| Inorganic semiconductor quantum dots(QDs)possess very unique absorption and luminescence characteristics due to the quantum size effect,which have attracted extensive research interest in biology,optoelectronics,electronics and other fields,and have obtained great research progress.Quantum-dot-based light-emitting diodes(QLED)have greater competitive advantage than organic light-emitting diodes(OLED)for the new generation flat-panel display and solid-state lighting because of their high color purity and saturation,good photochemical stability as well as solution processability.At present,researches of spherical or quasi spherical colloidal QDs in light-emitting diodes are most widely,studies on light-emitting devices based on nonspherical anisotropic semiconductor QDs are rare.However,shape anisotropic QDs(such as nanoplatelets,nanorods)with polarized light emission and high molar extinction coefficient different from the traditional spherical QDs,which have attracted the attention of researchers in recent years.Semiconductor nanoplatelets(NPLs),quasi-two-dimensional quantum wells with the narrowest known photoluminescence(PL)linewidth on the order of 10 nm due to one-dimensional(1D)confinement,which have potential to build QLED with super color purity(Full Width at Half Maximum,FWHM < 15 nm)because of far superior color purity than traditional Cd-based spherical or like-spherical QDs(PL FWHM more than 20 nm).Colloidal nanorods(NRs)with polarized light emission and self-assembly characteristics have a certain advantage in the application of QLED,which may be expected to break the limit of 25% light outcoupling efficiency of traditional QDs and to achieve performance breakthrough.In this paper,we firstly synthesize anisotropic sheet and rod-shaped CdSe/CdS core/shell structural quantum dots through optimizing the experimental program,and mainly study the related optical properties of QDs.The effects of shape anisotropic QDs on the electroluminescent properties of QLED fabricated by all solution processing are investigated,the work of this paper mainly include the following two parts.(1)The high quality green CdSe/CdS nanoplatelets with super color purity were prepared according to successive ionic layer adsorption and reaction by thermal cycling(TC-SILAR)method.The synthesized NPLs with PL located at 556 nm show ultra-narrow PL FWHM of 12 nm and high PL quantum yields about 60%.QLED containing the emissive layer of NPLs were fabricated with a multilayer sandwich structure by all solution processing,these devices with ultra-high color saturation and EL FWHM as narrow as 14 nm are superior to the using spherical QD-based LED.These devices demonstrate maximum brightness up to 33000 cd/m2,external quantum efficiency(EQE)and current efficiency(CE)of 5% and 12.5 cd/A,respectively.Compared with the previous reports,our as-prepared QLED not only exhibit a several hundred times increase of brightness,but also greatly improve efficiency.The significant improvements are mainly due to the high QYs of the CdSe/CdS core/shell NPLs and the excellent structure of QLED.With such a structure,the injection of electrons and holes into CdSe/CdS core/shell NPLs enables the balanced exciton formation and efficient exciton recombination within QD active layers and leads to high efficiency and brightness.All these results indicate that colloidal NPLs can improve color purity of QLED to a great extent,and provides the possibility for the application of QLED in ultra HDTV and other high definition display.(2)Two kinds of CdSe/CdS core/shell nanorods have been prepared by means of seed growth method by adjusting the growth rate of Cd S shell materials with one step fast growth and second slow growth.Comparing various properties of these two kinds of nanorods,the results show that the optical parameters of nanorods by second slow growth are better than nanorods of one step rapid growth,and PL intensity and quantum yields are improved obviously.Our as-prepared QLED by employing NRs using a slow injection second growth(S-NRs)show higher performance than NRs through one step fast growth(F-NRs).These devices exhibit maximum luminance of 32900 and 24300 cd/m2,corresponding to the maximum EQE of 7.1 and 3.7%,and CE up to 8.7 and 4.7 cd/A,respectively.The efficiencies of devices based on S-NRs are overwhelmingly higher,specifically yielding 91% and 85% increase than F-NRs based devices in EQE and CE.The improvement is attributed to the fact that the nanorods have more size-uniformity and better crystallinity after a second slow shell growth,which can effectively suppress the nonradiative Auger recombination caused by surface defects. |
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