紫外臭氧处理增强溶液法MoO3薄膜的空穴注入能力

空穴注入层对有机发光二极管的性能有重要的影响，尤其是当器件中的空穴传输材料的最高占据分子轨道能级较深的时候。近年来有许多关于新型的溶液法空穴注入材料的研究。在本文中，我们对溶液法MoO₃薄膜使用了三种不同的处理方法来研究其对空穴注入性能的影响，即：在空气中150 ℃退火；在空气中150 ℃退火再紫外臭氧处理(UVO) 15 min；只进行UVO处理15 min。结果发现当MoO₃薄膜在空气中150 ℃退火后，器件的电流最小，空穴注入能力最差。而当MoO₃薄膜经过UVO处理后，器件的电流显著增大，工作电压大幅下降，器件性能接近于蒸镀的MoO₃薄膜的器件。更惊喜的是，这种改善在MoO₃薄膜仅作UVO处理后也可获得。经定量计算发现MoO₃薄膜经过UVO处理后的空穴注入效率能提高到约0.1。XPS分析表明通过UVO处理后，MoO₃薄膜中Mo⁵⁺成分减少并且薄膜表面的富氧吸附物被有效地消除，使得其化学计量基本与蒸镀的MoO₃薄膜相同。基于这种经UVO处理的溶液法MoO₃作为空穴注入层，器件的最大电流效率可达到48.3 cd?A^?1。

Improved Hole Injection Property of Solution-Processed MoO3 with UV-Ozone Treatment

The hole injection layer (HIL) plays a significant role in determining the performances of organic light-emitting diodes (OLEDs), especially when hole transport materials with deep highest occupied molecular orbital levels (HOMOs) are employed. Intensive efforts have been devoted to exploring novel hole injection materials with good solution-processing abilities in recent years. In this study, the solution-processed molybdenum trioxide (s-MoO₃) is prepared via an ultra-facile method. Three different s-MoO₃ layers prepared by three different methods, viz. layers annealed at 150 ℃ (s-MoO₃ (150)), layers annealed at 150 ℃ and then processed in UV-ozone for 15 min (s-MoO₃ (150, UVO)), and layers processed in UV-ozone for 15 min without annealing (s-MoO₃ (UVO)), are obtained to investigate their influences on hole injection. The device with the s-MoO₃ (150) layer has the lowest current density and the largest driving voltage, showing poor hole injection ability. In contrast, with the s-MoO₃ (150, UVO) layer as HIL, the OLED produces a greatly enhanced current and sharply reduced driving voltage, comparable to the device using vacuum-evaporated MoO₃. Similar results are obtained for the device with the s-MoO₃ (UVO) film, suggesting that high-temperature annealing is not essential for the s-MoO₃ film with UV-ozone treatment. Hole injection efficiencies of MoO₃ films are quantitatively characterized by analyzing the space-charge-limited current of hole-only devices; the hole injection efficiencies of s-MoO₃ (150, UVO) and s-MoO₃ (UVO)-based devices are ~0.1, far exceeding that of the s-MoO₃ (150)-based device (10^?5). XPS analysis is performed to detect the impact of the above treatments on the surface electronic properties of the s-MoO₃ films. A typical characteristic of Mo⁵⁺ species is obtained for the s-MoO₃ (150) film and a high-binding-energy shoulder appears in the O 1s peak of the s-MoO₃ (150) film, indicating the existence of oxygen vacancies and oxygen adsorbed at the surface of s-MoO₃ (150) film. When UV-ozone treatment is applied to this s-MoO₃ (150) film, it produces a decrease of Mo⁵⁺ state and elimination of oxygen-rich adsorbates, resulting in MoO₃ stoichiometry similar to that of the vacuum-evaporated MoO₃ film. Consequently, a maximum current efficiency of 48.3 cd?A^?1 is realized with the optimized UV-ozone treated s-MoO₃ HIL. It This UV-ozone treated s-MoO₃ should have widespread applications in low-cost solution-processed OLEDs as an excellent hole injection layer.