Study On Preparation And Properties Of Cu₂ZnSnS₄-based Nanocrystals And Thin Films

With the large consumption of fossil fuels,the shortage of energy supply and environmental pollution are two major problems facing society today.Therefore,finding a clean,abundant and sustainable energy supply as an alternative to fossil fuels has become an urgent problem facing mankind.Solar photovoltaic power generation is the best choice to solve the energy supply and environmental problems because of its inexhaustible and pollution-free characteristics.Multi-component thim film solar cells,represented by copper indium gallium selenide（Cu（In,Ga）Se₂）,have matured in preparation technology and also have been used for commercialization.Howerever,rare elements indium and gallium are used to prepare Cu（In,Ga）Se₂,whichlimitsthewideapplicationofthesesolarcells.Copper-zinc-tin-sulfide/selenium（Cu2ZnSn（S,Se）4,CZTS）semiconductor compound is considered to be an ideal optical absorption layer of thin film solar cells due to its advantages of abundant constituent elements in the earth and suitable band-gap.In this paper,the preparation process of Cu₂ZnSnS₄（CZTS）based nanocrystals and their films was explored.The effects of reaction temperature,molar ratios of metal precursors（Zn:Sn and Cu:（Zn+Sn））and anionic precursor（S:Se）on the crystal structure,morphology and optical properties of CZTS based nanocrystals were investigated,and the formation mechanism of CZTSSe nanocrystals was explored.The CZTS film was prepared by the coating method based on nanocrystalline"ink".The effects of concentration of CZTS"ink",annealing temperature and annealing atmosphere on the crystal structure and surface morphology of CZTS films was studied.The main research contents and experimental conclusions are as follows:（1）In the oleylamine-octadecene solvent system,the effects of reaction temperature and metal precursor ratios(Zn:Sn&（Cu:（Zn+Sn））on the crystal structure,chemical composition,coating method,morphology and optical properties of CZTS nanocrystals were investigated.And the CZTS nanocrystals were characterized by different tests.It was found that when the reaction temperature was 250°C and the metal precursor molar ratio meets the requirement of 1≤Zn/Sn≤1.2 and0.8≤Cu/（Sn+Zn）≤1,the nanocrystals were pure and the optical band gaps were suitable.（2）In the oleylamine-thiol solvent system,the effects of reaction temperature on the properties of CZTS and CZTSSe（S:Se=1:1）nanocrystals were studied,and the differences in the crystal structure and morphology of the two kinds of nanocrystals were compared.Secondly,CZTSSe nanocrystals with different contents of selenium were prepared by adjusting the precursor ratio of S:Se,and the changes of phase composition and morphology of nanocrystals with the increase of Se content were observed.Finally,we studied the formation mechanism of CZTSSe.The XRD analysis revealed that the formation mechanism was as follows:Cu and S/Se combines firstly to form Cu₂SexS_1-x.As the temperature increases,Sn⁴⁺enters the lattice of Cu₂Se_xS_1-x to form Cu-Sn-（S,Se）.Finally,Zn²⁺enters Cu-Sn-（S,Se）lattice and forms CZTSSe eventually.（3）The CZTS thin film was prepared using a coating method based on nanoparticle"ink".It has been found that the doctor-blade coating is an effective method for saving raw materials,the thin-films had less cracks and a moderate thickness（～1μm）when the"ink"with a concentration of 150 mg/mL was coated on the glass substrate for once and then annealed at 520°C for 30 min under sulfur atmosphere.A CZTS thin film with less cracks and thickness of about 1μm was obtained.