posted on 2019-12-02, 13:04authored byYusuf Kelestemur, Yevhen Shynkarenko, Marco Anni, Sergii Yakunin, Maria Luisa De Giorgi, Maksym V. Kovalenko
Semiconductor nanoplatelets (NPLs) have emerged as a
very promising class of colloidal nanocrystals for light-emitting
devices owing to their quantum-well-like electronic and optical characteristics.
However, their lower photoluminescence quantum yield (PLQY) and limited
stability have hampered the realization of their outstanding luminescent
properties in device applications. Here, to address these deficiencies,
we present a two-step synthetic approach that enables the synthesis
of core/shell NPLs with precisely controlled shell composition for
engineering their excitonic properties. The proposed CdSe colloidal
quantum wells possess a graded shell, which is composed of a CdS buffer
layer and a CdxZn1–xS gradient layer, and exhibit bright emission (PLQY
75–89%) in the red spectral region (634–648 nm) with
a narrow emission line width (21 nm). These enhanced optical properties
allowed us to attain low thresholds for amplified spontaneous emission
(down to ∼40 μJ/cm2) under nanosecond laser
excitation. We also studied the electroluminescent performance of
these NPLs by fabricating solution-processed light-emitting diodes
(LEDs). In comparison to NPL-LEDs with CdSe/CdS core/shell NPLs, which
exhibit an external quantum efficiency (EQE) value of only 1.80%,
a significantly improved EQE value of 9.92% was obtained using graded-shell
NPLs, the highest value for colloidal NPL-based-LEDs. In addition,
the low efficiency roll-off characteristics of NPL-LEDs enabled a
high brightness of up to ∼46 000 cd/m2 with
an electroluminescence peak centered at 650 nm. These findings demonstrate
the paramount role that heterostructure engineering occupies in enhancing
the optoelectronic characteristics of semiconductor NPLs toward practically
relevant levels.