Preparation And Photoelectric Application Of Cesium Lead Halide Perovskite Quantum Dots

Metal halide perovskite quantum dots(Pe QDs)are a new type of photoelectric nano materials due to their high photoluminescence quantum yield(PLQY),narrow emission bandwidth,high color purity,low-cost solution processing and high carrier mobility.Therefore,Pe QDs have a good development prospect in application of light emitting diodes(LED)and solar cells.Discovered in 2014 and during only 8 years,the efficiencies of LED and solar cells based on Pe QDs have been close to commercial standards.However,the structure of Pe QDs are easily transformed to non-perovskite phase under humidity,ray-radiation and heat.Therefore,how to improve the stability of Pe QDs is still a research focus.At present,although many studies mainly focus on deep-red single halogen Pe QDs,the pure red Cs Pb Br_xI_3-x Pe QDs with the best band gap,which produced by adjusted mixed halogen,are exploare to meet the requirement of the rec.2020 display application standard.However,the high efficiency and long-term stability of LED and solar cells based on Cs Pb Br_xI_3-x Pe QDs are still a change due to instability of Pe QDs and the defect of device structure.Consequently,we aim at improving the luminous performance and stability of Cs Pb Br_xI_3-x Pe QDs,and simultaneously increasing the efficiency and stability of LED and solar cells based on Cs Pb Br_xI_3-x Pe QDs.The details results are as follow:(1)Firstly,pure red mixed halogen Cs Pb Br_xI_3-x Pe QDs,which is close to Rec.2020standard,were synthesized by thermal injection method.Secondly,Cs Pb Br_xI_3-x perovskite quantum dot light-emitting diodes(Pe LEDs)showed the device structure of ITO/PEDOT:PSS/poly-TPD/Pe QDs/TPBi/Cs F/Al.According to the physical model of the device,the injection state and recombination state of electrons and holes are simulated.The thickness of PEDOT:PSS film adjusted by water dilution method can realize the injection balance between electrons and holes and improve the light extraction efficiency,leading to an improvement in the luminous efficiency of Pe LED.Accordingly,when the thickness of PEDOT:PSS film is close to be 40 nm,the Cs Pb Br_xI_3-x Pe LED showed a highest EQE of 3.44%and the maximum brightness of 2676 cd/cm~2.(2)In³⁺-doped Cs Pb Br_xI_3-x Pe QDs,which were synthesized by thermal injection method,exhibited a near-unity photoluminescence quantum yield(?100%)and superior stability for more than half a year in atmosphere,resulting from the partial substitution of Pb²⁺by the smaller radii of In³⁺ions.Since In³⁺-doped can effectively increase theformation energy of Cs Pb Br_xI_3-x Pe QDs based on first-principles calculations,In³⁺-doped Cs Pb Br_xI_3-x Pe QD solution exhibits superior stability for more than half a year in atmosphere,and simultaneously In³⁺-doped Cs Pb Br_xI_3-x Pe QD film can still maintain bright red light for 11 days.The pure red Pe LED based on In³⁺-doped Cs Pb Br_xI_3-x Pe QDs display CIE of(0.70,0.29),a high luminance of 423 cd m^-2 and EQE of 11.2%?In³⁺-doped Cs Pb Br_xI_3-x Pe LEDs exhibit a more stability compared to original Cs Pb Br_xI_3-x Pe LEDs.(3)The conductivity and water-stability of Cs Pb Br_xI_3-x(x?1)Pe QDs could be improved by a small sulfhydryl molecule 2-naphthalene mercaptan(2NT)with naphthalene ring.After soaking into water for 45 min,Cs Pb Br_xI_3-x Pe QDs passivated by 2NT ligand still maintain the luminesce properties and phase structure.Simultaneously,Cs Pb Br_xI_3-x Pe QDs passivated by2NT ligand exhibit PLQY of 85%and average PL lifetime of 134.26 ns,respectively,which is attributed to the decreased surface defects.Pe LEDs based on 2NT-Cs Pb Br_xI_3-x Pe QDs as light-emitting layer not only can maintain the original birght light under UV radiation for 10h but also show maximum brightness of 736 cd m^-2 and EQE of 13.07%,respectively.(4)Through the modified spin-coating process,the 2NT-Cs Pb Br_xI_3-x Pe QDs with excellent conductivity and stability can improve the stability of Cs Pb Br_xI_3-x(x?1)perovskite film.As for Cs Pb Br_xI_3-x perovskite solar cells 2NT-Cs Pb Br_xI_3-x Pe QDs used as crystal seed not only can improve the crystallization properties of Cs Pb Br_xI_3-x films and reduce the grain boundary and surface defects of perovskite film,but also can improve the stability by suppressing the phase separation of the Cs Pb Br_xI_3-x films in solar cell.Interestingly,the instruction of 2NT-Cs Pb Br_xI_3-x Pe QDs not only increases the PL intensity and carrier lifetime of Cs Pb Br_xI_3-x perovskite films but also keep the similar conductivity of the film,leading to the enhancement of open circuit voltage and short circuit current density for solar cells.The PCE of Cs Pb Br_xI_3-x solar cell modified by 2NT-Cs Pb Br_xI_3-x Pe QDs is significantly increased from 7.57%to 12.45%.