Solution synthesis of Al doped ZnO transparent electrodes: mission impossible or need for more insight?

Abstract

Transparent conducting oxides (TCO) are gaining increasing importance, not only in solar panels, but also in various new thin film based technologies. The herewith increasing demand of TCO’s is associated with immense scientific and economic challenges: First, reliable synthesis routes have to be developed that allow mass production at justifyable cost. Solution based deposition is in this respect an appealing alternative to standard physical deposition methods, as they often require (high) vacuum conditions. Second, the vastly increasing demands for TCO materials motivate the search for cost-effective alternatives to Indium doped Tin Oxide (ITO). (Doped) zinc oxide (ZnO) is among such promising alternatives. Although the proposed routes and materials have important advantages, some crucial challenges jeopardize their success in e.g. energy applications.

We investigate and compare various approaches to synthesize Aluminium doped ZnO. A first route is based on precursor solutions comprising the dissolution of molecular precursors followed by solution deposition on a substrate. A thermal treatment is then needed to transform the molecular precursor into the desired (crystalline) oxide. A second route starts with the preparation of TCO nanoparticles, which are consecutively dispersed in a liquid before deposition and thermal treatment. We address experimental strategies towards optimizing the molecular precursor routes in terms of less toxic solvents and lower temperature thermal treatments [1]. For the nanoparticle based routes we assess the intrinsic potential of a nanoparticle precursor by focussing on the effectiveness of the doping [2,3].

Through in depth characterisation we correlate synthesis conditions to the TCO properties. Hence we try to understand and tackle the intrinsic challenges of solution processing and progress towards functional printed TCO layers competitive with sputtered coatings.