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.

中文翻译：

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揭示电荷载流子，激子和缺陷对ZnO非平衡光学性质的竞争性贡献。

由于其宽带隙和高载流子迁移率，ZnO除其他透明导电氧化物外，还是一种用于光收集和光电应用的有吸引力的材料。但是，其功能效率受到与缺陷相关的间隙内状态的强烈影响，这些状态会打开外部衰减通道并修改弛豫时间尺度。结果，几乎每个样本的行为都不同，从而导致无法再现甚至矛盾的观察结果。在这里，一套互补的时间分辨光谱学被应用于两个不同缺陷密度的ZnO样品，以解开电荷载流子，激子和缺陷对光激发后非平衡动力学的竞争贡献：时间分辨光致发光，激发态透射和电子和频生成。值得注意的是 缺陷在超过8个数量级的时间内影响ZnO的瞬态光学特性，首先是飞秒级时通常占据的缺陷状态的光损耗，然后是皮秒内缺陷位置的自由激子发射和激子俘获的竞争，缺陷的光致发光纳秒级的束缚和自由激子，以及深陷于微秒级寿命的空穴。这些发现不仅提供了ZnO中电荷和激子弛豫途径的第一张综合图，而且还揭示了这种具有挑战性的材料中先前相互矛盾的观察结果的微观来源，从而提供了克服其困难的手段。值得注意的是