量子点发光二极管中的电荷注入与转移

量子点发光二极管（Quantum dot light-emitting diode，QLED）因其发射波长可调，半峰宽窄，稳定性高，可溶液加工的特点，已成为下一代平板显示技术的有力竞争者。自1994年V. L. Colvin等人首次报道量子点（Quantum dot，QD）电致发光器件以来，在研究者的不断努力下，QLED的器件效率和工作寿命飞速发展。目前红绿蓝QLED的最大外量子效率（External quantum efficiency，EQE）分别达到了31.9%，28.7%和21.9%。最长的工作寿命T95（@ 1000 cd/m2）分别达到了7668小时，7200小时和227小时。尽管如此，QLED距离商业化应用还有一段距离。目前掣肘QLED量产的主要原因有QD的图案化，蓝光QLED的工作寿命等。同时QLED器件的一些基本物理问题尚未厘清（如失效机理，正向老化机理等）。本文立足于QLED的基本工作原理，重点关注QLED电荷注入和复合过程的器件物理，主要研究了蓝光QLED的电荷注入以及QLED的界面电荷转移。

为了辨识蓝光QLED的多数载流子，我们分析了蓝光单载流子器件的阻抗特性，发现单电子器件的内建电压小于单空穴器件的内建电压，表明蓝光QLED中，电子注入比空穴注入更有效。另外，我们使用红色QD作为荧光探针监测蓝光QLED的激子复合区域，发现激子复合区域更靠近空穴传输层（Hole transport layer，HTL），而且随着电压的增大，激子复合区域向电子传输层（Electron transport layer，ETL）方向迁移，证实了蓝光QLED是一个电子主导的器件。该工作解决了蓝光QLED是否是电子主导的器件的争论。

同时，我们观察到蓝光QLED中缺陷辅助电子注入的现象。通过对蓝光QLED的阻抗分析，我们建立了蓝光QLED器件的等效电路图，计算了缺陷捕获的电子数以及缺陷态密度的分布。缺陷不仅可以促进电子的注入，同时也是电子转移的受体，会造成QD内激子解离，降低器件的EQE。因此，缺陷对蓝光QLED电荷注入和复合过程都有重大的影响。

当QD与ETL直接接触时，电荷会自发的转移到ETL上，使QD带电，降低QLED的辐射复合效率。因此，QLED的器件效率受QD-ETL界面的激子解离制约。但目前高效率（EQE>20%）的QLED并未对QD-ETL界面做任何修饰。也就是说，即使面临严重的界面激子解离，QLED仍能维持高效率，这是QLED尚未解决的器件物理问题。为了解开这个疑问，我们对比了封装与不封装的QD-ETL样品的光学性质，发现封装的QD-ETL样品不会发生界面激子解离，只有不封装的QD-ETL样品会发生由O2诱导的电荷转移导致的界面激子解离。进一步地，我们研究了QD-ETL界面的电荷转移机制及其对QLED器件性能的影响。该工作确认了QD-ETL界面不会发生激子解离，解决了QLED中界面激子解离与高性能之间的矛盾，同时强调了封装对QLED的重要性。

最后，我们发现基于ZnMgO ETL制备的QLED器件在连续电压扫描下效率逐渐降低，而基于ZnO ETL制备的QLED器件效率却在逐渐增加。为了解释ZnMgO和ZnO器件效率变化趋势相反的原因，我们分析了ZnMgO和ZnO器件的电学性能参数以及ZnMgO和ZnO纳米颗粒表面的缺陷密度，发现ZnMgO器件的效率衰减是由ZnMgO表面缺陷介导的空穴转移导致的，而放置老化可以消除ZnMgO器件在连续电压扫描下的效率衰减。同时，ZnMgO器件的工作寿命在恒定电流老化的条件下会快速衰退，经放置老化之后，ZnMgO器件的工作寿命显著提升。

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