Fabrication And Performance Of The PbS Colloidal Quantum Dot Photodetectors Based On Halide Anion Surface Passivation

Colloidal quantum dots?CQDs?have driven dramatic interest in high-performance,low-cost semiconductor functional devices capitalizing on their unique properties such as size tenability,surface effects and quantum confinement effects,as well as good solution processability.The lead sulfide?PbS?process a band gap of 0.4 eV and its exciton Bohr radius is large up to 18 nm,enabling strong quantum confinement.PbS CQDs photodetector offers flexibly tunable response wavelength from 600 nm to 3000 nm.Its film preparation can be compatible with various substrates such as silicon,glass and ceramics,allowing for great application prospects.However,owing to the high defect density,surface passivation is essential to improve the photoelectric properties.It has been proved that surface passivation is one of an effective strategy that making use of halide anion ligand to enhance electronic transport and successfully passivate surface defects in PbS CQD films.In this work,focusing on the influence rules of halide anions ligand on performance of PbS CQDs photodetector,we contrive an in-depth research on device fabrication and performance optimization.As detailed,we system compares and optimizes the types of halogen ions and their introduction methods,which has effectively improved the performance of PbS colloidal quantum dot photodetectors.Firstly,we synthesized PbS quantum dots using PbO as a lead precursor in traditional syntheses routes,then,introduced iodide ions by ligand exchange,lastly,fabricated the film photodetector devices by coating onto a cleaned glass substrate at room temperature.Combined Fourier Transform Infrared Spectroscopy?FTIR?characterization with photoelectric performance analysis,we compared the effect of halide anions introduced in the process of solid-state ligand exchange?SSE?and phase-transfer exchange?PTE?on the optoelectronic properties of photodetector.As the result showed,the devices based on the PTE method exhibited better optoelectronic properties.As detailed,they have higher photocurrent and faster recovery time,and the device responsivity and detectivity reach27.119 mA/W and 2.81×10¹¹Jones under 940nm illumination with a power density of 0.0257?W/cm².In order to explore the impacts of the halogen ion species and their introduction types,we attempted to introduce halogen to passivate surface defects in site during the synthesis process using the PbCl₂,PbBr₂ and PbI₂ as lead precursors.The morphology and size of PbS quantum dots could be controlled by changing the reaction temperature.Subsequently,PbS quantum dot photodetectors were fabricated by PTE method.The comparison of photoelectric performances based on four lead precursors showed that under the same conditions,the device employing PbCl₂ as lead precursor has better performance,and the device responsivity and detectivity reach 69.6852 mA/W and 2.22×10¹² Jones,respectively.Further,we analyzed the mechanism of halogen ion species on the performances.Chloride ion has stronger binding ability to lead ion,which can form chloride terminated effectively,improving the oxidation resistance.Finally,we investigated the influences of the metal cation introducing on the photoelectric performance.Using PbCl₂ as a lead precursor,PbS was synthesized via cation exchange reaction where the Zn²⁺cation is exchanged for the Pb²⁺cation to further improve PbS quality and surface passivarion.The possible residual Zn²⁺cation in site passivate the surface defects together with the Cl^-anions.The device responsivity employing cation exchange synthesis reach 83.245 mA/W,which is higher than that synthesized above by PbCl₂ as lead precursor while the detectivity is slightly lower,corresponding to be 1.07×10¹²Jones.