Synthesis,Phase Conversion And Optoelectronic Properties Of Cu₂ZnSnS₄ Nanostructures

Cu₂ZnSnS₄ ?CZTS? has been regarded as a promising light harvesting semiconductor for next-generation solar cells. It has advantages of suitable direct bandgap ??1.5 eV?, high absorption coefficients in the visible lightregion ?>104 cm-1?,phase-tunable nature, naturally abundance and nontoxicity of the constituent elements.However, nanostructures of CZTS and related multinary compounds were typically prepared by high-temperature colloidal solution methods. Organic solvents and long-chain capping agents that are expensive and harmful to environment are often involved. Therefore, exploring green approaches to synthesize CZTS nanostructure with controllable morphology and tunable phases is highly desired. In this thesis, a hydrothermal approach without using any organic solvents is developed to synthesize CZTS and Cu₂SnS₃ ?CTS? nanostructures. Optical and optoelectronic properties are investigated. Main results are mummarized as follows:?1? A top-down strategy associated with green chemistry to prepare metastable wurtzite CZTS nanocrystals is presented. The wurtzite CZTS nanocrystals with diameters of 5±2 Dm are synthesized by reacting Cu₂O nanocubes in a tin metal chalcogenide complex ?Sn-MCC? solution with the addition of Zn²⁺ ions. No organic solvents and long-chain ligands are used. Conversion processes from Cu₂O nanocubes to Cu₂-xS nanoboxes, to wurtzite CZTS nanoboxes, and to CZTS nanocrystals are systematically investigated. It is revealed that formation of CZTS nanoboxes is resulted from a Kirkendal effect while evolution into CZTS nanocrystals is due to an etching result. The pH value of the growth solution plays a key factor in determining the CZTS with a wurtzite or kesterite structure. Fast reaction rate in the neutral growth solution favors the formation of a metastable hexagonal wurtzite CZTS while slow reaction rate in the alkaline solution prefers to produce a stable kesterite structure. The wurtzite CZTS nanocrystal exhibits remarkable photoluminescence and photoresponse properties. Our work opens up a new opportunity to synthesize green colloidal nanocrystals for promising low-cost thin-film solar cell applications.?2? Wurtzite Cu₂SnS₃ ?CTS? nanoboxes are prepared by reacting the Cu₂O nanocubes in the tin metal chalcogenide complex ?Sn-MCC? solution under hydrothermal condition. Chemical evolution from Cu₂O to Cu₂-xS,CuS and CTS is observed. Formation of the nanoboxes is resulted from the Kirkendal effect. Phase conversion of wurtzite CTS to tetragonal CTS is achieved by a simple annealing.Bandgap is estimated to be 1.4 eV for the wurtzite CTS and 1.2 eV for the tetragonal CTS. The photoelectrochemical properties of CTS nanoboxes are also studied.