Improved Photoactivity of Pyroxene Silicates by Cation Substitutions.

We investigated the possibility of band structure engineering of pyroxene silicates with chemical formula A+1 B+3 Si2 O6 by proper cation substitution. Typically, band gaps of naturally formed pyroxene silicates such as NaAlSi2 O6 are quite high (≈5 eV). Therefore, it is important to find a way to reduce band gaps for these materials below 3 eV to make them usable for optoelectronic applications operating at visible light range of the spectrum. Using first-principles calculations, we found that appropriate substitutions of both A+ and B3+ cations can reduce the band gaps of these materials to as low as 1.31 eV. We also discuss how the band gap in this class of materials is affected by cation radii, electronegativity of constituent elements, spin-orbit coupling, and structural modifications. In particular, the replacement of Al3+ in NaAlSi2 O6 by another trivalent cation Tl3+ results in the largest band-gap reduction and emergence of intermediate bands. We also found that all considered materials are still thermodynamically stable. This work provides a design approach for new environmentally benign and abundant materials for use in photovoltaics and optoelectronic devices.

Surface orientation effects in crystalline-amorphous silicon interfaces.

In this paper we present the results of empirical potential and density functional theory (DFT) studies of models of interfaces between amorphous silicon (a-Si) or hydrogenated amorphous Si (a-Si:H) and crystalline Si (c-Si) on three unreconstructed silicon surfaces, namely (100), (110) and (111). In preparing models of a-Si on c-Si, melting simulations are run with classical molecular dynamics (MD) at 3000 K for 10 ps to melt part of the crystalline surface and the structure is quenched to 300 K using a quench rate of 6 × 10(12) K s(-1) and finally relaxed with DFT. Incorporating the optimum hydrogen content in a-Si to passivate undercoordinated Si, followed by DFT relaxation, produces hydrogenated amorphous silicon on crystalline surfaces, a-Si:H/c-Si. The (100) surface is the least stable crystalline surface and forms the thickest amorphous Si region, while the most stable (110) surface forms the smallest amorphous region. Calculated radial distribution functions (RDF) in the amorphous and crystalline layers are consistent with a-Si and c-Si and indicate a structural interface region one layer thick. The electronic density of states shows an evolution from c-Si to a-Si (or a-Si:H), with a larger electronic interface layer, suggesting that the electronic properties are more strongly perturbed by interface formation compared to the atomic structure. The computed optical absorption spectra show strong effects arising from the formation of different a-Si and a-Si:H regions in different Si surfaces.