The present disclosure relates to a nanocrystal mixture and a light-emitting device using the nanocrystal mixture.
In recent years, semiconductor-based white light-emitting diodes have attracted more and more attention as next-generation replacements for current light-emitting devices due to their excellent characteristics, including prolonged lifetime, prospect of miniaturization, reduced power consumption and environmental friendliness (e.g., mercury free). White light-emitting diodes are currently used in a wide variety of applications, such as backlights for liquid crystal displays and display instrument panels for automotive vehicles.
Particularly, many attempts have been made to manufacture backlights for liquid crystal displays using three-color light-emitting diodes and having a high luminescence efficiency and good color purity. However, disadvantages associated with the use of three-color light-emitting diodes are significant, for example, high fabrication costs and complex driving circuits causing a loss in cost competitiveness. Thus, there exists a need to develop one-chip solutions capable of reducing the fabrication costs and simplifying the structure of devices while maintaining excellent performance characteristics of the devices in terms of luminescence efficiency and color purity.
A combination of YAG:Ce phosphors and a blue light-emitting diode has been employed as a one-chip solution to develop white LEDs. The white LEDs operate in such a way that a portion of blue light from the blue light-emitting diode excites the YAG:Ce phosphors to produce yellow-green light and the blue light is combined with the yellow-green light to emit white light. However, these white LEDs suffer from problems that include a low color rendering index and poor color purity because light from the white LEDs has some spectra in the visible range. Thus, these white LEDs cannot be easily applied to high definition display devices.
In recent years, an attempt has been made to fabricate a white LED that uses a UV light-emitting diode, which is expected to have high energy efficiency, as an excitation source instead of a blue light-emitting diode. This white LED is fabricated by applying green emitting nanocrystals and red emitting nanocrystals to a blue light-emitting diode, as shown in FIG. 1. Specifically, as shown in FIG. 2, the white light-emitting diode comprises a blue LED 20 mounted on a packaging frame 10 and a light-emitting layer 25 formed on the blue LED wherein the light-emitting layer 25 is formed of a mixture of red emitting nanocrystals, green emitting nanocrystals and a base (e.g., a matrix resin).
Semiconductor nanocrystals are advantageous in terms of energy conversion due to their small Stokes shift, but may undergo absorption-reabsorption when both red emitting nanocrystals and green emitting nanocrystals are used for the fabrication of a white LED. This absorption-reabsorption takes place between the semiconductor nanocrystals. FIG. 3 is a graph showing UV-VIS absorption spectra and UV-excited photoluminescence spectra of light-emitting semiconductor nanocrystal materials. Particularly, when red emitting nanocrystals absorb light from green emitting nanocrystals, energy transfer between the nanocrystals causes consecutive light down-conversion, reducing efficiency of a light source. This energy transfer occurs more frequently as the distances between the nanocrystals decrease. Therefore, an increase in the concentration of the semiconductor nanocrystals or aggregation of the semiconductor nanocrystals renders the problem more serious.