Preparation Of Light-Emitting Diodes Based On Novel Quantum Dots

This work was supported the National Science Fund for Distinguished Young Scholars “Preparation of semiconductor nanomaterials and quantum dot optoelectronic devices” Grant No.61225018,the National Natural Science Foundation of China “Research on Preparation High Efficiency White LED of Carbon Dots” Grant No.61475062,the National Natural Science Foundation of China“Research on Preparation High Efficiency LED of Inorganic Perovskite Quantum Dots” Grant No.61675086 and he National Natural Science Foundation of China“Research on Preparation LED of ZCIS Quantum Dots” Grant No.61106039.Light-emitting diodes(LEDs)are widely applied in display and lighting.With the development of technology,the power consumption of LEDs is getting lower and the brightness is getting higher.The quantum dots(QDs)phosphors as the most promising LEDs phosphor have also developed very quickly.In recent years,perovskite quantum dots and carbon dots(CDs)have become star materials among quantum dots,and they have been widely used in lasers,catalysis,batteries and biology.However,there are lots of problems in their applications toward LEDs.Due to the high quantum yield,narrow line width and simple synthesis methods,perovskite quantum dots are superior to traditional Cd-based QDs.However,they possess poor stability.The carbon dots are non-toxic and stable,besides,the preparation methods are rather simple.However,the carbon dots' fluorescence usually quenched in their solid-state,and the emission spectrum of the carbon dots mainly located in the blue-green region,which are not appropriate for White LEDs(WLEDs).In view of the above problems,we analyze the characteristics of these novel QDs,and find solutions to settle down these problems.In the end,high effeicency and stable LEDs have been prepared.The detailed results are shown as follow:(1)Air stable inorganic perovskite QDs powder were one-pot synthesized by embedding QDs into silica matrix.The as-prepared QD/silica composites not only possessed high quantum yields(QY),but also exhibited narrow emission line widths in various colors.The silica matrix protects the delicate perovskite QDs from air and water contact and suppresses the common anion exchange reactions between the QDs,which makes the stability of perovskite QDs enhanced.The WLED devices with Commission Internationale de l'Eclairage(CIE)color coordinates of(0.33,0.33)were fabricated with a power efficiency of 61.2 lm/W at a driven current of 20 m A and emission peaks of the WLED was stable after working continuously for 10 h.Thehalf-lifetime of the WLED was 227 h,demonstrating a great improvement in the perovskite materials for display and lighting applications.(2)By controlling the hydrolysis of alkoxysilanes,highly luminescent,transparent and flexible perovskite QD/silica gels were synthesized.The gels could maintain the structure without shrinking and exhibited excellent stability comparing to the QDs in solution.This in situ fabrication can be easily scaled up for large-area/volume gels.The gels integrated the merits of the polymer matrices to avoid the non-uniformity of light output,making it convenient for practical LED applications.WLEDs were fabricated using these QD/silica gels;the LEDs exhibited broader color gamut,demonstrating better property in the backlight display application.(3)Due to the excitation-dependent photoluminescence(PL)of canbon dots,a series of monochromatic light-emitting diodes based on monodisperse carbon dots with an emission color ranging from blue to red were fabricated,which is determined by the thickness of the downconversion layers and the carbon dot doping concentration in the polymer matrix.The highest luminance and luminous efficiency reached 11890 cd m-2 and 1.21 cd A-1,respectively,which is still higher than that reported in our previous work for electroluminescent CD-based LEDs.We further demonstrate the potential of CDs for fabrication of relief graphical patterns with anti-counterfeiting security.(4)Carbon dots are non-toxic fluorescent nanomaterials and very stable.However,the emission spectrum of the carbon dots mainly located in the blue-green region,resulting in low CRI of WLEDs.Therefore,we combined blue emissive carbon dots with green and red emissive zinc copper indium sulfide(ZCIS)core/shell QDs to achieve WLEDs with a high color rendering index of 93.The color temperature could be tuned by changing the volume ratio of these constituting luminophores.Their electroluminescence characteristics including color rendering index,CIE color coordinates,and color temperatures were evaluated as a function of forward current.The CIE-1931 chromaticity coordinates of the as-prepared WLEDs,exhibiting good stability,were slightly shifted from(0.321,0.312)at 10 m A to(0.351,0.322)at 30 m A,which was mainly caused by the different thermal quenching coefficients of carbon dots and ZCIS QDs.(5)By dispersing the CDs in a highly transparent polyvinyl pyrrolidone(PVP)matrix,solid state PL quenching effect was eliminated.Polymer dots phosphors were also prepared using the same method,and the thermal stabilities of polymer dots were elevated.We realized WLEDs with high color rendering index(85–96)and widely variable color temperatures(2805–7786 K)by combining three phosphors based oncarbon dots and polymer dots.All three phosphors exhibited dominant absorption in the UV spectral region,which ensured the weak reabsorption and no energy transfer crosstalk.The WLEDs showed excellent color stability against the increasing current because the tricolor phosphors only absorb the UV light.