The Effect of Aging Processes on OLED Performance

Sernicrystalline polymers and amorphous network materials, where some degree of order persists (either in domains or over a short range), have been shown to exhibit trap-charge-limited (TCL) conduction in many cases.59,80 These traps 

FIGURE 3.19. J-V characteristics taken at various times for a device stressed at 150 /xA/cm2. Successive curves begin to deviate from the original at about 3 Y and then reconverge to the same square-law behavior at higher bias. This corresponds qualitatively to the simulations where traps are generated —0.25 eY below the Alq3 LUMO, shown by the solid curves. 

The calculated curves in Fig. 3.19 were obtained using a traps depth of 0.25 eV below the conduction band. (The low-current discrepancies between the model and experiment are either due to some field-effect component of the mobility or else to a non-negligible energy barrier for electron in ection.) This value is consistent with independently measured trap levels determined by thermally stimulated luminescence experiments.84 It is also corresponds well with the difference in reduction potential of Hq and Alq3, about 0.2 eV, as suggested in Figs. 3.7 and 3.8. 

For example, Hq provides a trap for electrons in Alq3, but as shown earlier, this reduction is unstable. If oxygen is present, the easily oxidized q anion which results eventually produces species which efficiently quench the photoluminescence of Alq3.18,47-48 The breadth of the reduction peak in Fig. 3.8b is due to the reduction of Hq and the subsequent reduction of q. Most of the other species produce very deep electron traps and should are not expected, at a first approximation, to affect the J-V or efficiency characteristics by trapping. 

FIGURE 3.20. Simulated decay curves for devices stressed at 1 mA/cm2 and 150 /zA/cm2. Evolution of —0.3-eV trap states is represented with a conversion. 

---