Study On Quantum-dot-assisted Fabrication Of Perovskite Films And Their Electroluminescent Devices

Metal halide perovskite has attracted wide attention due to its excellent optical properties,such as high color purity,turnable luminescence wavelength and high quantum yield,which has been widely used in solar cells,photodetector and lightemitting diodes.The light-emitting diodes(Pe LEDs)based on perovskite film as an emitting layer have opened a new door to the industry and been considered to be the most potential next-generation display technology.In 2014,Friend et al.fabricated the first Pe LED at room temperature with external quantum efficiency of 0.1%.Up to now,the external quantum efficiency of Pe LEDs have got rapid development.The external quantum efficiency of green and red Pe LEDs have exceeded 20%,meeting the requirements of commercial application.And the external quantum efficiency of blue Pe LEDs have reached 17.9%,to move towards greater performance.Although Pe LEDs have made great progress in current efficiency,their short operational stability and fabrication reproducibility remain major challenges on the path to commercialization.Therefore,improving the stability and repeatability of Pe LEDs is an urgent issue.In this paper,we propose a method for fabricating high-quality perovskite films to achieve excellent operational stability and fabrication reproducibility of Pe LEDs.In addition,we explore the mechanism of sub-bandgap electroluminescence(EL)and carrier dynamics of Pe LEDs.The main contents of this article are as follows:1.We propose a method that Cd Se/Zn S quantum dots(Cd-QDs)were introduced into anti-solvent toluene to fabricate FAPb Br3 perovskite films by on-chip reprecipitation process.Due to lower solubility of Cd-QDs toluene solution in N,Ndimethylformamide(DMF,the solvent of perovskite precursor),in the process of perovskite film formation,the mixing of Cd-QD toluene solution and perovskite solution firstly induces the self-assembly of Cd-QDs into a monolayer.Then the aspreformed Cd-QD monolayer can act as a template to initialize the crystallization.By X-ray diffraction,scanning electron microscopy,UV-vis absorption and photoluminescence spectra(PL)measurements of perovskite films,it is found that the FAPb Br3 film with Cd-QDs has smooth and dense surface morphology.The crystallinity of the perovskite film is significantly improved.The grain size is larger and the particle size distribution is more uniform.Besides,the quantum yield of perovskite film with Cd-QDs is increased from 65%±26% to 89%±7%.The reproducibility gets improved by Cd-QDs-induced crystallization processes during insitu fabrication of perovskite film.2.The high current efficiency and stability of Pe LED with Cd-QDs assisted method is reached.The device structure is ITO/poly(3,4-ethylenedioxythiophene:poly(styrenesulfonate)(PEDOT:PSS)/perovskite film/4,6-Bis(3,5-di(pyridin-3-yl)phenyl)-2-methylpyrimidine(B3Py MPM)/Li F/Al.Compared with the Pe LED with toluene as anti-solvent,the maximum luminance and current efficiency of Pe LED with Cd-QDs increases from 6807 cd/m2 and 10.6 cd/A to 86670cd/m2 and 31.0 cd/A,respectively.Astoundingly,the operational stability is highly improved with T50 lifetime exceeding 83 minutes under initial luminance of 1021 cd/m2.Combining electrical field simulations and transient EL measurements,the enhanced stability effects of Cd-QD monolayer can be attributed to the improving quality of perovskite film and the electrical field redistribution in Pe LEDs.The introduction of Cd-QDs layer can reduce the electric field and inhibit ion migration for perovskite layer,ultimately improving the device stability.3.The working mechanism of sub-bandgap EL in Pe LED assisted by Cd-QDs was investigated.By changing the electron and hole transport layer materials in Pe LED,it is clearly demonstrated that turn-on voltage of Pe LEDs with Cd-QDs layer depends neither on the carrier injection nor on the energy levels of charge-transport layers.The recombination of diffused and thermally generated charge carriers is believed to be responsible for the sub-bandgap EL behavior.In addition,we fabricated an electronlimited device with the structure of ITO/PEDOT:PSS/perovskite film/Mo O3/Al to measure EL characteristics by photoexciting with different wavelengths.It is proved that the Cd-QDs layer at interface between perovskite film and electron transport layer has a good limiting effect on charge carriers of perovskite layer,which can effectively reduce the electron injection barrier through Coulombic attraction interaction to promote electron injection.Thus,the insertion of Cd-QDs layer between perovskite and electron transport layers is a necessary method to exhibit a sub-bandgap EL behavior.4.The influence of carrier dynamics on Pe LEDs assisted by Cd-QDs for different electron transport layer was analyzed.By changing the material of organic electron transport layer with different electron mobility,the performance difference of Pe LEDs with 2,4,6-tris(3′-(pyridin-3-yl)biphenyl-3-yl)-1,3,5-triazine(Tm PPPy Tz)and B3 Py MPM electron transport layer(ETL)is discussed.It is found that Tm PPPy Tz material with high electron mobility is used as electron transport layer,making more balanced charge injection and high current efficiency in device.Using transient current system,it is proved that ion migration occurs in Pe LEDs driven by electric field,and migrating ions can reduce the injection barrier of perovskite layer to promote hole injection.Combined with ion migration processes,the carrier dynamics of Pe LEDs with different electron transport layers are obtained by measuring transient electroluminescence spectrum.In addition,by the transient current of Hole-only and Electron-only devices,we consider that the migrating ion of perovskite film is halogen anion.