(418bu) Vibrational Spectroscopy Investigation of the Giant Surface Potential of Organic Semiconductors

A phenomenon known as the giant surface potential (GSP), where the surface potential of organic films display linear growth with increasing film thicknesses in the absence of light wasfirst reported by Ito et al (JAP 2002) on (8 hydroxyquinoline)aluminum(Alq₃), a prototypical fluorescent material used in OLEDs. It has been shown that the surface potential of Alq₃ has reached 28 V for a 560 nm thick film by Kelvin probe measurements in vacuum in the absence of light.  Since then this phenomenon has been observed for a broad range of molecules thermally evaporated on varying substrates under similar conditions. The effect is independent of the substrate, dependent on the film thickness and decays quickly with illumination at the normal mode of the respective molecule and in elevated temperature. The spontaneous buildup of the GSP cannot be explained by any classical interfacial phenomena.  Investigations into the cause of GSP, including the analysis of light and heat on the surface potential are not fully understood.

In this study we use vibrational spectroscopy to understand the nature of the GSP buildup, where we have found a significant change in the vibrational structure of the organic material in thick films where the GSP is present as compared to the thin films.  The vibrational spectra of the most commonly studied light-emitting material, Alq_3, on indium tin oxide (ITO) is investigated as a function of thickness using high resolution energy electron loss spectroscopy (HREELS), Raman spectroscopy, high resolution x-ray photoelectron spectroscopy (HR-XPS), attenuated total reflectance infrared spectroscopy (ATR-IR), and density functional theory (DFT) calculations.  In order to provide a holistic understanding of the GSP, the results are compared to the vibrational spectra of 1,3,5-tris(N-phenylbenzimiazole-2-yl)benzene (TPBi) on ITO, an electron transporter host material with a measured GSP of 0.07 V/nm, and bis(triphenylsilyl)-dibenzofuran (BTDF) on ITO, a typical electron-conducting host used in combination with hole-conducting deep-blue emitter with a measured GSP of 0.08V/nm.  The observed spectra show significant changes with the presence of the GSP in the organic material on ITO, which can be explained in terms of different symmetries of the isomers as well as between complexes and isolated anions.  Additionally, it’s been found that the surface phase differs from the bulk phase, where a structured layer is evident at the interface of the organic semiconductor, and this layer shifts with increasing thickness and in the presence of the GSP. The present work has provided direct evidence that a different molecular orientation exists at the interface than in the bulk, where the GSP exists.