Double Interface Passivated Two-dimensional Perovskite Photodetectors With Fast Response

In recent years,two-dimensional perovskites have been widely used in optoelectronic devices due to their better stabilities compared with traditional threedimensional perovskites and excellent optoelectronic properties.High efficiency has been achieved for the solar cells based on two-dimensional perovskites.In recent years,great progress has been made in two-dimensional perovskite-based photodetectors.Single crystal two-dimensional perovskite-based photodetectors have exhibited very high performance.These devices are based on the photoconductive structure,which has high external quantum efficiency,high light current and high responsivity and other advantages,but also has a large dark current,long response time and other shortcomings.Besides,the single crystal perovskite has a lower defect density and no boundary which result in the improved performances compared the polycrystalline perovskite,but the preparation of single crystal perovskite is complex.Compared with single crystal perovskite,polycrystalline perovskite thin films are easier to prepare.However,polycrystalline perovskite thin films have more defects,which will cause non-radiative recombination and seriously affect the photoelectric performance of the devices.Compared with photoconductive photodetector,photovoltaic photodetector has the advantages of fast response speed,lower dark current,larger linear dynamic range and higher light-dark current ratio.In this work,we adopted the photovoltaic structure.The active layer of two-dimensional perovskite in the device is polycrystalline film.Double passivation was carried out at the top and bottom of the perovskite layer in order to reduce the carrier recombination at the interface and prevent the entry of water molecules.Thus,the photoelectric performance and stability of the photodetector were improved.There are two main parts in this work:(1)The alumina passivation at the bottom of perovskite layer.Specifically,a thin alumina layer was inserted between hole transport layer and perovskite layer by atomic layer deposition which can accurately control the thickness of alumina.By optimization the thickness of alumina,when the thickness of alumina is 1.4nm,device performs best.At-1V bias,the maximum light-dark current ratio reached 10^7.The maximum external quantum efficiency,responsivity,detectivity reached 91%,0.38A/W,9.5×10^12 Jones,respectively.The linear dynamic range reached 160d B,and the response speed was less than 600ns.(2)The polymethyl methacrylate(PMMA)passivation on the top of perovskite layer.PMMA passivation has a wide range of applications in perovskite solar cells.On one hand it can passivate perovskite and decrease the recombination of carries at the interface.On the other hand,PMMA is a hydrophobic material,which can block water molecules into the perovskite and improve the stability of the device.Here,we adopt the method of spin-coating to prepare a thin PMMA layer between perovskite and electron transport layer.We found that there was no significant changes of the light and dark currents after introduction of PMMA.We also compared the stability of the device with or without PMMA,we found that the stability of the detector was improved after the introduction of PMMA.By comparing the change of bright and dark current,it is found that the decay rate is further slowed down,so the introduction of PMMA can improve the stability of the detector without reducing the photoelectric performance of the detector.To sum up,the bottom passivation can passivate the perovskite and reduce the recombination of carries at the interface to improve the photoelectric performance of the detector.The introduction of hydrophobic material at the top of perovskite can block the entry of water molecules and greatly improve the stability of the detector.