Color Stability of Blue Phosphorescent Organic Light-Emitting Diodes with Undoped Layer in Emissive Layer

Abstract

We analyzed the color stability of blue phosphorescent organic light emitting diodes (phOLEDs) as device operation. In the blue phOLEDs, color stability is one of the most important factors during operation for commercialization. Therefore, we measure the electroluminance spectrum in the as-dep. state and after the lifetime test. The reference device with the whole emission layer doped shows a severe color change during the device operation owing to the degradation of the hole blocking property of the electron transport layer (ETL) and a shift of the emission region to ETL. After the lifetime test, the green peak increased and the width of the peak increased. The adoption of an undoped layer near the interface of ETL/emitting layer (EML) impeded the degradation of ETL thus so these devices show a stable color and a long lifetime. On the other hand, in the case of the undoped layer, the interface of hole transport layer (HTL)/EML shows an increased efficiency owing to the blocking of the movement of triplet energy from the doped layer to HTL, but does a color change that is similar to that of the reference device.