Oxidation of rubrene thin films: an electronic structure study

Area of Science:

• Materials Science
• Organic Electronics
• Surface Chemistry

Background:

• Organic semiconductor device performance relies heavily on stability in ambient conditions.
• Understanding degradation pathways is crucial for reliable electronic devices.

Purpose of the Study:

• To investigate the electronic structure evolution of rubrene films upon exposure to ambient air.
• To identify the chemical transformations occurring in rubrene under atmospheric conditions.

Main Methods:

• Utilized UV and X-ray photoemission spectroscopy (UPS and XPS) for electronic structure analysis.
• Employed near edge X-ray absorption fine structure (NEXAFS) spectroscopy to study unoccupied electronic states.
• Performed density functional theory (DFT) calculations for theoretical validation.

Main Results:

• Identified rubrene-epoxide and rubrene-endoperoxide formation via reaction with ambient oxygen.
• Observed complete reaction within the probed depth of 20 nm thick films in approximately 120 minutes.
• Noted the disappearance of the highest occupied molecular orbital (HOMO) peak and an increased HOMO-LUMO gap, indicating enhanced insulating properties.
• Demonstrated the destruction of the delocalized electron system in the rubrene backbone due to oxidation.

Conclusions:

• Rubrene's electronic structure is significantly altered by ambient oxygen exposure within a short timeframe.
• Oxidation leads to increased insulating behavior and degradation of charge transport properties in rubrene films.
• These findings are critical for assessing the performance and reliability of rubrene-based organic semiconductor devices in real-world environments.