- Quantum Dot LEDs Incorporate Thermally Polymerized Hole Transport Layer
Anne L. Fischer
Colloidal semiconductor quantum dots show promise for use in optoelectronic devices such as LEDs and solar cells. Now a group led by David S. Ginger and Alex K.-Y. Jen at the University of Washington in Seattle has fabricated multilayer nanocrystal LEDs by spincoating a monolayer of colloidal CdSe/CdS nanocrystals atop polymerized hole transport layers.
The hybrid quantum dot LED was made with a thermally polymerized hole transport layer and a spin-coated monolayer of orange-emitting CdSe/CdS quantum dots.
Ginger, an assistant professor of chemistry, said that the best devices use a single monolayer of quantum dots. The challenge was controlling the thickness of the quantum dot layer independent of the organic one.
The investigators found that spincoating the quantum dots from a chloroform solution onto the solventresistant, thermally cross-linked layer yields nearly pure electroluminescence from the quantum dots. By using a second solution-processible hole transport layer, they improved the efficiency to approximately 0.8 percent at a brightness of 100 cd/m2.
Ginger believes that the LEDs show such pure electroluminescence even with imperfect quantum dot layers because the carriers are confined to recombine very near the quantum dot layer. Alternatively, it is possible that the cross-linked hole transport polymer is less intrinsically electroluminescent than its more common molecular analogue.
The researchers believe that the performance can be further improved. They will work to optimize the energy levels and will create devices with multiple hole transport layers. They also expect to improve the photonic structure of the LED.
Nano Letters, online Feb. 1, 2006, doi: 10.1021/nl052417e.
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