Research On Preparation Of Cu₂ZnSnS₄ Self-Powered Nitrogen Dioxide Gas Sensor By Solution Method

Photovoltaic self-powered gas sensors typically use the photovoltaic effect to harvest light energy to power the gas sensor.Therefore,the basic functions of photovoltaic self-powered gas sensors are the power supply function（solar cell）and the sensing function（gas sensor）.Copper zinc tin sulfur（Cu₂Zn Sn S₄,CZTS）is very suitable for solar cells and gas sensing materials due to its high optical absorption coefficient(>10~4 cm^-1),tunable direct bandgap（1.45～1.6 e V）,and p-type conductivity.Therefore,CZTS is a potential multifunctional material for photovoltaic self-powered gas sensors.Solar cells with high photoelectric conversion efficiency and gas sensors with high sensitivity are the goals pursued by self-powered gas sensors.The cationic disorder defects and the difficulty in controlling the surface morphology and structure of CZTS materials are the key problems to be solved in the preparation of high-performance self-powered gas sensors.This thesis aims at the development of photovoltaic self-powered gas sensors based on CZTS,and conducts researches on the performance optimization of solar cells and gas sensors.Firstly,using magnesium acetate as the magnesium source,a small amount of Mg is doped to replace part of Zn by solution method,and the Mg-doped CZTS thin film solar cell with less cation disorder defects and less impurity phase were prepared.The introduction of magnesium plays a quadruple role in CZTS thin-film solar cells.First,the CZMTS absorber prepared by the introduction of magnesium atoms has denser grains,larger grain size,and even runs through the entire absorber,and the voids at the interface with Mo are less and relatively tight.Second,the replacement of part of Zn by Mg atoms into the CZTS lattice reduces cation disorder defects and impurity phases such as Zn S and improves the crystal quality of the CZTS absorber.Third,the introduction of magnesium atoms directly affects the carrier transport properties of the CZTS absorber,such as reducing the carrier concentration and increasing mobility.Fourth,the introduction of magnesium atoms enhances the charge-collecting ability of the CZTS absorber in the long-wavelength region.These four effects make the open-circuit voltage of the prepared CZTS thin-film solar cell reach 679 m V,the short-circuit current density reaches 15.32 m A/cm~2.Secondly,we developed a solution drop coating and low-temperature annealing method to prepare CZTS nanocrystals with cauliflower-like spherical stacking structures by controlling the annealing time.The grain growth process of CZTS synthesized by solution method shows that this structure is the result of spontaneous aggregation of nuclei before grain growth.On the one hand,the sensing surface of CZTS,a cauliflower-like spherical packing structure,enables efficient gas diffusion and fast charge transfer,thereby improving its sensitivity and response/recovery time characteristics;On the other hand,the particle size of the CZTS nanocrystals of this structure is about 50 nm,and the nanoscale grain size can realize the enhanced modulation of the electron depletion layer,thereby enhancing the sensitivity and reducing the limit of detection.Therefore,the gas sensor based on this morphology has a response value of 1.287（50 ppb）and 4.04（1 ppm）for NO₂ gas at room temperature,and the response and recovery time are 73/131 s（50 ppb）and 122/231 s（1 ppm）,the testable limit of detection is 50 ppb,and the theoretical limit of detection is 1.454 ppb.Finally,the advantages of the solution preparation method are verified,and the optimized device basically meets the needs of the CZTS-based photovoltaic self-powered gas sensor for power supply and sensing,which lays a solid foundation for its successful development in the future.