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

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.