  The issue of the quantitative analysis of time-resolved photoluminescence
experiments is addressed by developing and describing two approaches for
determination of unimolecular lifetime, bimolecular recombination coefficient,
and equilibrium free-carrier concentration, based on a quite general
second-order expression of the electron-hole recombination rate. Application to
the case of band-edge emission of ZnO single crystals is reported, evidencing
the signature of sub-nanosecond second-order recombination dynamics for optical
transitions close to the interband excitation edge. The resulting findings are
in good agreement with the model prediction and further confirm the presence,
formerly evidenced in literature by non-optical methods, of near-surface
conductive layers in ZnO crystals with sheet charge densities of about
3{\div}5*10^13 cm^-2
