We studied electroluminescence degradation in phosphorescent organic light-emitting devices (PHOLEDs) and found that two distinctive mechanisms are responsible for device degradation depending on the device structure. For a device without a hole blocking layer (HBL), excess holes penetrate into the electron transport layer (ETL) and lead to the deterioration of the ETL adjacent to the interface of the emitting layer. The lower electron transport capacity of the degraded ETL alters the balance in hole/electron injection into the emitting layer and results in a decrease in the luminescence efficiency of the PHOLEDs. For a device with a HBL, on the other hand, holes accumulate and become trapped in the emitting layer, and result in a decrease in the luminescence efficiency of the PHOLEDs, likely due to their role in acting as exciton quenchers or as nonradiative charge recombination centers.
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