| Recently, polymer blend and organic multilayer LEDs have attracted increased interest since they allow for the study of energy transfer as well as aggregation effects and enhanced charge carrier balance, respectively. First, experiments with a polymer blend system, where energy transfer from host to guest is allowed, are presented. We find a bias-tuned change in the emission spectrum that can be interpreted in terms of a reduction of aggregation-induced self-absorption. Furthermore, results on polymer blend LEDs with inert hosts and nanoparticle composite LEDs emphasize the importance of interchain interactions and offer novel ways for color tuning. Second, self-consistent numerical simulations, based on intrinsic material properties, in both steady-state and pulsed operation are discussed. For single layer polymer LEDs, temperature dependent current-voltage curves and electroluminescence pulses are calculated. The simulation results on multilayer devices allow for a deeper understanding of charge accumulation, electric field and recombination zone confinement effects due to internal interfaces. These results compare favorably with transient data for Alq 3 based bilayer LEDs, confirming that the electric field is predominantly dropped across the Alq3 and that the delay time is determined by the electron transit time therein. Furthermore, our simulations indicate that only high concentrations of traps affect the space-charge-limited currents in organic LEDs. Trilayer design concepts are formulated in order to achieve optimized charge balance and power efficiency. |
