Effects of charge generation layers on multiple guest/host bilayer-based tandem OLEDs

Compared to OLEDs with a guest/host system as the light emitting layer (EML), in which guest molecules are doped or mixed into the host molecule layer, those with multiple guest/host bilayers as the EML has the advantage of uniform distribution of guest-to-host molecule ratio throughout the entire emission layer thickness. Herein, we report on the tandem OLED based on multiple guest/host bilayers as the EML, and the effects of charge generation layers (CGL) on device performance for the first time. With four pairs of vacuum evaporated host of 4,4 '-Bis(N-carbazolyl)-1,1 '-biphenyl (CBP) and guest of bis(1-phenylisoquinoline) (acetylacetonate) iridium(III) (Ir(piq)2(acac)) as the EML, two types of tandem OLEDs with 8-Hydroxyquinolinolato-lithium (Liq)/MoO3 or Liq/2,3,6,7,10,11-hexacyano-1,4,5,8,9,12hexaazatriphenylene (HAT-CN) bilayer as the charge generation layer, respectively, were investigated. The results showed that the device with Liq/MoO3 as CGL (T1) exhibited higher current efficiency but lower power efficiency than the one with Liq/HAT-CN as the CGL (T2). Single carrier devices with either Liq/MoO3 or Liq/ HAT-CN as the charge injection layer were fabricated to investigated charge injection ability of the CGLs. The results show that Liq/MoO3 CGL exhibit both lower electron and hole injection ability than Liq/HAT-CN, which is the origin of higher turn-on volage and lower power efficiency of T1 than T2. However, the balance degree of electron and hole injection of Liq/MoO3 is better than Liq/HAT-CN, resulting higher current efficiency of T1 than T2. These results provide a novel strategy towards fabricating high performance tandem OLEDs.