Preparation Of PbS Colloidal Quantum Dot Films Employing Solution Phase Ligand Exchange And Their Photoelectric Properties

Lead sulfide（PbS）,a narrow bandgap（0.4 eV）semiconductor material,has been widely used since it was found to be a good infrared optoelectronic devices at the beginning of last century.PbS colloidal quantum dots is a low cost,large sacle synthesis,quantum effect significant,physicochemical properties adjustable and solution-processable nanomaterial.The absorption of colloidal quantum dots could be continuously adjusted by size change ranging from 800-3000 nm.Through surface modification with different ligands,colloidal quantum dots was with potential application value in the fields as photodetectors,solar cells,light-emitting diodes,upconversion luminescence and biological imaging.In this thesis,fabrication method of PbS CQD films was optimized via solution phase ligand exchange.The mechanism of PbS CQD film optical and electrical properties influenced by surface ligands was studied.Combining with structure design and optimization,high sensitivity PbS CQD thin film photodetector was fabricated.Comparing with solid state ligand exchange,solution phase ligand exchange was a high ligand exchange efficiency and relatively stable process with ease film depositon method.First,CQD film was fabricated via solution phase ligand exchange and one-step spin coating method.X-ray energy spectroscopy（XPS）and other characterization taken place for analysis of devices to study the influence of halogen salts.Fourier infrared spectroscopy（FTIR）shows that oleic acid（OA）on surface of CQDs was almost replaced by halogen ligands.Temperature spectroscopy results show that PbS CQDs has two trap levels and corresponds to surface group from XPS.The performance is related to the deep trap,and response speed of the device is related to the shallow trap.PbI₂capped PbS CQDs shows the highest dark current and photo response.Under 950 nm LED,the response reaches 6.47 mA/W and the corresponding normalized detection rate is 2.08×10¹⁰Jones under 1 V bias.Organic ligands capped CQDs has longer coupling distances,higher tunneling barriers and lower dark current comparing with halogen ligands.Here,lead halogen salts and3-mercaptopropionic acid（MPA）were used for hybrid solution phase ligand exchange.The composition of CQD surface ligands was controlled by MPA concentration.CQD thin film photoconductive devices were fabricated.FTIR shows C-H vibration peak come back proving MPA ligand passivation,and XPS show that MPA introduces a new IO₂^-group on CQD surface.Dark current,response and recovery speed increased with the MPA concentration rising.Photo response of 0.2%MPA hybrid ligand exchange device was doubled to 12.75 mA/W.Temperature spectroscopy shows MPA increase trap depth leading to response rising.At the same time,IO₂^-group is introduced a new shallow trap onto CQD surface fasting response speed.Dark current of zninc halogen capped CQD photoconductive devices is low.Based on the research in previous chapter,zinc halogen ligands and MPA hybrid solution phase ligand exchange taken place to introdcue defects actively for performance improving.XPS results show that group are mainly connected to zinc cations.Trap depth was found reduced by MPA concentration rising via temperature spectroscopy.Performance of devices increases at first then decreases with increasing of MPA concentration.Photo response of0.1%MPA-ZnI₂ capped CQD device is 1.07 mA/W with D*of 3.44×10¹⁰Jones.Surface ligand engineering of CQD taken advantage to the performance improving,but could not dissociate the relationship between dark and photo current.Carrier transport layer is introduced to improve the performance of the device by changing the device structure.Comparing the band structure of devices,space charge layer taken place between different layers.It reduce the dark current via blocking carrier transportation.Built-in field under light speed photogenerated carriers to corresponding direction and improving photo response.Photovoltaic photodetector shows a 6 times response of 40.9 mA/W and a magnitude D*of 3.69×10¹¹Jones.