Luminescent Halide Perovskite Used For High Efficient And Stable Light Emitting Diode

This article is mainly working on perovskite Light-Emitting Diode(LED)based on cesium lead bromide(CsPbBr3).Inorganic perovskite like CsPbBr3 can be used to fabricate efficient and stable LED since it has high stability.Now,light-emitting devices based on CsPbBr3 develop rapidly.However,there are still a lot of problems for CsPbBr3,which limits the improvement of efficiency.Three dimensional(3D)CsPbBr3 has lots of traps inside the perovskite itself and the crystallinity of CsPbBr3 is not so good that the film morphology is relatively poor.Meanwhile,the deep mechanism behind perovskite materials based on CsPbBr3 is worth investigating.Here,by adding ligands into the CsPbBr3 solution,we made low-dimensional materials based on CsPbBr3.Through comparison and analysis by using ultraviolet-visible(UV-Vis)absorption spectrum,photoluminescence spectrum(PL),synchrotron radiation grazing incident X-ray diffraction(GIXRD)and X-ray diffraction(XRD),the composition inside the low-dimensional perovskite has been studied.In addition,by using scanning electron microscopy(SEM),atomic force microscopy(AFM),the improvement of film morphology has been confirmed.The photoluminescence quantum yeild(PLQY)has been largely improved as proved by measurement.At the same time,we also fabricated LED device for test and analysis in order to prove the improvement.Besides,some alkaline salts have been incorporated into CsPbBr3 in order to reduce the traps inside the CsPbBr3.The aim of this method is to improve the efficiency of CsPbBr3 based LED and remain the stability of this device at the same time.PL and PLQY measurements have been used to analysis different perovskite films.The LED devices based on these materials have been fabricated to prove original purpose.The details of this article are as follows:1.By incorporating organic ligands,low-dimensional CsPbBr3 perovskite has been made to improve the luminescent efficiency.Although excitonic binding energy of low-dimensional perovskite has been enhanced a lot so that its luminescent efficiency can be very high,the over-amount incorporation of organic ligands can induce the formation of perovskite nanoplatelets with low layers and the conductivity of the whole perovskite film can be very low.To solve this problem,we have achieved the best luminescent efficiency by precise control of the ratio of organic ligands.Meanwhile,we found that the formation of perovskite nanoplatelets with low layers negatively influenced the luminescent efficiency.Therefore,we have added another small organic molecule,formamidinium bromide(FABr).By tuning the forming energy of different perovskite,FABr effectively suppress the formation of perovskite nanoplatelets with single layer and enhance the crystallinity of perovskite.The LED device based on this perovskite can reach an external quantum efficiency(EQE)of 14.9%.2.Nowadays,most low-dimensional perovskite is made from organic ligands.However,organic materials are vulnerable to Joule heat and have very low conductivity.Most organic ligands are insulating materials.The conductivity of perovskite will be deteriorated.To solve this problem,we fabricated a two-dimensional(2D)and three-dimensional(3D)hybrid perovskite based on sodium bromide.Sodium ions can be ligands to restrict the growth of CsPbBr3 crystal so that Ruddlesden-Popper(RP)perovskite can be formed.Meanwhile,there are still some non-crystaline perovskite-like NaPbBr3 formed outside the CsPbBr3.Since the dielectric constants of NaPbBr3 and CsPbBr3 are quite different,the mixed particles can have strong dielectric confinement effect,which leads to a large improvement of excitonic binding energy and luminescent efficiency.Besides,by incorporating another kind of organic molecule called 18-crown-6,the morphology can be improved a lot.The LED based on this perovskite film can achieve a high EQE of 15.9%.3.Although 2D perovskite can have high luminescent efficiency,the heat tolerance of 2D perovskite is still poor.In order to solve this problem,we focused on 3D CsPbBr3.By incorporating lithium bromide(LiBr)into the perovskite film,the traps inside CsPbBr3 can be reduced a lot.The luminescent efficiency can reach a high value by tuning the amount of LiBr.Besides,a little amount of polyethylene oxide(PEO)have been added into the perovskite.PEO can effectively suppress the ion migration in the perovskite film.The LED device based on this perovskite can achieve an EQE over 15%.The stability of LED device has been largely improved.The half-life time for LED at a high luminance can be 2.3 hours.In addition to that,it was found that this method can be a general way to fabricate multi-colored LEDs.By incorporating LiX(X=Cl,Br,I)into CsPbX3(X=C1,Br.I),perovskite LEDs with different spectrum can be achieved,which can cover a large area of CIE coordination.