Activated Singlet Exciton Fission in a Semiconducting
Polymer
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Abstract
Singlet exciton fission is a spin-allowed
process to generate two
triplet excitons from a single absorbed photon. This phenomenon offers
great potential in organic photovoltaics, but the mechanism remains
poorly understood. Most reports to date have addressed intermolecular
fission within small-molecular crystals. However, through appropriate
chemical design chromophores capable of intramolecular fission can
also be produced. Here we directly observe sub-100 fs activated singlet
fission in a semiconducting poly(thienylenevinylene). We demonstrate
that fission proceeds directly from the initial 1B<sub>u</sub> exciton,
contrary to current models that involve the lower-lying 2A<sub>g</sub> exciton. In solution, the generated triplet pairs rapidly recombine
and decay through the 2A<sub>g</sub> state. In films, exciton diffusion
breaks this symmetry and we observe long-lived triplets which form
charge-transfer states in photovoltaic blends