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

Abstract

A numerical study of space charge effects in multilayer organic light-emitting diodes ͑OLEDs͒ is presented. The method of solving the coupled Poisson and continuity equations, previously established for single-layer polymer LEDs, has been extended to treat internal organic interfaces. In addition, we consider the transient current and electroluminescence response. We discuss the accumulation of charges at internal interfaces and their signature in the transient response as well as the electric field distribution. Comparison to experimental transient data of a typical bilayer LED based on tris͑8-hydroxyquinolinato͒aluminum (Alq 3) is provided and good agreement is found. 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 electroluminescence delay time is determined by electrons passing through Alq 3 to the internal interface are self-consistently supported by the results of the simulation. Moreover, the creation of emissive singlet excitons is found to be strongly confined to the Alq 3 side of the internal interface and the emission zone width is dictated by the exciton diffusion length. Design principles for trilayer LEDs with improved power efficiency are also discussed.

Pulsed magnetic resonance of Alq3 OLED detected by electroluminescence

“…Bipolar transport and light emission in OLEDs can be described using space charge limited current models [21,22]. These models have proved to be very reliable in simulating transient electroluminescence experiments [22].…”

“…These models have proved to be very reliable in simulating transient electroluminescence experiments [22]. In this paper a similar approach is used to highlight how the light emission deviates if the mobility of one of the two carriers suddenly changes.…”

Multi-Scale Modeling of Bulk Heterojunctions for Organic Photovoltaic Applications

“…For charge transfer at internal interfaces between donor and acceptor domains, a transfer rate that depends exponentially on the energy difference between the donor and acceptor HOMO levels (for hole transport) or LUMO levels (for electron transport) is often assumed (Ruhstaller et al, 2001). Device characteristics are generally calculated under steady-state conditions, in which the time derivatives on the left-hand sides of Equations 4, 5, and 9 are set to zero.…”

Numerical simulation of charge transport in disordered organic semiconductor devices

“…Generally, the drift-diffusion problem is solved time-dependently in a decoupled manner. [10][11][12][13] Pflumm 14 and deMello 15 adapted or replaced Poisson's equation with a time-dependent version and solved the system in a coupled and time-dependent manner. Additionally, deMello applied an adaptive mesh as opposed to the widely known Scharfetter-Gummel discretization at the cost of having additional criteria to be evaluated.…”