Numerical Study on Exciton Transport and Light Emission for Organic Light Emitting Diodes with an Emission Layer

Abstract

This paper reports the results of a numerical study on carrier injection and exciton transport in an organic light emitting diode (OLED) structure based on tris (8-hydroxyquinolinato) aluminum (Alq3). Because charge accumulation at the interfaces between the emission layer (EML) and transport layer are believed to increase the recombination rate, which also increases the exciton density, a numerical study was performed on the effect of inserting an EML in the bilayer structure. In the first case considered, the lowest unoccupied molecular orbital (LUMO) of the EML was aligned with the LUMO of the hole transport layer (HTL), whereas the highest occupied molecular orbital (HOMO) of the EML was aligned with the HOMO of the electron transport layer (ETL). In the second case, the LUMO of the EML was aligned with the LUMO of the ETL and the HOMO of the EML was aligned with the HOMO of the HTL. In case of a charge-blocking device, most of the recombination appeared to occur at both edges of the EML because the electric field exhibited a peak in these areas. On the other hand, in the case of the charge-confining device, the electric field was confined at the interface between the EML and ETL. This paper also discussed the effect of the insertion of a doping layer as transport layer.

The excitation mechanism of btp2Ir(acac) in CBP host

“…In conclusion, hole injection from the HTL to EML is difficult in the red‐doped device. This situation causes hole accumulation on the NPB side of the NPB/CBP interface, which enhances the field across the EML and contributes to carriers in the EML crossing over the EML interfaces (for electrons, the CBP/NPB interface and for holes, the CBP/BAlq interface), leading thus to the appearance of NPB and BAlq emissions. The difficult hole injection from the HTL to EML in the red‐doped device is illustrated in Figure (a).…”

“…Consequently, hole accumulation on the NPB side of NPB/CBP interface is eliminated in the co‐doped devices. This situation decreases the field across the EML (compared with that in the red‐doped device) and makes the process of carriers crossing over the EML interfaces more difficult . So carriers are confined within the EML, leading to the disappearance of NPB and BAlq emissions in the co‐doped devices.…”