Will nanoplatelets displace quantum dots in solution-processed LEDs?
Thanks to their efficient and narrowband luminescence and their relative ease of mass production (compared to quantum dots), the researchers note that nanoplatelets are promising candidates for solution-processable ultrahigh colour purity light-emitting diodes (LEDs). Colloidal semiconductor NPLs behave like two-dimensional quantum wells and their luminescence can be controlled through particle thickness (during their elaboration), they do not suffer from Auger recombination.
ITO/PEDOT:PSS/PVK/NPLs/P1/metal cathode.
In a paper titled “Efficient Solution-Processed Nanoplatelet-Based Light-Emitting Diodes with High Operational Stability in Air” published in Nano Letters, the researchers relate how they engineered a charge-regulating layer and achieved an External Quantum Efficiency of 5.73% at 658nm (colour saturation 98%) in completely solution processed LEDs. This is over one order of magnitude over the best reports of red NPL-LEDs designed so far, the authors write, while setting a new record for quantum-dot LEDs of any colour embedding solution-deposited organic interlayers.
The researchers used conjugated polymers with polar and electrolytic side-chains (also known as conjugated polyelectrolytes) as electron-transport materials to maximize carrier balance in the device and improve the compatibility with the metal cathode. Through functionalization, the polymer becomes easily soluble in polar solvents such as methanol or water, which are orthogonal to the nonpolar media typically used for processing the active layer in QD- and NPL-LEDs. This means one can deposit the electron-transport layer (ETL) using wet methods such as spin-coating and roll-to-roll or inkjet printing without damaging the underlying active layer.
What’s more, the authors report, using conjugated polyelectrolytes as the electron-transport layer allows NPL-based LED designers to tune the electron-injection barrier between the metallic cathode and the active QD layer by creating interface dipoles, which contribute to an improved carrier balance and higher EQEs.
Interestingly, planar LEDs built around CdSe/CdZnS core−shell NPLs with conjugated polyelectrolytes for the HTL do not seem to require some sort of encapsulation nor suffer from air exposure. In fact, the LEDs’ EQE increased when exposed to ambient air (with 50% humidity), reaching a record EQE of 8.39% after 5.5 hours of air exposure. This was achieved with NPLs whose photoluminescence quantum yield was only about 40%, notes the paper, hinting that optimized NPLs with a unity emission yield could probably help design fully solution-processed NPL-LEDs with an external quantum efficiency as high as 21%, without engineered light outcoupling.
The authors also stress-tested their devices, demonstrating a remarkable operational stability with no measurable efficiency drop even after over four hours of continuous operation in humid air (without encapsulation).
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