Phase Transition of Npb Molecule Under Vacuum
Authors: Kim Jin-Tae , Shim Seob , Yun Ju-Young , Kwon Ohyun 
Published: February 2020

Abstract
The Phase transition characteristics of N,Nʹ-diphenyl-N,Nʹ-bis(1-naphthyl)-1,1ʹ-biphenyl-4,4ʺ-diamine (NPB: C44H32N2) used in organic light emitting diodes (OLEDs) was evaluated under vacuum. The phase transition was indicated by a plateau in the temperature curve of the NPB molecule upon heating upto its melting and boiling temperature under various pressure in a vacuum chamber. As pressure decreased from 1 atmosphere to ~ 10-1 Torr, the boiling temperature rapidly decreased whereas the melting temperature did not change. As pressure decreased further, both melting and boiling temperatures showed no change.

Keywords
: phase transition, NPB, vacuum, OLEDs
1. Introduction
OLEDs have attracted considerable commercial and academic interest due to their great potential for full-color, flat-panel display and solid-state lighting applications. However, OLEDs suffer from performance degradation caused by the formation of dark spots, the occurrence of significant exciton quenching, etc. [1-2] Among the factors which influence on the quality and yield rate of OLEDs, the phase transition of the OLED molecule plays a key role in optimizing evaporation process under high vacuum. In this study, the phase transition characteristics of the widely used hole transporting material, NPB [3] were evaluated under vacuum.
2. Experiment
The vacuum chamber was evacuated to ~ 10-7 Torr using cryopump and turbo molecular pump before the desired vacuum chamber pressure (760 ~ 10-6 Torr) was maintained using the N2 gas, MFC, and throttle valve. During the experiments, various condition of heating temperature and vacuum pressure was systematically controlled. To verify the reproducibility of the phase transition measurement system, fresh samples were used during the experiments and every expriment was repeated 3 times. Using the vacuum system, the phase transition of NPB was measured through the variation in temperature (25°C ~ 500°C) at vacuum (10-6 Torr ~ 10 Torr). At the pressure of 1 atm, the temperature of the NPB powder increased with heater temperature and then plateaued at 280°C. At this point, the melting of NPB was observed with the naked eye. The melting point obtained from the differential scanning calorimeter (DSC) and the phase transition measurement system were almost equivalent at 280°C. Melting and boiling above 10-6 Torr were also observed by both visual observation and an increase in the rate of change of NPB temperature. The melting temperature showed no change at vacuum from 10-6 Torr to ~ 10 Torr, shown in Fig. 1. However, the boiling temperature rose with pressure above 10-1 Torr. Below 10-1 Torr, there was no obvious change in the boiling temperature. This phase diagram of NPB under vacuum is expected to contribute to understanding of the thermal evaporating nature of materials for OLEDs.
References
[1] S. Schmidbauer, A. Hohenleutner, and B. König, Adv. Mater. 25, 2114 (2013).
[2] T.-H. Han, M.-R. Choi, S.-H. Woo, S.-Y. Min, C.-L. Lee, and T.-W. Lee, Adv. Mater. 24, 1487 (2012).
[3] S. A. Van Slyke, C. H. Chen, and C. W. Tang, Appl. Phys. Lett. 69, 2160 (1996).