Analyzing Surface Characterization of Organic LED Material with XPS, REELS and UPS

The display industry is expected to undergo a new level of transformation with organic LED (OLED) displays. Compared to commercially available displays OLED displays consume less power, and enable portable screens to run for a longer period of time from just a single battery charge. Poly (9,9-dioctylfluorene), also known as PFO, is one example of an OLED material. It emits a bright blue light with a low turn-on voltage. A major drawback of PFO is that it has a huge optical gap, which means extreme care must be taken to develop and control the design of the OLED device, to prevent undesirable interaction with the charge carriers in PFO films.

Due to composition of materials such as PFO an analytical tool with more than one technique reveals a more comprehensive picture of the material’s electronic structure, such as understanding the interactions between the charge carriers and the PFO films. Thermo Fisher Scientific XPS tools can be used to investigate the PFO’s electronic structure using multiple technique options.

The techniques used to characterize OLED are ultraviolet photoelectron spectroscopy (UPS), reflected electron energy loss spectroscopy (REELS), and X-ray photoelectron spectroscopy (XPS). The material’s surface composition was investigated using XPS, and its full energy level diagram was generated with the combined data obtained from REELS and UPS.

Experimental and Results

A PFO film measuring 30 nm was placed on a glass substrate and stored in a fluoroware container for a number of days. The film was then investigated using the Thermo Scientific ESCALAB 250Xi X-ray Photoelectron Spectrometer that can be configured with multitechnique options.

Elemental analysis of PFO surface

Figure 1. Elemental analysis of PFO surface

Figure 1 shows the XPS elemental analysis of the surface of PFO film, exhibiting trace amounts of oxygen at the surface of the film. A strong peak of carbon is observed in the spectrum because PFO only contains carbon, however the small amount of oxygen (0.6 %at) present in the film could be due to contamination of the surface during transit or storage. High energy resolution XPS was employed to a carry out comprehensive chemical analysis of the carbon (Figure 2).

High resolution C1s XPS spectrum of the surface

Figure 2. High resolution C1s XPS spectrum of the surface

The surface purity of the PFO films is determined using XPS. The core-level transitions in the aliphatic and aromatic carbon of the PFO polymer are indicated by the strongest peak in the spectrum. The spectrum also shows small peaks which are the result of valence transitions in the PFO. In order to understand the complete electronic structure of PFO, the data in these peaks can be used. However the peaks are not only relatively weak, but they are convoluted with core-level peaks. Nevertheless valence transitions can be easily investigated using a multi-technique approach.

A large number of Thermo Scientific XPS tools, including ESCALAB 250Xi, provide REELS. Carbon aromaticity and unsaturation can be analyzed efficiently using REELS, which determines the electrons from an incident beam spread by the top surface. The aromatic polymers at valence levels can also be analyzed without any interference from core-level carbon transitions. REELS is a surface-sensitive technique, so data can be obtained from the top 1 nm of the surface.

REELS data obtained from a high quality polystyrene film is shown in Figure 3. Polystyrene consists of a long stretch of aliphatic polymers with phenyl side groups, which are chemically identical to each other. A sharp peak at 6.6 eV and a broad hump at approximately 20 eV are observed in the REELS spectrum. The interaction between the primary electron beam and lattice plasmons create the broad hump, while π to π*transitions in the aromatic valence levels produce the single sharp peak, which reflects the chemical environment of the phenyl groups.

REELS spectrum of polystyrene film

Figure 3. REELS spectrum of polystyrene film

The two π-π*peaks, which are formed from the two varied aromatic bonding environments of the 5 and 6 membered carbon rings, are shown in Figure 4.

UPS spectra of polystyrene and PFO

Figure 4. UPS spectra of polystyrene and PFO

The peaks seen in the REELS analysis of PFO, and the small peaks in the carbon XPS spectrum are caused by the same transitions, however the former peaks are not convoluted or hidden by core-level transitions. At 3.7 eV, the energy loss of the π-π1* peak is the result of valence transitions, to the lowest unoccupied π* anti-bonding level from the highest occupied bonding π level. The second peak is due to valence transitions from the same π level but to a slightly higher lying π* level, around 2.2 eV above the lowest unoccupied level. The band gap of the PFO film can be calculated later by using the data from the peaks’ energies.

UPS is used to obtain additional data on the PFO’s valence levels. It is suitable for analyzing valance band transitions, as the helium discharge source used has lower energy, compared to the monochromatic aluminum K- α X-rays utilized for XPS.

Ionization potential of OLED films and other valence parameters can be measured using UPS (Figure 5). The energy of the highest occupied molecular orbital can be directly calculated using the UPS data, given that fermi level position can be measured from a gold sample UPS. The molecular orbital measured using UPS and the p-bonding level involved in the π-π* transitions detected in REELS were found to be the same.

UPS spectrum of the valence level of PFO material

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

The PFO’s energy level diagram can be created, since the p-level energy is known from UPS and also the energy gap from this level to the p* levels was derived from the REELS data. The calculation of band gap of 3.3eV for PFO is achieved using this energy level diagram and is found to be in agreement with the published value. Information on the valence electronic structure is required when optimal PFO-based OLED devices are to be produced.

When manufacturing OLED devices, other materials are added to the PFO to adjust and control the band structure, altering the light emitting properties of the OLED device. Analysis and characterization of doped films can be carried out using the multi-technique approach shown for undoped PFO films.

Conclusion

The various characteristics of the OLED material can be investigated by using the multitechnique tool, Thermo Scientific ESCALAB 250Xi X-ray Photoelectron Spectrometer. Details about film purity and surface contamination are obtained through XPS, while the valence electronic structure can be analyzed with REELS and UPS techniques.

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.

The combination of REELS and UPS data is employed to generate an energy level diagram for PFO (Figure 6).

This information has been sourced, reviewed and adapted from materials provided by Thermo Fisher Scientific – Materials & Structural Analysis.

For more information on this source, please visit Thermo Fisher Scientific – Materials & Structural Analysis.

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