Investigation On Performance Of Electroluminescence White Light Emitting-diodes Based On Cu-In-Zn-S Semiconductor Nanocrystals

White light-emitting diode（WLED）is considered as one of the most potential candidates in the next-generation solid-state light source due to their adjustable color temperature,energy-saving,environmental-benign,large-area and flexible preparation.To date,much attention has been paid to the WLEDs based on organic emissive layer and colloidal semiconductor nanocrystals（NCs）.However,the former needs vacuum evaporation to prepare devices,which is not conducive to the preparation of large area and flexible substrate.In contrast,the latter solves the problem of solution preparation,but they are limited further development in industy by the intrinsic toxicity of heavy metal in most of NCs,which is not beneficial to the environmental protection.To solve these problems,in this thesis,multinary copper chalcogenide compound semiconductor NCs were selected to blend with different types of blue-emitting materials,which was used as emissive layer for fabrication of WLEDs with high color rendering index（CRI）.The multinary copper chalcogenide compounds exhibit the advantages including low-toxicity,broad emission spectrum and simple preparation process,which show popential prospects in the applications in fabrication of high CRI and eco-friendly WLEDs.In this thesis,we focused on the performance of the WLEDs based on Cu-In-Zn-S（CIZS）/ZnS NCs blending with different types of blue-emitting materials.An all-solution-processed technique was adopted to fabricate WLED devices with high CRI,highly-stable electroluminescent（EL）spectra as well as low efficiency roll-off.The detailed contents and research results are summarized as follows:1.An all-solution-processed technique was employed to fabricate double-layered WLED based on yellow CIZS/ZnS NCs and organic layer TFB with hole-transporting characteristics,in which the CIZS/ZnS NCs were insered between two TFB layers to form a sandwiched structure.This structure makes the two TFB layer keep clear of each other and avoids the exciton recombination in the hole-transporting TFB layer,which prohibits the phenomenon of carrier tunneling quenching.Through optimizing the thickness of the second TFB layer,the WLED device exhibited a high CRI value of 91and Commission internationale de l’Eclairage（CIE）of（0.33,0.33）.Moreover,the maximum brightness of the device could reach up to 680 cd/cm² and the turn-on voltage is only 2.2 V.More importantly,the EL spectra of the device were stable under different appling voltages,and the CIE coordinates were almost unchanged.This work showed the WLEDs with highly-stable EL spectra under different applying voltages,which provided a new idea of fabrication of CIZS/ZnS-based WLEDs.2.In order to obtain high-performance WLED,an all-solution-processed WLED was fabricated based on Cd-free CIZS/ZnS NCs blending with polyfluorene derivative PODPF,which exhibited a stable white light emission with a CIE of（0.31,0.34）and a CRI value of 85.Moreover,the device exhibited the maximum brightness of 1495 cd/m²and peak EQE of 0.14%.To further improve the device performance,PC₉O₄ with two oxygen-containing side chains was used to replace PODPF,which presented a better solubility and smoother film-forming properties.Through futher optimizing the carrier transport layer,the WLEDs based on the blends of PC₉O₄ and CIZS/ZnS NCs were fabricated by using poly-TPD as the hole-transporting layer and Zn O:Mg nanoparticles as the electron-transporting layer,respectively.As a result,the as-obtained WLED displayed a maximum brightness of 2154 cd/m² and a peak EQE of 1.03%.3.In order to resolve the larger efficiency roll off of the device due to the aggregation of the organic compounds and NCs blends,the aggregation-induced emission（AIE）molecules tetraphenylethylene-4Cl（TPE-4Cl）were selected to blend with CIZS/ZnS NCs,which were used as EML to fabricate WLEDs.The optimized devices exhibited a CIE coordinate of（0.30,0.33）and the CRI value of 87,and the maximum brightness could reach up to 265 cd/m².Since TPE-4Cl could reduce the aggregation quenching phenomenon,and the EL spectra of the device maintained stable under different operating voltage,and the efficiency roll-off was reduced greatly.To further enhance the performance of the WLEDs,the double TPE-based BTPE materials with higher efficiency were introduced to replace TPE-4Cl,and the device with the same device structure was fabricated through the all-solution-processed technique.By tuning the mass ratios of the emissive layer,a variety of white light devices with controllable color temperature were obtained,in which the maximum brightness of the warm white light device could reach up to 3502 cd/m²,and the maximum brightness of the standard white light device was 1100 cd/m².Moreover,the standard white light device exhibited a very low efficiency roll-off.This work could solve the efficiency roll-off due to the aggregation of CIZS/ZnS NCs through the introduction of AIE molecules.Moreover,the all-solution-processed WLEDs were fabricated based on the blends of AIE molecules and Cd-free CIZS/ZnS NCs for the first time,and this work offers a new idea for fabrication of WLEDs based on CIZS/ZnS NCs.4.In order to explore the fabrication of WLEDs based on all-inorganic emissive layers,the WLEDs were fabricated by using the blends of blue-emitting CGZS/ZnS and yellow-emitting CIZS/ZnS NCs as an emissive layer.When the mass ratio of CGZS/ZnS to CIZS/ZnS was 25:1,the optimized devices exhibited a CIE of（0.30,0.32）and a high CRI value of 93.The maximum brightness could reach up to 1602 cd/m² and the maximum EQE was 1.15%.It is worth mentioning that the EQE of the device could maintain 1.09%at 500 cd/m².This work presented the WLEDs based on all-inorganic multinary Cu-based NCs as emissive layer exhibited a high luminance efficiency at high current-density and high brightness,which indicated the advantage of the application of inorganic NCs in the WLEDs.This work layed the foundation for the optimization of the WLEDs based on all inorganic NCs as emissive layers,which showed a very low efficiency roll-off.This proved synthesis system effectively solved the problem of surface defects of CGZS/ZnS NCs.