Preparation Of Lead Sulfide/Copper Sulfide Core/shell Structure Quantum Dots And Their Applications

Lead sulfide quantum dots(PbS QDs)have quantum size effects and quantum confinement effects.The absorption wavelength of PbS QDs is tunable in the near-infrared region by controlling their size.In addition,PbS QDs also have a large exciton Bohr radius and a narrow band gap.Therefore,PbS QDs have great application prospects in photovoltaic devices such as solar cells,photodetectors and light-emitting diodes,and so on.At present,the most common method for synthesizing PbS QDs is the hot injection method,and the sulfur source used is usually hexamethyldisilazane,thioacetamide or sulfur powder.However,hexamethyldisiloxane is expensive and the reaction conditions require strictly controlled,which is not suitable for large-scale production.In addition,the absorption wavelength of PbS QDs synthesized by using sulfur powder as the sulfur source is longer so that it is not conducive to the application of solar cells.Therefore,the most widely used sulfur source is thioacetamide,but the product has a wide particle size distribution of which the performance needs to be improved.In this study,PbS QDs were synthesized by high gravity technique,and then were passivated by copper sulfide(CuS)to form the core/shell structure and applied to solar cells.The effects of synthesis conditions on the morphology,particle size distribution,optical properties and application properties of PbS QDs and PbS/CuS core/shell QDs were investigated.The main contents and conclusions are as follows.Lead chloride was used as lead source,thioacetamide as sulfur source,and oleylamine as surfactant and solvent to synthesize PbS QDs in a rotating packed bed(RPB).The effects of synthesis conditions on the morphology,particle size distribution,optical properties of PbS QDs were studied.The optimum conditions are determined as follows:RPB rotating speed is 1500 rpm;RPB reaction time is 5 min;RPB reaction temperature is 60 0C;the Pb2+precursor and S2-precursor preparation temperature is respectively 130? and 40?;the ratios of Pb2+to S2-is 3:1.The PbS QDs have a cubic crystal structure and good crystallinity,with an average size of 4.1 nm and excellent dispersibility.Compared with the PbS QDs synthesized by the traditional stirring method,the particle size distribution of PbS QDs synthesized by the high gravity method is narrow,and the stability is high,with less surface defects.Furthermore,the PbS QDs solar cell prepared by the high gravity method has higher open circuit voltage and fill factor,and the energy conversion efficiency is improved by 22.2%.PbS QDs were passivated with copper chloride as a copper source by cation exchange method in the RPB to form quantum dots of PbS/CuS core/shell structure,which is applied to solar cells.The effects of synthesis conditions on the optical properties and application properties of PbS/CuS core/shell QDs were studied.The optimum conditions are determined as follows:the amount of Cu2+addition is 20%;the reaction time is 5 min;the concentration of Cu2+precursor is 0.6 mol/L;RPB rotating speed is 2000 rpm;the reaction temperature is room temperature(25?).The average size of the PbS/CuS core/shell QDs is 4.1 nm,and the products have an excellent dispersibility,uniform morphology and high purity.The open circuit voltage,short circuit current and fill factor of PbS/CuS core/shell QDs solar cells are higher than those of PbS QDs solar cells,and the energy conversion efficiency is improved by 74.2%.It indicates that the passivation of the CuS shell improves the surface defects of the PbS QDs.It also improves the optical properties and the application performance.