7 research outputs found
A Solution-Processed Heteropoly Acid Containing MoO<sub>3</sub> Units as a Hole-Injection Material for Highly Stable Organic Light-Emitting Devices
We report hole-injection
layers (HILs) comprising a heteropoly acid containing MoO<sub>3</sub> units, phosphomolybdic acid (PMA), in organic light-emitting devices
(OLEDs). PMA possesses outstanding properties, such as high solubility
in organic solvents, very low surface roughness in the film state,
high transparency in the visible region, and an appropriate work function
(WF), that make it suitable for HILs. We also found that these properties
were dependent on the postbaking atmosphere and temperature after
film formation. When the PMA film was baked in N<sub>2</sub>, the
Mo in the PMA was reduced to MoÂ(V), whereas baking in air had no influence
on the Mo valence state. Consequently, different baking atmospheres
yielded different WF values. OLEDs with PMA HILs were fabricated and
evaluated. OLEDs with PMA baked under appropriate conditions exhibited
comparably low driving voltages and higher driving stability compared
with OLEDs employing conventional hole-injection materials (HIMs),
polyÂ(3,4-ethyleneÂdioxyÂthiophene):polyÂ(4-styreneÂsulfonate),
and evaporated MoO<sub>3</sub>, which clearly shows the high suitability
of PMA HILs for OLEDs. PMA is also a commercially available and very
cheap material, leading to the widespread use of PMA as a standard
HIM
Two-Dimensional Ca<sub>2</sub>Nb<sub>3</sub>O<sub>10</sub> Perovskite Nanosheets for Electron Injection Layers in Organic Light-Emitting Devices
We report in this
article the application of calcium niobate (CNO)
perovskite nanosheets for electron injection layers (EILs) in organic
light-emitting devices (OLEDs). Four kinds of tetraalkylammonium hydroxides
having different alkyl lengths were utilized as the exfoliation agents
of a layered compound precursor HCa<sub>2</sub>Nb<sub>3</sub>O<sub>10</sub> to synthesize CNO nanosheets, including tetramethylammonium
hydroxide, tetraethylammonium hydroxide, tetrapropylammonium hydroxide
(TPAOH), and tetrabutylammonium hydroxide. CNO nanosheet EILs were
applied in fluorescent polyÂ[(9,9-di-<i>n</i>-octylfluorenyl-2,7-diyl)-<i>alt</i>-(benzoÂ[2,1,3]Âthiadiazol-4,8-diyl)] (F8BT) organic light-emitting
polymer-based devices. The effects of dispersion concentrations and
alkyl chain length on the devices’ performances were investigated.
The results demonstrated that OLEDs’ performances were related
to the coverage ratio of the CNO nanosheets, their thicknesses, and
their work function values. Among the four exfoliation agents, the
device with CNO nanosheets exfoliated by TPAOH showed the lowest driving
voltage. The OLEDs with the CNO nanosheet EILs showed lower driving
voltages compared with the devices with conventional EIL material
lithium 8-quinolate
Solution-Processed Inorganic–Organic Hybrid Electron Injection Layer for Polymer Light-Emitting Devices
A lithium quinolate complex (Liq) has high solubility
in polar
solvents such as alcohols and can be spin-coated onto emitting polymers,
resulting in a smooth surface morphology. A polymer light-emitting
device fabricated with spin-coated Liq as an electron injection layer
(EIL) exhibited a lower turn-on voltage and a higher efficiency than
a device with spin-coated Cs<sub>2</sub>CO<sub>3</sub> and a device
with thermally evaporated Ca. The mixture of ZnO nanoparticles and
Liq served as an efficient EIL, resulting in a lower driving voltage
even in thick films (∼10 nm), and it did not require a high-temperature
annealing process
Efficient Electron Injection by Size- and Shape-Controlled Zinc Oxide Nanoparticles in Organic Light-Emitting Devices
Three
different sized zinc oxide (ZnO) nanoparticles were synthesized as
spherical ZnO (S-ZnO), rodlike ZnO (R-ZnO), and intermediate shape
and size ZnO (I-ZnO) by controlling the reaction time. The average
sizes of the ZnO nanoparticles were 4.2 nm × 3.4 nm for S-ZnO,
9.8 nm × 4.5 nm for I-ZnO, and 20.6 nm × 6.2 nm for R-ZnO.
Organic light-emitting devices (OLEDs) with these ZnO nanoparticles
as the electron injection layer (EIL) were fabricated. The device
with I-ZnO showed lower driving voltage and higher power efficiency
than those with S-ZnO and R-ZnO. The superiority of I-ZnO makes it
very effective as an EIL for various types of OLEDs regardless of
the deposition order or method of fabricating the organic layer, the
ZnO layer, and the electrode
High-Efficiency Perovskite Quantum-Dot Light-Emitting Devices by Effective Washing Process and Interfacial Energy Level Alignment
All
inorganic perovskites quantum dots (PeQDs) have attracted much attention
for used in thin film display applications and solid-state lighting
applications, owing to their narrow band emission with high photoluminescence
quantum yields (PLQYs), color tunability, and solution processability.
Here, we fabricated low-driving-voltage and high-efficiency CsPbBr<sub>3</sub> PeQDs light-emitting devices (PeQD-LEDs) using a PeQDs washing
process with an ester solvent containing butyl acetate (AcOBu) to
remove excess ligands from the PeQDs. The CsPbBr<sub>3</sub> PeQDs
film washed with AcOBu exhibited a PLQY of 42%, and a narrow PL emission
with a full width at half-maximum of 19 nm. We also demonstrated energy
level alignment of the PeQD-LED in order to achieve effective hole
injection into PeQDs from the adjacent hole injection layer. The PeQD-LED
with AcOBu-washed PeQDs exhibited a maximum power efficiency of 31.7
lm W<sup>–1</sup> and EQE of 8.73%. Control of the interfacial
PeQDs through ligand removal and energy level alignment in the device
structure are promising methods for obtaining high PLQYs in film state
and high device efficiency
Addition of Lithium 8‑Quinolate into Polyethylenimine Electron-Injection Layer in OLEDs: Not Only Reducing Driving Voltage but Also Improving Device Lifetime
Solution-processed
electron injection layers (EILs) comprising lithium 8-quinolate (Liq)
and polyethylÂenimine
ethoxylated (PEIE) are highly effective for enhancing electron injection
from ZnO to organic layers and improving device lifetime in organic
light-emitting devices (OLEDs). Doping of Liq into PEIE further reduces
the work function of zinc oxide (ZnO) by enhancing dipole formation.
The intermolecular interaction between Liq and PEIE was elucidated
by UV–vis absorption measurement and quantum chemical calculation.
The OLEDs with ZnO covered with PEIE:Liq mixture exhibited lower driving
voltage than that of the device without Liq. Furthermore, as doping
concentration of Liq into PEIE increased, the device lifetime and
voltage stability during constant current operation was successively
improved
Conjugated Polyelectrolyte Blend with Polyethyleneimine Ethoxylated for Thickness-Insensitive Electron Injection Layers in Organic Light-Emitting Devices
Electron injection
layers (EILs) based on a simple polymer blend of polyethyleneimine
ethoxylated (PEIE) and polyÂ[(9,9-bisÂ(3′-((<i>N</i>,<i>N</i>-dimethyl)-<i>N</i>-ethylammonium)-propyl)-2,7-fluorene)-<i>alt</i>-2,7-(9,9-dioctylfluorene)] (PFN-Br) can suppress the
dependence of organic light-emitting device (OLED) performance on
thickness variation compared with single PEIE or PFN-Br EILs. PEIE
and PFN-Br were compatible with each other and PFN-Br uniformly mixed
in the PEIE matrix. PFN-Br in PEIE formed more fluorene–fluorene
pairs than PFN-Br alone. In addition, PEIE:PFN-Br blends reduced the
work function (WF) substantially compared with single PEIE or PFN-Br
polymer. PEIE:PFN-Br blends were applied to EILs in fluorescent polymer-based
OLEDs. Optimized PEIE:PFN-Br blend EIL-based devices presented lower
driving voltages and smaller dependences of device performance on
EIL thickness than single PEIE or PFN-Br-based devices. These improvements
were attributed to electron-transporting fluorene moieties, increased
fluorene–fluorene pairs working as channels of electron transport,
and the large WF reduction effect of PEIE:PFN-Br blends