Transient and steady-state behavior of space charges in multilayer organiclight-emitting diodes

A numerical study of space charge effects in multilayer organic light-emitt ing diodes (OLEDs) is presented. The method of solving the coupled Poisson and continuity equations, previously established for single-layer polymer L EDs, has been extended to treat internal organic interfaces. In addition, w e consider the transient current and electroluminescence response. We discu ss the accumulation of charges at internal interfaces and their signature i n the transient response as well as the electric field distribution. Compar ison to experimental transient data of a typical bilayer LED based on tris( 8-hydroxyquinolinato)aluminum (Alq(3)) is provided and good agreement is fo und. Our results are consistent with commonly assumed operating principles of bilayer LEDs. In particular, the assumptions that the electric field is predominantly dropped across the Alq(3) layer and that the electroluminesce nce delay time is determined by electrons passing through Alq(3) to the int ernal interface are self-consistently supported by the results of the simul ation. Moreover, the creation of emissive singlet excitons is found to be s trongly confined to the Alq(3) side of the internal interface and the emiss ion zone width is dictated by the exciton diffusion length. Design principl es for trilayer LEDs with improved power efficiency are also discussed. (C) 2001 American Institute of Physics.