Origin of exciplex degradation in organic light emitting diodes: Thermal stress effects over glass transition temperature of emission layer

Exploiting exciplexes in organic light-emitting diodes (OLEDs) to enhance electroluminescence lifetime and quantum efficiency is of considerable interest to researchers. The presence of exciplexes has been demonstrated in a host material interface in a multiple stacked OLED; therefore, understanding the degradation mechanism of exciplexes and host materials is essential to develop highly reliable and uniform OLEDs. Herein, we report thermal stress-driven exciplex degradation in a blue OLED, which comprises 4,4′-bis(N-carbazolyl)-,1′-biphenyl (CBP) as a host material without dopants. The device structure dependent-electroluminescence clearly confirms the formation of exciplexes surrounding the CBP interfaces at 398 and 450 nm, respectively. During the thermal stress over the glass transition temperature (Tg) of the CBP, the spectral intensity of the exciplex decreased significantly, and the ideality factor and characteristic trap energy increased abruptly when the thermal stress temperature was higher than the Tg of CBP, signaling the origin of thermal degradation effects on the exciplex and host material in our OLED.