Modulating The Magnetic And Stability Properties Via Alteration Of Electronic Structure Of Halide Perovskites

The solution-grown lead halide perovskites(LHPs)have the advantages of low production costs,high defect tolerance,tunable bandgaps,and etc.,which have broad application prospects in the next-generation optoelectronic devices.For example,the power conversion efficiencies(PCE)of LHPs-based solar cells has reached over 25%,which is expected to replace the current expensive silicon-based solar cells to achieve low-cost power generation.At the same time,LHPs nanocrystals have the characteristics of near-unity photoluminescence quantum yields(PLQYs),narrow full width at half maximum(FWHM),and wide color gamut(400-800 nm).LHPs light-emitting devices are expected to find important applications in the next generation of flat panel displays and lighting products.However,despite the many advantages of LHPs,there are still tough obstacles to overcome before commercialization.Although solution-processing methods can greatly reduce the cost of material preparation,it brings a lot of vacancy defects to the crystal lattice.Under the action of an external field(such as electric field,light field,thermal field,and etc.),ions can achieve long-range migration by means of vacancies,resulting in segregation of ions and phase separation within grains,Finally,a sharp decline of optoelectronic properties is expected.Therefore,it is one of the most critical issues to suppress the field-induced ion migration of LHPs to ensure robustness and reliability of optoelectronic properties under working conditions.In order to achieve this goal,it is necessary to understand how vacancy defects affect band edge states and search effective strategies to modulate band structures(such as spin structure,orbital hybrid,charge transfer,etc.).The main works of this paper are as follow:(1)This chapter studies the d⁰ ferromagnetism induced by vacancies in nominally non-magnetic LHP semiconductors(including Cs Pb Cl₃,Cs Pb Br₃,Cs Pb I₃ and CH₃NH₃Pb Br₃).First-principle calculations reveal that bromine vacancies in defective Cs Pb Br₃lattice can generate spin-splitting states.Experimentally,the ferromagnetism can be observed at temperatures far above 300 K and can be enhanced via the coupling between bromine vacancies and orbitals of doped 3d metal cations.These experimental results can be described by a model based on exchange-coupled magnetopolaron.In addition,room temperature ferromagnetism has also been found in other 3d magnetic ion-doped LHPs.this indicates that it may be a common phenomenon for solution-processed LHPs materials possessing ferromagnetism caused by the coupling of vacancies,which is expected to have promising application future in spintronic devices.This research content in this chapter is helpful to deepen the understanding of the fine electronic structure of lattice defects in LHPs,and offer significant guidance for further improvement of optoelectronic properties and stabilities of LHPs.(2)This chapter puts forward a method of introducing equivalent 3d transition metal ions into the B site of LHPs to effectively control the migration kinetics of halogen vacancies.The extracted migration activation energy show that doping with 3d transition metal ions can significantly increase the ion migration energy barriers.The Ni²⁺doped halide-mixed NCs show much more stable emission under working conditions in comparison with undoped NCs.By studying the differential charge density maps,it is found that some benign impurity states are introduced into the band gap after Ni²⁺ion doping,which are formed by the hybridization between Ni 3d and Br 4p orbitals.Therefore,the migration of bromine ions requires more energy to break the Br-Ni bond,indicating the migration gets harder.The research content in this chapter is helpful to find a suitable way to adjust the electronic structure and dramatically enhance the stability of LHPs.(3)This chapter studies the synthesis of Cs₃Fe Cl₆ crystals using 3d metal iron atoms to completely replace Pb,and explores the relationship between 3d half-full orbits and ion mobility barriers.The study finds that the five unpaired electrons of Fe³⁺provide a local magnetic moment for the crystal and increase the energy barrier for the directional migration of ions.Cs₃Fe Cl₆ has a large absorption coefficience in visible light range.As a kind of low toxicity non-lead halide perovskite material,it has relatively high stability and suitable optical band gap,and can be used as a potential light-absorbing candidate in solar cells.(4)This chapter studies the synergistic inhibition of nickel ion substitution doping and halogen vacancy filling on ion migration in Cs Pb Br₃ nanocrystals.Through the measurement of ion migration activation energy and in-situ observation of high-resolution transmission electron microscope,the mechanism of the precursor dopant on the stability of LHPs is analyzed.Two kinds of doped LHPs nanocrystals are synthesized using nickel acetylacetonate and nickel bromide as doping sources,respectively.Subsequently,the ion migration activation energy of the nanocrystalline film is calculated from the slope of temperature-dependent conductivity tests.The in situ morphology evolution of LHPs NCsunder high-energy electron beam irradiation is observed using high-resolution electron microscope.The influence of doping on stability against electron beam irradiation can be revealed.The experimental results showed that the ion migration activation energy of the Ni²⁺doped Cs Pb Br₃ sample is significantly improved(0.24 e V for nickel acetylacetonate doped and 0.49 e V for nickel bromide doped sample)compared to the intrinsic Cs Pb Br₃ sample(0.07 e V),In addition,electron beam irradiation tests show that nickel bromide doped perovskite crystals exhibit higher structural stability,which is mainly due to the strong bonding of doped Ni²⁺to halogen and the synergistic passivation effect of halogen filling vacancy defects.Finally,it has been confirmed that the synergy of Ni²⁺doping and halogen vacancy filling can effectively inhibit ion migration in halide perovskite nanocrystals.