Low-density silicon allotropes for photovoltaic applications

Abstract

Silicon materials play a key role in many technologically relevant fields, ranging from the electronic to the photovoltaic industry. A systematic search for silicon allotropes was performed by employing a modified ab initio minima hopping crystal structure prediction method. The algorithm was optimized to specifically investigate the hitherto barely explored low-density regime of the silicon phase diagram by imitating the guest-host concept of clathrate compounds. In total 44 metastable phases are presented, of which 11 exhibit direct or quasi-direct band-gaps in the range of $\approx$1.0-1.8 eV, close to the optimal Shockley-Queisser limit of $\approx$1.4 eV, with a stronger overlap of the absorption spectra with the solar spectrum compared to conventional diamond silicon. Due to the structural resemblance to known clathrate compounds it is expected that the predicted phases can be synthesized

A fingerprint based metric for measuring similarities of crystalline structures

“…46,47 The first test set consists of clathrate like structures of low density silicon allotropes. 48 Low density silicon gives rise to a larger number of low energy crystalline structures than silicon at densities of diamond silicon and thus poses an ideal benchmark system. In the first line of the figure, J. Chem.…”

Formation of inclusion type silicon phases induced by inert gases

“…These low-density allotropes have a rather expanded crystal structure, usually containing open channels, or polyhedral cages. Although atoms are still tetrahedrally coordinated through sp 3 bonds, the open-framework structures yield different topologies from the DC structure. Particularly, while only six-membered rings are present in the DC structure, open-frameworks can contain a variety of ring structures, for example, five-membered rings.…”

“…Particularly, while only six-membered rings are present in the DC structure, open-frameworks can contain a variety of ring structures, for example, five-membered rings. Consequently, bonds in these low-density allotropes are distorted with respect to perfect tetrahedral geometry, and thus are expected to yield novel opto-electronic properties for solar applications 2,3 . For example, type II Si clathrate (Si 136 ) was demonstrated to have an optical band gap of 1.9 eV 4 , that is, about 0.7 eV above that of DC Si.…”

Novel silicon phases and nanostructures for solar energy conversion

“…Recently, Si based clathrates have drawn attention for photovoltaic applications as a result of their tunable quasi-direct band gap in the range needed for optimal performance according to the Shockley-Queisser (SQ) limit. [21][22][23][24] Si-based clathrates without guest atoms in the cages are neutral, and thus they are intrinsic semiconductors. The guest-free low density phases are metastable at ambient conditions.…”

“…This band gap is smaller than the optimal one (1.4 eV) and it might be improved at the nanoscale. 28 Although only certain types of clathrates were discovered in experiments, Amsler et al 23 recently conducted a computational search and explored 44 metastable low density silicon clathrates with potential use in photovoltaic applications. Solutions to the Kelvin problem, how to partition threedimensional space into cells of equal volume with minimal area, led to the discovery of another series of new low density Si allotropes that are structurally related to clathrates.…”