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

A material used as a component in an organic LED (OLED) was analyzed in order to characterize its surface chemistry and to analyze its LUMO and STATES to draw an energy level plot.

Introduction

OLED displays are predicted to become a key technology in the screens of the future. OLEDs tend to use less power than conventional display technologies, meaning they will allow the screens of portable devices to run for longer on battery power.

Poly (9,9-dioctylfluorene), or PFO, is a blue-light-emitting, high-brightness OLED material that has a low turn-on voltage. As PFO has a large optical gap¹ which means that in order to make the most of its potential for OLED displays, the overall design of the OLED device has to be carefully developed and controlled to avoid any adverse interaction with charge carriers in PFO films.

Thermo Scientific Nexsa Surface Analysis system

Thermo Scientific ESCALAB Xi⁺

To understand how PFO interacts with charge carriers its electronic structure must be understood. Developing this understanding requires a multi-technique analysis. Thermo Fisher Scientific provides XPS systems that can be configured to work with multiple techniques, such as Nexsa and ESCALAB, allowing the in-depth investigation of PFO’s electronic structure using a single instrument.

Figure 1: Elemental analysis of PFO surface

Experimental and Results

A film of PFO (30 nm) was deposited onto a glass substrate, stored for several days in a fluoroware container, and then analyzed using the Thermo Scientific™ ESCALAB™ Xi⁺. This instrument was chosen because it provides several options for sample preparation and multi-technique analysis.

Elemental analysis by XPS of the surface of the PFO film showed that there was a small concentration (0.6%) of oxygen present (Figure 1). As pure PFO only contains carbon (explaining the large carbon peak in Figure 1) this oxygen must be the result of contamination occurring during transit or storage.

High energy resolution XPS was used to carry out an in-depth chemical analysis of the carbon (Figure 2). This method can be used to determine how pure the surface of a PFO is. The largest peak in the spectrum is related to core-level (valence) electronic transitions in the aliphatic and aromatic carbon groups present in PFO.

Figure 2: High-resolution C1s XPS spectrum of the surface

These peaks contain key information that allows the electronic structure of PFO to be determined, however, unfortunately, here they are relatively weak and cross-over with core-level peaks. The use of a multi-technique approach allows these core level transitions to be analyzed more effectively.

The majority of XPS tools from Thermo Scientific, including ESCALAB, include electron energy loss spectroscopy (REELS) capability. REELS measures electrons resulting from an incident beam that is scattered on the top of the sample’s surface and is perfect for the analysis of aromatic and unsaturated carbon groups. The use of REELS allows valence level electronic transitions in aromatic polymers to be observed without interference from core-level transitions. In addition, the technique is highly surface sensitive and can collect data from the top 1 nm layer of a sample.

Figure 3 shows REELS data taken from the analysis of a high-quality polystyrene film. Polystyrene has a polymer backbone of aliphatic carbon with identical phenyl side groups projecting outwards. The REELS data shows a broad hump at 20 eV, the result of the electron beam interacting with lattice plasmons, and one sharp peak at 6.6 eV – the peak of interest. This peak is the result of aromatic π to π* transitions, there is only one peak because all of the phenyl groups are chemically identical.

Figure 3: REELS spectrum of polystyrene film

Figure 4 shows REELS data from the analysis of PFO. The spectrum has two different π-π* peaks, most likely because of the different chemical environments of the 6 and 5 membered aromatic carbon rings. These two peaks are the result of the same transitions that caused the smaller peaks in Figure 2, however they are present here without overlap with the core level transitions.

Figure 4: UPS spectra of polystyrene and PFO

The π-π^1* at an energy loss of 3.7 eV is the result of valence transitions from the π highest occupied molecular orbital (HOMO) to the π* lowest unoccupied molecular orbital (LUMO), which are therefore separated by an energy gap of 3.7 eV. The second peak is the result of transitions that are also from the HOMO but to a higher energy π* level that has an energy 2.2 eV above the lowest unoccupied level.

The information about the energies of the electronic transition peaks will be used later to determine the energy of the PFO film’s bandgap.

Additional information about PFO’s valence levels was collected using ultraviolet photoelectron spectroscopy (UPS). The low-energy helium discharge source is better suited for the investigation of electronic transitions in the valance band compared to XPS’s high energy aluminum K-a X-rays.

UPS can be used to measure an OLED film’s ionization potential and other valence level data (Figure 5). As UPS of a gold sample can be used to determine the Fermi level position UPS can also be used to determine the HOMO energy directly. The HOMO orbital is identical to the π-bond in the π-π* transitions in the REELS data.

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

USP and REELS allow the energy of the π HOMO to be known, and REELS provides the energy level of the π* LUMO, meaning an energy level diagram of PFO’s electronic structure can be drawn. This diagram makes it simple to determine that PFO’s bandgap is 3.3 eV, which is in agreement with values in the literature.¹

In order to produce optimized OLED devices using PFO information about its valence structure is required. When producing a PFO-based OLED device the PFO would be doped with other materials to fine-tune its electronic band structure and its light-emitting properties. The multi-technique approach described above for undoped PFO can also be used to analyze the electronic structure of doped PFO films.

Summary

The performance of OLED-based devices is highly dependent on the electronic and chemical behavior of the materials used. The use of multi-technique analysis methods allows researchers to characterize the materials used to construct OLEDS, investigate if the material’s surface is a contaminant, and develop a better understanding on how electronic structure impacts performance.

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

Acknowledgments

Thermo Fisher Scientific would like to acknowledge Harry M. Meyer III, Oak Ridge National Laboratory, USA, for supplying the samples and for helpful discussions.

Produced from materials originally authored by Paul Mack from Thermo Fisher Scientific East Grinstead, West Sussex, UK.

References and Further Reading

1. L. S. Liao, Applied Physics Letters, Vol. 76, No. 24, 12 June 2000

This information has been sourced, reviewed and adapted from materials provided by Thermo Fisher Scientific – X-Ray Photoelectron Spectroscopy (XPS).

For more information on this source, please visit Thermo Fisher Scientific – X-Ray Photoelectron Spectroscopy (XPS).