On
the basis of bluish-emitting double-shelled quantum dots (QDs)
of Zn–Cu–Ga–S (ZCGS)/ZnS/ZnS, Mn doping into
ZCGS host with different Mn/Cu concentrations is implemented via surface
adsorption and lattice diffusion. The resulting double-shelled Mn-doped
ZCGS (ZCGS/Mn) QDs exhibit a distinct Mn2+ 4T1–6A1 emission as a consequence
of effective lattice incorporation simultaneously with host intragap
states-involving emissions of free-to-bound and donor–acceptor
pair recombinations. The relative contribution of Mn emission to the
overall photoluminescence (PL) is consistently proportional to its
concentration, resulting in tunable PL from bluish, white, to reddish
white. Regardless of Mn doping and its concentration, all QDs possess
high PL quantum yield levels of 74–79%. Those undoped and doped
QDs are then employed as an emitting layer (EML) of all-solution-processed
QD-light-emitting diodes (QLEDs) with hybrid charge transport layers
and their electroluminescence (EL) is compared. Compared to undoped
QDs, doped analogues give rise to a huge spectral disparity of EL
versus PL, specifically showing a near-complete quenching of Mn2+ EL. This unexpected observation is rationalized primarily
by considering unbalanced carrier injection to QD EML on the basis
of energetic alignment of the present QLED and rapid trapping of holes
injected at intragap states of QDs.
In
contrast to a substantial progress of heavy metal-free green
and red emitters exclusively from indium phosphide (InP) quantum dots
(QDs), the development of non-Cd blue QDs remains nearly unexplored.
The synthesis of blue InP QDs with a bright, deep-blue emissivity
is not likely viable, which is primarily associated with their intrinsic
size limitation. To surmount this challenge, herein, the first synthesis
of blue-emissive ternary InGaP QDs through In3+-to-Ga3+ cation-exchange strategy is implemented. Pregrown InP QDs
turn out to be efficiently Ga-alloyed at a relatively low temperature
of 280 °C in the presence of Ga iodide (GaI3), and
the degree of Ga alloying is also found to be systematically adjustable
by varying GaI3 amounts. Such cation-exchanged InGaP cores
are surface-passivated sequentially with ZnSeS inner and ZnS outer
shells. As the amount of GaI3 added for cation exchange
increases, the resulting double-shelled InGaP/ZnSeS/ZnS QDs produce
consistent blue shifts in photoluminescence (PL) from 475 to 465 nm,
while maintaining high PL quantum yield in the range of 80–82%.
Among a series of QD samples, above 465 nm emitting InGaP/ZnSeS/ZnS
QDs are further employed as an emitting layer of an all-solution-processed
electroluminescent device. This unprecedented InGaP QD-based blue
device generates maximum values of 1038 cd/m2 in luminance
and 2.5% in external quantum efficiency.
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