Construction And Performance Modulation Of Bi-based Perovskite Photodetectors For Imaging Applications

Lead-based perovskite photoelectric materials（APb X₃,A=CH₃NH₃⁺,Cs⁺,X=halogen ion）have the advantages of high light absorption coefficient,long carrier diffusion length,and adjustable bandgap with wide range,which have been widely used in photoelectric conversion devices.However,the toxicity of the heavy metal lead will cause harm to the environment and humans,and how to maintain the stability of the material has become an obstacle to restrict the application of lead-based perovskite materials.Bismuth（Bi）has a similar atomic radius and outer electron arrangement to lead,and is environmentally friendly,non-toxic and stable.It can replace lead to form a Bi-based perovskite photoelectric materials with highly stable structure,which is expected to be further expanded for the development and application of non-toxic photoelectric materials.At present,the photoelectric conversion efficiency of Bi-based perovskite devices is much lower than that of Pb-based counterparts.The main limitations are as follows:First,the discontinuous perovskite film with pinholes is usually obtained,resulting in many defects in perovskites with.Second,the intrinsic properties restrict the carrier transport,such as strong exciton binding energy,low carrier separation efficiency.It is of great urgency to further improve the photoelectric performance of Bi-based perovskite devices.In this paper,a low-temperature controllable preparation method of high-quality Bi-based perovskite film is developed.By assistance of atomic layer deposition technology,the surface and interface modification of Bi-based perovskite films can be precisely controlled.Further contributing to the improved photodetection performance of photodetectors.The main research achievements are as follows:1.The FTO/Al₂O₃/Cs₃Bi₂I₉/Au photodetector is constructed to reveal the novel tunneling effect-assisted mechanism for detection performance enhancement,and to demonstrate the application of diffuse mode imaging.Firstly,ultra-thin Al₂O₃films are prepared on FTO substrate by atomic layer deposition（ALD）technology,the effects of Al₂O₃ layer on subsequent perovskite film growth and Al₂O₃ thickness on detection performance are investigated in details.The main findings are as follows:After Al₂O₃ modified FTO,the Bi-O bond is more favorable for the following growth of perovskite films.The optimal thickness of Al₂O₃ is 1.5 nm,contributing to the highest ON/OFF ratio,and the performance is improved by～19.3 times when compared with the bare device without Al₂O₃ layer.On the one hand,the introduction of Al₂O₃ layer improves the growth quality of Cs₃Bi₂I₉ film.On the other hand,ultra-thin Al₂O₃ allows some photogenerated holes tunneling to the FTO electrode.With the assistance of device configuration of energy band alignment,photogenerated holes and electrons can be collected by FTO and Au,respectively,and the dark current of the device is effectively suppressed.In addition,the optical and electronic bandgap of Cs₃Bi₂I₉ film is determined to be 2.19 e V and 2.55 e V respectively,which provides an indication for the research of strong exciton binding light detection materials.At the same time,we found that the spectral response range of the Cs₃Bi₂I₉ photodetector is significantly narrower than the cutoff edge of its absorption spectrum,namely,the response cutoff edge shows a blue shift,which is due to the strong exciton absorption of Cs₃Bi₂I₉ that binds photo-generated carriers to the[Bi₂I₉]^3-dimer.Finally,we demonstrate the application of Cs₃Bi₂I₉-based photodetectors in diffuse mode imaging.After optimization,the signal-to-noise ratio（SNR）of the device increases by 30.7 d B,showing higher imaging quality.2.A vapor-phase anion-exchange method is proposed to construct Cs₃Bi₂Cl_xBr_9-xphotodetectors with gradient energy band alignment,and the transmission mode UV imaging application is demonstrated.Firstly,based on previous finding,1.5 nm-Al₂O₃ layer was pre-deposited on FTO to assist the subsequent growth of high-quality Cs₃Bi₂Br₉film.Given the halogen can be precisely exchanged in perovskite materials,its bandgap can be easily modulated.By using gaseous precursors of chlorine source（Ti Cl₄）and oxidation source（H₂O）,the halogen reaction amount can be accurately controlled by high-speed pulse valve by the gradual substitution of Br with Cl.Additionally,the crystal structure and bandgap of Bi-based perovskite films can be gradually modulated.Along with the process of anion exchange,part of the precursor Ti Cl₄ and H₂O will undergo an oxidation reaction,forming a dense Ti O₂ layer on the surface of Cs₃Bi₂Cl_xBr_9-x film.By precisely controlling the reaction temperature and the cycles,we finally determine that when the number of cycles reaches 258,the heterojunction device shows the best UV detection performance.The ON/OFF ratio and linear dynamic range of the device are increased by 20 times and 2.6 times than those of the unoptimized devices,respectively.Finally,given the peak response of the photodetector is in the ultraviolet 350 nm（UVA region）,the transmission mode UV imaging of the two types of devices is compared.The signal-to-noise ratio（SNR）of the optimized device is improved by 21.9 d B.