9 research outputs found
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
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
Molecular Interdiffusion between Stacked Layers by Solution and Thermal Annealing Processes in Organic Light Emitting Devices
In
organic light emitting devices (OLEDs), interfacial structures
between multilayers have large impacts on the characteristics of OLEDs.
Herein, we succeeded in revealing the interdiffusion in solution processed
and thermal annealed OLEDs by neutron reflectometry. We investigated
interfaces between a polymer under layer and small molecules upper
layer. The small molecules diffused into the swollen polymer layer
during the interfacial formation by the solution process, but the
polymer did not diffuse into the small molecules layer. At temperatures
close to the glass transition temperatures of the materials, asymmetric
molecular diffusion was observed. We elucidated the effects of the
interdiffusion on the characteristics of OLEDs. Partially mixing the
interface improved the current efficiencies due to suppressed triplet-polaron
quenching at the interface. Controlling and understanding the interfacial
structures of the miultilayers will be more important to improve the
OLED characteristics
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 Ă— 3.4 nm for S-ZnO,
9.8 nm Ă— 4.5 nm for I-ZnO, and 20.6 nm Ă— 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
Colorful Squaraines Dyes for Efficient Solution-Processed All Small-Molecule Semitransparent Organic Solar Cells
Semitransparent organic solar cells
(ST-OSCs) exhibit highly promising
applications to develop integrated photovoltaics and power-generating
windows. However, the development of ST-OSCs is significantly lagging
behind opaque OSCs, especially for all small-molecule ST-OSCs. Here,
four unique squaraines dyes (IDPSQ, SQ-BP, D-BDT-SQ, and AzUSQ) were
successfully used as donors in ST-OSCs, whose colors can be tuned
from purple to blue, green, and dark green, respectively. While using
ultrathin Ag as a transparent electrode, the ST-OSCs fabricated using
IDPSQ:PC<sub>71</sub>BM, SQ-BP:PC<sub>71</sub>BM, D-BDT-SQ:PC<sub>71</sub>BM, and AzUSQ:PC<sub>71</sub>BM yield power conversion efficiencies
(PCEs) of 2.96, 4.36, 4.91, and 1.71%, respectively, and their colors
are purple, cyan, brown, and light brown, respectively. Compared to
their opaque OSCs (PCEs of 3.95, 5.45, 5.84, and 1.91%, respectively),
the reduction in the PCEs are as low as 25, 20, 16, and 10%, respectively.
Furthermore, each of these ST-OSCs exhibit good average visible transmittance
(AVT) of 25–30%, favorable CIE color coordinates, and a color
rendering index (CRI) of 71–97%. Finally, by changing the thickness
of the Ag electrode, an impressive PCE of 4.9% along with an AVT of
25% and a CRI of 97% can be obtained in D-BDT-SQ:PC<sub>71</sub>BM-based
ST-OSCs, which is the highest PCE among all small-molecule ST-OSCs
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
A Series of Lithium Pyridyl Phenolate Complexes with a Pendant Pyridyl Group for Electron-Injection Layers in Organic Light-Emitting Devices
We report a new series
of lithium pyridyl phenolate complexes with a pendant pyridyl group,
Li2BPP, Li3BPP, and Li4BPP, in which the pendant pyridines are substituted
at the 2-, 3-, and 4-positions, respectively. The most important difference
between these complexes is their molecular planarity; Li3BPP and Li4BPP
adopt twisted bipyridine structures, whereas Li2BPP adopts a planar
structure owing to the steric hindrance and chelating effect of bipyridine
on the Li core. The planar structure leads to crystallization through π–π
stacking interactions, and the small differences in the molecular
structures of the pendant pyridine rings cause drastic differences
in the physical properties of thin solid films of these complexes.
We applied these complexes as electron-injection layers (EILs) in
IrÂ(ppy)<sub>3</sub>-based organic light-emitting devices. When thin
EILs were used, Li3BPP and Li4BPP afforded lower driving voltages
than Li2BPP; the order of the driving voltages followed the order
of their electron affinity values. Moreover, the dependence of driving
voltage on the EIL thickness was investigated for each complex. Among
the three LiBPP derivatives, Li2BPP-based devices showed almost negligible
EIL thickness dependence, which may be attributable to the high crystallinity
of Li2BPP. All LiBPP-based devices also showed higher stability than
conventional 8-quinolinolato lithium-based devices
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