Probing Phase Transitions and Interfacial Reorganization in Tapc/Cbp/BPhen OLED 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 minutes, simulating the ageing process and analysing 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 is attributed to the glass transition temperature Tg of BPhen, while another two at around 82°C and at around 112°C correspond 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 are explored in this study, with a focus on interphase layer formation and morphological changes that emerges during heating. These findings provide insights into the stability of OLEDs under thermal stress.