3 research outputs found
Multifunctional Dendrimer Ligands for High-Efficiency, Solution-Processed Quantum Dot Light-Emitting Diodes
We
present multifunctional dendrimer ligands that serve as the
charge injection controlling layer as well as the adhesive layer at
the interfaces between quantum dots (QDs) and the electron transport
layer (ETL) in quantum dot light-emitting diodes (QLEDs). Specifically,
we use primary amine-functionalized dendrimer ligands (<i>e</i>.<i>g</i>., a series of polyÂ(amidoamine) dendrimers (PADs,
also referred to PAMAM)) that bind to the surface of QDs by replacing
the native ligands (oleic acids) and also to the surface of ZnO ETL.
PAD ligands control the electron injection rate from ZnO ETL into
QDs by altering the electronic energy levels of the surface of ZnO
ETL and thereby improve the charge balance within QDs in devices,
leading to the enhancement of the device efficiency. As an ultimate
achievement, the device efficiency (peak external quantum efficiency)
improves by a factor of 3 by replacing the native ligands (3.86%)
with PAD ligands (11.36%). In addition, multibranched dendrimer ligands
keep the QD emissive layer intact during subsequent solution processing,
enabling us to accomplish solution-processed QLEDs. The approach and
results in the present study emphasize the importance of controlling
the ligands of QDs to enhance QLED performance and also offer simple
yet effective chemical mean toward all-solution-processed QLEDs
Impact of Morphological Inhomogeneity on Excitonic States in Highly Mismatched Alloy ZnSe<sub>1–<i>X</i></sub>Te<sub><i>X</i></sub> Nanocrystals
ZnSe1–XTeX nanocrystals (NCs) are promising photon emitters
with tunable
emission across the violet to orange range and near-unity quantum
yields. However, these NCs suffer from broad emission line widths
and multiple exciton decay dynamics, which discourage their practicable
use. Here, we explore the excitonic states in ZnSe1–XTeX NCs and their photophysical
characteristics in relation to the morphological inhomogeneity of
highly mismatched alloys. Ensemble and single-dot spectroscopic analysis
of a series of ZnSe1–XTeX NC samples with varying Te ratios coupled with computational
calculations shows that, due to the distinct electronegativity between
Se and Te, nearest-neighbor Te pairs in ZnSe1–XTeX alloys create localized
hole states spectrally distributed approximately 130 meV above the
1Sh level of homogeneous ZnSe1–XTeX NCs. This forms spatially separated
excitons (delocalized electron and localized hole in trap), accounting
for both inhomogeneous and homogeneous line width broadening with
delayed recombination dynamics. Our results identify photophysical
characteristics of excitonic states in NCs made of highly mismatched
alloys and provide future research directions with potential implications
for photonic applications
Colloidal Spherical Quantum Wells with Near-Unity Photoluminescence Quantum Yield and Suppressed Blinking
Thick
inorganic shells endow colloidal nanocrystals (NCs) with
enhanced photochemical stability and suppression of photoluminescence
intermittency (also known as blinking). However, the progress of using
thick-shell heterostructure NCs in applications has been limited due
to the low photoluminescence quantum yield (PL QY ≤ 60%) at
room temperature. Here, we demonstrate thick-shell NCs with CdS/CdSe/CdS
seed/spherical quantum well/shell (SQW) geometry that exhibit near-unity
PL QY at room temperature and suppression of blinking. In SQW NCs,
the lattice mismatch is diminished between the emissive CdSe layer
and the surrounding CdS layers as a result of coherent strain, which
suppresses the formation of misfit defects and consequently permits
∼100% PL QY for SQW NCs with a thick CdS shell (≥5 nm).
High PL QY of thick-shell SQW NCs is preserved even in concentrated
dispersion and in film under thermal stress, which makes them promising
candidates for applications in solid-state lightings and luminescent
solar concentrators