Research On Optical Properties And Applications Of Lead-free Halide Perovskite Nanocrystals

Lead halide perovskite nanocrystals,as a new emerging semiconductor material,have attracted wide attention due to their excellent optical properties,such as high photoluminescence quantum efficiency（PLQY）,narrow full-width at half-maximum（FWHM）,tunable spectra.In recent years,lead halide perovskite nanocrystals have made some significant breakthroughs in the field of light-emitting diodes（LED）,and the external quantum efficiency（EQE）of LED devices based on them as the luminescence layer has been increased from the initially reported 0.1%to more than20%.However,the lead toxicity and instability of lead halide perovskite nanocrystals are the two major obstacles to its large-scale commercialization.Therefore,developing lead-free halide perovskite nanocrystals with excellent luminescent performance is an inevitable trend in this field.Halide double perovskite nanocrystals have received much attention due to their non-toxicity,stability,and adjustable composition.However,the optical properties of reported double perovskite nanocrystals are not ideal,manifested in generally low PLQY,difficult regulation of the emission spectrum,and little research in the near-infrared（NIR）emission.This is mainly because the regulation of the luminescent properties of lead-free halide perovskite nanocrystals by their structure and composition remains unclear.In view of the above problems,this paper starts with the design and preparation of double perovskite nanocrystals,optimizes the luminescence characteristics of nanocrystals though component regulation,doping of rare earth ions and transition metal ions,and structure control,and reveals the influence of nanocrystal composition,doping and structure on their luminescent properties.Based on this,lead-free perovskite nanocrystals with excellent performance,tunable spectra,and spectral coverage up to the NIR region are designed.The specific research contents are as follows:（1）Firstly,it is proposed to utilize alkali metal components to regulate and optimize the luminescence properties of Cs₂M（In/Sb）Cl₆（M=alkali metal）double perovskite nanocrystals.The M position was adjusted by replacing with Na,Na_0.5K_0.5,Na_0.25K_0.75,K,K_0.5Rb_0.5,and Rb.By using X-ray diffraction and its refinement technology combined with TEM measurement,it was determined that as the radius of alkali metal ions increases,the lattice constant and unit cell volume of the nanocrystals gradually increase,and the bond length of Sb-Cl decreases gradually.At the same time,the study of steady-state fluorescence spectra of the nanocrystals indicated that as the radius of alkali metal ions increases,the local crystal field environment around Sb³⁺ions become stronger,and the emission spectrum of the nanocrystals demonstrates a significant red shift,achieving continuous tuning of emission from 455 to 530 nm.Furthermore,Mn²⁺doping was carried out in the above series of host to achieve tunable color temperature white light emission,with the highest white light PLQY of 56.7%.Combined with UV chips,WLEDs were fabricated,with a CRI of about 82,a luminous efficiency of about 31.9 lm/W,and color coordinates of（0.31,0.31）.Meanwhile,white light sources with different color temperature ranging from 3276 K to 6900 K were obtained.（2）Subsequently,on the basis of Cs₂M（In/Sb）Cl₆（M=alkali metal）host,rare earth ion doping was utilized to further optimize the luminescence properties of Cs₂Na（In/Sb）Cl₆ nanocrystals.A series of rare earth ions(Sm³⁺,Eu³⁺,Tb³⁺,Dy³⁺)doped Cs₂Na（In/Sb）Cl₆ nanocrystals were prepared by the thermal injection method.By using XRD and XPS combined with high-resolution TEM measurement,it was experimentally confirmed that rare earth ions substituted the In³⁺sites.The luminescence mechanism of doped nanocrystals was revealed by steady-state spectra and time-resolved spectra techniques,demonstrating that multicolor emission is a combined contribution of double perovskite host and rare earth ions.The population of excited-state electrons of the rare earth ions originates from energy transfer of the host,and a level model for white light emission was constructed.Furthermore,by systematically controlling the doping concentration of rare earth ions and combining femtosecond transient absorption（fs-TA）technology,the regulation of rare earth ion on nanocrystal defect states was clarified,suppressing non-radiative transition channels and achieving rare earth ion-doped double perovskite nanocrystals with a PLQY of80.1%,which is the currently reported highest value among lead-free double perovskite nanocrystals.At the same time,the rare earth ion doped nanocrystals also showed good stability,and after placed in air for 180 days,the luminescence intensity could be maintained at over 90%of its original value.Finally,utilizing the highly efficient and stable rare earth ions doped double perovskite nanocrystals,research was conducted on anti-counterfeiting,temperature sensing,and WLEDs.（3）Further,in order to construct efficient near-infrared（NIR）emission in double perovskite nanocrystals,rare earth ions with rich NIR energy level Sm³⁺,Nd³⁺,Er³⁺were selected to incorporate into Cs₂M（In/Sb）Cl₆（M=alkali metal）double perovskite nanocrystals.Efficient single-component NIR emission（NIR PLQY=20.3%）covering spectral range of 850 to 1650 nm were achieved,which is the widest spectral coverage reported for single-component NIR fluorescent materials to date.Furthermore,by utilizing femtosecond ultrafast pump-probe spectroscopy technology,the regulation of metal halide octahedral structural units on the energy level structure of nanocrystals in the host Cs₂M（In/Sb）Cl₆（M=alkali metal）was revealed,thereby enhancing the energy transfer efficiency from the host to the rare earth ions.In addition,the Judd-Ofelt theory was used to determine the regulation of alkali metals on the metal halide octahedral structural units,reducing the local lattice symmetry of rare earth ions and increasing the radiation transition rate of f-f transitions of rare earth ions.Finally,combining UV chips,wide-coverage NIR LED were prepared and their potential applications in gas detection and night vision fields were demonstrated.（4）Finally,to obtain lead-free perovskite light-emitting materials with higher PLQY,we turned our attention to zero-dimensional（0D）materials with stronger quantum confinement effects.We first prepared 0D Cs₃Ce X₆（X=Cl,Cl/Br,Br）nanocrystals by hot injection method.Experimental results showed that as the halogen source was replaced from Cl to Br,the crystal phase of the material gradually changed from the monoclinic phase of Cs₃Ce Cl₆nanocrystals to the orthorhombic phase of Cs₃Ce Br₆ nanocrystals,and the luminescence properties of the material changed significantly.The emission wavelength shifted from 373 nm to 388 nm,and the luminescence efficiency of the material gradually increased.Based on experimental results,we believe that the PLQY of the material is influenced by the defect density of the material.Finally,we achieved a violet light emission with a PLQY of up to 91.7%in Cs₃Ce Br₆nanocrystals.The excellent luminescence performance was achieved by optimizing the reaction temperature,the ratio of reaction ligands,and the ratio of precursor.We further analyzed the reasons for achieving such high PLQY in low-phonon halide perovskite materials,which can be summarized as follows:（1）allowed Ce³⁺4f-5d transition with a large absorption cross-section;（2）zero-dimensional materials have larger exciton binding energy;（3）low defect state density.In order to further expand the spectra,we selected 0D Cs₃Ce Br₆nanocrystals as"antennas"and doped rare earth ions with rich infrared energy levels.As expected,we obtained effective NIR emission.We further optimized the doping concentration of Yb³⁺ions in Cs₃Ce Br₆ nanocrystals and achieved a NIR PLQY of 11.4%.Finally,we combined the optimized Yb³⁺-doped Cs₃Ce Br₆ nanocrystals with UV chips to prepare NIR LEDs and explored their potential in night vision applications.The innovations of this thesis are as follows:（1）A strategy was proposed to address the problem of wavelength regulation in most lead-free perovskite nanocrystals,by controlling the crystal field environment through alkali metal regulation,which achieved continuous tuning of the emission wavelength of the nanocrystals.Furthermore,spectral regulation of double perovskite nanocrystals was further achieved via codoping Mn²⁺ions,and ultimately a highly efficient white light emission with different CCT was constructed.（2）To address the issue of low PLQY in most double perovskite nanocrystals,rare earth ion doping was employed to fill material defects and serve as an activator ion,which not only regulated the nanocrystal’s emission but also improved its PLQY.As a result,a PLQY as high as 80.1%was achieved,which is the highest value among double perovskite nanocrystals.（3）To achieve wide-coverage near-infrared（NIR）emission,we prepared Sm³⁺,Nd³⁺,and Er³⁺-codoped double perovskite nanocrystals.By regulating the metal halide octahedral structural elements,highly efficient single-component NIR emissions were constructed with a spectrum covering range from 850 to 1650 nm and a NIR PLQY of20.3%.This is the currently reported widest-coverage and highest PLQY of single-component infrared emission.（4）To obtain lead-free halide perovskite nanocrystals with higher PLQY,we prepared zero-dimensional Cs₃Ce X₆（X=Cl,Cl/Br,Br）nanocrystals with stronger quantum confinement effect through hot injection method for the first time.Due to the high quantum confinement effect,5d-4f allowed transition,and lower defect state density,the violet-emitting Cs₃Ce Br₆ nanocrystals exhibited a high PLQY of 91.7%,which is currently the highest reported efficiency in the field of lead-free violet-emitting perovskite materials.