| In recently years,cesium lead halide perovskite nanocrystals have been actively studied in the application of light-emitting diodes(LEDs)for next generation display and lighting,due to their excellent optoelectronic properties such as narrow emission bandwidth,high photoluminescence quantum yield and wide color gamut.The external quantum efficiency(EQE)of monochrome red and green LEDs has exceeded 20%in just a few years.The performance for white perovskite LEDs(WPLEDs)with great potential for illumination and backlight applications to date has far fallen short of that of state-of-the-art white organic light-emitting diodes(WOLEDs).Only a few reports are about electroluminescent white light emitting devices because of the ion exchange between perovskite nanocrystals with different halide components and the photoluminescence quantum yield reduction caused by high annealing temperatures.Perovskite white light-emitting devices without intercalation layers have not been achieved due to the ion exchange.Although the intercalation layers prevent ion exchange between perovskite nanocrystals,it also creates a new problem of charge imbalance and the structure becomes more complex.To solve the problem,in this study,blue emitting ZnCdS/ZnS nanocrystals with high photoluminescence quantum yield and stability are introduced to work with the yellow emission from CsPb(Br1.65/I1.35)perovskite nanocrystals for white light-emitting diodes.The valence bands are derived to be-6.19 eV and-7.0 eV and thus the conduction bands are calculated to be-3.81 eV and-4.3 eV for CsPb(Br1.65/I1.35)and ZnCdS/ZnS,respectively.1.Firstly,two kinds of nanocrystals materials,CsPb(Br1.65/I1.35)and ZnCdS/ZnS,were synthesized.Their optical properties were characterized,such as luminescence spectra,absorption spectra,TEM and XRD;in addition,the optical properties of CsPb(Br1.65/I1.35)nanocrystals solutions in different days were characterized for test the stability of nanocrystals material.It is also verified that the ion exchange reaction will take place between different component perovskite films by using blue light emitting CsPb(C11.5/Br1.5)nanocrystals and CsPb(Br1.65/Il.35)nanocrystals.The electroluminescent monochrome LEDs based on CsPb(Br1.65/I1.35)and ZnCdS/ZnS nanocrystals were fabricated by using the above two kinds of nanocrystals materials,and the properties of the two kinds of electroluminescent LEDs were characterized,the maximum brightness of the two LEDs are 209 cd·m-2 and 198 cd·m-2,respectively.The turn-on voltages are 2.8 V and 3.3 V2.The white light-emitting diodes are constituted of ITO/ZnO/PEI/ZnCdS/ZnS NCs/CsPb(Br1.65/I1.35)NCs/TCTA/MoO3/Au,this design avoids ion exchange between different perovskites nanocrystals,and realizes white light emission by simple fabrication.The film exhibits two photoluminescence peaks at 473 nm and 578 nm,resulting from the photoemission of ZnCdS/ZnS and CsPb(Br1.65/I1.35),respectively.The energy transfer mechanism from the ZnCdS/ZnS nanocrystals to CsPb(Br1.65/I1.35)nanocrystals is revealed through the analysis of the time-resolved photoluminescence decays of nanocrystals films,the calculated energy transfer efficiency is 77%.The white light was confirmed by the CIE 1931 coordinates of(0.34,0.34).The white light have a correlated color temperature(CCT)of 5153 K and color rendering index(CRI)of 75.The devices have a low turn-on voltage of 2.8 V(defined with 1 cd·m-2).The luminance reaches a maximum value of 275 cd·m-2 at 10 V.A maximum external quantum efficiencies(EQE)value of 0.015%is obtained at a current density of 214 mA·cm-2.Overall,our structure beneficial effects are novel,relatively simple and easy to fabricate. |
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