Studies On The Synthesis Of Cu₂Zn（Cd）SnS₄ With The Heterojunction For Photocatalysis And Photovoltaic Performance

Cu₂Zn（Cd）SnS₄ is a novel quaternary semiconductor material based on copper chalcogenide,which contains the elements with high abundance and low-cost.The excellent photosensitiveness contributes to its application in many fields.As a narrow band gap semiconductor material of 1.1-1.5 e V,Cu₂Zn（Cd）SnS₄ can absorb sunlight effectively for photocatalysis and photovoltaic performance.The photocatalysis correlates directly with the properties of the materials,for example,the large specific surface area is beneficial to electron-hole separation for excellent photocatalysis.An effective method is to synthesize hierarchical structure to enhance the specific surface area.Normally,lots of defects in Cu₂Zn（Cd）SnS₄ synthesized by solution method results in high recombination of photogenerated electron-hole pairs,which affects the photocatalysis directly.The construction of its heterojunction with other catalysts can increase the lifetime of photogenerated charges effectively.As a material with the broad-spectrum absorption,Cu₂Zn（Cd）SnS₄ has been widely used as absorber layer in inorganic thin film solar cells with high cost.The construction of p-n junction thin film in the nano-dimension by solution methodis an effective method to fabricate solar cells with low-cost.In this paper,the main contents are consisted of three parts as followings:（1）In this paper,500-600 nm hierarchical flower-like Cu₂CdSnS₄（CCTS）was synthesized by two-step method,which was applied in the photodegradation to malachite green（MG）for the first time.With spherical CdSn（OH）₆ prepared by co-precipitation method as the precursor,flower-like CCTS micro/nano structured crystals were synthesized by microwave method.The formation of CCTS experienced CdSn（OH）₆ coordinating with sulfur source,then dissolving partly to combine with Cu²⁺in the existence of PVP as the capping agent,finally to form flower-like structure.The characterization of products showed that stannite CCTS had narrow bandgap and large special interface area,contributing to excellent photodegradation to MG up to 90.29%.The decomposition of MG may originate from N-demethylation,removal of the benzene ring and ring-open reactions.The deep understanding of the photodegradation mechanism may provide the guidance in the long-term study of MG.（2）Cu₂ZnSnS₄/SnO₂ heterojunction was synthesized by three-step reaction.Firstly,Cu₂ZnSnS₄（CZTS）nanoparticles were synthesized by solvothermal method.After its surface modification,Cu₂ZnSnS₄/SnO₂heterojunction with the size of about 10μm was synthesized successfully for the photocatalysis of rhodamine B（RhB）,with the comparation of the single material.The crystalline phase,microstructure,optical and surface properties of the CZTS/SnO₂ heterojunction,CZTS and SnO₂ were characterized.The results showed that CZTS/SnO₂ heterojunction synthesized after the surface modification of CZTS with mercaptopropionic acid can improve light-harvesting ability significantly,decrease the rapid recombination of photogenerated electron-hole pairs,and increase the negative charge on its surface,contributing to the highest photodegradation efficiency of RhB to 99.9%under 120 min illumination.CZTS/SnO₂ heterostructure displayed better photocatalysis,which was 3.1times of that of pure CZTS and 1.5 times of that of pure SnO₂.（3）Inorganic thin film solar cells were fabricated with Cu₂ZnSnS₄（CZTS）and Cu₂CdSnS₄（CCTS）as the light-harvesting materials prepared by sol-gel method,respectively.In comparison with CZTS,homogeneous and smooth CCTS thin film had stronger absorption in the range of 400-800 nm.With such CCTS as light-harvesting layer,we optimized the solvent for precursor,the synthesis method of buffer layer and the category of hole transport layer.The results showed that solar cells obtained higher device performance with ethylene glycol methyl ether as the solvent,CdS buffer layer obtained by microwave method followed by water bath and Spiro-OMeTAD as the hole transport materials.