XPS Depth Profile of OLED Thin-Film

Organic light emitting diodes, also known as OLEDs, have attracted a great deal of interest, thanks to their promising applications in solid-state lighting and flat-panel displays. OLEDs are made up of an organic material, which positioned between two electrodes, emits light in response to an electric current; one of the electrodes is generally transparent. Two things are fundamental to the OLED’s performance – the compositional chemistry of this organic thin-film and its chemical characteristic at the interface with the electrodes. This article looks at the compositional change of an OLED layer deposited on indium-tin oxide. Tris(4-carbazoyl-9-ylphenyl)amine doped by the novel material was deposited on ITO to a thickness of about 150 nm as characterized by ellipsometry.

Gas-cluster ion source (GCIS) in cluster mode was used to perform depth profiling. Currently, monatomic argon is routinely replaced by argon clusters to reduce the surface damage of delicate organic analytes¹. Before and after each etch cycle, XP spectra were obtained to see quantitative changes concerning the chemical and elemental state of the surface. HOMO electronic states and work function of the material were examined simultaneously using UPS.

For modern analytics, performing more than one analytical technique on a sample is more suitable to negate the variables of sample handling and treatment and to aid direct comparison of data. Using the automated methods on the AXIS Supra, a multi-technique experiment can be created without intervention during acquisition. This article demonstrates the capability by depth profiling an OLED device acquiring both UPS and XPS data sequentially without gas handling or manual intervention.

Results

The XPS/UPS/GCIS depth profiling method was selected in the software following introduction into the analysis chamber of the AXIS Supra. UPS and XPS spectra were obtained before and between each etch cycle. For this sample, 5 kV Ar₁₀₀₀+ were the selected etch conditions, typical for delicate polymer materials. Survey spectra were acquired from which a relative depth profile was eventually obtained (Figure 1). It can be observed that the surface concentrations of N, O, and C initially change after the first etch and subsequently, the concentration of the component elements remain steady-state all through the bulk of the material.

At the interface, an increase in tin and indium from the substrate material corresponding to a rapid decrease in organic elements is seen. The composition evolved to the expected stoichiometric ratio of ITO. With this multimodal method, UPS spectral structure can be directly compared with the surface composition and relative depth into the film. Figure 2 shows the evolution of the HOMO states of the surface as a function of etching – the spectra are offset for clarity, with the virgin non-etched surface the lowest vertically.

On completion of the first etch cycle, there is a slight change with 3 distinct features at 9.25 eV, 6.93 eV and 4.01 eV denoting different densities of states for the OLED material. These features continue to be constant until the interface is reached where there is a sudden change in distribution. The two higher energy features decrease in intensity with a noticeable increase in the gradient of slope down to the HOMO edge point. This change is constantly maintained into the ITO substrate. The change in on-set of photoemission with each etch cycle is shown in Figure 3. At first, a KE of 4.35 eV was recorded for the surface. This decreased to 4.26 eV after the first etch cycle and continued to decrease slightly with every cycle. For the bulk ITO, the value decreased to 3.9 eV at the interface.

Figure 1. 5 kV Ar₁₀₀₀+ depth profile of thin-film.

Figure 2. HOMO region of thin film (spectra are offset vertically with etch cycle).

Figure 3. Cut-off region (spectra are offset vertically with etch cycle).

Conclusion

This article demonstrates how a prescribed method can be run to perform XPS-UPS depth profiles of novel organic electronic devices using Argon cluster ions. The method enables users to directly compare the changes in electronic character and the changes in surface composition as seen in the UPS spectra. The prescribed method is automated, needing zero manual or computational intervention by the analyst. It is truly a ‘click-and-go’ hyphenated technique.

Acknowledgments

The author records his gratitude to Pusan University for supplying the sample preparation and instrument time.

References and Further Reading

1. A. G. Shard, S. Ray, M. P. Seah, L. Yang, Surf. Interface Anal. 2011, 43, 1240–1250. DOI 10.1002/sia.3705

This information has been sourced, reviewed and adapted from materials provided by Kratos Analytical, Ltd.

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