H₂O-Induced Crystallization of Organic Luminescent Thin Films by Direct Film Storage in a High Vacuum

Abstract

Atmospheric gas molecules, such as H2O and O2, are well-known to crucially affect the performance of various organic devices, and are practically avoided by device fabrications in a vacuum followed by device encapsulation in a glovebox filled with inert gases. However, it is often unclear whether the post-growth films can retain their intrinsic properties without any gas exposure and post-growth time under high-vacuum conditions, such as 10–3–10–5 Pa, where residual H2O is postulated to absorb on the device components and degrade device performance. Here we developed in situ photoluminescence measurement apparatus and used it for organic luminescent thin films. H2O-induced crystallization was observed for tris­(2-phenylpyridinato)­iridium­(III) (Ir­(ppy)3) films grown on various silicon oxide surfaces. The crystallization occurred not only by ambient air or H2O vapor exposure but also by just maintaining the films under high-vacuum conditions (10–5 Pa) directly after film fabrication. Furthermore, we investigated substrate dependence, surface wettability, post-growth treatment, and H2O exposure for other organic luminescent films, namely, 8-hydroxyquinoline aluminum and rubrene, to understand the crystallization phenomenon. Based on our findings, the crystallization phenomenon correlated with the surface roughness of the underlying substrates and the chemical nature of the molecules. Our results provide a comprehensive understanding of the deviations that can potentially occur during the photophysical characterization of organic thin films and the performance of organic devices fabricated under different vacuum environments, thus enabling the improved performance of the organic devices.

Chemical reactions of graphitic carbon nitride films with glass surfaces and their impact on photocatalytic activity

“…PL measurements were performed in a homemade vacuum deposition system combined with time-resolved PL spectroscopy (FluoroCube, HORIBA) at room temperature (297 K). 22 The PL spectra and TR-PLs were acquired at a measurement port in the vacuum chamber using a Duran glass jar (EVAC), which was placed at the center of the spectrometer sample chamber. A pulsed laser diode (NanoLED, HORIBA; wavelength: 342 nm, pulse width: 1.2 ns, pulse energy: 1–2 pJ, repetition rate: 100 kHz) was used as the excitation light source.…”

Plasmonic Metasurfaces as Surface-Enhanced Infrared Absorption Substrates for Optoelectronics: Alq3 Thin-Film Study

“…15,16 One of the recent studies describes the development of an in situ photoluminescence measurement apparatus that provides insight into the crystallization phenomenon of Alq further improvement of the performance of various organic devices. 17 Also, the performance of OLEDs made from Alq 3 layers strictly depends on the deposition rate. 18,19 Nowadays, new methods for analyzing the properties of the OLEDs based on the Alq 3 compound are being actively developed.…”

“…The development of the first high-luminance, low-voltage-driven device based on Alq 3 in 1987 marked the beginning of a new era in organic electronics . Because of the light emission generation by electron–hole recombination in Alq 3 thin films, this compound has been used as a standard emission layer and electron-transporting layer in heterojunction devices such as OLEDs, OLETs, and photovoltaic cells. , One of the recent studies describes the development of an in situ photoluminescence measurement apparatus that provides insight into the crystallization phenomenon of Alq 3 thin films and allows the further improvement of the performance of various organic devices . Also, the performance of OLEDs made from Alq 3 layers strictly depends on the deposition rate. , Nowadays, new methods for analyzing the properties of the OLEDs based on the Alq 3 compound are being actively developed .…”