Synthesis Of Cu-Zn-Sn-S Nanocrystals By Solution Chemical Method And The Thin Film Application

I-II-IV-VI semiconductors especially for Cu₂ZnSnS₄with ideal energy band gap of1.5eV have drawn many attentions recently. Cu₂ZnSnS₄has many advantages such aslow cost, little toxicity, high absorption coefficient of4x104cm-1in visible light rangeand high carrier concentration of5x10¹⁵-6x10¹⁶/cm³. It is a promising light-absorbermaterial for thin film solar cells. The highest conversion efficiency of Cu₂ZnSnS₄based thin film solar cells reached to10.1%, that made it to be one of the mostpromising materials for replace of CuIn_1-xGa_xSe₂.Single phase Cu-S, Zn-S and Sn-S nanocrystals were synthesized by an ambientpressure, solution chemical process based on solvents of diethylene glycol ortetraethylene glycol. Three different nanocrystals above were respectively dispersedin absolute ethanol solvent as colloidal inks, and the thin films were deposited bylayer-by-layer dip-drawing colloidal inks in turn with the three inks for80times. Thethin films were treated in different conditions for the synthesis of Cu-Zn-Sn-Scompounds. The advantages of the present experiment research were that thechemicals with less toxicity were used, and preparation process was low cost, simpleand easy to control, and stochiometric could be controlled by adjusting concentrationof inks. Influence of refluxing temperature, refluxing time, adding amounts andsolvents on the resultant features were studied systematically, and well dispersednanocrystals with different controllable morphologies were obtained. Influence ofdifferent inks prepared with different compounds on the synthesis of Cu-Zn-Sn-Scompounds was also studied.For the synthesis of Cu-S nanocrystals, single-phase, well-dispersed Cu1.75Snanocrystals were synthesized by an ambient pressure, hydrazine hydrate andethylenediamine co-assisted diethylene glycol based solution chemical process usingcopper chloride and thioacetamide as precursors. The crystal size, crystallization andsize distribution were controllable by adjusting the amount of additives and refluxingtemperature from180to210℃. Single-phase of CuS nanoflakes and Cu1.8Snanoparticles were also obtained with an ambient pressure solution chemical processusing tetraethylene glycol as solvent, by injecting Cu（II） precursor solution intothioacetamide precursor solution at below220℃and up260℃respectively. Theinfluence of refluxing time on the morphology of CuS nanoflakes and Cu_1.8S nanoparticles were studied, and the size distribution of Cu_1.8S nanoparticles becamenarrow with increase of refluxing time.For the synthesis of Zn-S nanocrystals, well-dispersed ZnS nanocrystals weresynthesized by diethylene glycol based solution chemical method. Influence ofNH₃·H₂O, PVP and precursor concentration on crystal size and size distribution werestudied. Single-phase, well dispersed and narrow size distributed ZnS nanocrystalswere obtained with0.5ml NH₃·H₂O,0.5g PVP and0.02mol/l precursor concentrationadded.For the synthesis of Sn-S nanocrystals, SnS₂nanocrystals were synthesized usingdiethylene glycol based solution chemical method and SnCl₄·2H₂O as precursor. SnS₂nanoflakes were obtained when adding small amount of PVP, and PVP-enclosedquantum dot SnS₂nanospheres with size of about100nm were synthesized withincrease of PVP. The crystallization of SnS₂was weakened with addition ofethylenediamine. SnS nanoparticles were obtained when using SnCl2·2H2O but notSnCl4·2H2O as precursor and the crystal size increased with increase ofethylenediamine amount or refluxing temperature when NH₃·H₂O added.For the synthesis of Cu-Zn-Sn-S compounds by dip-coating method, three differentnanocrystals of Cu1.75S, ZnS and PVP-enclosed SnS₂were dispersed in ethanolsolvent as inks respectively and glass substrates were dipped into each ink for a fewseconds and then drawn out in return for80times. The films were treated withdifferent treated temperature and time. Primary experimental results showed that itwas diffcult to obtain single phase Cu₂ZnSnS₄due to chemical nonreactivity of PVP-enclosed SnS₂, and part of Cu₂ZnSnS₄phase was obtained but some other phases werealso detected when replaced with the SnS₂nanocrystals synthesized with addition ofethylenediamine. When SnS but not SnS₂was used to prepare the mixed colloidal inkwith Cu_1.75S and ZnS nanocrystals together for the synthesis of Cu₂ZnSnS₄, noelement Zn was detected in this thin film, which may be caused by the participation ofSnS as a hindrance for the absorption of ZnS nanocrystals onto glass substrates.Further optimization of the ink-coated Cu-Zn-Sn-S thin films was required forformation of single phase Cu₂ZnSnS₄.