Spatially Resolved Functional Group Analysis of OLED Materials Using EELS and ToF-SIMS

Abstract

Electron energy-loss spectroscopy (EELS) is widely used in analyzing the electronic structure of inorganic materials at high spatial resolution. In this study, we use a monochromator to improve the energy resolution, allowing us to analyze the electronic structure of organic light-emitting diode (OLED) materials with greater precision. This study demonstrates the use of the energy-loss near-edge structure to map the nitrogen content of organic molecules and identify the distinct bonding characteristics of aromatic carbon and pyridinic nitrogen. Furthermore, we integrate EELS with time-of-flight secondary ion mass spectrometry for molecular mapping of three different bilayers composed of OLED materials. This approach allows us to successfully map functional groups in the by-layer OLED and measure the thickness of two OLED layers. This study introduces spatially resolved functional group analysis using electron beam spectroscopy and contributes to the development of methods for complete nanoscale analysis of organic multilayer architectures.

Figure 1

(a) Schematics of STEM/EELS equipment and the method for visualizing ELNES. (b) Organic materials used in OLED for bilayer cross-sectional samples. (c) Illustrations of bilayer architectures alternatively composed of the targeted organic materials.

Figure 2

(a) General scheme of ToF-SIMS analysis for organic bilayers. Overall secondary ion mass spectra recorded for (b) TPBi/TAPC, (c) TPBi/Alq3, and (d) Alq3/TAPC. Depth profiles recorded for (e) TPBi/TAPC, (f) TPBi/Alq3, and (g) Alq3/TAPC according to the number of ion beam irradiations sputtered with 2.5 keV Ar₁₀₀₀⁺ and analyzed with 30 keV Bi₃⁺ projectiles.

Figure 3

HAADF and LAADF images of organic bilayers for (a) TPBi/TAPC, (b) TPBi/Alq3, and (c) Alq3/TAPC. Corresponding EELS spectra of C K edges and N K edges are shown, illustrating the chemical information for (d) TPBi/TAPC, (e) TPBi/Alq3, and (f) Alq3/TAPC.

Figure 4

Mapping results of nitrogen components, aromatic carbon (C1), and pyridinic nitrogen (N1) for bilayer structures: (a) TPBi/TAPC, (b) TPBi/Alq3, and (c) Alq3/TAPC. The intensity profiles shown alongside the maps assist in layer separtion.