Sustainable Transparent Conducting Oxides: Insights from Amorphous SnO_x 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-) SnO 2 thin films as sustainable alternatives to conventional In 2 O 3 -based TCOs. The a-SnO x 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 SnO 2 -like to a mixture of SnO 2 -like and SnO-like properties. In particular, a-SnO x 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 SnO 2 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.