  Kinetics of singlet fission in organic semiconductors, in which the excited
singlet state (S_1) spontaneously splits into a pair of triplet (T) excitons,
is known to be strongly influenced by back geminate annihilation of TT-pairs.
We show that this influence can be properly described only by taking into
account the diffusive exciton migration. The migration effect is treated in the
model of two kinetically coupled states: the intermediate state of interacting
TT-pairs and the state of migrating excitons. Within this model the singlet
fission (including magnetic field effects) is studied as applied to the
fluorescence decay kinetics (FDK) I_{S_1}(t) for S_1-state. The analysis shows
that migration strongly affects the FDK resulting, in particular, in the
universal long-time dependence I_{S_1}(t) \sim t^{-3/2}. The model accurately
describes the FDK, recently observed for a number of systems. Possible
applications of the considered model to the analysis of mechanisms of
migration, using experimentally measured FDK, are briefly discussed.
