Revealing the competing contributions of charge carriers, excitons, and defects to the non-equilibrium optical properties of ZnO

Due to its wide band gap and high carrier mobility, ZnO is, among other transparent conductive oxides, 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 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 nonequilibrium 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 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. Noteworthy, a similar competition of intrinsic and defect-related dynamics could likely also be utilized in other oxides with marked defect density as, for instance, TiO2 or SrTiO3