In multiple LED chips emitting different colors;In multiple LED chips emitting different colors;

In terms
of saving energy, lighting has significant potential as artificial lighting constitutes approximately 20% of the
global electrical energy
consumption 1; in some regions of the world, this amount even
increases above 30% 2. As a means
of decreasing this number, solid-state lighting (SSL) offers
great potential, with a projected reduction of 50% in the electricity consumption occurring due to light-
ing 3. As the US Department of Energy reports,
replacing all the existing light sources with light-emitting diodes
(LEDs) will result in saving 133 TWh of electrical energy annually in the
USA 4.

General lighting applications require white light, which is generated
using sources like incandescent, flu- orescent, high-pressure sodium lamps,
mercury vapor lamps, and LEDs.
Because of their
energy-saving potential,
LEDs have been attracting great of attention
for a decade, both in the research community and in industry. Differ- ent techniques for generating a white light
spectrum using LEDs have been proposed. One approach is the
simulta- neous use of multiple LED chips emitting
different colors; however, important drawbacks of this
method have pre- vented its widespread preference. First, no efficient
mate- rial system suitable
for green LED is currently available. In
addition, using multiple chips at the same time requires complicated electrical
circuitry, which increases the cost
of the final LED lamp.
Therefore, the desired
efficiency and
cost-effectiveness cannot be obtained using
this approach. As an alternative method,
white LEDs can be obtained
by integrating color-converting materials
on a blue- or near- UV-emitting LED
chip. Today,
most of the
white LEDs are produced by employing
rare-earth-ion-based phosphors as color-conversion materials. These materials
exhibit high absorption at short wavelengths and a broad emis- sion covering
the whole visible
spectrum. Significant pro- gress has been made to improve their
energy efficiency. Currently, other commercial light sources like fluorescent
and incandescent lamps have already
been surpassed in terms of energy efficiency. However,
current phosphor- based white LEDs have limitations, especially on the

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