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

초록

We report our numerical study on the mechanism of carrier injection for organic light emitting diodes (OLEDs) structure with material basis on tris (8-hydroxyquinolinato) aluminum (Alq3). Our simulation model covers the key physical processes in OLEDs, namely charge injection, transport and recombination, exciton diffusion, transfer and decay as well as light coupling, and thin-film-optics [1]. The exciton model includes generation, diffusion, and energy transfer as well as annihilation. Our simulation results demonstrate the accumulation of charges at internal interfaces in the transient response as well as the electric field distribution for space charge effects. The accumulated charges at the interfaces between emission layer (EML) and transport layer seem to be the cause for accomplishing the peak of recombination rate density. Consequently, the numerical simulation revealed that the exciton density also increases in this region. In addition, we investigated the effect of inserting the EML in bilayer structure. For comparison, we call a basic 2-layer as device A. Then, we insert the EML between transport layers to make tri-layer devices B and C. In case of device B, the LUMO level of the EML aligns with the LUMO of the ETL while the HOMO level of the EML aligns with the HOMO of the HTL. In Device C, the LUMO level of the EML aligns with the LUMO of the HTL while the HOMO level of the EML aligns with the HOMO of the ETL. If the device structure is charge-blocking device B, the recombination occurs at both edge of the EML as the electric field peaks at these areas. But if the structure is charge-confining device C, electric field is confined at the interface between EML and electron transport layer (ETL). We will also discuss an effect of insertion of the doping layer for transport layer in this paper.