Study On Photoinduced Charge Transfer In All-inorganic Perovskite Solar Cell Systems

All-inorganic perovskite materials have become a new focus of researchers of solar cell materials because of their advantages such as tunable band gap width,high light absorption ability,long carrier diffusion length and high carrier mobility.At present,there are many researches on the material preparation and application of all-inorganic perovskite cells.However,there are few researches on the key microphysical processes affecting the photoelectric conversion efficiency of perovskite solar cells.In particular study of mechanisms of the separation,relaxation,transport and recombination processes of interfacial photoinduced carriers are far from clear.These transient processes occur on time scales ranging from femtosecond to microsecond.Ultrafast spectroscopy is a very sensitive experimental technique to study the generation and transfer of photoinduced carriers.By using the time-resolved pump-probe technique,the transient absorption spectra and its changes at different delay times of perovskite solar cell materials after photoexcitation can be detected to realize the study of the photoinduced transient effect and ultrafast dynamic process.In this paper,the femtosecond time-resolved ultrafast spectroscopy is used as the technical means,the wavelength ranges from visible light to near-infrared light,the all-inorganic perovskite Cs Pb Br₃ assembled with titanium dioxide systems are used as the research object.We regulate and control photoinduced carrier dynamics of all-inorganic perovskite cell systems by changing the preparation temperature of perovskite samples and doping metal ion to regulate energy gap and charge transfer interface conditions,to study the interfacial photoinduced carrier transfer processes of all-inorganic perovskite cell materials under the laser pulse excitation,then to discuss the key factors and physical mechanisms that affecting the formation,separation,transmission and recombination of photoinduced carriers,we will provide dynamic experimental supports for exploring new solar cell materials.The specific contents are as follows:(1)We prepared a series of Cs Pb Br₃ perovskite nanocrystals at different preparation temperatures by high temperature thermal injection method,and assembled with TiO₂ films to form Cs Pb Br₃/TiO₂ systems.The femtosecond ultrafast pump-probe technique was used to excite the above samples respectively,and the transient absorption spectra were collected.Then the femtosecond transient absorption spectra were analyzed by using singular value decomposition and global fitting,and three kinetic components were obtained,which were classified as hot carrier cooling,charge transfer,and charge recombination process.By comparing the time constants of the above three components of Cs Pb Br₃/TiO₂ systems prepared at different temperatures,we found that the size of Cs Pb Br₃ perovskite nanocrystals become longer with the increase of sample preparation temperature,which leads to time constants of the charge transfer and charge recombination become longer.We also propose a possible model of charge transfer mechanisms for the above Cs Pb Br₃/TiO₂ systems.(2)We doped the Mn ions of different concentrations into Cs Pb Br₃ nanocrystals by post-synthesis doping method,and assembled with TiO₂ to form a series of Mn:Cs Pb Br₃/TiO₂ composite systems.We characterize the optical properties of the samples by steady-state absorption spectra and steady-state fluorescence spectra,and the transient processes of the systems were measured by time-resolved fluorescence spectra and femtosecond transient absorption spectra.The femtosecond transient absorption spectra of a series of Cs Pb Br₃/TiO₂ composite systems doped with different concentrations of Mn ions were analyzed by singular value decomposition and global fitting,and three transient components were obtained,which were classified as hot carrier cooling,charge transfer,charge recombination process,respectively.The experimental data of time-resolved fluorescence spectra and femtosecond transient absorption spectra were combined to analyze,it was found that the doping concentration of Mn ions had effect on all three processes.There were two effects with the change of the doping concentration of Mn ions,one is band gap width of perovskites,the other is defect state density.When the doping amount of Mn is 30?l,the inflexion points appear in curves of luminescence lifetime of Mn:Cs Pb Br₃/TiO₂ composite systems,and the inflexion points appear in curves of hot carrier cooling time constants.When the doping amount of Mn is 20?l,the appear inflexion points in curves of time constants of the charge transfer and charge recombination processes.We believe that it is the result of the simultaneous influence of the above two effects.The detailed discussion and analysis will show in Chapter 4.We propose the possible charge transfer mechanisms model,and further elucidate the microscopic mechanism of generation,separation,transport and recombination process of interfacial photoinduced carriers in a series of Cs Pb Br₃/TiO₂ composite systems with doping different content of Mn ions.Our experimental results are expected to provide further and valuable information for optimizing and improving the stability and efficiency of new-type perovskite-based devices.