Degradation mechanism of blue thermally activated delayed fluorescent organic light-emitting diodes under electrical stress

摘要

The degradation mechanism of blue thermally activated delayed fluorescence (TADF) organic light-emitting diodes (OLEDs) has been elucidated, particularly, focusing on the cause of the degradation in the emitting layer (EML) by different host materials. In two different host materials used as the EML host, the operation stability against the constant electrical stress was investigated. The degradation mechanism of blue TADF OLEDs is described in terms of two parameters of the photoluminescence (PL) quantum yield and the exciton generation efficiency. In the early stage, the degradation in the PL quantum yield of the EML is dominant whereas the long-term degradation is attributed to the decrease in the exciton generation efficiency. Before and after the electrical stress, non-destructive measurements on the OLEDs with different host materials are performed to determine the main origin of the decrease in the exciton generation efficiency. Under the electrical stress, charge traps are formed inside the EML and the charge transport is hindered, leading to the charge imbalance. Such charge traps are generated by the exciton-polaron interactions in the EML. It is suggested that degradation products (nonradiative recombination centers or exciton quenchers), formed inside the EML, reduce the exciton generation efficiency.