Time-Resolved Electroluminescence Study for the Effect of Charge Traps on the Luminescence Properties of Organic Light-Emitting Diodes

The electroluminescence characteristics of pristine and degraded organic light-emitting diode (OLED) devices are studied using the time-resolved electroluminescence (TREL) technique. It is found that materials degradation results in notable changes in the temporal profile of TREL curve, which involves a shorter onset time, a longer rise time, and a longer decay time of electroluminescence. It is found that these temporal characteristics are affected by trapped charges formed during the OLED operation. More detailed analysis reveals that there are two types of charge traps. The longer decay time is found to be due to the excitons produced by weakly bound charges trapped in the organic layer that are released by thermal energy even without applying the voltage pulse. In contrast, the shorter onset time of electroluminescence is due to the excitons from strongly bound charges that can be released only when a voltage pulse is applied. It is demonstrated that TREL can be used to investigate the underlying mechanisms of OLED degradation. The presence of different types of charge traps found in this study may prove useful for more elaborate design of OLED devices toward enhanced durability against degradation and higher duty cycle of device operation.