Single-Layer Organic Light-Emitting Devices with C60 and MoO3 Mixed Materials as Hole Injection Layer

Multilayer phosphorescent organic lighting-emitting diodes (PHOLEDs) with complicated device configurations have greatly increased the complexity of manufacturing and the fabrication cost. Therefore, there is strong incentive to develop simplified OLEDs, such as a single-layer device that has the structure of anode/hole injection layer (HIL)/emissive layer/electron injection layer/cathode. However, because of the absence of a carrier transport layer, the single-layer device suffers from severe charge injection difficulties and unbalanced carrier transport. Hence, the performances of single-layer devices reported so far have not been satisfactory. It has been proved that the modification of the electrode/organic interface could influence carrier injection to improve the device performance in multilayer PHOLEDs. Modification of the electrode/organic interface is more essential for achieving high-performance single-layer OLEDs. In this work, efficient green phosphorescent single-layer OLEDs based on the structure of indium tin oxide (ITO)/C-60 (1.2 nm):MoO3 (0.4 nm)/1,3,5-tris(1-phenyl-1H-benzimidazol-2-yl)benzene (TPBi):fac-tris(2-phenylpyridine)iridium [Ir(ppy)(3)]/LiF (0.7 nm)/AI (120 nm) were fabricated. C-60, MoO3, and C-60:MoO3 were applied as the HILs, respectively, for comparison. The layer of TPBi played a dual role of host and electron-transporting material within the emission layer. Thus, the properties of the HILs play an important role in the adjustment of electron/hole injection to attain transport balance of the charge carriers in single-layer OLEDs with electron-transporting hosts. It is found that appropriate adjustment of the HIL is a key factor to achieve high-efficiency single-layer OLEDs. The large affinity of MoO3 (6.37 eV), inducing electron transfer from the highest occupied molecular orbital of C-60 to MoO3, results in the formation of C-60 cations and induces the decrease of the valence from Mo+6 to Mo+5 ; therefore, C-60:MoO3 can adjust the hole injection properties well. Finally, a single-layer OLED with a maximum current efficiency of 35.88 cd.A(-1) was achieved. Compared with devices with MoO3 (28.99 cd.A(-1) ) or C-60 (10.46 cd.A(-1)) as HILs, the device performance was improved by 24% and 243%, respectively. Overall, a novel and effective method of using different mixed ratios of C-60 and MoO3 as the HIL to realize effective charge carrier regulation is proposed, and it is of great significance for fabricating high-performance single-layer OLEDs.