2D Layered Perovskite Narrowband Photodetectors Enabled By Self-trapped Excitons

Narrowband photodetectors can achieve both wavelength discrimination and intensity measurement of light simultaneously,which is of great significance in the fields of machine vision,image recognition,biological imaging,etc.Currently,narrowband photodetectors are suffering from the complicated structure,high cost and low external quantum efficiency.To this end,a low-cost narrowband photodetector with great architectural simplicity and excellent responsivity is much desired.Charge collection narrowing is an effective method to realize narrowband photodetections.However,in order to eliminate the collection of surface charge,a thick light-absorbing layer or an additional barrier layer is required,and the bias voltage to maintain the narrowband response cannot be too large.In addition,the responsivity of narrowband photodetector is usually low due to the low absorption efficiency of band tail.2D perovskites equipped with natural quantum well structures are a family of layered materials.Compared with 3D perovskites,2D perovskites exhibit an improved stability,remarkable exciton and electrical conductivity anisotropy.In addition,there are self-trapped excitons below the free excitons in 2D perovskites due to the deformable lattice.In this dissertation,we propose the preparation of pure phase two-dimensional perovskite by mechanical exfoliation,and systematically studies the properties of self-trapping excitons in two-dimensional perovskite.Utilizing the characteristics of anisotropic conductivity and self-trapped exciton-assisted absorption,a high-performance narrowband detector was fabricated,which sucessfully addressed the drawbacks of current narrowband photodetectors such as high thickness,low working bias and low efficiency.Specific contents include:(1)Synthesis and characterizations of(C₄H₉NH₃)₂(CH₃NH₃)_n-1Pb_nX_3n+1 2D perovskites.2D perovskite single crystals were synthesized by solution method and characterized by X-ray diffraction spectroscopy,micro-area absorption and photoluminescence spectroscopy.We proposed the mechanical stripping method to separate the impurity phases in the crystals with a large n-value to realize the preparation of 2D perovskite with pure phases,which paves the way for studying its optical and electrical properties.(2)Optical properties of self-trapped excitons in(C₄H₉NH₃)₂(CH₃NH₃)_n-1Pb_nX_3n+12D perovskites.The exciton-phonon coupling strength of 2D perovskites was extracted by fitting temperature dependent absorption with Urbach tail theory,which verifies the existence of self-trapped excitons in 2D perovskites.The optical properties of free excitons and self-trapped excitons were studied through temperature-and power-dependent photoluminescence measurement.Furthermore,the enhanced absorption arise from self-trapped excitons were confirmed by means of polarization-resolved photoluminescence and photoconductivity measurement.(3)n=2 perovskite narrowband photodetectors with high performance were fabricated based on charge collecrting narrowing.The detector has a switching ratio of about 10³,a detection rate of 10¹¹ Jones,an external quantum efficiency of more than 300%,and a full-width at half-maximum about 30 nm.By changing the halide ion and n value,the bandgap of 2D perovskite was adjusted,and a series of tunable narrowband photodetectors in visible range were fabricated.Detectors all possess an external quamtum efficiency(EQE)over 200%(an improvement of 1-2 orders of magnitude compared to 3D counterparts)and a full-width at half-maximum smaller than 60 nm.(4)Narrow dual-band photodetector based on 2D perovskite heterostructures.2D perovskite heterostructures were prepared by a modified solution synthesis method.The structure and interface quality of heterostructures were characterized by photoluminescence and reflection spectroscopy,and the heterojunction junction depth was less than 100 nm.Taking advantage of exciton ionization and charge separation in the heterostructure region,the EQE of the narrow dual-band photodetector was further improved by a factor of three compared to 2D perovskite narrowband photodetectors.