Photodetectors Based On Perovskites Thin Film

In contemporary society,photodetectors that can convert optical signals into electrical signals play an important role in military and daily life.Compared with traditional photovoltaic materials,organic-inorganic perovskite materials（such as CH₃NH₃PbI₃）have the advantages of adjustable bandwidth,high carrier mobility,long diffusion length,strong light absorption capability,and high luminous efficiency.Because of the success of perovskite solar cells,the perovskite photodetectors such as photodiodes,and phototransistors become a research hotspot accordingly.Perovskite materials have very high external quantum efficiencies in photoelectric conversion between 300 nm and 780 nm.At the same time,the perovskite photodiode device can detect very low light intensity（incident light power density can be as low as 1 pW/cm²）,and the phototransistor has a high switching ratio（about 3×10⁵）and a very low dark current(about 1×10^-9 A).However,the stability and lifetime of perovskite film materials and their optoelectronic devices is an unsolved key issue:perovskite materials are difficult to store in the air for a long time,because when they are exposed to the atmosphere with water and oxygen,perovskite will decompose,the device eventually lost the photoelectric response.To solve problems of the perovskite thin film,this thesis carries out related research and achieved the following results:1.This thesis proposes an anhydrous solvent method to solve the problem of decomposition in the air and be unable to be stored for a long time:The thesis firstly performs reflux distillation and water removal operation on the two main solvents.Then,the two solvents are mixed and used as a solvent to dissolve CH₃NH₃I and PbI₂ to prepare a precursor solution.When a film is prepared by one-step spin coating,toluene is used as an anti-solvent,and uses a solvent vapor annealing method to reduce the surface defects of the film.Eventually perovskite thin films become more stable in the air and can be stored for a longer period of time.There is no significant change in the film after exposed to air for half a month,and after the unpackaged detector is stored in the atmosphere for half a month,the device still has a certain responsivity（about 29%of the initial value）.2.Fabrication of a heterogeneous vertical photodiode structure for good photodetection performanceIn this thesis,CH₃NH₃PbI₃ perovskite film is used as the light absorbing layer to prepare detectors of ITO/CH₃NH₃PbI₃/Ag and ITO/PEDOT:PSS/CH₃NH₃PbI₃/PCBM/Ag structure.The responsivity of the 3-layer structure detector reaches 500mA/W,the specific detectivity is5.4×10⁶ Jones,and the response time is about 1ms.In order to overcome the shortcoming of big dark current(1×10^-6 A/cm²)of the 3-layer structure device,the 5-layer structure detector is designed.The dark current density of the device can reach 1×10^-9 A/cm²,which greatly reduces the dark current of the device and the device responsivity reaches 50 mA/W,the specific detectivity was 2.7×10⁷ Jones.3.Extending the detection wavelength to the near-infrared band by selenium doping of the perovskite filmBecause of the absorption cutoff wavelength of CH₃NH₃PbI₃ is about 780 nm.In order to expand its detection range to make it realize the detection function in both visible light and infrared,Se powder is added to the perovskite precursor solution and fully stirred to react completely to expand its infrared detection capability in this thesis.After Se doping,the absorption capacity of the film in the 800¹000 nm band is obviously enhanced due to the formation of the Pb-Se impurity level.When the 850 nm infrared LED is used as the light source,the responsivity of the device can reach 600 A/W,and the specific detectivity is1.7×10⁷ Jones.4.Exploring the performance of perovskite thin film photodiodes on flexible substratesIn order to apply the perovskite detector to the flexible and wearable field,this thesis made a flexible device on the film electrode based on the CH₃NH₃PbI₃ film.After testing,when the device is not bent,the responsivity is 40 mA/W.When the radius of curvature of the device is 0.5 cm,the responsivity becomes 25%of the initial value,and when the radius of curvature is 0.3 cm,the responsivity becomes 2.5%.When the radius of curvature is 0.1 cm,the responsivity is 0,so the detection capability is lost.After 500 bends of the perovskite flexible device,the responsivity was dropped from 40 mA/W to 22 mA/W.