  The work concerns the analysis of experimental time-resolved ESR spectra in
photoexcited polymer:fullerene blend, consisting of poly(3-hexilthiophene) and
fullerene [6,6]-phenyl C_{61} -butyric acid methyl ester (at low temperature T
= 100 K). The spectra are assumed to be determined by spin-coherent pairs of
charged polarons P^{+} and P^{-} generated in the singlet state. The analysis
is made within simple model of a set of first order processes, in which
P^{+}P$^{-}-pair spin evolution is described by the stochastic Liouville
equation, allowing for fairly accurate description of experimental results.
Simple analytical interpretation of obtained numerical results demonstrates
that trESR spectra can be represented as a superposition of antiphase and CIDEP
contributions together with the conventional thermal one. These contributions
are shown to change their signs with the increase of time in agreement with
experimental observations.
