TCOs

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Transparent Conducting Oxides (TCOs)

Transparent Conducting Oxides (TCOs) are used in many (opto)electronic applications, e.g. as electrodes in solar cells or light emitting diodes. Recently TCOs are also developed for thin film transistors (TFT). Further important applications are gas sensors and coatings in energy efficient windows. The basic TCO materials are indium oxide (In2O3), tin oxide (SnO2), and zinc oxide (ZnO). They can be degenerately doped n-type with tin (In2O3:Sn also known as ITO), fluorine (SnO2:F) or aluminium (ZnO:Al). The performance of TCOs is largely determined by their electrical and optical properties. When used as electrodes for semiconductors or as sensors, their surface and interface properties become important.

CuInSe2 Solar Cell
Schematic arrangement and band alignment for a Cu(In,Ga)Se2 thin film solar cell with a transparent ZnO front contact

General Information on TCOs

  • H. L. Hartnagel, A. L. Dawar, A. K. Jain, C. Jagadish, Semiconducting Transparent Thin Films, (Institute of Physics Publishing, Bristol, 1995).
  • MRS Bulletin Vol. 25 [August] (2000)
  • C. Jagadish, S. J. Pearton (Eds.), Zinc Oxide: Bulk, Thin Films and Nanostructures (Elsevier, Oxford, 2006).
  • K. Ellmer, A. Klein, B. Rech (Eds.), Transparent Conducting Zinc Oxide: Basics and Application in Thin Film Solar Cells (Springer-Verlag, in press)

Research

We are investigating surfaces of transparent conducting oxides and their interfaces in thin film solar cells (e.g. CdTe and CIGS) and organic light emitting diodes. Our focus is on chemical and electronic properties of the surfaces and interfaces. We obtain on information chemical composition (segregation, interdiffusion, chemical reactivity), band alignment, and surface potentials (Fermi level position, work function) using photoelectron spectroscopy (XPS and UPS). The TCOs are mainly prepared as thin films using magnetron sputtering, but we are also analysing ceramic samples. Thin film deposition and XPS/UPS analysis are performed in-situ, which is possible due to the use of integrated systems like the DAISY-MAT.

Defect properties are essential to understand the electronic properties of oxides. We are extracting defect properties by measuring the surface Fermi level position. The energetics and kinetics (diffusion) of defects are investigated in cooperation with other groups (see below).

UPS spectra of In2O3 showing shifts of the Fermi level and the work function after heating in oxygen
UPS spectra of In2O3 showing shifts of the Fermi level and the work function after heating in oxygen
  • Surface composition, Fermi level position, work function and ionization potential of basis TCOs (In2O3, ITO, SnO2, SnO2:Sb, ZnO, ZnO:Al)
  • Surface properties of ceramic TCOs with complex composition (co-doped bixbyite, CuAlO2, and others)
  • Interfaces between ZnO and CdS, Cu(In,Ga)Se2 and In2S3
  • Surface and interface properties of (Zn,Mg)O
  • Interfaces between ITO and organic molecules (ZnPC)
  • Interface properties between SnO2 and CdS
  • Interface properties between In2O3 and CdTe
  • Energetics of intrinsic point defects in ZnO and In2O3
  • Energy band structure of In2O3

Experimentally determined energy band alignment at different TCO interfaces
Experimentally determined energy band alignment at different TCO interfaces

Cooperations

Publications

A list of publications concerning this field is soon to follow.