Transparent conducting oxides (TCO) are used as electrodes in solar cells and flat-panel displays. The high electrical conductivity is usually obtained by typical doping with heterovalent substitutional impurities like in Sn-doped In2O3 (ITO), fluorine-doped SnO2 (FTO) and Al-doped ZnO (AZO). For future application in solid state lighting materials with higher conductivities than those available today are desirable. Moreover, the TCO with the best performance to date is ITO, which suffers from limited resources of indium. The lower conductivites of FTO and AZO are caused by lower dopant solubility or grain boundary barriers, which are a direct consequence of the applied substitutional doping.
This project will elucidate the practical limits of an alternative doping strategy, namely modulation doping. Modulation doping is already used successfully in semiconductor (opto-)electronics and has also been discussed as a potential route to overcome conductivity limits. The conventional approach for modulation doping, using the contact between an undoped material with a lower energy gap and a doped material with a higher energy gap, is not suitable for TCOs. This is related to the intrinsic defect formation, which determines the maximum possible carrier concentrations. We will therefore use a novel strategy for modulation doping, which relies on defect related Fermi level pinning in insulators as dopant phase. The viability of this approach has already been demonstrated. In the project, electrically conducting materials shall be obtained by preparation of multilayer thin films using different techniques and of composite materials with nominally undoped TCO hosts and embedded dopant nanoparticles. The transferability to an industrial production process will be studied at the industry partner.
Surface Science Group, Department of Materials Science, TU Darmstadt, Germany: thin film deposition by magnetron sputtering and atomic layer deposition, synthesis of composite materials, interface analysis using XPS and UPS, electrical and optical characterization
Laboratoire des Matériaux et du Génie Physique, Grenoble INP, France: thin film synthesis with atomic layer deposition, chemical vapor deposition, spray pyrolysis, spin-coating, structural, electrical and optical characterization
Industry Partner: Lotus Synthesis, France: Inorganic precursors design and evaluation for scale-up production potential for TCO multilayer fabrication
Host Country (employment): Germany
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