The Surface Passivation And Interface Modification For CsPbBr₃ Perovskite Quantum Dots Based Light-emitting Diode

Metal-halide Perovskite Quantum Dots(PQDs or PNCs)are typical semiconductor nanocrystals which exhibit tunable bandgap,excellent photoluminance efficiency and facile synthesis process.Thus,they have been widely used in the fields of photodetectors,photovoltaics,light-emitting diodes(LED)and field-effect transistor.Although PNCs have excellent optoelectronic properties,several issues remain to be addressed.The photoluminance quantum yield(PLQY)of PNCs is seriously limited by the surface trap-states which are originated from the dangling bonds and vacancy atoms.They act as the non-radiative recombination centers and reduce the fluorescence quantum yield seriously.Moreover,the large specific surface area and under-coordinated sites make them vulnerable to water/oxygen/photo/heat,which may change the original size,component and crystal structure of PNCs.In device level,the poor carrier transport of PNCs solids film caused by long-chain organic ligands and carrier recombination loss at interface between different functional layers severely influence the optoelectronic performance of the PNCs LED(PNCs-LED).As a result,the low optoelectronic efficiency and short lifetime become the stumbling blocks for further application of PNCs-LED.As an atomic scale controllable surface treatment technique,atomic layer deposition(ALD)meets the above requirements of PNCs-LED fabrications.ALD is a gas phase deposition method relying on a sequence of self-limiting surface reaction steps to deposit ultrathin,uniform and conformal films.In this thesis,aimed at the problems which the efficiency and stability of PNCs LED can not be achieved at the same time,the ALD is applied for surface defects passivation,dense thin film encapsulation,interstitial infilling and interface functional layers fabrication of PNCs-LED from nanoscel to pixel cell due to the characteristics of self-limiting reactions dependent on energy,dense uniform thin film fabrication,gas-phase infiltration and atomic level thickness control.The main contents and results of the research are listed as follows:(1)In the nanoscale aspect of PNCs,a similar colloidal layer-by-layer deposition of Al O_x on Cs Pb Br₃ PNCs dispersed in solution has been performed with controllable thickness at the nanoscale.By surface modification and dense coating,the PNCs/Al O_x composite keeps60%photoluminescence(PL)intensity of initial value even when immersed into water for120 min.The Al O_x coated Cs Pb Br₃ PNCs is used as green emitting source for a prototype of white light emitting diode and it is found the mixing among different colors of PNCs upon irradiation is well suppressed due to the blocking of ion exchange by Al O_x shell.When it is used in green electroluminescence(EL)light-emitting diode,the maximum luminescence of PNCs/Al O_x LED increases from 22863 cd/m² to 32747 cd/m² and the maximum EQE is improved from 2.20%to 6.05%compared with the pristine PNCs LED.Moreover,the EQE T₅₀lifetime at an initial value of 1.3%is prolonged about 40 times.Additionally,the luminance T₅₀ lifetime of the fabricated device is prolonged by more than 4 times.Compared with the traditional core shell structure-based PNCs LED,the brightness,efficiency and life of the device are the best.The improved efficiency is attributed to the carriers'balance by the optimized interface band alignment.(2)In terms of the interface of the luminous pixel unit,by applying ALD Al₂O₃ treatment on Cs Pb Br₃ PNCs emission layer,the inorganic electron transport layer-based PNC-Zn Mg O LED device with ALD atomic level interface engineering is obtained for the first time.The introduction of precursor water ensures enough protective Al₂O₃ production and also preserve the optical properties of PNC thin film from the TMA destruction.Comparing to the PNC-TPBi LED device without ALD,the EQE value reaches up to 1.50%which is twice that of PNC-TPBi LED device.Moreover,the operation lifetime is prolonged about two orders magnitude of the PNC-TPBi with a T₅₀ of 955s under 23-26°C/40-50%relative humidity(RH).It is found that the introduction of Al₂O₃ ALD layers significantly improve the tolerance of Cs Pb Br₃ PNCs thin film to polar solvents ethanol of Zn Mg O with more than 60%of PNCs content retained after spin-coating.Moreover,the infilling of Al₂O₃ into the PNCs layer boosts the in-plane carrier transport with carrier mobility enhancement by more than40 times.On the other hand,the introduction of ALD Al₂O₃ sandwiched structure hinders the interfacial carrier transport between different functional layers in device due to the insulated property.The insulated Al₂O₃ provide an effective barrier layer for electrons and facilitate the carrier balance of the device,which is confirmed by the analysis of device modeling.With the ALD interface engineering,both effective carrier balance in the emission layer and long lifetime duration of the device are achieved.Our works throw a light on the mechanism of ALD treatment on the stability and carrier transport properties of PNCs-LED device and provide a facile method to fabricate highly efficient QLED device with long-term lifetime.(3)In the PNCs-LED device structure level,the growth process and interaction mechanism of ALD Zn O on the surface of PNCs thin film are studied.It is demonstrated that ALD Zn O ETL alone is not suitable for PNCs-LED with high performance and high stability.At the same time,it is found that the growth of ALD Zn O at the defect sites of PNCs thin film reduces the fluctuation and the surface roughness of the PNCs thin film.By adjusting the PNCs concentration and the thickness of ALD Zn O,the carrier balance of PNCs-LED is achieved,and the external quantum efficiency is increased by 210%.Based on the control of the unstable PNCs/ETL interface and the introduction of inorganic ETL Zn O,the stability of the device is improved,and the half-life of the device at 100 nit and 1000 nit is extended by285%and 1452%respectively.the ultra-thin Zn O interface layer is introduced by ALD to passivate some defect sites on the surface of PNCs,and finally PNCs-LED with high performance(EQE_max=7.21%)and highly stability(T_50@100nit=31 days)which is one of the highest values repoteed so far is obtained.In summary,the main significance of this paper is as follows.This work presents a facile collide alternative layer-by-layer coating of the PNCs with controllable thickness in nanoscale,and firstly demonstrates its advantages in PNCs based-LED with excellent efficiency,brightness and long lifetime.The optimal performance of the same type of core-shell PNCs-LED is obtained.Besides,our works throw a light on the mechanism of ALD treatment on the stability and carrier transport properties of PNCs-LED device and provide a facile method to fabricate highly efficient QLED device with long-term lifetime.A new hybrid structure of PNCs-LED based on Zn Mg O fabricated by solution process is realized for the first time.Moreover,this work confirmed that the PNCs/ETL interface control and the selection of ETL could improve the luminous efficiency and lifetime of PNCs-LED.As a result,the PNCs-LED with the longest equivalent working life has been achieved.