Insights into thermal degradation of organic light emitting diodes induced by glass transition through impedance spectroscopy

Highly sensitive alternate current (ac) impedance measurements with variable temperature have been performed to investigate the optical and electrical failure mechanisms during the glass transition phenomena in the archetypal $\text{ITO}/\text{TPD}/{\text{Alq}}_3/\text{Al}$ organic light emitting diode (OLED) structure. Since the device degradation is mainly related to the lower glass transition temperature $\left(T_g\right)$ of the $N,N^{\prime }$ -Bis(3-methylphenyl)-$N,N^{\prime }$ -diphenylbenzidine (TPD), this study is focused on the frequency response of thin TPD films approaching the glassy region. The related experimental data are discussed in the framework of the universal dielectric response model. By ac measurements, TPD glass transition temperature is located and temperature regions with different OLED behaviors are evidenced. The relation between the behaviors of TPD frequency response and of the OLED electro-optical response, while the temperature approaches the glass transition region, is discussed.