Controllable Synthesis Of CuInS₂ Quantum Dots And Its Application In Solar Cells

Quantum Dots(QDs)are generally considered to be quasi-zero-dimensional nanocrystals with sizes ranging from 2 nm to 10 nm and composed of hundreds to thousands of atoms.Since the size of quantum dots is close to the exciton Bohr radius,electrons and holes are confined in a three-dimensional space,so quantum dots exhibit a strong quantum confinement effect.Unlike macroblock materials,quantum dots have many peculiar properties in terms of light,electricity,and magnetism and have important applications in biomarkers,displays,and photovoltaics.As a green and direct bandgap semiconductor material,the band structure of CuInS₂ can be adjusted in a wide range with size and composition,and its bulk material has a band gap about 1.53e V,which exhibit an extremely large molar extinction coefficient in the ultraviolet-visible region.These features making the CuInS₂ an excellent photovoltaic material.The controllable synthesis of CuInS₂ quantum dots and exploring its application in photovoltaics devices is a very important subject.This paper started with the synthesis of CuInS₂ QDs with different crystal structure,size and shape.The influence of oleylamine the growth process of the reaction system was studied;We applied the CuInS₂ QDs as hole transport material and explored their applications in perovskite solar cells.By optimizing the preparation process of perovskite film and CuInS₂ QDs,we prepare the perovskite solar cell devices with photoelectric conversion efficiency as high as 12.33%.The CuInS₂/ZnS core/shell QDs coated with different thickness of ZnS layers were synthesized by epitaxial self-assembly method which realized the conversion of core/shell structure from type I to quasi-type II.The generation and recombination process of electrons and holes during the conversion of core/shell structure are studied,and the influence on the photoelectric performance of quantum dot-sensitized solar cells was studied.The main research content of this paper includes the following aspects:(1)The CuInS₂ QDs with different structures,sizes and shapes were synthesized by thermal injection method,just simply adjust the dose of oleylamine in the reaction system.The mechanism of oleylamine affects the formation of CuInS₂ QDs was revealed.As an organic ligand,oleylamine not only increases the activity of the monomer,accelerates the process of QDs nucleation and growth,but also competes with the DDT ligand in the reaction system.When the volume fraction of oleylamine are 0%,10%and 15%,the zincblende CuInS₂ QDs are obtained;when the volume fraction of oleylamine is 20%,the pseudo-zincblende and wurtzite mixture was obtained;when the dose of volume fraction of oleylamine in the reaction system was further increased to 25%,the wurtzite CuInS₂ QDs were obtained.Surprisingly,in the wurtzite CuInS₂ QDs,the wavy and five-fold symmetric twin structure were observed.This was because the oleylamine can improve the activity of the monomer,which prevented the atoms from being sufficiently rearranged and annealed,resulting in stacking errors.(2)The energy band positions of CuInS₂ QDs and Cs_0.17FA_0.83Pb(Br_0.2I_0.8)₃perovskite materials were determined by UV-Vis absorption spectroscopy and ultraviolet photoelectron spectroscopy respectively,which theoretically that the CuInS₂QDs can serve as hole transport material for perovskite solar cells.Through optimizing the preparation procedure of perovskite film and the concentration of CuInS₂ solution,the perovskite solar cells with a photoelectric conversion efficiency of 7.44%were prepared.By further optimizing the size and surface ligands of CuInS₂ QDs,the photoelectric conversion efficiency of perovskite solar cell devices was increased to12.33%;The ultra-fast spectra technology was applied to analyzed the transport process of carriers at the CuInS₂/Ti O₂ interface.The results show that the CuInS₂ quantum dots can strongly quench the fluorescence of the perovskite film and increase the photobleaching recovery rate of the perovskite,indicating that the addition of CuInS₂contributes to the photo-cavity transport and collection process.(3)The ZnS was continuously grown on the CuInS₂ QDs by epitaxial self-assembly method to realize the conversion of core/shell structure from type I to quasi-type II.The fluorescence lifetime and carrier recombination process of CuInS₂/ZnS QDs were studied by ultrafast spectroscopy.The results show that the fast lifetime of QDs fluorescence decay originates from its surface defects rather than internal defects.When the ZnS thickness of core/shell QDs,its quantum yield increases from 2.3%to32.2%and then decreases to 26.5%,and its fluorescence bleaching recovery rate increases first and then decreases.In quasi-type II core-shell QDs,the electron wave function is extended from the core region to the shell region,which facilitates the separation and collection of charges.The CuInS₂/ZnS QDs coated with different thicknesses of ZnS were used as light harvesters to study their effects on the photoelectric properties of QDs sensitized solar cells.When CuInS₂/ZnS 5 was used as the light absorbing layer,the prepared QDs sensitized solar cell had an optimum photoelectric conversion efficiency of 2.07%.This was due to the sufficient passivation of QDs surface defects.When CuInS₂/ZnS 10 was used as a sensitizer for the solar cells,the photoelectric conversion efficiency of the device was decreased to 1.71%.This was because thicker ZnS acts as a physical barrier that hindered the transfer of electrons from the conduction band of the QDs to the conduction band of TiO₂.