δ-CS: A Direct-Band-Gap Semiconductor Combining Auxeticity, Ferroelasticity, and Potential for High-Efficiency Solar Cells

We propose a two-dimensional material, δ-CS, and show that it is a direct-band-gap semiconductor with strong absorption of solar radiation. Power conversion efficiencies of 7.0% to 20.1% are predicted for solar cells with δ-CS as a donor and a transition metal dichalcogenide as an acceptor. δ-CS also excels in terms of having an exceptionally large negative in-plane Poisson’s ratio. Its ferroelasticity with moderate switching barrier is promising for applications in shape memory devices.

Figure 1

(a) Atomic structure, (b) phonon dispersion, and (c) total potential energy fluctuation during an AIMD simulation at 300 K (inset: atomic structure after 5 ps) of δ-CS.

Figure 2

(a) Band structure and partial densities of states (PDOS) calculated by the Heyd-Scuseria-Ernzerhof functional (VBM set to zero energy and indicated by a black dashed line), charge distribution (isovalue 0.1 e Å⁻³) at the (b) VBM and (c) CBM, (d) direction dependences of the electron and hole effective masses, and (e) absorption spectrum of δ-CS.

Figure 3

(a) Band alignment in δ-CS and the TMDs. (b) PCE_max (in %) [Eq. (1)]. Gray, black, and white squares mark δ-CS/${\mathrm{Mo}\mathrm{S}}_2$, δ-CS/${\mathrm{Mo}\mathrm{Se}}_2$, and δ-CS/${\mathrm{Mo}\mathrm{Te}}_2$, respectively.

Figure 4

(a) Response of the lattice constant in the x(y) direction to uniaxial strain in the y(x) direction. (b) Strain-stress curves for the x and y directions.

Figure 5

Energy profile of the ferroelastic switching in δ-CS between equally stable states S and S′ with different orientations through a paraelastic state T.