Computational Design of Mixed-Valence Tin Sulfides as Solar Absorbers

Area of Science:

• Materials Science
• Solid State Chemistry
• Computational Materials Design

Background:

• Binary tin sulfides (SnS, SnS₂) are promising for optoelectronics due to their semiconducting properties.
• The multivalency of tin (Sn²⁺, Sn⁴⁺) allows for intermediate compositions (SnₓSᵧ) with unique structures and properties.
• Sn₂S₃ is recognized as a mixed-valence semiconductor, while other compositions like Sn₃S₄ and Sn₄S₅ remain less explored.

Purpose of the Study:

• To comprehensively investigate the phase stability of the SnₓSᵧ series compounds.
• To identify novel, stable or metastable mixed-valence tin sulfide compositions.
• To explore the structure-property relationships of these tin sulfide materials for potential applications.

Main Methods:

• Employed a swarm-intelligence crystal structure search method.
• Utilized first-principles energetic calculations to assess phase stability.
• Analyzed structural, energetic, and electronic properties of predicted tin sulfide phases.

Main Results:

• Mixed-valence SnₓSᵧ compounds exhibit weaker stability against decomposition compared to SnₓOᵧ analogs.
• Confirmed the stability of the known Sn₂S₃ phase and identified Sn₃S₄ as marginally stable.
• Discovered two low-energy structures for Sn₃S₄, featuring 1D chain fragments and exhibiting phonon stability; one shows a quasi-direct band gap of 1.43 eV.

Conclusions:

• The Sn₃S₄ phase is a potentially viable mixed-valence semiconductor with properties suitable for solar absorbers.
• Metastable SnₓSᵧ compositions with slightly positive formation enthalpies may exist under ambient conditions.
• Low-dimensional Sn-S motifs and van der Waals interactions contribute to diverse structures and properties in mixed-valence tin sulfides.