Tuning the colour of white polymer light emitting diodes

Colour tuning of white polymer light emitting diode (LED) light sources can be attained by various methods at various stages in the production process of the lamps and/or by the design of the active material incorporated in the LEDs. In this contribution we will describe the methods and discuss the physical background of colour tuning. Furthermore, the material design has led to polymers which are more stable during electrical stress, so that colour shift during lifetime can be excluded for white polymer LEDs.     

High color rendering index white LED based on nano-YAG:Ce3+ phosphor hybrid with CdSe/CdS/ZnS core/shell/shell quantum dots

To improve the poor color rendering index (CRI) of YAG:Ce-based white light-emitting diode (LED) due to the lack of red spectral component, core/shell/shell CdSe/CdS/ZnS quantum dots (QDs) were synthesized and blended into nano-YAG:Ce³⁺ phosphors. Prominent spectral evolution has been achieved by increasing the content of QDs. A white LED combining a blue LED with the blends of nano-YAG phosphors and orange- and red-emission QDs with a weight ratio of 1:1:1 was obtained. This kind of white LED showed excellent white light with luminescent efficiency, color coordinates, CRI and correlated color temperature (CCT) of 82.5?lm/W, (0.3264, 0.3255), 91 and 4580?K, respectively.     

Optical properties of the InAs/InAlGaAs quantum dots subjected to thermal treatments

The inter-diffusion kinetics of group-III elements at the interface between self-assembled InAs quantum dots (QDs) and InAlGaAs barriers were investigated indirectly by post-growth annealing treatments and photoluminescence (PL) spectroscopy. The emission wavelength of the InAs/InAlGaAs QDs subjected to thermal annealing at 550 °C was 1444 nm at 10 K, which indicated a 57 nm red shift compared to the as-grown sample (1387 nm). The emission wavelength was blue-shifted with further increases in annealing temperature to 650 °C. Although there was a blue shift in the emission wavelength at an annealing temperature of 600 and 650 °C, the emission peak was still longer than that of the as-grown sample. These results were explained by the difference in inter-diffusion probability between group-III elements at the interface between the InAs QDs and InAlGaAs barrier.     

Tunable color emitting of CaAl2Si2O8: Eu,Tb phosphors for light emitting diodes based on energy transfer

A series of single-phased CaAl₂Si₂O₈: Eu, Tb phosphors have been synthesized at 1400 °C via a solid state reaction. The emission bands of Eu²⁺ and Eu³⁺ were observed in the air-sintered CaAl₂Si₂O₈: Eu phosphor due to the self-reduction effect. Tb³⁺ ions that typically generated green emission were added in CaAl₂Si₂O₈: Eu phosphor for contributing for a wider-range tunable emission. Energy transfer from Eu²⁺ to Tb³⁺ and the modulation of valence distribution of Eu²⁺/Eu³⁺ that contributes to the tunable color emitting were elucidated. More importantly, a white emission can be obtained by controlling the codoped contents of Li⁺ as well as suppressing the self-reduction degree of Eu. The white light emitting with the color coordinate (0.326, 0.261) was obtained, which indicates that CaAl₂Si₂O₈: Eu, Tb is a promising tunable color phosphor for application in ultraviolet light emitting diodes (UV-LEDs).     

Effect of blue doping concentration on the light emission of two-color phosphorescent white organic light emitting diodes

The effect of the doping concentration of the blue dopant on the light emission of two color phosphorescent white organic light-emitting diodes (PHWOLEDs) doped with a blue dopant and a red dopant was investigated. The red emission was intensified at high blue doping concentration due to direct energy transfer from the blue dopant to the red dopant by charge trapping. The changes in the electroluminescence spectra could be well correlated with the light emission mechanism of the PHWOLEDs.     

Optical spectroscopy of single Cd0.6Zn0.4Te/ZnTe quantum dots on Si substrate

Microphotoluminescence (μ-PL) measurements were carried out to investigate the optical properties of single Cd_0.6Zn_0.4Te/ZnTe quantum dots (QDs) grown on Si (001) substrate by using molecular beam epitaxy. The high quality of single Cd_0.6Zn_0.4Te/ZnTe QDs is witnessed by resolution-limited emission, negligible background and absence of measurable spectral jitter or blinking. Polarization-dependent and power-dependent μ-PL spectroscopy measurements were performed to identify the exciton, the biexciton, and the charged exciton in the emission spectra of single QDs. Furthermore a weak linearly polarized line is observed on the low energy side of the neutral exciton and is ascribed to dark exciton recombination.     

Photoluminescence properties of emission-tunable Ca9Y(PO4)7: Tm,Dy phosphor for white light emitting diodes

A white-emitting Ca₉Y(PO₄)₇: Tm³⁺, Dy³⁺ phosphor has been successfully prepared by conventional high-temperature solid-state reaction. X-ray diffraction (XRD) and fluorescence spectrophotometer were used to characterize the as-synthesized phosphors. The excitation and emission spectra show that all the Tm³⁺ and Dy³⁺ co-doped Ca₉Y(PO₄)₇ samples can be effectively excited by UV light and then emit blue and yellow light simultaneously. Furthermore, the emission and color coordinate of as-obtained samples pumped by 365 nm are able to be adjusted around white light by varying the doping concentrations of Tm³⁺ and Dy³⁺. So, the as-fabricated single-composition Ca₉Y(PO₄)₇: Tm³⁺, Dy³⁺ phosphor will have a promising application in the area of white light emitting diodes.     

结温对高显指低色温白光LED光谱特性的影响

将MEH-PPV混合入YAG荧光粉中,并利用实验室自主研发的一种集成封装LED散热支架替代传统支架,该支架可利用热电偶直接测量出结温。研究了结温对混合MEH-PPV的白光LED光谱特性的影响。实验表明,随着结温的不断升高,蓝光光谱峰值随着结温的升高而不断下降,发生红移;荧光粉的吸收增强,黄光波段和红光波段的光谱峰值先增后减,并且红光波段的光谱发生蓝移;器件的显色指数从90.3下降至78.6,色温从3265K先快速升高至3672K,随后又缓慢下降至3612K,而光通量和发光效率则表现为先上升后下降,且它们的下降趋势基本吻合。     

Admittance spectroscopic analysis of organic light emitting diodes with the CFX plasma treatment on the surface of indium tin oxide anodes

Admittance spectroscopic analysis was used to examine the effect of a CF_X plasma surface treatment on indium tin oxide (ITO) anodes using CF₄ gas and model the equivalent circuit for organic light emitting diodes (OLEDs) with the of ITO anode surface treated with CF_X plasma. This device with the ITO/N,N′-diphenyl-N,N′-bis(3-methylphenyl)-1,1′-diphenyl-4,4′-diamine/tris-(8-hydroxyquinoline) aluminum/lithium fluoride/Al structure was modeled as a simple combination of two resistors and a capacitor. The ITO anode surface treated with the CF_X plasma showed a shift in the vacuum level of the ITO, which resulted in a decrease in the barrier height for hole injection at the ITO/organic interface. Admittance spectroscopy measurements of the devices with the CF_X plasma treatment on the surface of the ITO anodes showed a change in the contact resistance, bulk resistance and bulk capacitance.