Prospects of Colloidal Nanocrystals for Electronic - Computer Science

Colloidal Nanocrystals in Electronic Applications Chemical Reviews, 2010, Vol. 110, No. 1 421
Figure 36. White light emitting LED utilizing a mixture of red, green, and blue emitting semiconductor nanocrystals. (a) Normalized EL
spectra of devices containing only blue-, green-, and red-emitting nanocrystals. (b) Device cross section, and (c) photograph of white
QD-LED. Reprinted with permission from ref 93. Copyright 2007 American Chemical Society.
core in emitting CdSe/ZnS core-shell NCs from 3.2 to 5.8
nm; ηext was improved to ∼1.1% by optimizing the thickness
of ZnS shell. 407 Optimization of packing and ordering of NCs
allowed enhancing ηext to over 2% (>1 lmW -1 ) with a
maximum brightness of over 7000 cd m -2 . 402 The electroluminescence
spectrum was dominated by the NC emission
with excellent color purity. 402 Steckel et al. reported LEDs
utilizing CdxZn1-xSe/CdyZn1-yS core-shell NCs integrated
into a four-layered HTL/QDs/HBL/ETL structure. 403 The
color saturated green emission was achieved with a peak ηext
of 0.5% at low operating voltages (1100 cd m -2 ). 405 Uniform and defect-free
EL emission from the QDs over a large surface area (1.5
cm × 2.5 cm) was displayed, promising for the application
of QD-LEDs in large-area displays.
Other potential areas of application for thin film LEDs
are large area lighting systems and backlighting for liquid
crystal displays. Such applications require LEDs emitting
white light. Here, colloidal NCs with their tunable emission
colors and the possibility of inexpensive solution-based
device fabrication have a potential to successfully compete
with other technologies such as OLEDs. Li et al. fabricated
white organic/NCs LEDs by combining blue emitting poly[(9,9dihexyloxyfluoren-2,7-diyl)-alt-co-(2-methoxy-5-phenylen-
1,4-diyl)] (PFH-MEH) polymer doped with red-emitting QDs
and green-emitting Alq3. 413 The key for balanced white
emission in the hybrid ternary systems PFH-MEH/QDs/Alq3
was the charge-transfer processes from PFH-MEH and Alq3
to the QDs. Maximum external quantum efficiency of 0.24%
at1mAcm -2 and 11 V during operation in air was reported.
Anikeeva et al. reported white QD-LEDs with the emitting
layer assembled of a balanced mixture of red, green, and
blue emitting NCs integrated into ITO/PEDOT:PSS/TPD/
QDs/TAZ/Alq3 device (Figure 36). 93 The mixed-monolayer
QD-LED revealed uniformly white luminescence with the
international commission on illumination (CIE) coordinates
(0.35, 0.41) at an applied voltage of 9 V. At 5 V applied
bias (1.51 mA/cm 2 ), the peak ηext of white QD-LED was
0.36%, corresponding to 0.9 cd A -1 . The independent
processing of HTL and emitting layer of NCs provided
precise tuning of the emission spectrum by simply changing
the color ratio of QDs without altering the device structure.
In contrast to significant efficiency improvements of QD-
LEDs, the lifetime of hybrid organic/NC devices is limited
by the instability of the metal contacts and degradation of
organic components under high current operation conditions.
In an attempt to overcome these limitations, the organic hole
transport and electron transport layers have been replaced
with much more robust inorganic materials. Caurge et al.
reported QD-LEDs with p-type inorganic NiO films as
HTL. 266 Inorganic NiO films demonstrated higher chemical,
thermal, and electrical stability as compared to TPD and other
tested organic materials. A maximum ηext ≈ 0.18% and
brightness ∼3000 cd m -2 were achieved by optimizing
resistivity of the NiO layer. In the next step toward
all-inorganic QD LEDs, same authors fabricated inorganic
ETL by replacing TAZ and Alq3 layers with sputtered
amorphous ZnO:SnO2 semiconductor. 266 The all-inorganic
QD-LED combined HTL of p-type NiO, luminescent
Cd1-xZnxSe NCs as the emitting layer, and n-type ZnO:SnO2
ETL as shown in Figure 37. These devices demonstrated
pure QD emission with the peak luminance of 1950 cd m -2
and enabled very high injection currents of 3.5A cm -2 . The
maximum ηext was about 0.1%. The ability of inorganic HTL
and ETL to drive high currents into QD layers may one day
open up bright perspectives for electrically pumped lasers
on colloidal NCs.
Highly luminescent spectrally tunable core-shell NCs can
also be coupled to bright and efficient single-crystal LEDs.
This approach utilizes NCs as the down-converting medium,
which efficiently absorbs blue light emitted by, for example,