SiC2 siligraphene and nanotubes: novel donor materials in excitonic solar cells.

In excitonic solar cells (XSC), power conversion efficiency (PCE) depends critically on the interface band alignment between donor and acceptor materials. Graphene or silicene is not suitable for donor materials due to their semimetallic features (zero band gaps); it is therefore highly desired to open an energy gap in graphene or silicene to extend their application in optoelectronic devices, especially in photovoltaics. In this paper, based on the global particle-swarm optimization algorithm and the density functional theory methods, we predict a novel SiC2 siligraphene (g-SiC2) with a direct band gap of 1.09 eV showing infinite planar geometry, in which Si and C atoms adopt sp(2) hybridization and C atoms form delocalized 4 C-domains that are periodically separated by Si atoms. Such a g-SiC2 siligraphene (with a global minimum of energy) is 0.41 eV/atom lower and thermally stabler than the isomeric pt-SiC2 silagraphene containing planar 4-fold coordinated silicon (3000 K vs 1000 K). Interestingly, the derivative (n, 0), (n, n) nanotubes (with diameters greater than 8.0 Å) have band gaps about 1.09 eV, which are independent of the chirality and diameter. Besides, a series of g-SiC2/GaN bilayer and g-SiC2 nanotube/ZnO monolayer XSCs have been proposed, which exhibit considerably high PCEs in the range of 12-20%.