Blue Perovskite Light-emitting Diodes Based On Molecular Regulation

In recent years,perovskite materials have presented a rapid development because of their excellent photoelectric properties,solution processing capacity,cost-effective payment and so on.At present,perovskite light-emitting diode(Pe LED)has attracted intriguing research attention due to its distinctive advantages,such as the narrow full width at half maximum,the adjustable optical band gap and wide color gamut.In the current research field of Pe LED the external quantum efficiency of red and green Pe LEDs have achieved the records of 20%,and blue Pe LED which is relatively backward,has a record of external quantum efficiency exceeded10%.Therefore,Pe LED has been regarded as a alternative competitor of next-generation electroluminescent display devices.To push forward the commercialization process of Pe LED,there are still series of key hindrances that need to be solved urgently,including poor operational stability,numerous defects within active layer caused by halogen ion migration,and phase separation challenges in mixed halogen perovskites.The introduction of specific functionalized organic small molecules into perovskite can effectively passivate the surface defects of perovskite crystals,modify the emission wavelength of the films,and improve long-term structural stability of devices.This thesis mainly focuses on the research about the relationship between molecular manipulation and device performance of Pe LEDs.The precise defect control of perovskite luminescence layer and the improvement of device stability have been realized by combining varied molecular configuration design.According to the changed structures of perovskite crystals,our research can be divided into the following aspects:1.Based on the Quasi-2D perovskite structure,we used two new-type small molecules named isobutylamine hydrobromide(IBABr)and 1-Phenylbiguanide hydrochloride(1-PBGCl)to adjust the emission wavelength and defects density of the perovskite film.The absorption spectra,steady-state photoluminescence(PL)spectra and XRD patterns proved that IBABr can assist the crystallization process of perovskite to form a 2D crystal structure with adjustable optical band gap.The photoluminescence quantum yield(PLQY)and time-resolved photoluminescence(TRPL)spectra proved that 1-PBGCl can effectively passivate defects from crystalline surface and improve the recombination efficiency of excitons.The optimal device P₁₈₆ exhibited a blue light emission wavelength of 480nm with a maximum current efficiency of 6.28 cd·A^-1,a maximum brightness 728 cd·m^-2,and a maximum EQE of 5.80%,attribute to the pure bromine perovskite system,the perovskite film has good component stability,and the device does not have wavelength red shift during operation.We introduced 1,8-octanediamine hydrobromide(ODADBr)to construct the Dion-Jacobson(DJ)perovskite structure.By a variable-temperature PL experiment,we testified the obtained DJ phase perovskite films occupied a better structural stability compared to the traditional Ruddlesden-Popper phase analogues.The optimal device exhibited a blue emission wavelength of 479 nm with a maximum EQE of 1.09%,a maximum brightness of 63 cd·m^-2,and no red shift of light emission wavelength under 8 V operating voltage.2.We incorporated 1-PBGCl into the pure-Br system perovskite system to obtain a 3D mixed-halogen perovskite light-emitting film.The light-emitting device using this film delivered a blue light emission wavelength of 487 nm with a maximum EQE is 2.0%.Attribute to the high charge injection efficiency of the 3D structure perovskite,the device present a high brightness of 1635 cd·m^-2 and a low turn on voltage of 2.8 V.The mixed-halogen device with functional additive showed a 5 nm red shift of the emission wavelength at the higher voltage.The value was significantly smaller those of previously reported mixed-halogen blue Pe LEDs prepared by the traditional method.Based on the these experiments,we further explored the effects of molecular manipulation on the mixed-cation perovskite system.By replacing part of the A-site cesium(Cs)ions with the formamidine(FA)cation,we obtained a 3D multiple-cation and mixed-halogen perovskite film.The atomic force microscopy(AFM)and TRPL results proved that the deposited perovskite film with composite composition exhibiter the significantly low surface roughness and mitigated crystal defects.The light-emitting device prepared based on this film exhibited a blue light emission wavelength of 493 nm with a maximum EQE of 6.68%,and a maximum brightness of 12668 cd·m^-2.Benefiting from the additional incorporation of FA cation,the charge injection efficiency of Pe LED was further improved,and the corresponding turn on voltage was reduced as low as 2.6 V The possible hydrogen bonding between FA and 1-PBGCl may further contribute to the retard the wavelength red shift during operational state,indicating the desirable phase stability of the decorated films and devices.