Moisture-resistant solution-processed LEDs: built around nanoplatelets

May 14, 2018 //By Julien Happich
Moisture-resistant solution-processed LEDs: built around nanoplatelets
An international team of researchers from Italy's ISMac-CNR, Milan University and France's CNRS has taken the advances reported in the use of organic interlayers with spherical colloidal quantum dots (QDs) to boost the quantum efficiency of colloidal nanoplatelets (NPLs)-based 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.

Multilayered-device architecture for the NPL-LEDs:
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.

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