Alkali metal doping and energy level shift in organic semiconductors Huanjun Ding, Yongli Gao * Department of Physics and Astronomy, University of Rochester, Rochester, NY 14627, USA Available online 18 November 2005 Abstract We have investigated Cs and Na doping in copper phthalocyanine (CuPc) and tris(8-hydroxyquinoline) aluminum (Alq) using photoemission spectroscopy. We observed valence and core level spectra changes at different doping levels, and found that the doping induces an energy level shift that can be seen in two different stages. The first stage is predominantly due to the Fermi level moving in the energy gap as a result of the doping of electrons from the alkaline metal to the organic, and the second stage is characterized by a significant modification of organic energy levels, such as the introduction of a new gap state, new core level components and a change of binding energies. Furthermore, we observed that the energy level shift in the first stage depends in a semi-logarithmic fashion on the doping concentration, whose slope cannot be explained by the conventional model used in inorganic semiconductors. These results indicate that the molecular nature and strong correlation must be considered for doping in organic semiconductors. # 2005 Elsevier B.V. All rights reserved. PACS: 71.20.D; 73.20; 79.60.F Keywords: Organic semiconductors, Doping, Interface electronic structure, Energy level shift 1. Introduction Doping plays an important role in semiconductor device applications, and the possibility of doping in organic semiconductor devices has been extensively explored recently. One example is that doping of conjugated polymers can increase the conductivity by many orders of magnitude [1]. Moreover, doping has also been widely used in organic light emitting diodes (OLEDs), and proven to be one of the most effective methods to improve the performance of the devices. There have been numerous examples of using dopants to control the luminescence of the emitter and to enhance the stability [2–5]. In addition, doping can be used to modify the properties of the metal–organic interface by adjusting the energy level alignment [6–8]. Despite these exciting observa- tions, the fundamental relationship between the guest and host energy levels is not yet clearly understood. In this paper, we report our investigation of the energy level dependence on the doping concentration in alkali metal (Na and Cs) doped copper phthalocyanine (CuPc) and tris(8-hydro- xyquinoline) aluminum (Alq). Using X-ray and ultraviolet photoemission spectroscopy (XPS and UPS), we measured directly the energy level position and doping concentration, and observed that doping induces an energy level shift which can be seen as in two stages. Initially, all the energy levels shift simultaneously in a semi-logarithmic fashion whose slope cannot be explained by the conventional model used in inorganic semiconductors. As the doping level increases, the energy level shifts become saturated and a new gap state appears, which indicates that the electronic structure of the organic materials has been modified. 2. Experiment Measurements were made in two separate instruments. All spectra involving Cs doped CuPc were recorded in an ESCA Lab Mark II and these measurements have been previously described in detail [9]. A custom-designed ultrahigh vacuum (UHV) system was used to collect the data for Cs and Na doped Alq. The organic film was thermally evaporated in an evaporation chamber onto a Au-coated Si wafer cleaned with Ar + sputtering, and then transferred directly into a spectrometer chamber without breaking the vacuum. The base pressure in the spectrometer chamber was typically 8  10 11 Torr. Cs or Na was evaporated from a SEAS getter source in the spectrometer www.elsevier.com/locate/apsusc Applied Surface Science 252 (2006) 3943–3947 * Corresponding author. E-mail address: ygao@pas.rochester.edu (Y. Gao). 0169-4332/$ – see front matter # 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2005.09.071