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Full-color quantum dot displays are getting within reach

(Nanowerk Spotlight) Quantum dots (QDs), because they are both photo-active (photoluminescent) and electro-active (electroluminescent) and have unique physical properties, will be at the core of next-generation displays. Compared to organic luminescent materials used in organic light emitting diodes (OLEDs), QD-based materials have purer colors, longer lifetime, lower manufacturing cost, and lower power consumption. Another key advantage is that, because QDs can be deposited on virtually any substrate, you can expect printable and flexible – even rollable – displays of all sizes.
Back in 2009, a team led by researchers from the Frontier Research Laboratory at the Samsung Advanced Institute of Technology, demonstrated a functioning monochromatic QD display, the first embodiment of QD light emission in a display ("High-performance crosslinked colloidal quantum-dot light-emitting diodes").
However, the lack of size-selective QD patterning by conventional methods has hindered the realization of full-color QD displays so far.
"To date, the integration of QDs into a full-color LED structure has not been possible due to the difficulty in patterning individual red–green–blue (RGB) QDs onto the pixelated display panel," Byoung Lyong Choi, a researcher at the Frontier Research Lab, tells Nanowerk. "Spin-coating QDs – the method used previously to fabricate monochrome displays – and other existing semiconductor processing methods result in cross-contamination of the RGB pixels. More recently, inkjet, contact printing and mist deposition processes have been proposed as alternative methods for QD patterning; however, they produce a non-uniform film with a rough surface morphology, leading to disordered QD arrays and/or poorly defined interfaces with the hole transport layer."
Now, the Samsung team has demonstrated a novel transfer printing approach which enables fine patterning of high-quality QD films for large-area (4-inch diagonal), full-color displays mounted on glass as well as on flexible plastic substrates.
Presenting their findings in the February 20, 2011 online edition of Nature Photonics ("Full-color quantum dot displays fabricated by transfer printing"), the team's new approach is associated with kinetic control and interfacial modification, which results in uniform QD films and well-ordered QD network structure.
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Schematic illustration of solvent-free transfer printing. a, Schematic of transfer printing process for patterning of quantum dots. The first step of the process begins with surface modification of the donor substrate by the introduction of a chemically bound SAM. After printing the red-emissive QDs, green- and blue-emissive QDs are printed by the same process, with precision alignment. (i) Modification of the donor surface with SAM, and spin-coating of QDs. (ii) Application of an elastomer stamp to the QD film with appropriate pressure. (iii) Peeling of the stamp, quickly, from the donor substrate. (iv) Contacting the inked stamp to the device stack, and slowly peeling back the stamp. (v)–(vii) Sequential transfer printing of green and blue QDs. b, Fluorescence micrograph of the transfer-printed RGB QD stripes onto the glass substrate, excited by 365 nm UV radiation. (Reprinted with permission from Nature Publishing Group).
"We developed a nano-transfer printing process of QD films with excellent morphology, well-ordered structure and clearly defined interfaces required to achieve a full-color display over a large area, with high resolution," says Choi. " We also achieved optimum QD film condition and 100% transfer yields through control of interfacial condition and kinetic control. QLEDs fabricated by our novel approach showed improved electroluminescent properties with more balanced charge behaviors due to the high quality films and well-defined interface resulted from solvent-free transfer process."
Compared to the previously applied technique of spin-coating the quantum dots, the researchers found that a key difference of the transfer-printed films is the degree of ordering in the structure.
"After crosslinking, QDs in both the spin-coated and transfer-printed films are rearranged and close-packed in all directions" explains Choi. "In the spin-coated film, the peak sharpness is significantly reduced in-plane and disappears out-of-plane. In the transfer-printed film, the peak height is slightly reduced in-plane and remains constant out-of-plane. This suggests that the ordered structure of QD packing remains after crosslinking in the printed QD films."
The approaches reported by the Samsung team represent a further step towards the application of colloidal quantum dots in full-color displays, solid state lightings, photovoltaics, and large-scale optoelectronic devices.
Choi notes that a fundamental understanding of the charge transport behavior of the printed QD films is still something that needs to be developed before commercial applications of highly efficient displays, LEDs and solar cells application can be expected.
He also cautions that the development of cadmium-free quantum dots is necessary for solving and preventing environmental issues in the long run.
By Michael is author of two books by the Royal Society of Chemistry: Nano-Society: Pushing the Boundaries of Technology and Nanotechnology: The Future is Tiny. Copyright © Nanowerk

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