White-light LEDs (Light Emitting Diodes) have advantages of low energy consumption, long service life, and environment friendliness. Along with the improvement of the luminescent efficiency and the decrease of the production cost, the white-light LED is predicted to become a new generation of illuminating light source after incandescent lamp, fluorescent lamp, and high intensity discharge lamp. Similar to the replacement of conventional vacuum tubes with transistors, the replacement of conventional vacuum light sources with solid light source white-light LEDs is a disruptive technical innovation, and will initiate a revolution in the illuminating field.
Currently, the most commonly used method to achieve white light in an LED is to combine a blue-light LED chip with a fluorescent powder which can be effectively excited by blue lights and emits yellow lights to form a white-light LED. Although a relatively high luminescent efficiency can be obtained by such a method, a white-light LED with warm white light and high color rendering cannot be obtained due to the lack of red lights in its emission spectrum. Another method to achieve a white-light LED is the combination of a blue-light LED chip with green-light and red-light fluorescent powders to obtain a white-light LED with high color rendering and low color temperature. However, the green-light fluorescent powder and the nitride red-light fluorescent powder have low luminescent brightness and high production cost. The other main disadvantages of the green-light fluorescent material for a white-light LED are narrow full width at half maximum, and the currently used luminescent materials having wide emission spectra show low luminescent efficiencies.
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