Investigation On The Photoelectronic Properties And High-performance Photodetectors Of Lead Methylamine Iodide Thin Films

Photodetector is a type of device that can convert optical signals into electrical signals,which are widely used in many fields such as industrial control,optical communications,medical imaging,environmental monitoring and aerospace.In recent years,the organic-inorganic hybrid perovskite,especially the most representative CH₃NH₃PbI₃,has shown excellent optical and electrical properties.It is suitable for the preparation of optoelectronic devices because the suitable and adjustable band gap of 1.55 eV,high light absorption coefficient of 10⁴?10⁵ cm^-1,long carrier lifetime and diffusion length of 10²?10³ ns,and good mechanical flexibility.A large number of researches have shown that CH₃NH₃PbI₃ photodetectors can have excellent photoelectric detection capabilities,and have unique advantages in terms of transparency and mechanical flexibility.However,we should find that the understanding of the physical properties of CH₃NH₃PbI₃ is not sufficient,and photodetectors based on CH₃NH₃PbI₃ also have some drawbacks such as unsatisfactory performance,low stability in the air environment,complex manufacturing processes and high manufacturing costs.Therefore,it is necessary to further study the optical and electrical properties of CH₃NH₃PbI₃,optimize and improve the device performance and stability,and propose new device structures.First of all,the optical constants of semiconductor materials,the forbidden band width and the Urbach band tail greatly affect the performance of optoelectronic devices.Most of the existing studies focus on the changes of these parameters with the environmental temperature after the CH₃NH₃PbI₃ film has been formed,we need to have a deeper understanding of the influence of the precursor temperature on the optical properties.Second,among the common methods to improve the performance and stability of photodetectors,the use of other materials for compounding has proven to be an effective way.On the basis of full research on the physical and chemical properties of CH₃NH₃PbI₃ thin films,we can choose appropriate materials and processes to construct heterojunctions,thereby improving device performance and stability.Third,the self-driving photodetector can be prepared based on a variety of principles.Under the premise of ensuring the performance of the device,seeking simpler process and lower cost to construct the CH₃NH₃PbI₃-based photodetector can better meet the needs of different application scenarios.The main contents and innovations of this paper are as follows:?1?The CH₃NH₃PbI₃ thin films with a thickness of about 140 nm on Si/Al₂O₃ substrates using precursors at different temperatures?30?,50?,70?,90??by the evaporation-dipping method.The influence of precursor temperature on the refractive index,absorption coefficient,forbidden band width,Urbach tail and dielectric function of CH₃NH₃PbI₃ thin films have been deeply studied through ellipsometry.The ellipsometric spectra of the prepared samples were tested in the range of0.496?4 eV photon energy,and the obtained data were fitted.The optical constants,absorption coefficients and dielectric functions of the CH₃NH₃PbI₃ films were extracted and analyzed.The study found that the temperature of the precursor greatly affects the refractive index,absorption coefficient and dielectric function of the films.The band gaps of the CH₃NH₃PbI₃ thin films obtained by the Tauc polt method are1.594 eV,1.577 eV,1.516 eV,and 1.634 eV when the precursor temperature is 30?,50?,70?and 90?,respectively.With the increase of the precursor temperature,the Urbach band tail energy changes from 107 meV to 129 meV,indicating that the defects,impurities and disorder of the CH₃NH₃PbI₃ thin films are also obviously affected by the precursor temperature.Among them,the film sample prepared by the precursor at 70°C has the most suitable band gap for the preparation of solar cells.The results of this study show that adjusting the precursor temperature can effectively adjust the optical constants,band gap and Urbach band tail energy of the perovskite material,which helps to understand the influence of the preparation process of the perovskite film on its intrinsic physical properties.It provides theoretical reference for constructing optoelectronic devices based on perovskite thin film.?2?The CH₃NH₃PbI₃/IDT-BT composite film was prepared by the sol-gel method,and the material was characterized.Then a high-performance photoconductive CH₃NH₃PbI₃/IDT-BT/Mo O₃ photodetector was constructed and its electrical properties were studied.It was confirmed that the polymer IDT-BT effectively filled and repaired the grain boundaries of CH₃NH₃PbI₃ and covered the perovskite layer by scanning electron microscope,X-ray diffraction and X-ray photoelectron spectroscopy.The CH₃NH₃PbI₃/IDT-BT composite film maintains a high light absorption capacity,comparing to pure CH₃NH₃PbI₃ films,the absorption coefficient is enhanced in the wavelength range of 590 nm?710 nm.The prepared CH₃NH₃PbI₃/IDT-BT/Mo O₃photoconductive photodetector has excellent performance,its response time is less than 7 ms,the on-off ratio is greater than 3×10⁴,the response rate reaches 11.6 A W^-1,and the detection degree reaches 5.2×10¹³ Jones.This is mainly caused by three reasons:Firstly,the filling of the CH₃NH₃PbI₃ grain boundary by IDT-BT can reduce the recombination center of photogenerated electron-hole pairs.Secondly,IDT-BT and Mo O₃ can enhance the separation and transmission efficiency of photogenerated electron-hole pairs cause the energy bands of IDT-BT and Mo O₃ match the CH₃NH₃PbI₃ film.In addition,because the IDT-BT layer can isolate CH₃NH₃PbI₃from oxygen and water,the stable service life of the photodetectors exceeds 20 days.?3?Self-driving photodetector based on CH₃NH₃PbI₃ thin film was constructed with asymmetric Au electrodes,and self-driving CH₃NH₃PbI₃/VO₂photodetector was prepared.The electrical characteristics of photodetectors were studied,and the cause of self-driving effect is analyzed and discussed.Due to the thermal effect in the thermal evaporation process,during the preparation of the top electrode on the right,some Au particles will penetrate into the perovskite film.But for the Au electrode at the bottom on the left,since only the CH₃NH₃PbI₃ film is spin-coated on top of it,no diffusion effect will be produced,which makes the contact between the two electrodes and the film present different states.The infiltration of Au will change the Schottky barrier for the contacts of upper electrode because the screening effects of image charges on metal surfaces.Moreover,part of the light can irradiate through the film to the interface between the CH₃NH₃PbI₃ and the lower electrode,which can decrease the Schottky barrier.Thus,the Schottky barrier heights of the two electrodes are not consistent,thus forming a built-in electric field.Electrical tests show that our self-driving CH₃NH₃PbI₃photodetector based on asymmetric Au electrodes has good performance.Without an applied bias,the photodetectors exhibit an ultralow dark current of?3 p A.Under the635 nm laser of 2.92 m W cm^-2,the photodetectors present high performance with the responsivity of 10 m A W^-1,detectivity of 2.88×10¹¹ cm Hz^1/2W^-1,external quantum efficiency of 2.1%,and On/Off ratio of 10⁴.The variation of electrical properties with temperature of CH₃NH₃PbI₃/VO₂ photodetector is studied in the temperature range from 100 K to 360 K.The results show that the photocurrent of CH₃NH₃PbI₃/VO₂photodetector increases with the increase of temperature.The dark current of device is opposite to the photocurrent at low temperature and shows a tendency of first increasing and then decreasing.When the temperature is higher than 306.3 K,the dark current of the device changes into the same direction with the photocurrent and increases rapidly with the increasing temperature.At the power intensity of 100?w cm^-2 under 650 nm illumination,the photocurrent is about 32 n A?room temperature?,and the responsivity is 448 m A W^-1,which is higher than reported self-driving photodetector based on CH₃NH₃PbI₃ film.The results of electrical tests and surface morphology characterization suggested that the self-driving effect is due to the inhomogeneity of perovskite films caused by spin coating.