Computational Design Of New Binary Tin Sulfides As Optoelectronic Semiconductor Materials

The increasing energy demands with the social development have become a bottleneck in traditional energy sources,such as fossil fuels,bring serious environmental problems.So,scientists have paid considerable attention to new energy materials without exhaustible and polluting,e.g.nuclear energy,hydropower,wind energy,solar energy,thermal energy,and so on.Especially,solar cell and thermoelectric materials attract more attention because of the flexible usability.Usually,semiconductor materials can realize energy conversion from the solar energy.The design of eco-friendly green semiconductors with low-cost and higher energy conversion efficiency has become the key to further development of new energy materials.However,the main issues are the low efficiency and high-cost of the conventional“trial and error”approach to screen the potential candidates from a huge of materials space.With the rapid development of computer technology and theoretical calculation methods,the advantages of developing and designing functional materials based on accurate theoretical simulation technology are gradually highlighted.Tin sulfides are composed of earth-abundant and nontoxic elements.Three stoichiometric compounds are generally obtained with discrete phases,i.e.SnS,SnS₂,and Sn₂S₃,which are widely studied and attracted extensive interests.In 2014,approaching 4.4%photo-electric conversion efficiency of Pnma-SnS photovoltaic was reported.At 878 K or 12.6 GPa,the Pnma-SnS phase translates to one Cmcm phase,of which every Sn atom is five-fold coordinated by S atoms,with the phenomenon of abrupt increase in carrier concentration and mobility and decrease in electrical resistivity and Hall coefficient.Rocksalt SnS?Fm-3m?is one topological crystalline insulator in its native phase without any alloying or applied strain/pressure.It shows great promise for potentially revolutionary applications in quantum computing and spintronics and has attracted interest as a new physical paradigm.SnS₂ is a 2D?two-dimensional?n-type semiconductor with a moderate band gap of 2.18–2.44 eV and suitable band-edge positions for photoexcited H₂ evolution,which is of interest for several applications such as window layer for thin-film solar cells,photodetector,and water-splitting photocatalyst.Tin sesquisulfide Sn₂S₃ has a band gap of 0.9-2.2eV with semiconductor behavior and exhibits n-type conductivity,whose optoelectronic properties are dependent on its crystalline structure.So the properties optimization for tin sulfides are very widely and the phase stability diagram is also elusive.This work has focused more in-depth and extensive research on the Sn-S binary system.We herein carried out a comprehensive structure search for Sn-S system by combining swarm-intelligence and first-principles calculations.Our findings are as follows:1.We have identified several new-type metastable SnS phases.We identify a series of never-found-before SnS new phases with the energies comparable to the reported metastable phases.The majority of low-lying energy structures are found and contain Sn coordinated mainly by 3/4/5/6 S atoms.They show widely varied electronic band gaps in the range of 0-2.2 eV and most of them have reasonably small and comparable electron and hole carrier effective masses.Especially,four new lattice-dynamically stable phases,Pnma',Cmcm',P21/m andPnma'.They are narrow-gap semiconductors,with the direct band gaps?1.18-1.55 e V?compatible with the solar spectrum.Therefore it is very promising to synthesize them in experiments and explore the possibility of using them as solar absorbers or thermoelectric materials.2.We carried out a systematic crystal structure search and design for SnxSy compounds.A systematic crystal structure search for Sn_n+mS_n+2m compounds with thirteen Sn-S stoichiometric ratios were carried out by combining swarm-intelligence and first-principles calculations.Two Sn₃S₄ crystal structures are predicted having formation enthalpy about 1.3×10^-2 eV/atom above the convex hull.They are dynamically stable.Notably,we find that the C2/m phase Sn₃S₄ shows the indirect gap value E_id=1.34 eV and the minimal direct bandgap is E_d=1.39 eV,matching with the optimal value for solar cell determined by the Shockley–Queisser limit.By imitating the Sn₃S₄ chain structure,we have studied the optoelectronic properties in 1D line-defective SnS₂ with different the chain width?means different the Sn?+2?fraction?.We conclude that a suitable bandgap,as well as a high efficient onset excitation in Sn₃S₄,can be attributed to the moderate Sn?+2?fraction.Finally,the Sn-S allotropes vdW binding can be engineered by the basic Sn-S units with varied component ratios.Local charge transfer from Sn?+2?gives rise to a fluctuated potential step between the interfaces,which can affect the band alignments and induce band renormalization.In conclusion,our designed Sn-S compounds shown excellent thermodynamic stability and optoelectronic properties,which provides new theoretical insights for the developing of novel semiconductors.