Investigation On The Control Of Optical Properties Of Perovskite Nanocrystals And Their Optoelectronic Applications

In recent years,all inorganic perovskite CsPbX₃ nanocrystals?NCs?as an outstanding fluorescent material have been widely developed because of their high photoluminescence quantum yields?PLQY?,narrow PL emission spectra?FWHM?and excellent photophysical properties based on quantum confinement effect.These extraordinary properties make them as one promising material for next generation optoelectronic devices,such as solar cells,photo-detection,micro-lasers and light-emitting diodes?LED?.This thesis mainly focus on the preparation,properties control,surface modification,doping and coating of perovskite NCs as well as their applications on electroluminescence QLED and white LED.The main contributions were as follows:?1?Preparation of the color-full CsPbX₃ NCs and their application on monochrome solid-state LED devices.It is found that the the recation factors such as halogen atoms and reaction temperatures have significant influences on the morphology and optical properties of CsPbX₃ NCs.By optimizing the reaction temperature,the CsPbBr₃ NCs with good crystallinity,uniform size and high PLQY could be obtained.The exciton binding energy?38 meV?and the optical phonon energy?15.8 meV?of CsPbBr₃ NCs can be calculated from the temperature-dependent PL spectra.By controlling the ratio of halogen elements,the PL spectra of CsPbX₃ NCs can be controlled from 400 nm to 700nm.Furthermore,CsPbX₃ NCs with different emissions were selected as phosphors to prepare solid state LED devices with red,yellow,green and blue emissions,and their corresponding luminous efficiencies could achieve 31.07,18.64,47,10.55 lm/W,respectively,under the driving voltage of 3 V.We also explored the relationship between the emission peaks and FWHM as the increase of driving voltage.The corresponding results demonstrate that the emission bandwidth of perovskite NCs is insensitive to the temperature variations,indicating that these perovskite NCs keep great stability and are suitable as down-conversion materials for the application on solid state LED lighting.?2?Preparation of didodecyldimethylammonium bromide?DDAB?modified CsPbBr₃ NCs and their application on electroluminescent QLED devices.The conductivity of CsPbBr₃ NCs can be improved by the DDAB molecule with short carbon chains.Bsides,the Br^-ions contained in DDAB can fill the suspended bonds or Br-defects that occur on the surface of NCs,and DDA⁺ions can also help to maintain the solubility of NCs in organic solutions.Compare with oleate group,Br^-ion displays strong affinity with positive ions(Pb²⁺and Cs⁺).Furthermore,because the DDA⁺with branched chain structure shows large space steric hindrance,which makes a small amount of DDA⁺ions could be adsorbed on the surface of NCs,inducing the formation of negative polarized NCs with Br^-rich surface.The larger steric hindrance of DDA⁺with a large posititve zeta potential could lead to improve the stability of perovskite NCs.The PLQY of the unmodified CsPbBr₃ NCs solution and film are 78%and 8.2%,respectively,while that for DDAB modified CsPbBr₃ NCs increase to 85%and 23.6%,respectively.It can be concluded that short chain ligands covered on the surface of NCs can shorten the distance between NCs and NCs and induce their wave functions overlap between electrons and holes,which can increase the energy transfer efficiency between the NCs film.The UPS data demonstrate that the position of conduct band for DDAB-treated NCs decrease 0.28 eV,which contribute to the electrons injection from ZnO layer into the perovskite active layer.In addition,an inverted QLED device based on CsPbBr₃ NCs was prepared.The turn-on voltage for the QLED is 2.5 V,and the highest EQE is 0.58%,almost 9 times than that of QLED based on untreated NCs.The CIE coordinate of this device is?0.1,0.79?,corresponding to the color purity of 91%,which is higher than that of high-performance commercial LED?84%?.This research method of ligand modified NCs provides a new idea for further preparation of efficient,stable and colorful inverted QLED devices.?3?Preparation of CsPbX₃?X=Br,I?/ZnS heterodimers NCs and their application on QLED devices.During the synthesis of the CsPbX₃ NCs,zinc and sulfur sources were added into the reaction solution.Thus,the ZnS NCs gradually grew on the perovskite NCs surface,finally forming as the heterodimer structure.The CsPbX₃/ZnS heterodimers present high crystalline quality and enhanced chemical stability,which can be demonstrated by the density functional theory calculation based on first principles.The?001?surface of CsI?CsBr?and PbI₂?PbBr₂?terminations were selected as representatives,corresponding to the Zn or S atoms adsorbed on the top of?001?direction of ZnS.When S plane of ZnS is connected with CsI plane and PbI₂ plane of CsPbI₃ surface,the binding distances are 2.9?and 2.3?,respectively.When Zn plane is connected with CsI plane and PbI₂ plane,the binding distances are 2.1?and 3.1?,respectively.It is well known that the short binding distance is corresponding to large binding energy.The calculated binding energy of the above four interfaces are 32,33,45 and 12 meV/?²,respectively,which are much larger than the surface energies of CsPbI₃?001?,i.e.,?3 meV/?²,demonstrating that the stability of CsPbX₃ NCs is improved.In addition,obvious potential variations occurred at the connected interface region can be observed,and the decrease or increase of the potential can induce the generation of in-built electric field,which may greatly promote the separation of electrons and holes pairs in the heterodimers.Furthermore,when the S plane is contact with CsI or the PbI₂ planes of CsPbI₃,about 0.01 e/?² of electrons is transfer to the ZnS,resulting in p-type doped CsPbI₃.However,when Zn atom is connected with CsPbI₃,a small number of electrons?0.002 e/?²?is transferred to CsPbI₃,resulting in n-type doped CsPbI₃.The result indicates that the formation of p-type or n-type doped perovskite materials can be controlled by adjusting the initial growth conditions and interface structure.Experimentally,with the increase of sulfurization time,the PL spectra of the heterodimers are gradually blue-shifted,indicating that energy conversion between ZnS and CsPbX₃ NCs appears.This strategy provides an idea for the fabrication of color-full CsPbX₃/ZnS heterodimers NCs.Stability measurement shows that CsPbX₃ NCs are extremely unstable in the atmosphere,while the CsPbX₃/ZnS heterodimers could be stored in the atmosphere for more than 12 days.Taking the as-obtained heterodimers NCs as the active emission layer,an inverted QLED device with maximum brightness of 293 cd/m² and EQE of 0.35%was successfully prepared.?4?Preparation of CsPbMnCl₃@SiO₂ composites and their application on white LED devices.Firstly,the hot-injection method was adopted for the preparation of the Mn²⁺doped CsPbCl₃ NCs.When the reaction temperature was 200°C,we obtained the maximum PLQY of 60.5%for CsPbMnCl₃ NCs.Besides,the optimal nucleation time was 10 s for the Mn²⁺doped CsPbCl₃ NCs.While the nucleation time was less than or greater than 10 s,broad or multi-peaks for the exciton emission appeared.Besides,changes of Mn²⁺ions concentration was employed for the adjustment of CsPbMnCl₃NCs emission.To enhance the stability and prevent ion exchange for the doped NCs,3-aminopropyl)triethoxysilane was selected as silane precurs to wrap the CsPbMnCl₃NCs,and their morphologies,structures and surface properties were analyzed with some characterizations.Specifically,we calculated the energy transfer rate and energy transfer efficiency from perovskite band-edge to Mn²⁺in CsPbMnCl₃ NCs.The result shows that the energy transfer rate is 1.27×10⁸ s^-1 and energy transfer efficiency is 21.9%for the unwrapped NCs.For the SiO₂ coated NCs,both of the energy transfer rate and energy transfer efficiency are significantly improved,reaching 1.94×10⁸ s^-1 and 60.7%,respectively.The mixture of CsPbMnCl₃@SiO₂ composite and CsPbBr₃ NCs are utilized as phosphor for the fabrication of white LED device,which exhibits a luminous efficiency of 77.59 lm/W and CRI of 88.Furthermore,the EL spectra of the device show strong stability after continuous working 24 hours,which lays the foundation for their application in the field of lighting and display.?5?Preparation of ultra-thin SiO₂ coated CsPbMnX₃?X=Br,Cl?NCs and their application on white LED devices.The CsPbMnX₃ NCs were first prepared by employing the MnBr₂/PbX₂-DMSO composite as precursor at room temperature.In general,the Mn²⁺ions emission usually occurs at the red-emission region.While the exciton emission spectra can be adjusted between 400 and 510 nm according to the variation of Br/Cl ratios.The band gap of CsPbX₃ could directly affect the whole emission of Mn²⁺ions doped CsPbX₃ NCs.When the Br^-ions concentration is high,the probability of radiation transition of CsPbBr₃ NCs is much higher than that of CsPbCl₃NCs,and the effective energy transmission from ⁴T₁ to ⁶A₁ is effectively blocked,which results in a weak emission of Mn²⁺ions.With the increase of Cl^-concentration,the energy transport from the band edge to the excited state of Mn²⁺becomes stronger,leading to the enhanced Mn²⁺ion emission.The highest energy transfer rate reaches2.04×10⁸ s^-1 when the Br/Cl ratios equals to 0.4/0.6.Different emissions including green-white,cyan-white,orange-white and yellow-white emitting solid-state LED devices were prepared based on the CsPbMnX₃ NCs with different proportion of Br/Cl.In addition,the surface ligands such as trioctylphosphine?TOPO?and2-methoxyethanol as active agents and tetraethoxysilane?TEOS?as the silane precursor were selected to prepare SiO₂ coated CsPbMnX₃ NCs.The thickness of SiO₂ shell can be controlled as 2 nm with appropriate TEOS concentration.Moreover,the morphology and crystal structure of CsPbMnX₃ NCs can be well maintained,and 92.3%of PLQY can be kept after SiO₂ coating.The water stability and thermal stability are also improved simultaneously.By mixing the CsPbMnX₃@SiO₂ and CsPbBr₃ NCs,we can obtain white emissive NCs mixture,which can be directly applied to fabricate white LED.The as-fabricated tri-color white LED shows warm white light with a color rendering index of 91 and a luminous efficiency of 68.4 lm/W.These results suggest that the CsPbMnX₃@SiO₂ NCs can be used as down-conversion material for the next generation of solid-state lighting.