The performances of organic semiconductor devices are crucially linked with their stability at the ambient atmosphere. The evolution of electronic structures of 20 nm thick rubrene films exposed to ambient environment with time has been studied by UV and X-ray photoemission spectroscopy (UPS and XPS), near edge X-ray absorption fine structure (NEXAFS) spectroscopy, and density functional theory (DFT). XPS, NEXAFS data, and DFT calculated values suggest the formation of rubrene-epoxide and rubrene-endoperoxide through reaction of tetracene backbone with oxygen of ambient environment. Angle dependent XPS measurement indicates that the entire probed depth of the films reacts with oxygen by spending only about 120 min in ambient environment. The HOMO peak of pristine rubrene films almost disappears by exposure of 120 min to ambient environment. The evolution of the valence band (occupied states) and NEXAFS (unoccupied states) spectra indicates that the films become more insulating with exposure as the HOMO-LUMO gap increases on oxidation. Oxygen induced chemical reaction completely destroys the delocalized nature of the electron distribution in the tetracene backbone of rubrene. The results are relevant to the performance and reliability of rubrene based devices in the environment.

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A systematic experimental and theoretical study about substrate induced molecular conformation in rubrene thin films by varying film thickness from sub-monolayer to multilayer, which currently attracts substantial attention with regard to its application in organic electronics, is performed. The clean polycrystalline Au and Ag were used as noble-metals, whereas, H passivated and SiO 2 terminated Si (100) were used as dielectric substrates. Angle dependent near edge x-ray absorption fine structure spectroscopy (NEXAFS) was employed to understand the molecular conformation whereas atomic force microscopy (AFM) was used to investigate the surface morphologies of the films. X-ray absorption spectra (XAS) of rubrene molecules with flat and twisted conformations were calculated using density functional theory (DFT). All the observed NEXAFS spectra of rubrene thin films at various thicknesses and onto different substrates were explained in terms of different combination of the spectral intensity from the twisted and the flat molecules. In contrast to general findings, comparatively rough polycrystalline Ag surface is found to support growth of better quality rubrene films. The results have important implications for the understanding of the substrate induced molecular 2 conformations in rubrene thin films with its thickness and are beneficial for the improvement of the device performance.

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