Analysis of the dopant distribution in Co-deposited organic thin films by scanning transmission electron microscopy

• We present a methodology to analyze the dopant distribution in organic thin films.
• The method combines HAADF-STEM imaging and EDS X-ray spectroscopy.
• Ir(ppy)3 dopant was co-deposited into Spiro2-CBP organic matrix.
• The dopant was co-deposited with and without substrate vibration.
• Images and chemical information of the dopant were simultaneously obtained.

Organic light-emitting diodes using phosphorescent dyes (PHOLEDs) have excellent performance, with internal quantum efficiencies approaching 100%. To maximize their performance, PHOLED devices use a conductive organic host material with a sufficiently dispersed phosphorescent guest to avoid concentration quenching. Fac-tris(2-phenylpyridine) iridium, [Ir(ppy)3] is one of the most widely used green phosphorescent organic compounds. In this work, we used scanning transmission electron microscopy (STEM) equipped with HAADF (high-angle annular dark-field) and EDS (energy dispersive X-ray spectroscopy) detectors to analyze the distribution of the [Ir(ppy)3] concentration in the host material. This analysis technique, employed for the first time in co-deposited organic thin films, can simultaneously obtain an image and its respective chemical information, allowing for definitive characterization of the distribution and morphology of [Ir(ppy)3]. The technique was also used to analyze the effect of the vibration of the substrate during thermal co-deposition of the [Ir(ppy)3] molecules into an organic matrix.