Chemical insights into the formation of Cu₂ZnSnS₄films from all-aqueous dispersions for low-cost solar cells

Abstract

CuZnSnS (CZTS) shows great potential for photovoltaic application because of its non-toxic earth-abundant components and good optoelectronic properties. Combining low-cost and environmentally friendly routes would be the most favorable approach for the development of CZTS solar cells. In this context, development of CuZnSnS (CZTS) films from all-aqueous CZTS nanocrystals inks represents an interesting challenge. Here, we have highlighted a condensation regulation by the alkali ion size observed in the alkali series Li < Na < K < Rb < Cs, and demonstrated the chemical stability of CuZnSnS surfaces in basic aqueous dispersions. Data such as optimal nanocrystal size, critical cracking thickness and average thickness to fabricate micron crack-free films from all-aqueous chalcogenide nanocrystals dispersions were determined. From these results, a proof of concept for the formation of a crack-free film of 2.2 μm formed from an all-aqueous CZTS nanocrystals ink is given. When employing low-cost materials, removal of carbon impurities represents another important challenge. With the objective to fabricate residue-free films, a specific annealing strategy is proposed involving a high temperature purification step under Se partial pressure. Carbon removal is thus achieved via the CSe gas formation, simultaneously to the amorphous domains crystallization as demonstrated by Raman spectroscopy. These source data favoring the formation of residue-free, crack-free, annealed films should assist the large scale development of CZTS solar cells from low-cost and environmentally friendly, all -aqueous inks.

Photoconversion Optimization of Pulsed-Laser-Deposited p-CZTS/n-Si-Nanowires Heterojunction-Based Photovoltaic Devices

“…In the classical scheme, CZTS is basically integrated as an absorber layer in the rather complex SLG/Mo/p-CZTS/n-CdS/i-ZnO/ITO/Al multi-layered structure to fabricate solar cells, which have been shown to exhibit interesting PV performance [ 7 , 8 , 9 ]. However, the fabrication of this CZTS related multilayered architecture requires careful optimization of several laborious processing steps [ 10 , 11 , 12 ], namely: ( i ) post-sulfurization of the as-deposited CZTS layer, ( ii ) selective chemical treatment of CZTS surface (successive etching by KCN, NH 4 OH and HCl solutions), ( iii ) chemical deposition of the CdS buffer layer followed by soft thermal treatment, ( iv ) CZTS/CdS interface engineering, and ( v ) sputter-deposition of a ZnO/ITO top window contact layer. This rather complex and multi-step processing calls for alternative approaches aimed at exploiting the optoelectronic properties of the p-type CZTS films through their direct association with an appropriate n-type material to form a p-n junction-based PV devices.…”