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

We propose a two-dimensional material, \ensuremath{\delta}-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 \ensuremath{\delta}-CS as a donor and a transition metal dichalcogenide as an acceptor. \ensuremath{\delta}-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.