Polymeric hole-transport materials (HTMs) have been widely
used
in quantum-dot light-emitting diodes (QLEDs). However, their solution
processability normally causes interlayer erosion and unstable film
state, leading to undesired device performance. Besides, the imbalance
of hole and electron transport in QLEDs also damages the device interfaces.
In this study, we designed a bis-diazo compound, X1, as carbene cross-linker
for polymeric HTM. Irradiated by ultraviolet and heating, a poly[(9,9-dioctylfluorenyl-2,7-diyl)-alt(4,4′-(N-(4-butylphenyl))] (TFB)/X1 blend can achieve fast “electronically
clean” cross-linking with ∼100% solvent resistance.
The cross-linking reduced the stacking behaviors of TFB and thus led
to a lower hole-transport mobility, whereas it was a good match of
electron mobility. The carbene-mediated TFB cross-linking also downshifted
the HOMO level from −5.3 to −5.5 eV, delivering a smaller
hole-transport energy barrier. Benefiting from these, the cross-linked
QLED showed enhanced device performances over the pristine device,
with EQE, power efficiency, and current efficiency being elevated
by nearly 20, 15, and 83%, respectively. To the best of our knowledge,
this is the first report about a bis-diazo compound based carbene
cross-linker built into a polymeric HTM for a QLED with enhanced device
performance.
In this work, we report the finely controlled synthesis of Zn 1−x Mg x O nanoparticles (NPs) by a Corning advanced-flow reactor (AFR), which produced a uniform size distribution with 75% NPs in the range 3−4 nm with good reproducibility. The Zn 1−x Mg x O NPs have been applied as an electron transport layer (ETL) in red quantum light-emitting diodes (QLEDs). Different from the traditional method of one-pot synthesis which needs several hours to finish a synthetic cycle, the synthesis by AFR is much faster due to the fast heat and mass transport in the reaction chamber, and a typical synthetic cycle in the AFR needs only a few minutes. Optical properties of the NPs indicate that the Mg ion that substitutes the Zn ion in the ZnO matrix is almost linear with the doping concentration in the solution, which is consistent with the first-principles calculations. The Zn 1−x Mg x O NPs synthesized with the AFR were subsequently used as the ETL in fabricating red QLEDs, and the resultant performance exceeded that of the device based on the traditional one-pot synthesized NPs. For example, the external quantum efficiency, current efficiency, and power efficiency improved by 25.1%, 21.6%, and 26.9%, respectively. Moreover, better luminescence uniformity was observed in the QLED using the ZnMgO NPs synthesized with the AFR. The device reached an optimal performance when x is 15% in Zn 1−x Mg x O. The finely controlled synthesis of stable Zn 1−x Mg x O NPs has the potential in industry application to solve the critical issues of ETLs that impede progress in the mass production of QLEDs.
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