High Efficiency Tandem Organic Light Emitting Diode Using an Organic Heterojunction as the Charge Generation Layer: An Investigation into the Charge Generation Model and Device Performance

Organic heterojunctions (OHJs) are frequently used as charge generation layers (CGLs) in the construction of high efficiency tandem organic light emitting diodes (OLEDs). However, the charge generation mechanism still remains unclear. In this article, the working principle of a typical OHJ CGL composed of 1,4,5,8,9,12-hexaazatriphenylene-hexacarbonitrile (HAT-CN)/ 4,4′,4″-tris(N-3-methylphenyl-N-phenylamino)triphenylamine (m-MTDATA) is studied. We found that the charge generation process in this OHJ results from electron tunneling, which is shown through temperature-dependent J–V characteristics and model fitting. Additionally, we fabricated an ultrahigh efficiency green tandem OLED using only commercially available organic semiconductor materials for OLED. The maximum power efficiency reaches 120 lm/W, yet remains at 110.3 lm/W at 1000 cd/m2, which is greatly enhanced from when compared to the corresponding single-unit device. The maximum current efficiency and external quantum efficiency reach as high as 201 cd/A and 54.5%, respectively.