| CuInSe2 quantum dots(QDs),as one kind of narrow bandgap P-type semiconductor nanocrystals,have shown great potential in the field of photoelectric detection because of their unique advantages,such as tunable bandgap,high extinction coefficient,broad response spectrum(300-1100 nm wavelength)and environmental friendliness.However,long-chain organic ligands attached to the surface of the QDs,dramatically affect the charge transfer within QDs films.Besides,the high-density defect states also lead to serious non-radiative recombination.Therefore,the performance of CuInSe2 QDs based photodetector is low.Therefore,this article focuses on these challenges of CuInSe2 QDs,and summarizes strategies to optimize them,including ligand exchange method,manganese-ions doping method and double active layer structure design for QDs-based photodetectors.With these strategies,the conductivity of QDs films can be improved,the defect state density of QDs can be reduced,and the responsivity in the visible region can be enhanced as well.Therefore,it is expected to realize the high-performance and broadband response(ultraviolet to the near-infrared region)CuInSe2 QDs-based photodetectors.Noting that the resistive property of CuInSe2 QDs is unexpectedly discovered during the research process,a new type of CuInSe2 QDs memristor is fabricated,and its internal working mechanism is clarified.The overall research results are stated as follows.CuInSe2 QDs with a broadband photoresponse(300-1100 nm)was synthesized by the hot-injection method.Considering that the long-chain oleylamine(OAm)ligands at the surface of CuInSe2 QDs limit the charge transferring,we introduced the short-chain conductive mercaptopropionic acid(MPA)ligands,converting the OAm-CuInSe2 QDs to MPA-CuInSe2 QDs through the ex-situ ligand exchanging.Subsequently,the CuInSe2 QDs-based near-infrared photodetectors with a vertical photovoltaic structure were fabricated,which involved OAm-CuInSe2 QDs or MPA-CuInSe2 QDs as light-absorbing materials.The research results show that the carrier transfer ability and photocurrent response ability of the optimized QDs-based photodetectors significantly improved,and their detectivity(D*)at 1000 nm reaches 7.5×1010 Jones,which is larger than it of OAm-CuInSe2 QDs-based photodetector(3.6×109 Jones).The response speed of the MPA-CuInSe2 QDs based photodetector can be reduced from 0.15 s to 0.04 s.To solve the problem of poor crystalline and high defect density of CuInSe2 QDs,we introduced the transition metal manganese ions(Mn2+)into CuInSe2 QDs,and investigated the properties of Mn-CuInSe2 QDs.The results show that when the content of Mn2+ions increase,the lifetime of carriers increases as well.It is explained that introducing Mn2+reduces electron capture probability and guides excitons to migrate to long-lived Cux states,thereby increasing the carrier lifetime.Besides,doping Mn2+into CuInSe2 QDs can effectively tune the energy band position of the QDs and enhance the driving force of the carrier's migration to the transport layer.Compared to MPA-CuInSe2 QDs-based photodetectors,these Mn-CuInSe2 QDs-based photodetectors exhibit high responsivity(R)of 30 mA/W(a 15-fold increase)and D*of 4.2×1012 Jones(almost two orders of magnitude increase)in the NIR,respectively.Based on these successful optimizations of CuInSe2 QDs,we proposed a brand-new idea for photodetector structure design,which is assembling the QDs and the perovskite film together to form a double active layer for photodetectors.This structure is highlighted because of its dual advantages:the broadband response range of CuInSe2 QDs and the high light response of perovskite film in the visible region.The novel photodetector with CuInSe2 QDs and CH3NH3PbI3(MAPbI3)perovskite film(as the double active layer)was fabricated and we found that CuInSe2 QDs could promote the nucleation of the perovskite films,which thus assist the formation of high-quality perovskite films on them.This could be explained that the CuInSe2 QDs serve as the nucleation center during the crystal growth stage of the perovskite films,limiting the growth of the film and minimizing the grain size.For those photodetectors with double active layers,CuInSe2 QDs have been proved to benefit the active layer to effectively collect electrons because of their excellent light absorption properties.Besides,they provide the light response in the near-infrared region,while the MAPbI3 perovskite film provides the light response in the visible light region.Additionally,the CuInSe2 QDs layer acts as an electron blocking layer to effectively block electrons from entering hole transport layer(HTL),which reduces unnecessary recombination.These optimized photodetectors give excellent R which reaches more than 180 mA/W in the visible region,and 20 mA/W in the near-infrared region.Moreover,the D*of the device reached above 7.0×1012 Jones in the visible region and 7.7×1011 Jones in the near-infrared region,respectively.It is worth noting that we unexpectedly discovered the hysteresis of the current-voltage curve of CuInSe2 QDs,which results from its inherent memristive properties.Based on this point,we prepared a new-type CuInSe2 QDs-based memristor with the structure of FTO/CuInSe2 QDs/Au electrode,and deeply studied the generation mechanism of memristive properties.The results show that the memristor exhibits good reproducibility and stability during continuous current-voltage cycle scanning,and maintains a stable SET voltage(1 V)and RESET voltage(-0.8 V)for 70 consecutive cycles.The ratio of the high resistance to low resistance is 5.7.Meanwhile,the read-write memory function can be maintained for up to 104 s.The memorizing behavior of CuInSe2 QDs-based memristor is regarded as the result of the redox reaction of copper ions and their migration forced by the external electric field.In addition,we also fabricated a flexible CuInSe2 QDs-based memristor using a solution-processed method.This memristor can stably maintain SET and RESET transitions within 100 continuous scan cycles and show steady memory ability after 600 times fatigue bending test. |
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