Probing Phase Transitions and Interfacial Reorganization in TAPC/CBP/BPhen Organic Light-Emitting Diode Stacks by In Situ Ellipsometry

The thermal behavior of a three-layer structure—glass/ITO/TAPC/CBP/BPhen—in an OLED system was investigated using in situ spectroscopic ellipsometry during controlled heating from room temperature to 120 °C over 60 min, simulating the ageing process and analyzing degradation kinetics. Variations in Ψ and Δ spectra were observed across the entire 0.7–5.9 eV spectral range, with five distinct anomalies, particularly in the UV region. An anomaly at approximately 66 °C was attributed to the glass transition temperature Tg of BPhen, while another two at around 82 °C and at around 112 °C corresponded to the first-order phase transition of TAPC and Tg of CBP, respectively. The origins of the remaining anomalies at 91 °C and 112 °C were explored in this study, with a focus on interphase layer formation and morphological changes that emerge during heating. These findings provide insights into the stability of OLEDs under thermal stress.