Tunability of the bandgap of SnS by variation of the cell volume by alloying with AE elements

We clarified that the bandgap of inorganic materials is strongly correlated with their effective coordination number (ECoN) via first-principles calculations and experimental confirmations. Tin mono-sulphide (Pnma) and germanium mono-sulphide (Pnma) were selected as model cases since these materials successively alter the ECoN as the cell volume changes and show an uncommon relationship between cell volume and bandgap. Contrary to the common semiconductors, the bandgaps of SnS (Pnma) and GeS (Pnma) have a positive relationship with respect to cell volume. This unique phenomenon was explained by incorporating the concept of ECoN into the theoretical studies. The theory proposed in this study is widely applicable to semiconductors with low-symmetry structures. Further, we experimentally demonstrated that the bandgap of SnS (Pnma) can be broadly tuned by changing the unit cell volume via alloying with alkali-earth (A.E.) metals, which could allow SnS to be applied to Si-based tandem photovoltaics. Alloying with A.E. elements also stabilised Cl as an n-type donor, which enabled n-type conduction in the bandgap-widened SnS film in the SnS-based semiconductors.

Automated summary

We demonstrated the correlation between the bandgap of inorganic materials and their effective coordination number through theoretical calculations and experimental validations. The unique relationship between cell volume and bandgap of tin mono-sulphide and germanium mono-sulphide was explained by incorporating the concept of effective coordination number. This theory is applicable to semiconductors with low-symmetry structures. We also experimentally showed that the bandgap of tin mono-sulphide can be tuned by alloying with alkali-earth metals, and n-type conduction can be achieved in tin mono-sulphide based semiconductors.