Low cost, high performance spray pyrolysis grown amorphous Zinc Tin Oxide: The challenge of a complex growth process.

Transparent conductive oxides (TCOs) are important materials for a wide range of optoelectronic devices. Amorphous zinc-tin oxide (a-ZTO) is a TCO and one of the best non toxic, low-cost replacement for more expensive amorphous indium-gallium-zinc oxide. Here, we employ spray pyrolysis, an inexpensive and versatile chemical vapour deposition based technique, to synthesise a-ZTO with an as-deposited conductivity of ≈300 S/cm - the highest value hitherto among the reported solution processed films. Compositional analysis via X-ray photoelectron spectroscopy reveals a non-stoichimoetric transfer \rev{of Zn and Sn from the dissolved precursors into the film}, with best electrical properties achieved at a film composition of xfilm=0.3 ((ZnO)x(SnO2 )1-x (0 < x < 1)). The morphology of these films is compared to films synthesised by physical vapour deposition, and a strong correlation between morphology and electrical properties is revealed. A granular nature of the SP grown films, which seems like a drawback at a first glance, brings a prospect of using a-ZTO in ink-jet printed films from a nanoparticle suspension for the room temperature deposition. Brief post-anneal cycles in N2 gas improve the conductivity of the films by means of grain boundary passivation.