Regensburg 2013 – scientific programme

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DS: Fachverband Dünne Schichten

DS 23: Focus Session: Thin Film Photovoltaic Materials and Solar Cells II

DS 23.2: Talk

Wednesday, March 13, 2013, 15:00–15:15, H8

Work function determination of degenerately Al-doped ZnO by thermionic emission — •Christian Wilde, Bernd Schmidt, Mykola Vinnichenko, and Sibylle Gemming — Helmholtz-Zentrum Dresden-Rossendorf, Institute of Ion Beam Physics and Materials Research, Dresden, Germany

Degenerately Al-doped ZnO (AZO) is a transparent conductive oxide (TCO) widely used, especially as electrode material in solar cells. The work function of these electrodes is of crucial importance, because it determines the electronic barrier between the TCO and the semiconducting absorber. Therefore, this barrier directly affects the charge collection and thus solar cells efficiency. In this contribution we report the results of experiments carried out to determine the work function of AZO by using the thermionic emission theory. AZO, as a degenerately doped semiconductor with the Fermi level in the conduction band shows a metal-like behaviour, and, if it is brought into contact with a semiconductor, it forms a Schottky barrier. From measurements of temperature-dependent current-voltage characteristics the work function of AZO can be determined. We demonstrate that this model of metal/semiconductor contact is applicable to the contact between AZO and a non-degenerately doped substrate (silicon or germanium). The Schottky barrier formation is studied with respect to the substrate conductivity type and surface cleaning. The determined AZO work function variation will be discussed in relation to the film properties and process parameters of reactive and non-reactive DC magnetron sputter deposition.