Research On Ultrafast Dynamics Of Quasi-Two-Dimensional Semiconductor Materials

The advent of the post-Moore era has brought new challenges to the development of the semiconductor industry.Two-dimensional(2D)materials play a crucial role in the continuation of Moore's Law.Graphene is a well-known 2D material,but its zero-bandgap severely limits its application in optoelectronic devices.Two-dimensional transition metal dichalcogenides(TMDs)have a similar structure to graphene.TMDs are 2D semiconductor materials and exhibit optoelectronic properties such as tunable band gaps,which have attracted the attention of many researchers.In addition,the emerging quasi-2D layered perovskites in recent years have a natural quantum well structure,and they show better stability than traditional three-dimensional perovskites,which has become one of the research hotspots.These two materials have shown great application potential in various optoelectronic devices such as solar cells,light-emitting diodes,and photodetectors.In optoelectronic devices,the carrier dynamics of the material directly determine the performance of the device.Therefore,it is of great significance to study the carrier dynamics of materials and clearly understand the dynamic processes(such as hot carrier relaxation,charge/energy transfer,defect-related dynamics,etc.)for the improvement of optoelectronic device performance.Ultrafast spectroscopy is an effective means to explore these ultrafast kinetic processes.By observing the changes in the absorption or fluorescence of the sample over time after photoexcitation,relevant spectral and kinetic information can be obtained.Therefore,in this thesis,we have studied the ultrafast dynamic process of monolayer tungsten disulfide(WS₂)and quasi-2D layered perovskite(PEA)₂(MA)₂Pb₃I₁₀(PEA=C₆H₅(CH₂)₂NH₃;MA=CH₃NH₃)through various time-resolved spectroscopic techniques.The specific research results are summarized as follows:(1)Exciton dynamics in pristine and oleic acid(OA)treated monolayer WS₂ were comprehensively studied through various ultrafast experimental techniques.We demonstrate that OA effectively passivates the defect states in as-fabricated WS₂,resulting in trap-free exciton dynamics and exciton-exciton annihilation rate reduction,which leads to stronger steady-state photoluminescence and longer photoluminescence lifetime.In addition,comparing the kinetics of WS₂ before and after OA treatment helps us confirm that the long-standing controversial sub-picosecond kinetic process is the process of ultrafast exciton formation.These results provide valuable information on the intrinsic exciton dynamics in monolayer WS₂,which could also be applicable in other two-dimensional TMDs and help improve optoelectronic device performance.(2)Quasi-2D layered perovskite(PEA)₂(MA)₂Pb₃I₁₀(n=3)films were prepared by two-step spin coating,and we investigated the photo-generated carrier dynamics in quasi-2D perovskite films via femtosecond transient absorption and femtosecond time-resolved photoluminescence(fluorescence up-conversion)techniques.Our study reveals that the photogenerated carrier relaxation pathway is dominated by hot carrier relaxation on a time scale of 100–400 fs and the subsequent carrier transport from small n perovskite phases to their surrounding large n perovskite phases via energy transfer on the time scale of 2–300 ps.These results reveal the working mechanism of 2D layered perovskite solar cells from the perspective of microkinetics,which can help to further improve the device performance.