  Optical devices fabricated using conjugated polymer systems give rise to
singlet exciton yields which are high compared to the statistically predicted
estimate of 25% obtained using simple recombination schemes. In this study we
evaluate the singlet exciton yield in conjugated polymers systems by fitting to
a model that incorporates the Pauli exclusion principle. The rate equations
which describe the exciton dynamics include quantum dynamical components (both
density and spin-dependent) which arise during the spin-allowed conversion of
composite intra-molecular excitons into loosely bound charge-transfer (CT)
electron-hole pairs. Accordingly, a crucial mechanism by which singlet excitons
are increased at the expense of triplet excitons is incorporated in this work.
Non-ideal triplet excitons which form at high densities, are rerouted via the
Pauli exclusion mechanism to form loosely bound CT states which subsequently
convert to singlet excitons. Our derived expression for the yield in singlet
exciton incorporates the purity measure, and provides a realistic description
of the carrier dynamics at high exciton densities.
