CZTSSe Solar Cells from Nanoparticle Inks

Abstract

Future energy demand can be addressed by using renewable and inexhaustible solar energy, providing clean, unlimited, economical and green energy. Electricity generation from the sun employing PV technology is currently dominated by Si-based PV and requires expensive equipment and process and schemes for cost reduction on a large scale are limited. Thin film technologies such as CdTe and Cu(In,Ga)Se2, provide a lower cost alternative primarily due to the use of in-line and low-temperature processes. While considerable efforts have been made to increase efficiency and reduce costs, thin film PV currently relies on scarce and therefore expensive resources and/or toxic elements. Alternative thin film materials would therefore provide routes to reduce PV cost-per-watt while still exhibiting lower input energy requirements. Solar cells based on Cu2ZnSn(S,Se)4 (CZTSSe) absorber layers offer such an alternative. Despite its young history CZTSSe record efficiency stands at 12.6% and the major limitations are (i) a lower than expected open circuit voltage accompanied by a low efficiency at converting and collecting carriers from low energy photons; (ii) the difficulty in controlling the kesterite crystal structure throughout the fabrication process; and (iii) the use of hydrazine, a highly toxic chemical, in the fabrication process to achieve the record efficiencies. This project uses nanocrystal dispersions (inks) of CZTS synthesised from hot injection as the starting material. This technique can reliably control crystal structure, composition and doping and does not present any environmental risks. Inks are easily spin coated or sprayed on substrates and a heat treatment under selenium rich atmosphere promotes grain growth without loss of the crystal structure. In order to fabricate record efficiencies using this technique the microstructure of the absorber and back contact layers need to be engineered to provide large grains extending the full thickness of the absorber combined with a small interfacial layer to ensure a good ohmic contact.