PP2.2: Thin Film Inorganic (CZTS)

Project Coordinator: 
Chief Investigators: 
Xiaojing Hao

All successfully commercialised non-concentrating photovoltaic technologies to date are based on silicon or the chalcogenide (semiconductors containing Group VI elements, specifically Te, Se and S). As indicated by Figure PP2.2.1, the successful chalcogenide materials, CdTe and Cu(In,Ga)Se2, can be regarded as “synthetic silicon” where the balance between atoms in these materials provides the same average number of valence band electrons as in silicon, resulting in the same tetrahedral co-ordination. Cd and Se are toxic while Te and In are among the 12 most scarce elements in the Earth’s crust. These factors would seem to clearly limit the longterm potential of the established  chalcogenide technologies. However, as indicated in Figure PP2.2.1, by delving more deeply into the periodic table, an alternative option can be uncovered with the same number of valence band electrons on average but involving Earth-abundant, non-toxic elements.

Kesterite Cu2ZnSn(S, Se)4 (CZTSS(e)) compound semiconductor has emerged, based on such reasoning, as a promising candidate for use in absorber materials for thin-film solar cells. Analogous to the chalcopyrite structure of CIGS, Cu2ZnSnS4 (CZTS) shares similar optical and electrical properties. CZTS has a bandgap of 1.4–1.5eV, a large absorption coefficient of over 104 cm-1 and is composed of non-toxic and Earth-abundant elements. Notable is that the bandgap of the CZTS family can be tuned to span a wide range beyond 2.25eV, even above the accessible range of the highest efficiency III-V cells. This makes the material suitable for tandem cells (see Section PP1.3a). For thin-film solar cells, energy conversion efficiency up to 12.6% and 8.5% have been achieved so far for CZTS(e) and CZTS solar cells, respectively. However, the 12.6% record CZTSSe solar cell was obtained through a hydrazine solution method. Hydrazine is a highly toxic and very unstable compound that requires extreme caution during handling and storage.

The Centre’s work in the CZTS area takes a different fabrication direction, aiming for the development of a low-cost, highthroughput and up-scalable manufacturing process for highefficiency CZTS-based solar cells. Work in this strand includes the development of CZTS solar cells on soda lime glass as well as stainless steel and the continuation of established collaborations with and financial support from industry partners.