Operational lifetime improvement of solution-processed OLEDs: Effect of exciton formation region and degradation analysis by impedance spectroscopy

It is known that the lifetime of the organic light-emitting diode device manufactured by the solution process is deteriorated due to the problem of mixing the interface between the hole transport layer and the emitting layer. We found that moving the recombination area away from the interface mixing zone as above doubled the efficiency (22.8 cd/A → 51.5 cd/A, 7.6% EQE → 14.6% EQE) and tripled the lifetime (14 h → 42 h). The reason was mainly attributed to the suppression of degradation due to exciton-polaron quenching at the mixing interface. Especially, the degradation of solution-processed devices has not been widely discussed. Therefore, in this study, we investigated thoroughly the deterioration of those devices by impedance spectroscopy and molecular simulation. The trap sites were revealed to present at the mixing zone when a large amount of charges accumulating there. These traps potentially resulted from the fragment of molecules undergoing the bond dissociation due to quenching of exciton and negative charge. Furthermore, by fitting Cole-Cole plots, we observed that the most stress region after half-lifetime test was the mixing zone. To confirm which molecules have a high possibility to dissociate, we calculated the bond dissociation energy of the possible dissociated bonds. The dissociation of host molecules from the anionic excited state due to quenching of exciton and negative polaron played the main role in device degradation. Meanwhile, the dissociation of hole transport molecules in the anionic charged states could also affect device lifetime by slow degradation.