Preparation And Study Of Quasi-two-dimensional Tin Perovskite Light-emitting Diodes

Because of metal halides perovskite the advantages of continuous tunable wavelength,high quantum yield,high defect tolerance and solution processing,it has shown great application prospects in the fields of the third generation thin film solar cells,new luminescence and display,and ultra-high sensitivity space-time resolution radiation detectors.Currently,the external quantum efficiency（EQE）of perovskite light-emitting diodes（PeLEDs）has exceeded 20%in the near-infrared,red,and green light regions.However,the commercial application of PeLEDs still faces many challenges.One of the key scientific challenges is the biocompatibility of heavy metal lead（Pb）,and its use in solid-state lighting and display is severely limited.Therefore,the development of lead-free perovskite materials as luminescent layer has become one of the important scientific issues in industry and academia.Because tin（Sn）and Pb belong to the same main group,they have similar electronic structure and radius,and the degradation product of Sn²⁺is considered to be biosafe.Therefore,Sn-based perovskite is considered as one of the most ideal materials to replace Pb-based perovskite.However,Sn-based perovskite also faces some urgent scientific problems.Firstly,Sn²⁺in tin-based perovskite is unstable,and it is easy to be oxidized into Sn⁴⁺,which results in harmful p-type self-doping and deteriorates the performance of optoelectronic devices.Secondly,the tin-based perovskite crystallization rate is faster,poor film coverage,easy to produce leakage current in the device,reduce the efficiency of the device.Third,the non-radiative recombination loss caused by defects at the interface of sn-based perovskite light-emitting diode（Sn-PeLEDs）and the imbalance of electron and hole injection also restrict the efficiency and stability of devices.Based on this,the quasi-two-dimensional tin-based TEA₂SnI₄ perovskite was studied to improve the stability of Sn²⁺and prepare highly efficient and stable Sn-PeLEDs.Sn²⁺in tin-based perovskite was stabilized by tautomer coordination-induced electron localization function,which effectively solved the problem of poor stability of Sn²⁺and greatly improved the efficiency and stability of Sn²⁺.The main research contents are as follows:（1）Considering the nature of Sn²⁺oxidation in tin-based perovskite is due to the weak binding capacity of Sn nucleus to the outermost 5s² orbital electron,as a result,the electrons in the 5s² orbitals are easily lost and oxidized to Sn⁴⁺,which deteriorates the device performance.By introducing cyanuric acid（CA）into TEA₂SnI₄ lead-free tin-based perovskite,it was found that CA can spontaneously form quasi-vertical,highly stable H-bond tautomerization dimer and trimer superstructures on the perovskite surface.The superstructures can induce electron localization function around tin and improve the delocalization of lone pair electrons in the 5s² orbital of Sn.The Electron localization function induced by tautomer coordination gives the perovskite films a more stable and ordered crystal structure and weakens the adverse effects of Andson localization,the carrier lifetime and exciton binding energy of tin-based perovskite films are increased by 2 times,and the nonradiative recombination coefficient is reduced by two orders of magnitude.Based on this strategy,highly efficient and stable Sn-PeLEDs have been successfully prepared.The external quantum efficiency（EQE）of Sn-PeLEDs is 20.29%,which is 2.4 times of the highest EQE of Sn-PeLEDs reported so far.In addition,the strategy has been successfully applied to other quasi-2d and 3D tin-based perovskites,which proves the universality of the method.（2）The crystallization rate of tin-based perovskite was effectively controlled by adding urea and ammonium thiocyanate.The TEA₂SnI₄/TEA₂SnI_4-xSCN_x（TEA=2-thiopheneethyllamine）composite structure was formed and the stability of tin-based perovskite was greatly improved.The experimental results show that SCN^-can replace some iodine ions（I^-）and form a protective layer on the surface of tin-based perovskite thiocyanate.The addition of urea further improved the film-forming quality,and urea formed hydrogen bonds with ammonium ion（NH₄⁺）,resulting in more free SCN^-ions which can replace I^-,lead-free tin-based perovskite with TEA₂SnI_4-xSCN_x structure was successfully formed.Sn-PeLEDs device based on TEA₂SnI₄/TEA₂SnI_4-xSCN_xcomposite structure,the external quantum efficiency（EQE）reaches 20%and has a operating life of over 31.3 hours.（3）By introducing a double interface layer at the upper and lower interface of tin-based perovskite,the non-radiative recombination loss at the interface of the device is effectively reduced.A thin layer of formamidine acetate（CH₃COOFA）was grown on PEDOT:PSS,and the C=O functional group in the acetate as Lewis base could form a strong chemical bond with tin,which improved the stability of Sn²⁺in tin-based perovskite,the crystallization rate of the photosensitive films was effectively controlled and the oxidation of Sn²⁺was inhibited.On this basis,the defects on the surface of tin-based perovskite were further passivated by graphite phase carbon nitride（g-C₃N₄）to restrain the halogen vacancy defects caused by the annealing process.The nitrogen atom of sp² of g-C₃N₄ can effectively form strong interaction with the perovskite layer,further improving the crystallization and stability of perovskite.Based on this method,highly efficient Sn-PeLEDs were successfully fabricated,which greatly improved the balance between electron and hole injection and the roll-off characteristics of the device.