Quantum dot light emitting diodes (QD-LEDs) are a promising next-generation display technology that uses nano-scale semiconductor crystals called quantum dots. QD-LEDs offer advantages over traditional displays like OLEDs such as saturated colors, energy efficiency, and the ability to tune emission wavelengths by varying quantum dot size. They can also be produced using low-cost solution-based processes. Major challenges to the technology include preventing quantum dot charging and luminescence quenching. Potential applications of QD-LEDs include medical phototherapy devices, nanoscale microscopy, and solid-state lighting to replace incandescent bulbs.

1. Introduction to Nano
biotechnology
1 7 - D e c - 1 5
Assignment# 4
Quantum dot light emitting diodes
Submitted to: Dr. Zahra
Submitted by: Zohaib hussain & Irum

2. Quantum Dot Light Emitting Diode
Introduction
Quantum dots (QD) or semiconductor Nano crystals could provide an alternative for commercial
applications such as display technology. This display technology would be similar to organic
light-emitting diode (OLED) displays, in that light would be supplied on demand, which would
enable more efficient displays.
Quantum dots could support large, flexible displays. At present, they are used only to filter light
from LEDs to backlight LCDs, rather than as actual displays. Properties and performance are
determined by the size and/or composition of the QD. QDs are both photo-active (photo
luminescent) and electro-active (electroluminescent) allowing them to be readily incorporated
into new emissive display architectures.
Definition
QD-LED or QLED is considered as a next generation display technology after OLED-Displays.
“QLED means Quantum dot light emitting diodes and are a form of light emitting technology
and consist of nano-scale crystals that can provide an alternative for applications such as display
technology”. The light emitting centers are cadmium selenide (CdSe) nanocrystals, or quantum
dots.
Charactristics
❀QLEDs are a reliable, energy efficient, tunable color solution for display and lighting
applications that reduce manufacturing costs, while employing ultra-thin, transparent or
flexible materials.
❀Quantum-dot-based LEDs are characterized by pure and saturated emission colors with
narrow bandwidth.
❀Their emission wavelength is easily tuned by changing the size of the quantum dots.
Moreover, QD-LED offer high color purity and durability combined with the
efficiency, flexibility, and low processing cost of organic light-emitting devices. QD-
LED structure can be tuned over the entire visible wavelength range from 460 nm (blue)
to 650 nm
❀Due to spectrally narrow, tunable emission, and ease of processing, colloidal QDs are
attractive materials for LED technologies.

3. Why quantum dots for light-emitting devices (LEDs)?
Saturatedcolors
The electronic structure of colloidal QDs, which typically range from 3 to 12 nm in diameter,
is dominated by quantum size effects. This gives colloidal QDs their signature narrowband
emission that can be spectrally positioned by controlling the nanocrystal size during synthesis.
Furthermore, QDs can be used to tune the quality of white lighting, which can be evaluated by
color temperature and color rendering index (CRI).
The color temperature of a light source is the temperature of an ideal black-body radiator that
radiates light of the same hue. The CRI defines how well a particular artificial light source
illuminates an object compared to illumination by natural light, with a CRI of 100 indicating that
the artificial light source reproduces the lighting conditions achieved by a black-body light
source (such as the sun) with the designated temperature
Solutionprocessable
Since QD are synthesized from organometallic precursors, they retain a passivating layer of
ligands, making them solution processable. This facilitates a variety of low cost, large-area
deposition techniques, such as phase separation, inkjet printing, and microcontact printing. The
ligands used in the QD synthesis can be exchanged to make the QDs compatible with aqueous
solutions in addition to standard organic solvents
Stability
Because they have inorganic semiconductor cores, QDsare often more resistant than organic
dyes to degradation caused by effects such as photobleaching. Overcoating QDs suppress
blinking, which has been linked to suppressed Auger recombination , a key source of
inefficiency in electrically excited QDs. Recent work replacing organic aliphatic ligands on QDs
with metal chalcogenide ligands has enabled QD films that are entirely inorganic and exhibit
record electronic transport properties
Working principle
In QD-LED a layer of cadmium-selenium quantum dots is sandwiched between layers of
electron-transporting and hole-transporting organic materials. An applied electric field causes
electrons and holes to move into the quantum dot layer, where they are captured in the quantum
dot and recombine, emitting photons. The spectrum of photon emission is narrow, characterized
by its full width at half the maximum value.

4. By making an emissive layer in a single layer of quantum dots, electrons and holes may be
transferred directly from the surfaces of the ETL and HTL, providing high recombination
efficiency.
The array of quantum dots is manufactured by self-assembly in a process known as spin
casting: a solution of quantum dots in an organic material is poured onto a substrate, which is
then set spinning to spread the solution evenly.
Challenges in quantum dot light-emitting devices (QD-LED)
development
There are two key challenges facing the electrical excitation of colloidal QDs:
1. QD charging and QD luminescence quenching in thin film. QD charging can occur
whenever dc current passes through a QD film. As QDs become charged, it becomes
increasingly difficult to pass current through the device and maintain QD
electroluminescence (EL). Time scales associated with QD charging range from minutes to
days, making it challenging to obtain consistent luminescence from a QD film that
experiences significant QD charging.
2. QDs suspended in solution routinely have photoluminescence quantum yields of 95%, when
the QDs are deposited in a close-packed thin film, the luminescence efficiency decreases by
approximately an order of magnitude (to 5 or 10%). Embedding QDs in an insulating
polymer matrix decreases the amount of QD luminescence quenching observed in close-
packed QD structures, however, dc electrical conductivity through these QD-polymer
composites is impeded by the low conductance of the wide band gap polymers.
Fabrication process
Quantum dots are solution processable and suitable for wet processing techniques. The two
major fabrication techniques for QD-LED are called phase separation and contact-printing
Phase separation
Phase separation is suitable for forming large-area ordered QD monolayers. A single QD layer is
formed by spin casting a mixed solution of QD and TPD.
Contact printing
The overall process of contact printing:
❀Polydimethylsiloxane (PDMS) is molded using a silicon master.

5. ❀Top side of resulting PDMS stamp is coated with a thin film of parylene-c, a chemical-
vapor deposited (CVD) aromatic organic polymer.
❀Parylene-c coated stamp is inked via spin-casting of a solution of colloidal QDs
suspended in an organic solvent.
❀After the solvent evaporates, the formed QD monolayer is transferred to the substrate by
contact printing.
Contact printing allows fabrication of multi-color QD-LEDs. The demonstrated color gamut
from QD-LEDs exceeds the performance of both LCD and OLED display technologies.
Applications
1. Quantum dot light-emitting diodes for phototherapy
Quantum dots LED QDLEDs use in phototherapy. One expression is a medical dressing having
an occlusive layer and translucent layer. Quantum dot light-emitting diode chips are configured
within the occlusive layer to provide light of a specific wavelength for use in phototherapy.
Another embodiment is a medical dressing having an occlusive layer and translucent layer,
where in quantum dot material is embedded or impregnated within one or both layers.
Phototherapy, also known as heliotherapy, is the use of light to treat medical disorders.
Since then, phototherapy has been used to treat a wide range of conditions, including skin
disorders, circadian rhythm and seasonal affective disorders, neonatal jaundice, and tumours.
Treatment of skin conditions using phototherapy, e.g. psoriasis, eczema, dermatitis, acne
vulgaris, is largely reliant on radiation in the UV region, however red to infrared (IR) light can
be used to promote wound healing. Quantum dot light-emitting diode chips are configured within
the occlusive layer to provide light of a specific wavelength for use in phototherapy.
2. Near- field scanning optical microscopy (NSOM)
Near-field fluorescence excitation and imaging with a quantum dot (QD) light emitting diode
(QDLED) integrated at the tip of a scanning probe. The tip-embedded QDLED is employed in
a near-field scanning optical microscopy setup to directly excite a secondary colloidal QD
sample. Electrically pumped QDs enable multi-color, self-illuminating probes with no
conventional optics needed for light coupling. Monolayer QDs stamped at the very tip of a
micromachined silicon probe facilitates precise position control of the ultra-thin (10–15 nm) light
source. Sensitivity of fluorescence intensity to the QDLED–QD sample distance was measured
down to 50 nm order, demonstrating spatially resolved imaging

6. 3. Incandescent bulbs
“QD-LEDs can potentially provide many advantages over standard lighting technologies, such as
incandescent bulbs, especially in the areas of efficiency, operating lifetime and the color quality
of the emitted light,” said Victor Klimov of Los Alamos. Incandescent bulbs, known for
converting only 10 percent of electrical energy into light and losing 90 percent of it to heat,
are rapidly being replaced worldwide by less wasteful fluorescent light sources. However, the
most efficient approach to lighting is direct conversion of electricity into light using
electroluminescent devices such as LEDs.
Other applications
Quantum-dot based light emitting diodes (LEDs) fabricated on silicon have applications in
nanophotonics, optical micro/nanoelectromechanical systems (MEMS/NEMS) and
biomedical sensing and imaging. Control of both the thickness and the area of the nanoparticles
during deposition can be achieved via microcontact printing. QD-LED are also applied in novel
optoelectronic applications including near-field microscopy beyond the diffraction limit,
MEMS-basedmedical endoscopes for sub-cellular imaging, and compact light-on-chip
biosensors and biochips.
Reference
1. Wood, Vanessa, and Vladimir Bulović. "Colloidal quantum dot light-emitting devices." Nano
Reviews 1 (2010).
2. Hoshino, Kazunori, et al. "Nanoscale fluorescence imaging with quantum dot near-field
electroluminescence." Applied Physics Letters 101.4 (2012): 043118.
3. http://www.lanl.gov/discover/news-release-archive/2013/October/10.25-quantum-dot-
light-emitting-diodes.php
4. http://nanotechweb.org/cws/article/lab/39721
5. http://www.technologyreview.com/demo/405755/nanocrystal-displays/
6. Coe-Sullivan, Seth; Steckel, Jonathan S.; Kim, LeeAnn; Bawendi, Moungi G.; et al. (2005). "Method for
fabrication of saturated RGB quantum dot light emitting devices". Progress in Biomedical Optics and
Imaging
7. https://www.google.com/patents/WO2014177943A2?cl=en