Research On The Influence Mechanism Of Surface Ligands On Hot Carrier Dynamics In CsPbBr₃ Nanocrystals

In recent years,benefiting from the excellent carrier properties of perovskite mate-rials,the efficiency of perovskite solar cells has broken through from 3.8%to 25.7%.In order to increase the efficiency to the Shockley-Queisser limit and beyond,many studies have cast their sights on the initial process of the charge carriers,and found that phonons mediate the rapid cooling of hot carriers(hundreds of femtoseconds)is one of the major energy loss process in the photoelectric conversion process.And if this energy loss is mitigated,it is theoretically possible to achieve higher efficiency.Therefore,reducing the cooling rate of hot carriers in the material to increase lifetime is a key part of the realiza-tion of hot carrier solar cell.The emergence of all-inorganic perovskite nanocrystals with long hot carrier lifetime has attracted extensive attention in the field of hot-carrier opto-electronic devices.Compared with common methylamine or formamidine perovskites,the inorganic nanocrystals have a slower hot carrier relaxation process.The current re-searches also discuss and summarize the effects of different compositions and sizes of perovskite nanocrystals on the relaxation process of hot carrier,but there is still a lack of direct control methods for the hot carrier relaxation process in nanocrystals.Based on the nanocrystal surface effect,the study was carried out on how the surface organic lig-ands affect the carrier-phonon interaction in CsPbBr₃nanocrystals and thus the hot carrier relaxation process,and the following research results were obtained:1.This thesis synthesized CsPbBr₃nanocrystals with uniform size but modified by two ligands with different structural characteristics.3-Aminopropyl triethoxysilane molecules form a rigid shell of organic ligands around the nanocrystals.Through the self-designed and built femtosecond transient absorption spectroscopy system,the initial hot carrier dynamics process of nanocrystals was measured,and the results of global fitting of transient absorption spectroscopy using a more accurate Fermi-Dirac distribution model showed that compared with the traditional oleylamine CsPbBr₃,the hot carrier relaxation lifetime in the CsPbBr₃system utilizing the cross-linked silane ligand is increased three-fold,approaching the order of picoseconds.2.Through the cryogenic photoluminescence technology,the fluorescence intensity,the peak position of the emission peak and the full width at half maximum were calculated and analyzed to obtain the intensity of the carrier-phonon coupling.The main peak was separated by fitting the low-temperature fluorescence spectrum.Calculate the phonon energy and coupling strength of longitudinal optics,and establish the intrinsic relationship between the carrier-phonon interaction and the luminescence properties of nanocrystals.It shows that the CsPbBr₃with cross-linked silane ligands have weaker carrier-phonon coupling strength without changing the phonon properties.3.Combined with density functional theory calculations,on the basis of the tradi-tional polaron model,a damping factor related to the rigidity of the ligand was introduced,and a damped oscillation model was proposed to describe the mechanism of the ligand to carrier phonon coupling process.The damping effect further explains the observed linear dependence of the carrier-phonon coupling strength on temperature.Due to the stronger structural properties,the energy barrier of the change between different configurations is large,so the 3-Aminopropyl triethoxysilane molecule has a stronger damping factor,which inhibits the emission of perovskite phonons and delays the relaxation of hot carriers.