The electronic behaviour of polytypes. Relevance to energy conversion

Abstract

The polytypes of a chemical compound correspond to distinctly different crystallographic variants. In recent years detailed investigations embodying both the structural and physical behaviours of the polytypes have revealed the existence of a definite structure-property relationship. In this paper the relationship between the electronic behaviour e.g. the electronic band gap and polytype periodicity/structure has been discussed. The case of tin disulphide/diselenide has been dealt with in detail. The electronic band gaps of the SnS2 (and also SnSe2) polytypes have been found to vary within wide limits. As for example for SnS2 the band gap varies from 2H (2.18 eV) through 8H (1.20 eV) to 50H (0.81 eV). The results obtained suggest that the polytypes of a compound represent lattice matched variable band gap semiconductors. It is known that efficiency of photovoltaic solar cell is materials limited and extra high efficiency solar cell (tandem solar cells) require lattice matched variable band gap semiconductors. It has been shown that polytypes correspond to ideal ingradients for tandem solar cells. Models representing independent and integrated tandem configurations employing polytypes have been outlined. The case of polytypes of other MX2 type layered materials e.g. WSe2 in relation to possible tandem cell systems, has been discussed. The possible role of polytypes in the storage of energy (hydrogen and electrical energies) has been outlined.