Enhancing Tandem Organic Light-Emitting Diode Performance with Multiple Electroluminescent Units

Tandem organic light-emitting diodes (TOLEDs) which stack multiple electroluminescent units (EUs) via a charge-generation layer (CGL) are advanced device designs that improve the electroluminescent (EL) performance and operating lifetime. However, each CGL used in a device with multiple EUs is affected by electrodes and the other CGLs, resulting in a suboptimal ability to generate carriers and thus decreasing device efficiency and luminance values. Herein, four specific designs, including an effective CGL structure, an exciplex host for the emitting layer (EML), simplified device architectures, and nanoparticle-based diffusers, are proposed to realize highly efficient TOLEDs, aiming to sustain or even surpass the external quantum efficiency linearity with the number of stacked EUs. Aside from the heterojunction structure of the CGL, the n-type electron injection layer also plays a critical role in efficient charge generation. In addition, the exciplex host used in the EML could improve the carrier balance and reduce the operating voltages. The EL efficiency of the three-stacked tandem device reaches 78.7% (303.5 cd A –1 ), about 2.5 times that of the conventional device, which fails to meet the expected value. When a nanoparticle-based diffuser layer was inserted between the substrate and the anode of the three-stacked TOLEDs, the EL efficiency of the device rose to 123.9% (456.9 cd A –1 ), about 3.9-fold that of the conventional device. Furthermore, an ultra-high maximum luminance of 411,531 cd m –2 could be obtained, exceeding that of the conventional device by 5.8 times. Notably, the maximum current efficiency of device T3D reaches record highs compared to the reported green TOLEDs. These results demonstrate the viability of practical design strategies for TOLEDs with multiple EUs for realizing high EL efficiency and luminance values, thus fulfilling the requirements for OLED phototherapy and display applications.