Using XPS, REELS and UPS for the Surface Characterization of Organic LED Material

Organic LED (OLED) displays have been seen as one of the most promising technologies for the displays of the future. OLED displays have a lower power consumption than the other existing commercial displays and hence the battery life of portable screens is considerably increased from a single battery charge. Poly (9,9-dioctylfluorene) or PFO is a blue-light emitting OLED material with a high brightness, a large optical gap and a low turn-on voltage.

All these properties render PFO a potential candidate for OLED displays. In order to avoid any adverse reaction with PFO charge carriers, the overall design of the OLED device has to be developed with extra care.

An in-depth insight on the electronic structure of PFO is required to understand the PFO interaction with charge carriers. This demands a multi-technique analytical method. Thermo Scientific presents XPS tools capable of being configured with multi-technique options, allowing investigation of most of the electronic structure of PFO with a single tool.

Experimental

A glass substrate was deposited with a 30nm PFO film which is then stored in a fluoroware container for several days and analyzed. Thermo Scientific ESCALAB 250Xi was used for the analysis, and it can be configured with different options for sample preparation and multi- technique analysis.

XPS elemental analysis of the PFO surface is shown in Figure 1 with small quantities of oxygen at the film surface. As PFO contains only carbon, the observed oxygen is likely to be a contaminant deposited onto the surface during a transit or storage. As shown in Figure 2, detailed chemical analysis of carbon was performed through high energy resolution XPS. In this manner, XPS could be potentially used to evaluate purity of PFO film surface.

Elemental analysis of PFO surface

Figure 1. Elemental analysis of PFO surface

High resolution C1s XPS spectrum of the surface

Figure 2. High resolution C1s XPS spectrum of the surface

Results

The core-level transitions in the aliphatic and aromatic carbon of the PFO polymer resulted in the strongest peak in the spectrum. In addition, small peaks resulting from valence transitions in the PFO were also observed. The peaks were found to have information required for the complete understanding of PFO’s electronic structure. However, the peaks were relatively weak and convoluted with core-level peaks. The valence transitions can be easily analyzed using a multi-technique approach.

Most of the Thermo Scientific XPS tools, including ESCALAB 250Xi provide reflected electron energy loss spectroscopy (REELS) as standard. This technique involves the measurement of the electrons from an incident beam scattered by the top surface, and is suitable for analyzing aromaticity and unsaturation. It also studies the valence levels of aromatic polymers without any interference from core-level carbon transitions. In addition, the information related to the top 1nm of the surface can be obtained as the technique is extremely surface sensitive.

Figure 3 shows an example of REELS data from a high quality polystyrene film. Polystyrene consists of a long aliphatic polymer backbone with phenyl side groups where each group is chemically similar. The REELS spectrum exhibits a broad hump at around 20eV and a single sharp peak at 6.6eV. The broad hump is a result of the interaction between the primary electron beam and lattice plasmons. The sharp peak is due to π to π* transitions in the aromatic valence levels. A single peak indicates the single chemical environment of the phenyl groups.

REELS spectrum of polystyrene film

Figure 3. REELS spectrum of polystyrene film

Figure 4 shows the π to π* peaks resulting from the REELS analysis of PFO. The peaks are formed due to the transitions that caused the small peaks in the carbon XPS spectrum. However, they are not obscured by core-level transitions or convoluted. The valence transitions from the highest occupied bonding π level to the lowest unoccupied π* anti-bonding level results to the π to π1* peak at 3.7eV energy loss. Therefore, the energy separation between these levels is 3.7eV. The second peak results from the transitions from the same π level to a higher lying π* level, some of which is 2.2eV above the lowest unoccupied level. The measurement of the band gap of the PFO film can be carried out using the information related to the peak energies.

UPS spectra of polystyrene and PFO

Figure 4. UPS spectra of polystyrene and PFO

Using the ultraviolet photoelectron spectroscopy (UPS), more information on the PFO valence levels can be obtained. UPS consists of a helium discharge source having energy lower than the monochromatic aluminum K-a X-rays used for XPS. This makes the UPS an ideal source for studying the valence band transitions. UPS can also measure the ionization potential of OLED films and other valence level parameters as shown in Figure 5. The Fermi level position can also be determined from the UPS of a gold sample, which ensures measurement of the highest occupied molecular orbital energy from the UPS data. The molecular orbital resembles the p-bonding level involved in the π-π* transitions observed in REELS.

UPS spectrum of the valence level of PFO material

Figure 5. UPS spectrum of the valence level of PFO material

The energy level diagram of PFO can be generated with the known energy value of the p-level from UPS and the energy gap from this level to the p* levels obtained using REELS data. With this diagram, a band gap of 3.3eV for PFO can be calculated for PFO that is in compliance with the published value. For the development of optimal PFO-based OLED devices, information related to the valence electronic structure is required. The PFO can be doped with other materials to adjust and control the band structure to make OLED devices, thereby changing the light emitting characteristics of the device. Based on the multi- technique approach carried out for undoped PFO, the electronic structure of doped films can be analyzed and characterized.

Conclusion

Thermo Scientific ESCALAB 250Xi is a multi-technique analytical tool used for the analysis of different types of OLED materials. Information related to the film purity and surface contaminants can be obtained through XPS, while the valence electronic structure can be studied using the REELS and UPS. An energy level diagram for PFO can be produced with the combination of REELS and UPS data, as shown in Figure 6.

By combining the information from REELS and UPS the energy level diagram of PFO surface can be created

Figure 6. By combining the information from REELS and UPS the energy level diagram of PFO surface can be created

This information has been sourced, reviewed and adapted from materials provided by Thermo Scientific – Electron Microscopy Solutions.

For more information on this source, please visit Thermo Scientific – Electron Microscopy Solutions.

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