Structure And Spectroscopy Of Novel Low-Dimensional Lead-Free Perovskites

The next generation of solid-state lighting technology needs single-source white light emission materials with high photoluminescence quantum efficiency(PLQE)and strong thermal stability.Materials with self-trapped excitons(STEs)are suitable for achieving broadband white light emission,especially given their strong exciton binding energy and electro-phonon coupling.On the one hand,low-dimensional lead-free perovskites avoid the toxicity and instability of Pb-based materials.On the other hand,its low-dimensional structure has a stronger quantum confinement effect,providing favorable conditions for the generation of STEs.Therefore,exploring low-dimensional lead-free perovskites and realizing low-toxicity and high-efficiency broadband emission is a very meaningful subject.Recently,blue and white emission has been achieved in 3D perovskites with a PLQE of over 80%.Theoretically,the dimensional reduction will further enhance the exciton binding energy and electric-phonon coupling,which is very helpful for the improvement of PLQE.However,achieving broadband STE emission in layered double perovskites(LDPs)remains a challenge.Nowadays,in LDPs,STE emission can only be detected under extreme conditions(such as high pressure or low temperature),which is not suitable for daily needs.Hence,our works focus on exploring the low-dimensional lead-free perovskite(2D and 0D)with stability and highly efficient broadband emission.The main contents are as follows:1.Based on 3D Cs₂NaInCl₆,we obtained two LDPs by temperature lowering method:(PEA)₄NaInCl₈(n=1)and(PEA)₂Cs NaInCl₇(n=2).The structural changes caused by dimensional reduction are mainly on two aspects:the thickness reduction of the inorganic layer and the octahedral distortions.The former effect is mainly on the increase of the band-gap,and the latter breaks the crystal symmetry.By combining spectroscopy and theoretical calculations,we reveal the reasons for the absence of STE in LDPs.The reason is:the non-Jahn–Teller-like distortion of[In Cl₆]^3-in the excited state,providing a channel for the non-radiative recombination of STEs.2.Based on the previous chapter,we successfully achieved broadband STE emission by Sb doping,with PLQE up to 48.7%and 29.3%,respectively,which is the optimal value for LDPs.By combining spectroscopy and theoretical calculations,we demonstrate the origin of broadband STE emission.After the excitation,the[Sb Cl₆]^3-octahedron generates Jahn–Teller-like distortion and becomes a new self-trapped center,making the STE can emit photons by radiative recombination.3.In this work,we successfully improved the PLQE and thermal stability of LDPs by using histamine(HA)molecules as A-site ions.The inference of improved thermal stability was verified in the corrugated(HA)₂KIn Cl₈perovskite and 0D(HA)₂In Cl₇·H₂O perovskite.After a heating-cooling cycle,the photoluminescence(PL)intensity of the two materials only slightly decreases,showing excellent thermal stability.