Optimal Design Of CuInS₂ Based Quantum Dots And Their Application To Photovoltaic Devices

Quantum dots(QD)have been widely used in various display fileds due to their excellent photoelectric properties.However,Cd-based quantum dots are not in good accordance with the concept of green development mainly due to the toxic elements cadmium(Cd)in their constituent.Low-toxic cadmium-free quantum dots represented by Cu In S₂(CIS)have attracted many attentions from researchers,but there are still many problems for this kind of quantum dots including poor optical performance,low efficiency,and low brightness of the Quantum Dots Light-Emitting Diodes(QLED)devices derived from these quantum dots.In this paper,CIS nuclear quantum dots were first prepared by one-pot boiling method,and then the CIS/ZnS/ZnS quantum dots with thick shell structure were obtained by coating the CIS core with ZnS shell after long time high temperature growth process.Finally,according to the electroluminescent properties of quantum dots,high-performance CIS-based QLED were fabricated,and corresponding photoelectric properties of quantum dots and QLED were studied in detail.The main research contents are as follows:Firstly,CIS nuclear quantum dots were prepared via high temperature one-pot boiling methodby using copper iodide(Cu I)as copper source,indium acetate(In(o Ac)₃)as indium source and dodecathiol(DDT)as sulfur source,respectively.After the ZnS shell continuously coating process via extended high temperature reaction time was employed to coat the thick ZnS shell by using zinc acetate dihydrate(Zn(o Ac)₂·2H₂O)and zinc stearate as the Zn source,CIS/ZnS and CIS/ZnS/ZnS quantum dots were synthesized respectively.The effects of reaction time and precursor ratios on the performance of quantum dots were investigated in detail.When the molar ratio of Cu/In was equal to 1/4,the quantum yield of the as-prepared CIS quantum dots was 13%.When the molar ratio of Cu/In was equal to 1/4,the precursor of Zn was 8 mmol,two layers of ZnS shells were coated and the reaction time of the second shell was 8 h,the quantum yield of the as-prepared quantum dots was up to 76%.The quantum dots with the half width fluorescence spectrum of 95 nm,the diameter of 4.5nm,and uniform particle size were obtained,their TRPL was265ns.Secondly,by using the as-prepared CIS core/shell/shell quantum dots as the light-emitting layer,indium tin oxide(ITO)as the anode,poly(3,4-ethylenedioxythiophene)-polystyrenesulfonic acid(PEDOT:PSS)as a hole injection layer(HIL),TFB as a hole transport layer(HTL),zinc oxide(ZnO)as an electron transport layer(ETL),and Ag as a negative electrode,after optimizing the process parameters,one simple spin-coating method was used to fabricate the quantum dots light emitting diode with a structure of ITO/PEDOT:PSS/TFB/QDs/ZnO/Ag.The longer the reaction time of the quantum dots shell was,the higher the brightness,external quantum efficiency(EQE)as well as the current efficiency of QLED was.When the CIS/ZnS/ZnS quantum dots coated with two layers of ZnS shell and having 8h reaction time of the second shell were used as the light-emitting layer,the brightness of the device 5473cd/m² and the corresponding EQE 0.83%as well as the current efficiency 2.24cd/A were achieved,respectively,the turn-on voltage of the device is 2.5V.Finally,polyethylenimine(PEI)layers with different thickness were introduced between the QDs layer and the ZnO layer by high-speed spin-coating method to prepare QLED device with the structure of ITO/PEDOT:PSS/TFB/QDs/PEI/ZnO/Ag.The introduction of PEI reduced the fluorescence quenching effect of ZnO layer on QDs layer and effectively improved the balance between holes and electrons.With a PEI concentration of 1mg/ml,the device's EQE of 1.57%was achieved,which was1.96 times higher than that of the controlled QLED without PEI layer,while the device's current efficiency of 3.21cd/A was obtained,which was 1.62 times higher than the control group.