  Due to its wide band gap and high carrier mobility, ZnO is an attractive
material for light-harvesting and optoelectronic applications. Its functional
efficiency, however, is strongly affected by defect-related in-gap states that
open up extrinsic decay channels and modify relaxation timescales. As a
consequence, almost every ZnO sample behaves differently, leading to
irreproducible or even contradicting observations. Here, a complementary set of
time-resolved spectroscopies is applied to two ZnO samples of different defect
density to disentangle the competing contributions of charge carriers,
excitons, and defects to the non-equilibrium dynamics after photoexcitation:
Time-resolved photoluminescence, excited state transmission, and electronic sum
frequency generation. Remarkably, defects affect the transient optical
properties of ZnO across more than eight orders of magnitude in time, starting
with photodepletion of normally occupied defect states on femtosecond
timescales, followed by the competition of free exciton emission and exciton
trapping at defect sites within picoseconds, photoluminescence of defect-bound
and free excitons on nanosecond timescales, and deeply trapped holes with
microsecond lifetimes. These findings do not only provide the first
comprehensive picture of charge and exciton relaxation pathways in ZnO, but
also uncover the microscopic origin of previous conflicting observations in
this challenging material and thereby offer means of overcoming its
difficulties.
