Research On Near Infrared Light-emitting Diodes Besed On PbSe/PbS Core/shell Quantum Dots

Near-infrared light-emitting diodes have a wide range of applications in medical,military,and communications fields.Colloidal quantum dots(QDs)provide unique opportunities as NIR emitters due to their advantages,such as high photoluminescence efficiency(PLQE)in solution and size-tunable emission combined with convenient,low-cost,solution-based integration on any substrate using utilizing effective mass production methods including inkjet printing.Pb Se QD based NIR LEDs are the research object of this dissertation.The research content mainly focuses on the stability of Pb Se QDs,carrier transport in QD films,and defect passivation for QDs to optimize the key parameters of NIR LEDs performance.The specific research content and related results are as follows:Regarding the instability of Pb Se QDs Pb Se QDs are covered with the Pb S shell thickness greater than 1nm,which is used as a protective layer to try to delay the oxidation process of Pb Se QDs in ambient environment.In the process of shell growth for Pb Se QDs,the molar ratio between Pb and S adjusted by the two respective precursors injection of double pump,reaction time and temperature are together controlled to achieve the Pb Se/Pb S core/shell QDs with desired shell thickness.Among them,the double emission peak induced by Pb S nucleation can be resolved by increasing the Pb/S molar ratio and concentration of Pb Se QDs.To test whether Pb S shell can effectively improve the stability of Pb Se QDs,Pb Se QDs and Pb Se/Pb S core/shell QDs in toluene solution were measured in the air to obtain changes of absorption spectrum in three days,and it turn out that the surface Pb S shell of Pb Se/Pb S core/shell QDs cannot effectively prevent the blue shift of the spectrum under more than one day of air exposure.In addition,compared to Pb Se core QDs,the fluorescence lifetime of Pb Se/Pb S core/shell QDs is longer,and photoluminescence quantum efficiency is lower.As for the carrier transport efficiency in QDs film,the long carbon chains of oleic acid on the surface of as-synthesized Pb Se/Pb S core/shell QDs were replaced with substituted 8-hexyloctanoic acid ligands by means of solid-state ligand exchange.The maximum radiance obtained by the device at the light emission wavelength of 1280 nm is 0.65Wsr-1m-2,and the average and maximum of external quantum efficiency(EQE)is 0.6±0.26% and 1%,respectively.In terms of the trap states of QDs,the Cd Cl2 solution was used to passivate the surface defects of QDs film after the ligand exchange of short carbon chains(8-hexyloctanoic acid,MOA)was completed.Since the surface coverage of the short carbon chains on QDs is inefficient and the chemical stability of QDs is deteriorated,further atomic ligand exchange was used to fill the surface traps of QDs.QDs film with MOA/Cd Cl2 ligands were used to prepare LEDs with the same device structure.At the same emission wavelength,the maximum radiance of the device was increased to 0.95Wsr-1m-2,and the average and maximum of EQE is 0.73±0.52% and 1.3%,respectively.In addition,with the pure treatment of 8-hexyloctanoic acid ligands,the turn-on voltage of QD based NIR LEDs where MOA/Cd Cl2 ligands treated QDs film based NIR LED' turn-on voltage varied from 5V to 4V,and its breakdown voltage has been increased from 8V to 9V.