| The excellent optoelectronic properties of organic-inorganic hybrid perovskite materials make them widely used in photovoltaic cells(PVCs),lasers,light-emitting diodes(LEDs),and photodetectors.Especially for photodetectors based on perovskite materials,the detectable wavelength range has expanded from ultraviolet,visible,near-infrared to the X-ray region,and the device performance have been continuously improving.Although perovskite optoelectronic devices have made great achievements,some basic physical problems of perovskite materials are not clear.In addition,the instability of perovskite materials in humid air has severely restricted their development and practical applications in the field of optoelectronics.In this dissertation,in response to the above problems,the physical properties of two-dimensional(2D)Ruddlesden-Popper(C6H5CH2CH2NH3,PEA)2(MA)n-1Pbn Br3n+1 perovskite prepared by an anti-solvent vapor-assisted method(ASV)and the corresponding lateral photodetectors are systematically studied.The main results include the following three sections:1.2D(PEA)2Pb Br4 perovskite microcrystals with a natural quantum well structure areprepared,and the exciton photoluminescence(PL)characteristics of the perovskite with different numbers of layers are investigated.The coupling of singlet excitons and triplet excitons with acoustic phonons and optical phonons in the material are further explored,respectively.(1)2D(PEA)2Pb Br4 perovskite microcrystal flakes are prepared,and the morphology and optical properties are studied.It is found that the thick perovskite exhibits a strong triplet excitons emission with an ultranarrow line width of 839?e V.With the decrease of the number of layers,the PL intensity of triplet excitons decreases.For the single-layer perovskite,the PL can no longer be detected.The emission of singlet excitons shows a blue shift as the layer number decreases due to the expansion of the crystal lattice,while the emission peak position of triplet excitons is not affected.(2)The coupling strength between singlet or triplet excitons with longitudinal acoustic(LA)phonons is proved to be almost less than 0.1 in the(PEA)2Pb Br4 perovskite.The coupling strength of singlet and triplet excitons with longitudinal optical(LO)phonons is much stronger than that of these two excitons with LA phonons,reaching tens or hundreds of millielectron volts(me V).The coupling strength of triplet excitons with LO phonons is two to three times stronger than the one of singlet excitons with LO phonons,while the energy of LO phonons is almost the same,which is approximately 21 me V.The huge difference in the coupling strength between these two excitons with LO phonons may be due to the different locations of singlet excitons and triplet excitons in the 2D layered perovskite with a quantum well structure.Since singlet excitons exist in the potential well,the strong quantum confinement effect and the dielectric confinement effect inhibit its interaction with the LO phonon.Therefore,the coupling strength of singlet excitons with LO phonons is weaker than that of triplet excitons with LO phonons.2.2D perovskite materials have large exciton binding energy,which is not conducive to the separation of excitons,but it has high humidity stability.Although three-dimensional(3D)perovskites have small exciton binding energy,it is unstable in humid air.Therefore,it is necessary to choose perovskite materials with a low binding energy and high humidity stability for the fabrication of photodetectors.Based on this,the lateral photodetectors based on 2D(PEA)2Pb Br4 and 3D MAPb Br3 perovskite microcrystals are prepared,respectively.The performance of these two devices and their stability are studied,respectively.(1)The(PEA)2Pb Br4 microcrystal photodetector shows a small dark current.When the voltage is 15 V,the dark current is as low as 7.2×10-11 A.Under a voltage of 15 V and light intensity of 2.47 n W cm-2,the responsivity,gain,and specific detectivity of the device are 5.70 A W-1,17.43,and 3.50×1013 Jones,respectively.After the photodetector is placed in an environment with a humidity of 60%for 8 days,the responsivity,gain,and specific detectivity become 11.22 A W-1,34.33,and 5.09×1013 Jones,respectively,indicating almost no device performance degradation,thanks to the hydrophobic long-chain PEA+organic cations in the perovskite,which increases the humidity stability of(PEA)2Pb Br4.(2)For the MAPb Br3 microcrystal photodetector,the dark current is 8.2×10-7 A at an applied voltage of 15 V.When the light intensity is 158.60?W cm-2,the current is 1.6×10-4A,and the on/off ratio is 195.When the voltage is 15 V and the light intensity is 7.51n W cm-2,the responsivity,gain,and specific detectivity are 1.40×106 A W-1,3.34×106,and 4.92×1016 Jones.When the photodetector is placed in an environment with a humidity of 60%for 8 days,the on/off ratio of the device decreases to 8.When the light intensity is 7.51 n W cm-2,the device has no light response.As the light intensity increases to 206.78 n W cm-2,the device begins to show light response,and its responsivity,gain,and specific detectivity are 2.65 A W-1,6.31,and 3.09×1012 Jones,respectively.The results prove that the 3D MAPb Br3 perovskite has relatively good optoelectronic properties,but the stability needs to be improved.3.In order to make full use of the advantages of 2D and 3D perovskite,the(PEA)2(MA)2Pb3Br10 and(PEA)2(MA)4Pb5Br16 perovskite microcrystals are prepared,and proved to be mixed-dimensional microcrystals containing 2D,single-phase quasi-2D,and 3D perovskite,respectively.The performance and stability of the photodetectors based on these two mixed-dimensional microcrystals are evaluated.(1)For the photodetector based on the(PEA)2(MA)2Pb3Br10 perovskite microcrystal,when the voltage is 15 V,the dark current is 1.9×10-10 A.When the light intensity is 2.47n W cm-2 at 15 V,the responsivity,gain,and specific detectivity are 351.78 A W-1,1.16×103,and 2.56×1014 Jones,respectively.When the photodetector is placed in an environment with a humidity of 60%for 8 days,the dark current is 1.2×10-10A.The responsivity,gain,and specific detectivity become 620.81 A W-1,1.90×103,and 5.74×1014 Jones,suggesting the device performance is not degraded.(2)For the(PEA)2(MA)4Pb5Br16 perovskite microcrystal photodetector.The dark current is 9.4×10-11 A at 15 V.When the light intensity is 210.25?W cm-2,the current increases to 1.2×10-8 A,and the on/off ratio is 126.When the voltage is 15 V and the light intensity is 2.47 n W cm-2,the responsivity,gain,and specific detectivity of the device are 182.63 A W-1,5.58×102,and 2.51×1014 Jones,respectively.Being placed in an environment with a humidity of 60%for 8 days,the on/off ratio becomes 43,and the responsivity,gain,and specific detectivity are 364.34 A W-1,1.11×103,and 4.21×1014Jones.The device performance is not degraded.Therefore,the mixed-dimensional perovskite microcrystals retain the better humidity stability of 2D and quasi-2D perovskites to a certain extent,and retain the better optoelectronic properties of quasi-2D and 3D perovskites to a certain extent.This dissertation provides a basic understanding of the exciton-phonon coupling in 2D layered hybrid perovskite,which has important guiding significance for the applications of this series of perovskites in optoelectronic devices.At the same time,it provides a new strategy for the development of efficient and stable photodetectors.There are 61 figures,6 tables,and 262 references in this dissertation. |
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