Quantum dots (QDs) semiconductor nanocrystal have gained significant
interest because of their unique properties, which include high color purity,
high quantum efficiency, and tunable emission wavelength. They are being
considered to be alternative emissive materials for next-generati...
Quantum dots (QDs) semiconductor nanocrystal have gained significant
interest because of their unique properties, which include high color purity,
high quantum efficiency, and tunable emission wavelength. They are being
considered to be alternative emissive materials for next-generation of lightemitting
diodes (LEDs) in display, and lighting applications. QD chemistry
seems rather simple, but obtaining extremely tiny size is an big challenge. The
cost of QD manufacture and the device fabrication also needs to be minimized.
QD materials are unsuitable for conventional vacuum evaporation deposition
for making opto-electronic devices like organic light-emitting diodes (OLEDs).
Spin-coating is the most conventional solution process for the formation of QD
thin film in laboratories; however, its application is restricted in large-scale
patterning deposition. The relatively very high cost of QD materials and the
loss of more than 96% of the supply solution during spin-coat are the mostxi
reasons to search for an alternative deposition approach that is capable of
patterning and minimum consuming QD materials.
In the electrohydrodynamic (EHD) - jet printing process, a liquid jet breaks up
at tip head into fine droplets under the external electric field. There are various
jetting modes that is created from the QD supply solution, such as dripping,
spindle, Taylor cone-jet, multi-jet modes, or electrospray mode. Based on the
individual purposes of making electronic devices, an EHD jet printing can be
used for continuous patterning, dots patterning, thin film deposition, and
various printing modes.
Herein, by using the unique three spraying techniques: the big circular thin
film, the spiral line and the straight-line method, with the combination of EHD
jet printing parameters, we achieved a large, smooth and uniform printed-QD
thin films from the QD hexane solution, then all-solution processed QD-LEDs
with normal structure (ITO/PEDOT:PSS/PVK/EHD sprayed-QDs/ZnO/Al)
were successfully fabricated. The QDLED devices obtained low turn-on
voltage of 3.0 V, maximum current efficiency of 2.93 cd A-1
, and a maximum
luminance of 7801 cd m-2 at the voltage of 11.5 V.
Even though the device performance in our first work was quite good, the RMS
roughness was high due to the large size of sprayed-QD droplets, and there
were many QD droplet marks with the rough rings due to the quick solvent
evaporation, which is still a difficult task to be solved. Therefore, we tried to
figure out these issues by surveillance the rate of solvent evaporation and
reducing the size of sprayed-QD droplets by using EHD electrospray mode
with a solvent mixture of QDs, n-hexane and n-butanol solution.The formation
of a printed- QD thin film is based on the ordering and collection of individual xii
QD droplets by droplets. To understand the EHD electrospray mechanism, the
initial QD droplet size, spraying time, and solvent evaporation rate were
estimated by applying of Scaling law and Rayleigh law to find out the suitable
EHD parameters, as well as properties of the supply solution. We successfully
developed large and smooth electrosprayed- QD as an emissive layer by
controlling the EHD conditions and treating the QD supply solution to obtain
a homogeneous dispersion and maintain the stable EHD electrospray mode. A
QD-LED device based on electrodeposed- QD film showed a maximum
luminance of 12,082 cd m-2
, maximum EQE of 1.86%, and maximum current
efficiency of nearly 4.0 cd A-1
.
The architecture designing of QDLEDs can be separated into conventional and
inverted structures. Among them, inverted QDLED devices structure rather
than the regular structure is becoming popular, and it is very beneficial for
future display applications. The transparent bottom cathode of the inverted
QDLEDs can be directly linked to n-channel metal oxide or TFTs backplane.
Therefore, we carry on with research by making the inverted green QDLED
with a structure of (ITO Glass/ZnO/QDs/PVK/PEDOT:PSS/Al/Encap). To
solve problem of hydrophilic/hydrophobic- PEDOT:PSS/PVK interface, we
firstly developed the stable EHD electrospray mode by using the supply
solution of (PEDOT:PSS: Water:IPA: Triton X-100) then printed liquid
droplets on top of PVK pattern. The optimized electrosprayed-PEDOT:PSS
thin film obtained extremely smooth Ra roughness of 0.55 nm after 35 minutes
deposition. A Green-inverted-QDLED device made of the HIL layer by the
electrospray process exhibits a maximum luminance of 5692 cd m-2
, current
density of 3.76 cd A-1
, EQE of 2.23% which comparable to the performa
Quantum dots (QDs) semiconductor nanocrystal have gained significant
interest because of their unique properties, which include high color purity,
high quantum efficiency, and tunable emission wavelength. They are being
considered to be alternative emissive materials for next-generation of lightemitting
diodes (LEDs) in display, and lighting applications. QD chemistry
seems rather simple, but obtaining extremely tiny size is an big challenge. The
cost of QD manufacture and the device fabrication also needs to be minimized.
QD materials are unsuitable for conventional vacuum evaporation deposition
for making opto-electronic devices like organic light-emitting diodes (OLEDs).
Spin-coating is the most conventional solution process for the formation of QD
thin film in laboratories; however, its application is restricted in large-scale
patterning deposition. The relatively very high cost of QD materials and the
loss of more than 96% of the supply solution during spin-coat are the mostxi
reasons to search for an alternative deposition approach that is capable of
patterning and minimum consuming QD materials.
In the electrohydrodynamic (EHD) - jet printing process, a liquid jet breaks up
at tip head into fine droplets under the external electric field. There are various
jetting modes that is created from the QD supply solution, such as dripping,
spindle, Taylor cone-jet, multi-jet modes, or electrospray mode. Based on the
individual purposes of making electronic devices, an EHD jet printing can be
used for continuous patterning, dots patterning, thin film deposition, and
various printing modes.
Herein, by using the unique three spraying techniques: the big circular thin
film, the spiral line and the straight-line method, with the combination of EHD
jet printing parameters, we achieved a large, smooth and uniform printed-QD
thin films from the QD hexane solution, then all-solution processed QD-LEDs
with normal structure (ITO/PEDOT:PSS/PVK/EHD sprayed-QDs/ZnO/Al)
were successfully fabricated. The QDLED devices obtained low turn-on
voltage of 3.0 V, maximum current efficiency of 2.93 cd A-1
, and a maximum
luminance of 7801 cd m-2 at the voltage of 11.5 V.
Even though the device performance in our first work was quite good, the RMS
roughness was high due to the large size of sprayed-QD droplets, and there
were many QD droplet marks with the rough rings due to the quick solvent
evaporation, which is still a difficult task to be solved. Therefore, we tried to
figure out these issues by surveillance the rate of solvent evaporation and
reducing the size of sprayed-QD droplets by using EHD electrospray mode
with a solvent mixture of QDs, n-hexane and n-butanol solution.The formation
of a printed- QD thin film is based on the ordering and collection of individual xii
QD droplets by droplets. To understand the EHD electrospray mechanism, the
initial QD droplet size, spraying time, and solvent evaporation rate were
estimated by applying of Scaling law and Rayleigh law to find out the suitable
EHD parameters, as well as properties of the supply solution. We successfully
developed large and smooth electrosprayed- QD as an emissive layer by
controlling the EHD conditions and treating the QD supply solution to obtain
a homogeneous dispersion and maintain the stable EHD electrospray mode. A
QD-LED device based on electrodeposed- QD film showed a maximum
luminance of 12,082 cd m-2
, maximum EQE of 1.86%, and maximum current
efficiency of nearly 4.0 cd A-1
.
The architecture designing of QDLEDs can be separated into conventional and
inverted structures. Among them, inverted QDLED devices structure rather
than the regular structure is becoming popular, and it is very beneficial for
future display applications. The transparent bottom cathode of the inverted
QDLEDs can be directly linked to n-channel metal oxide or TFTs backplane.
Therefore, we carry on with research by making the inverted green QDLED
with a structure of (ITO Glass/ZnO/QDs/PVK/PEDOT:PSS/Al/Encap). To
solve problem of hydrophilic/hydrophobic- PEDOT:PSS/PVK interface, we
firstly developed the stable EHD electrospray mode by using the supply
solution of (PEDOT:PSS: Water:IPA: Triton X-100) then printed liquid
droplets on top of PVK pattern. The optimized electrosprayed-PEDOT:PSS
thin film obtained extremely smooth Ra roughness of 0.55 nm after 35 minutes
deposition. A Green-inverted-QDLED device made of the HIL layer by the
electrospray process exhibits a maximum luminance of 5692 cd m-2
, current
density of 3.76 cd A-1
, EQE of 2.23% which comparable to the performa
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