Phase behaviors of NPB molecule under vacuum

• We investigated the phase transition characteristics (melting and boiling behaviors) of NPB molecule under vacuum.
• The phase transition temperatures obtained under various vacuum condition allowed the phase diagram of NPB to be drawn.
• The resulting phase diagram under vacuum will be useful to understand thermal behaviors during vacuum evaporation.

Commercialization of organic light emitting diodes (OLEDs) with high stability and reproducibility remains challenging. Although many studies have evaluated the intrinsic (electrochemical, photochemical, and thermal degradation of the active layers) and extrinsic (encapsulation and layer adhesion) degradation mechanisms of OLED devices and have attempted to improve device stability, until recently little attention was paid to the phase behavior of the deposition materials used in OLEDs during evaporation. In addition to intrinsic and extrinsic factors, phase behaviors may also affect device efficiency and lifetime as they play key roles in optimizing the evaporation process under high vacuum and subsequently reducing manufacturing costs. In this study, the phase behavior of N,N′-diphenyl-N,N′-bis(1-naphthyl)-1,1′-biphenyl-4,4″-diamine (NPB: C44H32N2), a widely used as hole transporting material in OLEDs, was investigated under vacuum. The resulting phase diagram under vacuum will be useful to understand the nature of thermal evaporation for the NPB molecule for use in OLED manufacture.

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