Abstract:

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)/C₆₀ (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)/Al (120 nm) were fabricated. C₆₀, MoO₃, and C₆₀: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 C₆₀ to MoO₃, results in the formation of C₆₀ cations and induces the decrease of the valence from Mo⁺⁶ to Mo⁺⁵; therefore, C₆₀: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 C₆₀ (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 C₆₀ 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.

Key words: Single-layer, Organic light-emitting diodes, Hole injection layer, Mixed materials, Balance of charge carriers