Bright and stable quantum dots and their applications in full-color displays
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Introduction Following the discovery of new physics in semiconductor nanocrystals,1 many potential applications exploiting the advantages of quantum dots (QDs) have been suggested, such as optically or electrically pumped lasers, biosensors, printable thin-film transistors (TFTs), light-emitting diodes (LEDs), and photovoltaics.2–6 QD-LEDs seem the most attractive at the current stage because QDs have many advantages over conventional light-emitting materials such as rare earth based inorganic phosphors and fluorescent and phosphorescent organic polymers. First, they show easily tunable wavelengths, high quantum efficiency, and narrow spectra and hence can produce saturated colors. Moreover, the colloidal synthesis process is simpler, less expensive, and more easily scalable compared to equipment-intensive vacuum processes. Furthermore, scattering loss is absent in the luminescence since the nanocrystal size is much smaller than the optical wavelength. In addition, conventional device fabrication processes are also compatible with QD materials. Intensive effort has been devoted to prepare high-quality QDs with uniform sizes, controlled shapes, and defect-free passivation.7–10 For display applications, the emission wavelengths should be controlled in the ranges of 620–630 nm, 525–535 nm, and 445–455 nm for red, green, and blue colors, respectively, to obtain a more saturated color gamut (i.e., a complete
subset of colors). The emission wavelengths of QDs can be simply controlled through size variation. However, it is very difficult to prepare small-sized QDs with high quantum efficiencies (QEs), since most atoms in small-sized QDs are exposed on the surface or the interface, and defects can form easily even after inorganic shell coating (see the Introductory article). Therefore, relatively large QDs with appropriate passivation structures are more desirable to improve both the QEs and the tolerance to the processing conditions for device fabrication. Relatively large QDs can be obtained by using an alloy structure with mixed bandgap energies and applying a semiconductor nanocrystalline interlayer with staggered energy levels. In current driven QD-LEDs, however, optimization of the shell thickness of core–shell type QD structures is necessary to achieve direct charge injection into the QDs while maintaining the original optical properties.11,12 One or two monolayer shells could be better for direct charge injection than thick multilayered shells. The importance of color-converting white LEDs has expanded in a wide range of applications such as general lighting devices and displays. In particular, LCD displays with white LED backlights are currently in demand in the large-panel TV market because of their decent power consumption, reasonable price, and slim package size. However, their color gamut is degenerated to only 75% of the NTSC (National Television
Tae-Ho Kim, Samsung Advanced Institute of Technology, Samsung Electronics; [email protected] Shinae Jun, Samsung Advanced Institute of Technology, Samsung Elect
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