Study and preparation by Close-Spaced Vapor Transport (CSVT) of Cu₂ZnSnS₄ absorbers as polycrystalline thin films for a low cost solar cells realization

The kesterite Cu₂ZnSnS₄ (CZTS) is a p-type semiconductor material made from abundant and nontoxic chemical elements. These advantages in addition to a direct band gap, with energy between 1.45 eV and 1.5 eV, make it an excellent candidate for replacement of Cu(In, Ga)Se₂ and CdTe absorber layers currently used in thin film solar cells. It has although been highlighted that photovoltaic devices based on thin CZTS absorber layers are highly suffering from the presence of secondary phases in the thin films. So the work presented in this thesis describes thin CZTS layers deposition by a simple and low-cost process called Close Spaced Vapor Transport (CSVT). Its main objective is to realize a CZTS compound free of any secondary phase with the aim of improving conversion efficiencies of CZTS thin films based photovoltaic solar cells. For this purpose, the bulk CZTS material was first synthesized in the form of ingot by a slow cooling of a molten stoichiometric mixture of pure elements. Characterizations realized on this bulk material showed that it relates to a single phase, quasi-stoichiometric Cu₂ZnSnS₄ compound in the kesterite structure. The ingot was milled into powders and pressed to give 1 mm thick pellets. These pellets were therefore used as evaporating sources in a CSVT reactor with iodine as transport agent, for the thin CZTS layers deposition. Optimizations of the key deposition parameters that are substrate temperature and iodine pressure were performed. The Results of the investigations conducted on the CZTS layers deposited at substrate temperature in the range 460 deg. C-500 deg. C, under iodine pressure in the order of 2 kPa to 4 kPa, revealed excellent physico-chemical properties. (author)