Optical Properties And Applications Of Ion-doped Lead-free Perovskite Nanocrystals

In recent years,metal lead halide perovskites(APb X₃)have received extensive attention due to their excellent optoelectronic properties,such as high absorption coefficient,carrier mobility,defect tolerance,and photoluminescence efficiency(PLQY).These qualities show that they have great potential in the field of optoelectronic applications,such as solar cells,light-emitting diodes(LEDs),and photodetectors.However,the instability of the compounds and the toxicity of Pb are still two key problems hindering the commercialization of APb X₃ nanocrystals(NCs).Therefore,it is very meaningful to search for nontoxic candidate materials that replace Pb with nontoxic elements and at the same time have similar photophysical properties.Lead-free halide double perovskite A₂B⁺B³⁺X₆ NCs and ordered vacancy A₃?B₂³⁺X₉NCs have attracted extensive attention in solving the problems of toxicity and compound instability of perovskite Pb,becoming Pb-based halide promising alternatives to perovskites.However,since most double perovskite NCs exhibit indirect and wide band gaps,or the direct band gap with parity-forbidden optical transitions,which leads to the poor photoluminescence(PL)performance.In addition,the characteristic band gaps of Pb-free ordered vacancy NCs,only emits light in the blue spectral region.Ion doping can introduce luminescent centers to achieve multicolor luminescence and effectively improve optical properties.In view of this,this thesis mainly focuses on ion doping to improve the optical properties of lead-free perovskite nanocrystals.The specific research contents are as follows:(a)Lead-free halide perovskite NCs have caused widespread concerns for solving the inherent toxicity and instability problems of lead perovskites.Here,we synthesized Al³⁺-Cs₂AgBiCl₆ NCs by a modified hot-injection method.Through controlling the doping content of Al³⁺ions,the as-prepared Al³⁺-Cs₂AgBiCl₆ NCs exhibit bright self-trapped exciton(STEs)red emission.The PLQY increases significantly from 0.98%of the undoped sample to 17.2%of Al³⁺-Cs₂AgBiCl₆ NCs with doing content of 14.8%,which is higher than the record value(16%)of the currently reported red-emitting lead-free double perovskite NCs.The essential enhancement of PL performance of NCs is accomplished by the indirect to direct band gap transition assisted by doping of Al³⁺ions,as supported by the density functional theory(DFT)calculations.Moreover,the Al³⁺-Cs₂AgBiCl₆ NCs exhibit ultrahigh air stability and can maintain their original structure for more than 7 months,and PLQY merely decreases by 17%stored in ambient conditions for 8 months,which demonstrates the best stability comparing with the reported double perovskite NCs.Finally,the white light emitting diode(WLED)was fabricated by combining the as-prepared Al³⁺-Cs₂AgBiCl₆ NCs and blue-emitting K₃Sb Cl₆ perovskite NCs on a commercial 365 nm UV light chip.The prepared WLED has a Commission Internationale de L'Eclairage 1931(CIE)of(0.32,0.30),a correlated color temperature(CCT)of 6861 K,and a color rendering index(CRI)of 81.These results provide the opportunity for the future application of lead-free double perovskite NCs in LED and other optoelectronic fields.(b)Although the PL performance of lead-free perovskite NCs in the red region is improved by ion doping,the PLQY of lead-free perovskite nanomaterials in the green region is not ideal.In addition,because of the halogen ions easily exchanged,the controllable multicolor emission in perovskite NCs was difficult to realize in current reports.Therefore,the introduction of luminescent central ions is an effective measure to achieve multi-emission in lead-free perovskite NCs.Blue-emitting lead-free perovskite Cs₃Sb₂Cl₉ NCs were synthesized using a modified thermal injection method.By introducing lanthanide ions into Cs₃Sb₂Cl₉ NCs and introducing luminescent central,the lead-free perovskite Cs₃Sb₂Cl₉ NCs host can efficiently transfer energy to the intrinsic energy levels of Tb³⁺or Eu³⁺doped ions,and Cs₃Sb₂Cl₉:Eu³⁺/Tb³⁺NCs achieve multicolor emission.By adjusting the doping concentration of Eu³⁺/Tb³⁺,a single-component white light emission with a PLQY of 32.5%was achieved.A lead-free perovskite Cs₃Sb₂Cl₉:Eu³⁺/Tb³⁺NCs-based WLED with tunable CCT from warm(3158 K)to cold(5727 K)white light,CRI up to 91,and lumen efficiency up to 25.5lm W^-1 was prepared,indicating that Cs₃Sb₂Cl₉ NCs material has potential application value for future solid-state lighting.