Electroluminescent CsPbI₃ Perovskite Quantum Dot Light-Emitting Diodes

Perovskite quantum dots?QDs?are one of the most promising fluorescent materials in the field of display and lighting due to their excellent optoelectronic properties,such as high photoluminescence quantum yield?PL QY?,wide emissive wavelength covering the whole visible light range,narrow emission width,high carrier mobility,simple synthesis method.Compared with the organic and inorganic hybrid perovskite QDs,the all inorganic perovskite QDs?CsPbX₃,X=Cl,Br,I?have better thermal stability,making them suitable for the fabrication of highly efficient and stable electroluminescent light-emitting diodes?LEDs?.At the beginning of this study,there were few reports about the all inorganic perovskite quantum dot-based LEDs?QD-LEDs?,and their electroluminescence performances were poor.In addition,most studies focused on green emitting perovskite QD-LEDs.The balanced three primary colors?red,green and blue?luminescences are very important for the display application,so it was urgent to strengthen the research on red and blue emitting perovskite LEDs.Based on that background,this study is mainly on red emitting CsPbI₃ perovskite QD-LEDs.It is known that the surface defects and unstable lattice structure of CsPbI₃ QDs seriously influence the device performances.Moreover,the unbalanced carrier transfer in devices also affects the device performances.In view of those problems,this study aims to improve the optical properties and stability of CsPbI₃ QDs as well as the device performance of LEDs.Corresponding resear was carried out from two aspects of device structure design and material optimization,and excellent outcomes were achieved.?1?Top-emitting LEDs based on bottom Ag electrode and top semi-transparent MoO₃/Au/MoO₃ electrode were designed.The work function of Ag cathode is lower than that of the traditional ITO cathode,which can reduce the electron injection barrier from cathode to ZnO electron transport layer?ETL?and improve the electron injection efficiency.Besides,Ag electrode can be used as a diffusion source of Ag⁺ions.Some Ag⁺ions can diffuse into CsPbI₃ lattice to replace Cs⁺ions,thus stabilizing the lattice structure of perovskite.Other Ag⁺ions react with I^-ions on the surface of CsPbI₃ QDs to form AgI,then the AgI is easily decomposed into Ag⁰ under light conditions to passivate the surface defects of CsPbI₃ QDs.Finally,this device structure not only improved the optical properties and stability of perovskite QD films,but also increased the external quantum efficiency?EQE?of the corresponding LEDs from 7.3%to 11.2%.?2?Thioacetamide was used as a sulfur source to synthesize PbS coated CsPbI₃QDs,and the optimized QDs were used as emitters to fabricate LEDs.PbS coating can effectively passivate the surface defects of perovskite QDs to improve the PL QY and reduce the stokes shift and emissive line width of CsPbI₃ QDs.Moreover,PbS coating can also stabilize the surface of perovskite and suppress ion migration,thus improving the stability of perovskite.At the same time,CsPbI₃ QDs change from n-type to nearly ambipolar,which allows us to make electroluminescence LEDs using p-i-n structures.Thus-fabricated LEDs have good stability and maximum EQE of11.8%.These results suggest that shell coating is an effective method to improve the optoelectronic properties of perovskite QDs and device performances of corresponding LEDs.?3?SrCl₂ was used as a co-precursor to synthesize simultaneous Sr²⁺doped and Cl^-surface passivated CsPbI₃ QDs,and using this QDs as emitters to fabricate highly efficient and stable perovskite LEDs.Because the ion radius of Sr²⁺is slightly smaller than that of Pb²⁺ions,Sr²⁺cations can partially replace the Pb²⁺ions in the lattice structure of CsPbI₃ QDs and cause a slight lattice contraction,leading to a more stable lattice structure.The depth-resolved XPS of Cl element shows that the Cl^-ions are mainly located on the surfaces of CsPbI₃ QDs.These Cl^-ions can bond with exccesive Pb²⁺ions on the surface of CsPbI₃ to effectively passivate the surface defects of perovskite QDs,thus increasing the PL QY of CsPbI₃ QDs from 65%to 84%.The simultaneous Sr²⁺doped and Cl^-surface passivated CsPbI₃ QD-LEDs show a high EQE up to 13.5%.?4?NH₄SCN was added during the synthesis of CsPbI₃ QDs to passivate the surface defects caused by unbonded Pb²⁺ions on the surface of perovskite QDs,and the LED based on NH₄SCN passivated CsPbI₃ QDs were fabricated.This effective defect passivation prolonged lifetime and enhanced PL QY of CsPbI₃ QDs.Meanwhile,the overall energy level structure of NH₄SCN passivated CsPbI₃ QD films goes down compared with the pristine CsPbI₃ QD films,which contributes to a better electron injection.As a result,the NH₄SCN passivated CsPbI₃ QD-LEDs have higher luminance and EQE than pristine CsPbI₃ QD-LEDs.?5?Flexible CsPbI₃ QD-LEDs were prepared by using NOA 63 photopolymer with high solvent corrosion resistance as the flexible substrate,and a super-smooth Ag film as electrode obtained by template stripping method.The flexible LEDs maintained their good performance after 1000 times repeated stretch-release bending tests with a bending angle of 180 degrees.No bad points were seen,and luminance droped only 30%,which demonstrated excellent flexibility.This study lays a good research foundation for perovskite QDs in the flexible display.In conclusion,we have improved the optoelectronic properties of CsPbI₃perovskite QD materials and the device performances of corresponding LEDs from two aspects of device structure design and material optimization,and finally broadened the applicable range of perovskite LEDs.