Molecular aggregation and degradation mechanism of

The Joule heating effect on electroluminescent efficiency is important in the degradation origin of organic
light-emitting diodes (OLED). Scanning tunneling microscopy (STM) and photoluminescence (PL) measurements
were performed on the guest molecule BT (1,4-bis(benzothiazole-vinyl) benzene), host molecule TPBI
(2, 2¢,2¢¢-(1,3,5-phenylene)tris-[1-phenyl-1H-benzimidazole]), and their mixture deposited on an HOPG surface
to study the OLED degradation mechanism due to thermal heating. At room temperature, BT and TPBI in
the mixed layer show good compatibility and high PL intensity, but at higher temperatures, they show phase
separation and aggregation into their own domains and a concomitant decrease in PL intensity. The PL intensity
loss suggests ineffective energy transfer from TPBI to BT due to phase separation, which may cause OLED
degradation. Scanning tunneling spectroscopy (STS) results show that the band gaps of TPBI and BT remain
unchanged with the annealing temperature, suggesting that the heat-induced decay of OLED is related to the
interfacial structural change rather than the respective molecular band gap. The results provide direct evidence showing how the molecular structures of the mixed layer vary and affect the PL intensity due to temperature.