Sustainable Transparent Conducting Oxides: Insights from Amorphous SnOx Thin Films via Oxygen Stoichiometry Control

Transparent conducting oxides (TCOs) are essential in optoelectronic devices. However, the reliance on indium-based TCOs raises concerns about resource scarcity and environmental impacts, especially with the increasing demand for solar cell production. This study focuses on amorphous (a-) SnO2 thin films as sustainable alternatives to conventional In2O3-based TCOs. The a-SnOx thin films are synthesized via reactive plasma deposition at low process temperatures. The effects of the oxygen stoichiometry and film density on the optical and electrical properties are elucidated, revealing a range of semiconducting behaviors from SnO2-like to a mixture of SnO2-like and SnO-like properties. In particular, a-SnOx films with an O/Sn ratio of ∼1.99 and high mass density show excellent TCO properties, more than 1000 S cm–1, even for films with thicknesses of less than 100 nm. Conversely, when the O/Sn ratio and mass density are reduced, the TCO properties degrade, and extended lone-pair Sn 5s states appear in the upper valence band. This study not only shows that the rare-metal-free SnO2 system is promising as an amorphous TCO film with high conductivity but also provides valuable insights into the complex interplay between oxygen content and structural configuration in tuning the properties of amorphous Sn-based oxide semiconductors.