color band of QDs enables the repro- duction of

color quality
and spectral efficiency. These LEDs cannot simultaneously accomplish a good color
rendition, a good spectral match with the spectral sensitivity of the human eye, and a warm white shade,
although individual high performances are possible. This basically emanates
from the difficulties in the spectral tuning of phosphors. In addition,
concerns regarding the supply of and current commercial monopoly on phosphors have increased the demand for alternative color
converters 5. At this point,
quantum dots are rising as a promising candidate since they exhibit fine
spectral tuning, achieved by their size control and narrow-band emission 6.
Therefore, with optimized spectral designs,
the real colors
of objects can be
rendered properly while
achieving a warm
white shade and a good spectral
overlap with the human eye sensitiv-
ity function, which in turn increases the efficiency of the
light source; all of these improvements can be achieved at the same time with QDs employed in white LEDs 7, 8.
Furthermore, their high photoluminescence quantum
effi- ciencies can contribute to realizing the high electrical effi- ciency of the device 9, 10. Considering these
features of QDs, they
offer great potential for white LEDs
by possess- ing high color quality
along with photometric and electri- cal efficiency. In addition to general lighting applications, QD-based LEDs can easily respond
to the demands of the backlights used in liquid
crystal displays (LCD).
In partic- ular, the narrow emission
band of QDs enables the repro-
duction of high purity colors. Moreover, a much larger number of colors can be generated using these materials; in other words, the color gamut of the LCDs can be broad- ened beyond the industrial standards.

At this point it is useful to distinguish two types
of LEDs using QDs,
which rely on two
different means of exci-
tation. One is based on electrically exciting the QDs, which makes LEDs based on the direct
electroluminescence of QDs, and the other is through
optically exciting them, which makes color-conversion LEDs using QDs as
the nanophosphors. As the name implies, in the electrically excited QD based
LEDs, electrons and holes are directly
injected into the quantum dots, and the white light emis- sion is thus obtained through the radiative recombination of these injected carriers within QDs having different sizes and emitting different colors. Over the years to date,
the overall efficiencies of these LEDs typically remained
lower compared to those of color converting QD-WLEDs mainly because of the charge injection problem. The organic
ligands surrounding the QDs, whose main function
is the passivation of the QD surface, are poor conductive mate- rials and generate a large barrier that makes the charge injection difficult. Therefore, the injection of carriers into the
QDs is not an easy task which in the end decreases
the

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