| The research on the photoelectric conversion of hybrid organic-inorganic and all-inorganic perovskite semiconductor has been a hot topic in the field of thin-film solar cell because of their tunable bandgaps in the visible light range,and high electron and hole mobility.At present,the perovskite solar cell with the photoelectric conversion efficiency of 23%has been developed,and therefore perovskite is expected to be the third generation solar cell material.The related research is mainly focused on new perovskite material’s application,electron-hole transport and solar cell structure,etc.However,the preparation temperature of hybrid organic-inorganic perovskite material is relatively low,which makes it easy to be hydrolyzed and oxidized.This is the bottleneck that limits the applications of hybrid organic-inorganic perovskite material.How to improve the optical and thermal stability as well as photoelectric conversion performance of this kind of semiconductor material are the key problems to solve.In this regard,many researchers have turned their attention to the research of optical properties of perovskite material.The study of the photoluminescence mechanism of all-inorganic perovskite nanocrystals（NCs）by spectral techniques is evolving.These research will be beneficial to explore the mechanism of photoinduced exciton recombination of all-inorganic perovskite NCs,which may generate a fundamental interest that is useful for the application in solar cell,LED display,laser as well as photoelectric detection.In this dissertation,we investigate the optical properties of a series of Cs Pb X3 NCs with similar sizes,and the mechanism on the exciton recombination process are illustrated by use of steady-state photoluminescence,time-resolved photoluminescence,transient absorption and Z scan technique.The steady-state and time-resolved photoluminescence spectra of Cs Pb X3 NCs at variable temperature are studied.Under low-temperature conditions,it is found from steady-state photoluminescence spectra that photoluminescence intensity decreases rapidly with the increase of temperature,when decreases to a certain value,the photoluminescence intensity keeps invariant with the increase of temperature because of quenching by pyrolysis.Meanwhile,the linewidth and the emission peak of photoluminescence spectra are also changed with the temperature.The temperature dependence of photoluminescence peak shift as well as linewidth broadening mechanism due to exciton-phonon coupling of CsPb X3 NCs are analyzed.Moreover,photoinduced exciton recombination are discussed by using time-resolved photoluminescence spectra,the fitting results show that the mechanism of PL is composed by two components,intrinsic exciton recombination and surface trap-assisted exciton recombination.Some important parameters,such as exciton binding energy,exciton-phonon coupling constant and longitudinal optical phonon energy,were obtained from the temperature dependent photoluminescence spectra,which enrich the knowledge of the exciton recombination mechanism of CsPb X3 NCs.Exciton recombination mechanism under two-photon excitation in a series of Cs Pb X3 NCs are studied.Based on femtosecond open-apeture Z scan technique,it is found that two-photon absorption is the main cause of the reverse saturation absorbance of Cs Pb X3 NCs.The impact of halide anion replacement on the nonlinear absorption coefficient and two-photon absorption cross section is disccussed.Then,two-photon absorption induced upconversion luminescence spectra are studied.It is found that the peak position of upconversion luminescence has a red shift of13 me V than that of normal luminescence.The reason of the red shift is discussed by the time-resolved luminescence spectra,and an exciton recombination model of two-photon excitation is put forwarded.The ultrafast exciton recombination dynamics of CsPb Br3 NCs excited by single-photon and two-photon excitation is studied by femtosecond transient absorption spectra.Two spectral components corresponding to the excited-state absorption and photobleaching can be found in the transient absorption spectra under both single-photon and two-photon excitaion.The peak of photobleaching gradually moves to the long wavelength,indicating that there are multi-relaxation pathways in the relaxation process under the femtosecond laser excitation.Keep the same average numbers of absorbed photons under high excitation intensity,it is found that the relaxation rate of two-photon excitaton is faster than counterpart of single-photon excitaton.Three lifetimes can be obtained by triple exponential fitting of the dynamics,they are assigned to biexciton recombination,trion recombination,and single exciton recombination,respectively.This provide a new information of exciton recombination mechanism for all-inorganic perovskite NCs under high laser excitation intensity.In this dissertation,the photoinduced exciton generation and recombination of a series of all-inorganic CsPb X3 NCs are investigated by using of a variety of spectroscopic methods.Thermal effect on exciton recombination is analyzed through the calculation of exciton binding energy,exciton-phonon coupling constant,and optical phonon energy.Two-photon absorption coefficient for various Cs Pb X3NCs are obtained,and the mechanism of the normal and two-photon upconversion luminescence is analyzed.The exciton recombination model is proposed,and the peakshift between the normal and upconversion luminescence is ascribed to the intensity change between intrinsic exciton and surface trap-assisted exciton recombination.The ultrafast exciton recombination dynamics at the very begining of excitation is studied by femtosecond transient absorption spectra.This work shed new light on exploring the exciton recombination mechanism in all-inorganic perovskite NCs. |
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