Ultrafast Dynamics Study Of Highly Excited States Of Methylammonium Lead Iodide Perovskite And Azulene Molecules

With the rapid development of photovoltaic technology and light-emitting diodes(LED),it is imperative to improve the photoelectric conversion efficiency and luminescence quantum yield of materials.Organic-inorganic lead halide perovskites exhibit excellent optical and electronic properties among numerous perovskite materials,such as long carrier lifetimes and high photoluminescence quantum yields.At the same time,it has the advantages of easy solution processing and low cost of raw materials.These properties make organic-inorganic lead halide perovskites the most promising semiconductor materials for the construction of light-emitting diodes and solar cell devices.Azulene has fluorescence that violates Kasha’s rule.People have not been able to give a very clear description of the intramolecular vibrational energy redistribution process within azulene molecules.Therefore,a large number of theoretical and experimental studies have been triggered.Azulene has also become the core of understanding the non-radiative decay of macromolecules.In recent years,the unique photophysical properties and electronic structure of azulene make it gradually developed into a new type of optoelectronic material.Organic molecules with azulene as the backbone play an important role in the development of organic light-emitting diodes(OLEDs),solar cell dyes,conducting polymers,and nonlinear optical materials.The performance of optoelectronic devices is inseparable from the electronic excited transition process of materials.However,the relaxation process of electronic excited states is very fast,whether through radiation or non-radiative pathways,and the time scale can reach the sub-picosecond level.Due to the popularity of femtosecond laser technology,there are already very mature commercial femtosecond lasers that allow us to observe this ultrafast process in the laboratory,and many ultrafast spectroscopy techniques have been developed,promoting the rapid development in the fields of physics,chemistry,materials and life.In this paper,the generation of highly excited states is induced by femtosecond laser,then,the ultrafast excited-state dynamics of methylammonium lead iodide(CH3NH3PbI3)perovskite thin films and azulene molecules were studied by transient absorption spectroscopy.The main contents and research results of this paper are as follows:1.The CH3NH3PbI3 film was tested for transient absorption in the range of 360-670 nm with excitation at 330 nm.The relaxation and recombination dynamics of photogenerated carriers in CH3NH3PbI3 were studied.The effect of pump fluence on carriers recombination mechanism is explained.The causes of photobleaching and photoinduced absorption in transient absorption spectroscopy were analyzed.And the possibility of the existence of higher valence bands in CH3NH3PbI3 was speculated according to the spectral characteristics.2.The femtosecond transient absorption measurements on azulene in cyclohexane were performed upon 580 nm and 327 nm excitation.The dynamic properties of the S1 and S2 singlet excited singlet states of the azulene molecule were studied.The completely different relaxation mechanisms of the S1 and S2 states were explored.The effect of the vibrational excess energy of intramolecular deposition on the internal transition rate between the S1 and S0 states is elucidated.The process of transition from S1 and S2 states to higher energy states caused by excited state absorption is analyzed.In order to further obtain the time-and space-resolved dynamics of carriers,and to study the nonlinear optical properties of materials,our research group independently built a transient absorption microscopy imaging system and Z-scan system.In this paper,LabVIEW was used to design and develop automation control software for two optical systems,which realized the full automation of the test process,improved the efficiency of the test,and improved the accuracy of the test results.