Preparation And Photoelectrochemical Properties Of Hydrophilic Cu₂ZnSnS（Se）₄Nanoparticles

Cu2ZnSnS4(CZTS) and Cu2ZnSnSe4(CZTSe) have been considered as ‘next generation’photovoltaic materials, due to their excellent properties such as high absorption coefficients (1×105cm-1),suitable absorption band gap for solar spectrum, high radiation stability and considerable cell efficiency.Various methods have been used for the preparation of CZTSe materials, including physical methods andwet chemical routes. Physical methods are high cost and it is difficult to control the size and morphology ofnanocrystals and obtain monodisperse nanocrystals. Wet chemical routes are more prevalent due to theirconvenient operability, achievable by using traditional instruments and low cost. CZTSe NCs are usuallycovered with long alkyl chain ligands to shield the surface of the NC, which can realize homogeneousnucleation and enable easy solution processibility for fabrication. However, these ligands also act as aninsulating layer that impedes efficient charge transfer and the dissociation of photogenerated excitons andlead to poor conductivity, limiting the NCs application in electronic and photoelectronic devices. In thisthesis, we synthesized all-inorganic CZTSe NCs by ligand exchange strategy and one-step method.Photoelectrochemical properties of Cu2ZnSnSe4were systematically studied. This thesis includes thefollowing main aspects:(1) For the first time, we synthesized all-inorganic Cu2ZnSnSe4nanocrystals (CZTSe NCs) byligand exchange strategy using a metal-free chalcogenide compound [(NH4)2S] as the inorganic ligand.The results of I-V and Hall-effect measurement illuminated that all-inorganic CZTSe films reveal theexcellent electron-transfer performance. When the all-inorganic CZTSe NCs films used as counterelectrode (CE) in dye-sensitized solar cells, the photocurrent density increases significantly from4.48(OLA-capped CZTSe NCs) to14.17mA/cm2(all-inorganic CZTSe NCs). A higher efficiency of η=7.06%was achieved for all-inorganic CZTSe films as CE in dye-sensitized solar cells. The results showed thathigh temperature annealing can be avoided for all-inorganic CZTSe films as CE after exchange. Cyclicvoltammetry (CV) results and electrochemical impedance spectroscopy (EIS) showed the all-inorganicCZTSe films as CE exhibited good electrocatalytic activity and high conductivity, which demonstrated thatall-inorganic CZTSe NCs would serve as an effective CE material in dye-sensitized solar cells.(2) By optimizing the experimental conditions，we synthesized CZTSe nanosheets by one-step method in an excess selenium environment at relatively low temperatures. The excess-selenium situationenhanced the reaction of the metal chlorides with selenium. By dropping different volumes of the aqueousdispersion of CZTSe nanosheets, we prepared the films with different thickness. The effect on the qualityand crystallinity of the film of aqueous dispersion of CZTSe nanosheets after550oC selenization has beenstudied. The results showed the removal of large organic molecules improved the quality and crystallinityof the film after550oC selenization.(3) By using acetylacetonates, we developed a facile one-step synthesis method for synthesizingCu2ZnSnSe4(CZTSe) nanocrystals (NCs) in excess selenium environment. This excess-selenium situationenhanced the reaction of the metal acetylacetonates with selenium, resulting in the burst nucleation of NCsat relatively low temperatures. The phase morphology, surface and optoelectronic properties of NCs beforeand after ligand exchange were discussed in depth. It was found that pure tetragonal-phase structure CZTSeNCs with~1.7eV band gap could be synthesized. The removal of large organic molecules on CZTSe NCsafter ligand exchange by S2-decreased the resistivity of thin films effectively. For potential application inCZTSe solar cells, we constructed an energy level diagram to explain the mutual effect between absorptionlayer and CdS layer. Using cyclic voltammetry (CV) measurement, we found that the conduction band ofCdS layer is below that of CZTSe layer and a “Cliff” is formed at CdS/CZTSe interface before ligandexchange. This structure will cause recombination between majority carriers at the interface and the entirerecombination increases with increasing the absolute value of conduction band difference between CdS andCZTSe layer. However, we found HOMO and LUMO energy levels of CZTSe films shifted down afterligand exchange. After energy level alignment at CdS/CZTSe interface, a “Spike” structure was moreconveniently formed after ligand exchange. This structure acted as the barrier against injection electronsfrom ZnO to CZTSe layer and recombination would subsequently be depressed.(4) By simplifying the experimental procedure, we successfully synthesized ligand-freeCu2ZnSnS4NCs by a one-step synthesis through changing the experimental conditions. By XRD, UVabsorption and Raman analysis, the process of Cu2ZnSnS4NCs formation was systematically studied. Theresults showed that when the temperature increased to180oC, pure phase kesterite Cu2ZnSnS4NCs wereobtained. By the analysis of the morphology and electrochemical performance of Cu2ZnSnS4films at550oC under argon protection and after550oC selenization, the result showed kesterite Cu2ZnSnSe4film after 550oC selenization was obtained and has lower resistivity.