Synthesis Of Efficient And Stable Red CsPbI₃ Nanocrystals And Their LED Application

All-inorganic perovskite（CsPbX₃,X=Cl,Br,I）nanocrystals（NCs）have the potential to promote the development of display and lighting industry due to their high photoluminescence quantum yield（PLQY）,high color purity and good solution processability.Currently,the external quantum efficiencies（EQEs）of green,red and near-infrared perovskite light-emitting diodes（LED）are more than 20%,which meets commercial requirements.Whereas,the poor stability of CsPbI₃ limits its commercial promotion.The poor stability of CsPbI₃ mainly originates from crystal phase and environmental issues:1)The tolerance factor（τ）of CsPbI₃ is small,and CsPbI₃ is easy to transform from cubic phase（excellent optoelectronic properties）to orthorhombic phase（poor optoelectronic properties）at room temperature.2)Oleic acid（OA）and olamine（OLA）with long chain on CsPbI₃ surface have weak affinity to NCs surface.OA and OLA easily fall off to form surface defects,which ultimately accelerate CsPbI₃ degradation of photoluminescence（PL）performance and stability.Focusing on the above problems,we mainly carried out the following research works:（1）Mn²⁺doping to improve the stability of CsPbI₃ NCs.Doping of Mn²⁺with small size intoα-CsPbI₃ induces lattice contraction,increases theτofα-CsPbI₃ and improvesα-CsPbI₃ crystal phase stability.A blue shift of PL,a higher angle shift of X-ray diffraction（XRD）peak and the decrease of lattice distances demonstrated Mn²⁺doping.We measured the stability of the Mn²⁺doped CsPbI₃ NCs under extreme conditions such as UV,polar solvent and heating.1)Mn²⁺doped CsPbI₃ NCs could preserve 70%of the initial PL intensity（35%for CsPbI₃ NCs）under UV for 22 min.2)Mn²⁺doped CsPbI₃ NCs could maintain 58%of the initial PL intensity（10%for CsPbI₃ NCs）after adding ethanol.3)Mn²⁺doped CsPbI₃ NCs solid film could maintain the cubic phase at 120℃at least for 7 h（1 h for CsPbI₃ NCs）.Finally,we used Mn²⁺doped CsPbI₃ NCs to prepare LED,which achieved a brightness of 470 cd/m² and a peak EQE of 5.8%.（2）Ca²⁺doping to improve the stability of CsPbI₃ NCs and LED performance.Doping of alkaline earth metal ions(such as Sr²⁺)could effectively improve the stability and optoelectronic properties ofα-CsPbI₃NCs.We explored Ca²⁺（smaller size）doping method forα-CsPbI₃NCs,which could shrink the lattice,increase theτofα-CsPbI₃NCs and enhance the crystal phase stability.A blue shift of PL,a higher angle shift of X-ray diffraction（XRD）peak and the decrease of lattice distances demonstrated Ca²⁺doping.The PLQY of Ca²⁺doped CsPbI₃ NCs was above 90%.The stability of bothα-CsPbI₃ NCs solution and film were significantly enhanced.1)The PL intensity of the Ca²⁺doped CsPbI₃ NCs solution remained 83%of the initial PL intensity（36%for CsPbI₃ NCs）after 147days storage in ambient condition.2)The Ca²⁺doped CsPbI₃ NCs films could exhibit cubic phase after 58 days storage in ambient condition（14 days for CsPbI₃ NCs）.In addition,the hole-only devices data indicated significantly reduction of defect density and enhancement of carrier mobility for Ca²⁺doped CsPbI₃ NCs films.Ultraviolet photoelectron spectroscopy（UPS）results showed that Ca²⁺doping could lower the Fermi level of CsPbI₃ NCs films,which revealed the transformation from n-type to a more nearly ambipolar nature for CsPbI₃ NCs,and improved the hole injection efficiency.Finally,we used Ca²⁺doped CsPbI₃ NCs to prepare LED achieving a maximum brightness of 790 cd/m²（2.5 times enhancement）,and an EQE of 7.8%（3 times enhancement）.The half-lifetime for Ca²⁺-doped CsPbI₃ NCs LED was 85 min,which had 2.2 times enhancement.（3）Multi-dentate ligand tetramethyltulam disulfide（TMTD）boosting CsPbI₃ NCs LED performance.A short chain multi-dentate ligand（TMTD）was introduced to achieve ligand exchange via washing processes.TMTD could improve CsPbI₃ NCs optoelectronic properties,stability and LED performance.By optimizing TMTD amount,the PLQYs of CsPbI₃ NCs solution were increased from 42%to 70%after two-step washing process.The PL intensity of CsPbI₃ NCs solution maintained 70-90%of initial PL intensity after 26 days storge（42%for CsPbI₃ NCs）.In addition,Fourier transform infrared spectroscopy（FTIR）and X-ray photoelectron spectroscopy（XPS）results suggested that TMTD could combine with uncoordinated Pb²⁺to passivate surface defects.As the result,the LED achieved a maximum brightness of 954 cd/m² and a maximum EQE of 16.6%.