A novel method of fabrication has been developed for a Light Emitting Device (LED) based on the use of Quantum Dots (QDs). This new process, described in a paper published in late October, 2011, overcomes some of the common problems observed with QDs, such as interactions between the dots, and their possible agglomeration (sticking together in a mass).
Quantum Dots (QDs) are nanoparticles made of semiconductor materials, such as cadmium selenide (CdSe), cadmium sulphide (CdS), etc. Their particularity is that they show, at the same time, the properties of a semiconductor and of a nanomaterial.
QDs have the potential to be used in several fields, such as biology, solar energy and optoelectronics. QD technology made remarkable progresses in recent years; because of this, several devices are now commercially available and currently used, for applications in biology and in solar cells. QD-based optoelectronic devices, on the contrary, are not yet ready to reach the market, due to some issues affecting their behaviour and performances.
QD optoelectronic devices
Optoelectronic devices based on QDs work as Light Emitting Diodes (LEDs); this means that they emit light when an electric current passes through them. Compared with other LEDs, QDs could emit light with brigther colors; there is therefore, a great interest in the development of these devices. The production of these devices and their performances, however, has to be improved.
Standard Geometry of QD devices and Possible Problems
Generally QDs-based devices have a multi-layer structure; QDs, the active region that generates light, are sandwiched between layers made of different material, the most common being Indium Tin Oxide, or ITO, a transparent material that also conducts electrical current.
For the devices made in this way, a common problem that is observed is the interaction between the individual particles; this could cause, for instance, energy transfer, and lead to a faster degradation of the device itself.
Other possible issues are related to the ligands employed in the QD synthesis. Ligands are generally organic molecules used to control the dimensions of the nanoparticles. Although their use is essential during the preparation, their presence in the final device can cause problems. This is because, if any post-deposition processing is necessary, they can degrade; this can cause agglomeration of the dots and, therefore, a worsening of the device performance.
Novel geometry for QD device
A research group in the School of Engineering and Applied Science of Harvard University recently developed a new method to fabricate QD devices that overcomes some of these problems.
The geometry of their device is shown in the Figure below. The QDs are made of CdSe and ZnS, while the electrical contact is made through two layer of aluminium-doped zinc oxide (AZO). In the fabrication process, there are two key elements:
- The use of octadecylamine (ODA) as a ligand in the synthesis and deposition of the QDs; this is a hydrophobic molecule.
- The use of Atomic Layer Deposition process (ALD) instead of the standard evaporation technique, to deposit a layer of aluminium oxide on the dots.
The Steps in the Fabrication
Dr. Edward Likovich, leading author of this study, explains to us the various steps in their process:
“In the first step, we deposited the QDs on the AZO substrate. Then we went to the ALD process: here we used alternated pulses of metallic aluminium and water, to deposit an insulating layer of aluminium oxide. Because the ALD process involves water and the dot ligands are hydrophobic, the layer of aluminium oxide firstly filled the interstices among the QDs; only on a second phase went to cover the dots themselves.
In this way, we have two main advantages: (1) there is a robust mechanical and electronic in-plane barrier to the dots, which prevents interactions between the dots and energy losses; (2) this mechanical barrier allows several forms of post deposition processing that would otherwise be destructive. In this way we can manipulate the deposited dots without causing any agglomeration.”
Better Electric Conduction
The figure to the left shows the different mechanisms of electric conduction for the device made with standard evaporation technique (left) and the one made with the ALD (right). In the second case, the current goes directly through the QDs; this means that the system is more efficient, i.e. a better signal can be emitted using the same amount of electricity.
Furthermore, the same approach could be used in fabrication processes with different ligands and/or different QD materials; this could lead to novel QD devices, and eventually to their commercialization.
E.M. Likovich et al.: “High-Current-Density Monolayer CdSe/ZnS Quantum Dot Light-Emitting Devices with Oxide Electrodes.” Advanced Materials, 23, 4521-4525 (October 18, 2011). Accessed November 23, 2011.
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