Enhanced Performance Of Perovskite Light-Emitting Devices Using Thiocyanate Additives

Metal halide perovskite has the advantages of high absorption coefficient,high fluorescence quantum yield and low solution processable cost,etc.It is widely used and studied in the optoelectronic fields such as perovskite solar cells and perovskite light-emitting devices.At present,the research of perovskite materials in red and green perovskite light-emitting devices has been developed in the long term,and their maximum external quantum efficiency（EQE）has exceeded 25%,however,as an indispensable part of full-color display,the development of blue perovskite light-emitting diodes is relatively lagging behind a lot,and its poor performance and stability seriously hinder the commercial application and development of perovskite materials in the fields of lighting and display.In perovskite luminescent materials,by introducing halogen anions with different energy band characteristics or mixing different proportions of halogen anions,the energy band structure of perovskite materials can be changed and their band gaps can be adjusted,so that the luminescence spectrum（color）of perovskite can be continuously adjusted,and red,green and blue perovskite luminescent devices with high purity can be prepared.In addition,the long-chain amine cation can be embedded in the octahedral inorganic layer to adjust the dimensional structure of the perovskite material during the preparation of perovskite films by adding an appropriate amount of organic long-chain amine cation in the perovskite precursor solution,and through the quantum confinement effect,the band gap of quasi-2D perovskite can be gradually expanded as the n value decreases,thus achieving the blue shift of the perovskite emission spectrum.The preparation of mixed halogen quasi-2D perovskite devices by mixing halogen anions and introducing large organic amine cations is currently the most effective and widely used way to achieve blue luminescence from perovskite.However,the serious phase separation phenomenon of mixed halogen perovskite under the action of electric field will reduce the efficiency and color stability of the luminescent devices;the introduction of insulating large organic amine cations in perovskite will also hinder the charge transfer and affect the performance of the luminescent devices.Therefore,based on the blue luminescence achieved by using mixed halogen quasi-2D perovskite,how can the optoelectronic performance of blue perovskite devices be enhanced?In this regard,additive engineering has been proposed and widely applied to improve the performance of perovskite light-emitting devices.In this thesis,the chlorine/bromine ratio is adjusted by mixing lead bromide（PbBr₂）,cesium bromide（CsBr）and lead chloride（PbCl₂）,and the long-chain amine cation phenylethylammonium bromide（PEABr）is further introduced to adjust the dimension of perovskite.A quasi-2D layered perovskite light-emitting diode with mixed halogen is prepared by one-step solution spin-coating method.On this basis,we further added different proportions of thiocyanate additives to adjust the phase distribution of perovskite,passivate the defects of perovskite films,and improve the EQE of perovskite light-emitting devices.The specific research work is as follows:（1）Different perovskite light-emitting devices were prepared by introducing different ratios of ammonium thiocyanate（NH₄SCN）into the perovskite precursor solution,and the effects of NH₄SCN on the morphology,crystal structure,photophysics,charge transport and electroluminescence properties of the mixed-halogen quasi-2D perovskite films were investigated.The results show that the use of NH₄SCN additives can effectively passivate the defects in the mixed-halogen quasi-2D perovskite films,and reduce the non-radiative recombination.With the increase of NH₄SCN additive content,the crystallinity of the perovskite film gradually increases,and NH₄SCN effectively controls the crystal growth and improves the film quality,which is beneficial to the carrier transport in the film,promotes the radiation recombination and improves the device luminescence efficiency.After the optimization of the perovskite film by NH₄SCN,the ion migration in the film is suppressed and the stability of the devices is significantly improved.The target sky-blue mixed-halogen quasi-2D perovskite light-emitting diode with 20%NH₄SCN addition achieves the maximum electroluminescence spectrum at 486 nm with a maximum EQE of 5.83%and a maximum luminescence of 1258 cd m^-2,which are 6.7 and 3.6 times higher than those of the comparison devices without NH₄SCN addition,respectively.（2）In order to prepare deep blue perovskite light-emitting diodes,while ensuring the stability of device color and high efficiency,different perovskite light-emitting devices were further prepared by introducing different ratios of multifunctional guanidine thiocyanate（GaSCN）additives on the basis of mixed halogen quasi-2D perovskite,and the effects of GaSCN on the photophysical,crystallographic and morphological performance characteristics of mixed halogen quasi-dimensional perovskite films were investigated.By comparing the doped guanidine bromide（GaBr）with the doped GaSCN samples,the working mechanism of GaSCN in perovskite thin films was analyzed by fourier transform infrared spectroscopy（FTIR）,dynamic light scattering（DLS）and X-ray photoelectron spectroscopy（XPS）,etc.The results showed that SCN^-would inhibit the formation of n=3 and n=4 phases,increase the n=2 phase,and the redistribution of phases is beneficial to accelerate energy transfer in the film,promoting charge migration in the perovskite and enhancing the photoelectric efficiency of the device.The passivation effects of GaBr samples and GaSCN are similar,which proves that Ga⁺can mainly play the role of defect passivation,so as to improve the quality of the film and optimizing the luminescence efficiency of the device.With the increase of GaSCN content,the crystallinity of the crystals in the perovskite film is gradually improved,and the current transfer efficiency inside the perovskite is greatly enhanced,which promotes the stability of the perovskite device.Finally,the device with20%GaSCN content has the highest EQE of 8.39%,which is 4.26 times higher than that of the control device without GaSCN addition,and the stability of the device have also been improved.