Application Of Optical Pump THz Probe Spectroscopy In The Degradation Period Of Organic-Inorganic Hybrid Perovskites

Benefited from the outstanding optoelectronic properties and easy fabrication techniques,hybrid organic-inorganic lead halide perovskites(HOIPs)have been widely studied in photovoltaic and nano-photonic applications,with the power conversion efficiency(PCE)of perovskite solar cells reaching a phenomenal advancement.One primary problem which hinders the commercialization of HOIPs is their environmental instability.For example,it has been reported that pure FAPbI3(FAPI)perovskite exhibited poor stability after exposure to humidity,and the black α phase readily turned into the non-photoactive yellow δ phase.This unpleasant transformation severely hinders their further applications.In order to improve the stability of HOIPs,additive and compositional engineering has been actively explored.Additive engineering usually introduces a controlled amount of organics into the precursor.The added component can contribute to the reduction of trap-state density and improvement of morphology,and sometimes the hydrophobic additives are prone to distribute along grain boundaries and surfaces,which can protect the perovskite from moisture.By contrast,compositional engineering incorporates different ions into the structure and thus provides more freedom to improve the intrinsic properties of perovskite materials.Hence great strides developing the compositional engineering have been widely made in recent years.To reveal the effect of additive and compositional engineering on the performance of HOIPs,various methods have been employed.The maximum power point tracking method is frequently adopted to demonstrate the degradation process of solar cells based on HOIPs.However,when a perovskite film is incorporated in a layered solar cell,it has been shown that the adjacent layers and their interfaces also have an effect on the perovskite stability.It’s unfriendly for us to observe the degradation process clearly.Therefore,some other methods,including the anelastic and dielectric measurements,photoluminescence and absorbance spectra,are also introduced to observe the degradation process.Moreover,the ultrafast transient absorption spectrum and optical pump THz probe spectroscopy were performed to provide information on carrier dynamics of HOIPs.However,in a large number of studies,more attention is focused on the optimization of optoelectronic performance by additive and compositional engineering.Little understanding has been established on the charge carrier dynamics and photo-physical changes in the natural degradation period.In this report,the time-resolved spectroscopy,that is,optical pump THz probe spectroscopy,was employed to observe the long term degradation process of HOIPs.we compared the evolution of 2D PEA2PbI4 passivated(FAPEAPI)and Cs stabilized(FACsPI)films with pure FAPI perovskite film,and focused on the charge carrier dynamics and photo-physical changes in the natural degradation period.This thesis is mainly divided into four chapters,the specific content is as follows.The first chapter is the introduction,which is mainly divided into two parts.The first part introduces the essential properties of terahertz,the generation and detection methods and three commonly used terahertz spectra systems.Meanwhile,we also briefly describe several application prospects of terahertz.In the second part,organic-inorganic hybrid perovskite system is briefly introduced and we make a simple classification.In the end,the basis and foothold of our work are introduced.The second chapter is about experimental methods and data processing,which is mainly divided into sample preparation,characterization methods and data processing.Several commonly used HOIPs synthesis methods are introduced,including solution-based growth,colloidal growth,vapor and liquid-phase epitaxial growth and mechanical ball milling.Besides,several commonly used single crystal synthesis strategies are also briefly introduced.The next section mainly introduces several characterization techniques,focusing on the optical pump terahertz probe spectra.At the end of this chapter,the basis and specific methods for terahertz spectrum analysis are demonstrated in detail.The third chapter is the application of optical pump terahertz probe spectra in the decay period of HOIPs.It is the core part of the article.We find that photoconductivity and carrier mobility of the FA0.9Cs0.1PbI3 film decay steadily,while these of the 2D phenylethylammonium lead iodide(PEA2PbI4)passivated FAPbI3 film show an accelerated decay rate.Moreover,it’s also revealed that the localization of charge carriers increases with the transformation process from the fitted results by the Drude-Smith model.It has been pointed out that at small to moderate concentrations,the 2D PEA2PbI4 perovskite spontaneously forms at grain boundaries and surfaces to passivate the ingression pathway of moisture and suppress ion migration.Meanwhile,the incorporation of Cs optimizes the Goldschmidt tolerance factor,which favors the formation of a perovskite phase thermodynamically.What’s more,it’s energetically unfavorable for the unpleasant transformation reaction for the FACsPI film.These results indicate that the stabilized effect of Cs incorporation is continuous,while the passivated effect of PEA2PbI4 can be weakened by the newly emerged phase boundaries.This report proposes a new perspective and sheds light on the degradation process of HOIPs.The fourth chapter is the summary of this thesis,we raise the innovation points of the subject research,and prospects for the realization of terahertz ultrafast regulation by HOIPs are given.