Application Of Tin Dioxide As Electron Transportation Layer In Quantum Dot Light-Emitting Diodes

Contemporary display technology puts forward higher requirements for lightemitting diodes（LEDs）,which need to have the characteristics of high brightness,high emission efficiency,high color purity,wide color gamut,adjustable wavelength,solution processing,low-processing cost,and high stability.Quantum dot light-emitting diodes（QD-LEDs）and perovskite light-emitting diodes（PeLEDs）are light-emitting layer devices that meet the above requirements.At present,the external quantum efficiency（EQE）of CdSe-based QD-LEDs has exceeded 20%,and the performance of PeLEDs has also made great progress.At the same time,most high-efficiency LEDs use ZnO nanoparticle films as the electron transport layer（ETL）,but the chemical stability of ZnO is poor,easily leading to a degradation of LED device performance after a long-term operation.In contrast,the SnO₂,as a highly stable N-type semiconductor material with a similar bandgap to ZnO,is expected to replace ZnO and solve the problem of the poor chemical stability of ZnO.However,the conduction band position of bulk SnO₂ is lower than that of ZnO,and its energy band structure is not matched with the high-efficiency light-emitting layer.Although the energy band positions of commercial water-soluble SnO₂ nanoparticles may match the light-emitting layer,the aqueous solution has poor wettability on the surface of ITO conductive glass,making it difficult to prepare high-quality SnO₂ thin films.Given the scientific problems faced by SnO₂ in QD-LEDs and PeLEDs,we have synthesized SnO₂ nanocrystals that have the quantum size effect by the solvothermal method,where the position of its conduction band can be successfully increased to match the energy band structure with the light-emitting layer.Besides,we also presented the strategy of using organic ligands to modify the commercial SnO₂ nanoparticle aqueous solution,which can be dispersed in the organic phase and solve the problem of film formation.The details are as follows:1.The solvothermal method is proposed to prepare SnO₂ nanocrystals with a quantum size effect,which can be used as ETL for QD-LED.In this study,the SnO₂ nanoparticles with an average particle size of 3.7 nm were prepared by a solvothermal method,and the size of SnO₂ nanoparticles is smaller than that of the Bohr exciton diameter;thus the bandgap of SnO₂ nanoparticles is expanded from the bulk 3.5 eV to 4.1 eV.Also,the energy level of the conduction band is moved up from-4.50 eV to-3.84 eV,which is higher than the conduction band of ZnO nanoparticles,which is favorable for electron injection into the quantum dot light-emitting layer.The QD-LED devices based on SnO₂ nanoparticle films have the maximum current efficiency,maximum brightness,and maximum power efficiency of 11.8 cd·A^-1,12846 cd·m2,and 6.23 lm·W-1,respectively.The stability of the device is better than that of QD-LEDs based on ZnO as ETL.2.The quantum-sized SnO₂ was also proposed to use as an ETL for the PeLED,in which the perovskite thin-film light-emitting layer of PeLED is prepared by one-step spin-coating based on soluble precursors.In this study,the solid components such as Cs₂CO₃,PbO and CH3NH3X（X=Cl,Br,I）were dissolved in an-butylamine/n-butyric acid ionic liquid to prepare a CsPbX3 precursor solution.The as-prepared solution can be spin-coated on the surface of the conductive glass with ITO,and then annealed to form a dense CsPbX3 nanocrystalline film.Butyric acid acts as a capping ligand to modify the surface of CsPbX3 nanocrystals,which can reduce surface defects.The high-quality,multi-color tunable CsPbX3 nanocrystal thin films have PLQYs of 15.6%,11.7%,and 8.3%for the blue,green,and red light,respectively,enabling continuous tunability of wavelengths in the visible light range.QD-LED devices are assembled using blue perovskite material films and SnO₂ films,and the maximum brightness can reach 12.6 cd·m^-2.3.The organic ligand-modified commercial water-soluble SnO₂ nanoparticles were proposed,which successfully solved the difficulty of preparing high-quality thin films from commercial SnO₂ nanoparticle aqueous solutions,and applied them to QD-LEDs.In this study,the SnO₂ nanoparticle aqueous solution was treated with acetic acid to reduce the-OH groups on the SnO₂ surface and suppress the effect of-OH on device performance.At the same time,the ethanol solution of SnO₂ nanoparticles was successfully prepared by coating the nanoparticles with ethanolamine,which improved the wettability of the solution on the surface of the ITO conductive glass,and obtained a high-quality SnO₂ film.The film has better electron transport ability,higher carrier mobility,and a weaker quenching effect on quantum dots.The maximum brightness,maximum current efficiency,maximum power efficiency,and EQE of the inverted cadmium-based QD-LED composed of ETL are 14555 cd·m^-2,19.83 cd·A^-1,12.46 lm·W-1 and 15.69%,respectively.4.A dual-ligand synergistic modification of commercial water-soluble SnO₂ nanoparticles was proposed,and n-butyric acid/an-butylamine（BA）,isooctanoic acid/isooctylamine（IA）,and oleic acid/oleylamine（OA）were successfully designed.Ligand-coated SnO₂ nanoparticles.Through this study,the coordination of functional groups on the surface of SnO₂ can not only reduce the defects on the surface of SnO₂ nanoparticles but also optimize its interface with the light-emitting layer.Three kinds of double-ligand-coated SnO₂ thin films as ETLs were successfully applied to allinorganic cadmium-based QD-LEDs with an upright structure.13916 cd·m^-2,7.0 cd·A1,2.7 lm·W-1.In this dissertation,the SnO₂ nanocrystals prepared by the solvothermal method and commercial water-soluble SnO₂ nanoparticles modified by organic ligands were systematically studied.High-quality SnO₂ thin films were prepared and successfully applied as ETL in QD-LED and PeLED.This series of studies not only solves the problem that bulk SnO₂ is difficult to use as an ETL for QD-LEDs and PeLEDs,but also solves the difficult film-forming problem of commercial water-soluble SnO₂ nanoparticles.This dissertation provides a new solution for the application of SnO₂ nanoparticles in QD-LEDs and PeLEDs,and provides a new idea for the preparation of high-stability QD-LEDs and PeLEDs.