Reliable and reproducible determination of work function and ionization potentials of layers and surfaces relevant to organic light emitting diodes

Ultra violet photoelectron spectroscopy has been used in most of the research related to determination of the energy levels of the materials in multilayer devices, such as organic light emitting diodes, to measure the Fermi and HOMO levels of conductors and organic materials, respectively. The variation in the values reported in the literature often remains unexplained despite apparently similar experimental conditions. We report (i) on the methods that we have found are required to achieve reproducible and stable values of work function (WF) and ionization potentials (IP) of ITO and common organic layers, and (ii) on the origin of the variations. Not unexpectedly, we have found that hydrocarbon contamination arising from vacuum pumps (e.g. oil diffusion pumps) can cause significant changes in WF or IP. Of greater possible significance is the observation that continuous exposure to ultraviolet light can enhance these effects and can change the work function of an interface much faster than would occur in the absence of UV light. The major effect probably arises from effects of the secondary electrons produced by the 21.22 eV photons of the He resonance lamp. By dealing with these problems, we have been able to measure stable values for WF and IP over extended periods of time ranging from hours to days. [Copyright &y& Elsevier]