To analyze the lifetime difference based on the charge dynamics in the emitting layer (EML), we applied two electron transport layers (ETLs) with significantly different electron transporting characteristics to the same EML. Even with the same EML configuration, the device lifetime increased by approximately 4-fold, from 291 h to over 1000 h of LT50 (the time taken for the luminance to decrease to 50% of its initial value of 1000 cd/m2). Although trap/detrap of holes in the dopant molecules was observed through impedance spectroscopy, we found that the most significant difference in lifetime was caused by the quantity of electron current. Surprisingly, depending on the electron transporting layer, the primary bimolecular interaction in the EML (i.e., exciton-exciton, exciton-polaron interaction) dramatically changes even in the same EML configuration, which is theoretically analyzed by the numerical fitting of transient electroluminescence data and experimentally confirmed by magneto-electroluminescence (MEL) measurements. To the best of our knowledge, for the first time, the MEL measurements are demonstrated as a tool that can be utilized to intuitively discern the dominance of bimolecular interaction with respect to the operational stability of phosphorescent organic light-emitting diodes (PhOLEDs).
Keywords: blue phosphorescent organic light-emitting diodes; degradation; impedance spectroscopy; magneto-electroluminescence; transient electroluminescence.