spin zero exciton into a single low-energy spin one exciton. Indeed, the low energy exciton is a dark state, inaccessible by a direct transition from either the high-energy exciton or the ground state. Only the evolution of the high-energy state into two dark excitons is spin-allowed. Consequently, the efficiency of singlet exciton fission can approach unity even in the visible spectrum, harnessing photons of just twice the energy of the child excitons.
In theory, singlet fission can be exploited in solar cells to double the photocurrent from high-energy solar photons, ultimately boosting the efficiency of the silicon cell to 30% or more. The outstanding challenge is how to get the energy from the triplet excitons (energy ~ 1.1eV) from tetracene into silicon (bandgap 1.1eV). We report direct excitonic energy transfer from ‘dark’ triplets in the organic semiconductor tetracene to colloidal PbS nanocrystals, thereby successfully harnessing molecular triplet excitons in the near infrared. Steady-state excitation spectra, supported by transient photoluminescence studies, demonstrate that the transfer efficiency is at least (90±13)%. The mechanism is a Dexter hopping process consisting of the simultaneous exchange of two electrons. Triplet exciton transfer to nanocrystals is expected to be broadly applicable in solar and near infrared light-emitting applications, where effective molecular phosphors are presently lacking. We will conclude by demonstrating its usefulness in the reverse process of triplet exciton fusion – a promising approach to upconversion of incoherent light.