| Photodetectors have wide range of potential applications in civil and military fields,such as optical communications,biomedical imaging,remote sensing monitoring,night vision,and infrared guidance.Optoelectronic devices inevitably have noise.It is of great significance to identify and extract the effective information of weak signals from the noise,which requires the development of photodetectors towards high detectivity.In recent years,solution-processed organic-inorganic hybrid perovskite materials have attracted considerable attention due to their outstanding optoelectronic properties,such as high defect tolerance,superior light absorption coefficient,long carrier diffusion length,tunable bandgap,and low exciton binding energy.However,the organic components in organic-inorganic hybrid perovskites are sensitive to light,heat,and humidity,which thereby hinders their practical applications and commercialization.Compared with organic-inorganic hybrid perovskites,all-inorganic cesium lead bromide（CsPbBr3）perovskites can have an outstanding stability due to their lack of organic components,making them promising candidates for next-generation high-performance photodetectors.Taking into account the excellent photoelectric characteristics and stability,there still exists some challenges for CsPbBr3-based photodetectors to attain high detectivity,such as poor film quality,high defect density,insufficient carrier transport ability,and inferior carrier extraction.Meanwhile,CsPbBr3 perovskite has a wide bandgap and thus a limited spectral response range from ultraviolet to visible.To tackle these challenges,in this paper,effective strategies are proposed to improve the detectivity of CsPbBr3-based photodetectors,including ligand exchange,surface passivation,heterojunctions construction,etc..In-depth analyses have been employed to study the intrinsic mechanism of their impact on device performance.The main results are summarized as follows.（1）Due to the existence of excessive organic long-chain ligands on the surface of quantum dots（QDs）,QDs thin films prepared by solution process often have the problem of low carrier mobility.Herein,we adopt a ligand exchange method to substantially remove organic long-chain ligands,resulting in the enhancement of carrier transport properties in films.Then,we prepare a low-cost and high-detectivity vertical photodetector without HTL.The electron mobility of the QDs thin films after ligand exchange increases by two orders of magnitude and the charge transfer between the QDs thin film and the underlying Zn O layer becomes more effective,thus improving the photocurrent significantly.In addition,we find that the QDs thin films after ligand exchange are less sensitive to octane,and thick CsPbBr3QDs films could be prepared through multiple spin-coating and ligand exchange methods.The influence of the thickness of the QDs films on the performance and the intrinsic mechanism are explored.The maximum detectivity of the optimized device achieves 4.5′1012Jones.（2）Compared with QDs films,CsPbBr3 polycrystalline films have higher carrier mobility,which is more conducive to the transport of photogenerated carriers.Therefore,we also explore photodetectors based on CsPbBr3 polycrystalline films.Owing to the fast crystallization,a large number of defects would unavoidably be produced,which can lead to nonradiative recombination of carriers,thereby impairing the detector performance.The density of defect states at the surface/interface of the polycrystalline films are 1-2 orders of magnitude larger than that in the bulk of films,so the surface passivation is vital for CsPbBr3polycrystalline film photodetectors to improve the detectivity.A passivation strategy with congeneric CsPbBr3 QDs is proposed for the first time.Compared with the pristine device based on CsPbBr3 polycrystalline film,the dark current of the CsPbBr3 QDs-modified device is reduced by about two orders of magnitude,which greatly improves the performance of the device.The maximum detectivity can reach 7.53′1012 Jones.With detailed characterization methods,we find that the CsPbBr3 QDs can fill the holes,cracks and grain boundaries of the CsPbBr3 polycrystalline film,which is beneficial to improve the film morphology and provide intimate contact between the perovskite and Au.（3）High trap density on the surface of CsPbBr3 polycrystalline thin films can result in serious nonradiative recombination,especially for photodetectors without HTL.The charge at the interface cannot be effectively extracted,thus aggravating the nonradiative recombination at the interface.To solve the above problems,we construct a CsPbBr3/Pb S heterojunction photodetector.Pb S QDs were spin-coated on the surface of CsPbBr3 polycrystalline films as a surface passivator to passivate the surface defects.At the same time,Pb S QDs thin films treated with 1,2-ethanedithiol（EDT）exhibit weak p-type semiconductor properties,which can allow the effective hole extraction from the polycrystalline films.The results of high-resolution transmission electron microscopy and X-ray photoelectron spectroscopy reveal that Pb S QDs and polycrystalline films have a good interface and a coupling effect at the interface.Compared with the pristine devices based on CsPbBr3 polycrystalline film,the performance of the devices modified by Pb S QDs is improved.To further improve the detectivity,we propose a novel structure based on Pb S-EDT/Pb S-OA（oleic acid）hybrid QDs layers,in which the Pb S-EDT layer acts as an extraction layer for photogenerated holes,while Pb S-OA layer acts as a blocking layer for carriers.As a result,the dark current of the device is greatly reduced,and the maximum detectivity reaches 1.14′1013 Jones.The device also exhibits superior detection capabilities in the near-infrared region,with a detectivity over 1011Jones. |
