Photoelectric Properties Of CsPbBr₃ Single Crystals And Devices

In recent years,lead halide perovskites have emerged as a research hotspot due to their superior optoelectronic properties,such as adjustable bandgap,high light absorption coefficient,high carrier mobility,and long carrier diffusion length.However,the performances of organic-inorganic hybrid perovskites in practical industry application are crucially limited by the poor long-term stability resulting from water,heat,air,and radiation.On the other hand,all-inorganic perovskite,in which Cs⁺ions substitute organic groups,shows excellent photoelectric properties and superior water and thermal stability,and attracts intensive interest in the field of optoelectronic devices.Currently,perovskite devices are mainly fabricated based on polycrystalline thin films,and their efficiency depends strongly on the quality of thin films,especially on the defect content and grain boundary of thin films.In contrast,single crystals,with no grain boundaries,better surface condition,and 5～6 orders lower defect content than that of the polycrystalline film,play a key role in basic research and device optimization.CsPbBr₃,as a research hotspot of all-inorganic perovskite crystal,possesses three crystal structures,orthorhombic structure,tetragonal structure,and cubic structure.The orthorhombic CsPbBr₃ has been widely reported,covering the characterization,the photoelectric properties,and the transport mechanisms.However,studies about cubic CsPbBr₃ mainly focus on the characterization and the applications in high-energy ray detectors and light-emitting devices,while the transport properties such as hysteresis behavior and light response are rarely reported.In this thesis,high-quality CsPbBr₃ single crystals with different crystal structures were successfully grown,and their carrier transport properties were systematically investigated.Then,photodetectors with different structures were fabricated based on orthorhombic CsPbBr₃ single crystals,and their detection performance was compared and studied.The main contents of this article are described as follows:（1）Orthorhombic system and cubic system CsPbBr₃ single crystals were successfully grown through the inverse temperature crystallization（ITC）method by optimizing temperature,solution concentration,and heating rate.The main exposed surface of both crystals is（010）surface.The optical band gaps of orthorhombic and cubic CsPbBr₃ single crystals are 2.19e V and 2.21e V,respectively.The steady-state photoluminescence（PL）spectra show that the photoluminescence intensity of cubic CsPbBr₃ is three times that of orthorhombic CsPbBr₃.Transient photoluminescence（TRPL）spectra and space charge limited current（SCLC）calculations show that cubic CsPbBr₃ single crystal has lower trap charge density,larger carrier mobility（～1 order of magnitude larger than orthorhombic CsPbBr₃）and longer carrier diffusion length（～three times of that of orthorhombic CsPbBr₃）.（2）The planar structure photoelectric devices based on the cubic and orthorhombic CsPbBr₃ crystals were fabricated by evaporation of Au interdigital electrodes.The light-to-dark current ratio,detection performance,hysteresis phenomenon,and light response properties were systematically studied under various the applied bias voltage and light conditions.Due to the highly ordered arrangement of[Pb Br₆]^4-octahedron in the crystal lattice,cubic CsPbBr₃ exhibits better luminescent and transport properties,but it fails to suppress the dark current and to limit the production of photogenerated carriers.As a result,the maximum optical on-off ratio of cubic CsPbBr₃ is only 2.4 due to the light response influenced by ion migration.However,the lower trap charge density of cubic CsPbBr₃ makes the hysteresis and recombination loss weaker than that of orthorhombic CsPbBr₃.In contrast,due to the rotation of[Pb Br₆]^4-octahedron around the crystal axis,the trap charge density of orthorhombic CsPbBr₃ increases and the recombination loss increases,and the hysteresis phenomenon is more obvious.However,the orthorhombic CsPbBr₃ has lower dark current,which improves the light dark current ratio（10³～10⁵）,so it has higher responsivity（51.9μA/W）detection rate（3.98×10⁸ Jones）and optical on-off ratio（15.2）.The results show that the photoelectric detection performance of orthorhombic CsPbBr₃ is better than that of cubic CsPbBr₃.（3）The photodetectors with sandwich structure were fabricated from orthorhombic CsPbBr₃ single crystal.The photoelectric detection properties of sandwich structure and planar structure devices are compared under the same bias voltage and illumination intensity.Under the same conditions,the photocurrent generated by the sandwich structure is 5 times higher than that of the planar structure device,and the responsivity,detection rate and optical on-off ratio are increased by more than 3 times.The excellent performance of the sandwich structure device is due to the increase of the contact area between the crystal and the electrode,the increase of the charge extraction range,which enhances the carrier extraction ability,reduces the carrier recombination probability,and effectively inhibits the ion migration,so the photoelectric detection performance is significantly improved.