Strain Control Of Ultrafast Carrier Dynamics Of CH₃NH₃PbI₃ And MoSe₂/MoS₂ Heterostructure

With the rapid development of femtosecond laser technology,ultrafast time-resolved spectroscopy technology has become more and more widely used in basic scientific research.The ultrafast quantum(electron,hole,exciton and phonon)dynamic process in the photophysical or photochemical phenomenon that occurs in an ultra-short period of time can be detected via transient absorption spectroscopy technology.It is helpful to understand more accurately the material or the microscopic quantum mechanism.The organic-inorganic hybrid perovskite materials and two-dimensional transition metal sulfide materials,which have received extensive attentions in the past ten years,have unique advantages in the application of flexible optoelectronic devices.For flexible devices,in-depth exploration of the performance evolution of optical,mechanical,magnetic and other properties of materials subjected to bending and applied strain in the actual application process is important for understanding,regulating,and optimizing the design of materials and devices with different properties.This thesis discussed the effects of strain on the ultrafast carrier relaxation processes in organic-inorganic hybrid perovskite CH₃NH₃PbI₃ film,monolayer MoSe₂and MoSe₂/MoS₂ heterostructure on polyethylene terephthalate(PET)substrates.The main contents are as follows:(1)The carrier relaxation process of CH₃NH₃PbI₃ film on PET substrate under different strain states was studied using transient absorption spectroscopy technology and time-resolved fluorescence technology.The film is prepared by a one-step spin coating method.The experimental results show that the fluorescence lifetime of CH₃NH₃PbI₃film increases gradually with increasing tensile strain,while decreases with increasing compressive strain.However,the results of transient absorption spectroscopy show that different strain states have almost no effect on the carrier relaxation lifetime.This is because the CH₃NH₃PbI₃ film has a strain gradient in the vertical direction,the film surface and bottom show tension and compression respectively after bending.This lattice tensile and compressive strain makes the transient optical response to compensate each other,leading a relatively stable carrier relaxation process under different strain states.This is the reason why the flexible perovskite solar cell device can maintain high conversion efficiency even after thousands of large-scale bendings.(2)Using transient absorption spectroscopy,the relaxation dynamics of“hot”electron under different strains in monolayer MoSe₂ film,which is grown by chemical vapor synthesis(CVD)and transferred to a PET substrate states,was studied.In this experiment we resonantly excite the A exciton transition and detect the B exciton transition.The pure hot electron relaxation signal in conductoin band can be extracted from the transient absorption signal due to the B exciton transition.The results indicates that there are two dynamic processes in hot electron relaxation,corresponding to the hot electron cooling process and the thermal diffusion via phonons.For the former,there is no obvious change with increasing strains,while for the subsequent slow relaxation process is gradually accelerated.This could be due to the modulation of phonon frequency and the electric-phonon coupling strength under different strains.(3)Using transient absorption spectroscopy,the strain dependence of electron transport dynamics in MoSe₂/MoS₂ heterostructure,which is grown by CVD method and subsequently transferred to PET surface,was studied.We selectively excite the MoSe₂which is the electron donor layer,and detect the transient absorption signal of MoS₂which is the electron acceptor layer to obtain the electron transfer signals.By analyzing the transient absorption relaxation curve of A exciton corresponding to the acceptor layer MoS₂,we obtain three dynamic processes,corresponding to the electron transfer between two layers,the electron cooling relaxation or defect trapping process,and the interlayer excitons unradiation recombination process respectively.Using the same bending method described for perovskite film,we investigated the electron transfer dynamics under different bending states.The results indicates that as the strain increases,the time constant of the electron transfer process increases,which shows that the electron transfer process is inhibited.The two electron relaxation processes were also inhibited,that is,the cooling and the recombination lifetimes were both gradually increased,which could be due to the modulation of the electrical-phonon coupling at different the applied tensile strains.