(Via Harvard)
A Quantum Dot (QD) is a semiconductor where excitons (bound stat of an electron and hole) are confined in all directions. Quantum Dots are already being used in communications and solar panels, to name a few. Even though QDs have outstanding optical properties, high extinction co-efficient, efficiency is not their strong suit.
Since the dots are nano-sized individual semiconductor elements, it is difficult to direct the electrical flow within them. Electrons have a tendency to be dissipated among the quantum dots instead of travelling between electrodes.
Researchers at Harvard University have published a paper detailing a process to improve on this efficiency. The remedy comes in a coating applied to these quantum dots. The author of the paper, Edward Likovich, visually describe the new process as putting QD’s in insulator egg cartons. This insulator separates the individual QD’s and prevents conduction between them.
To accomplish this, they create a single layer of QD’s, made up of a CdSe core and a ZnS shell, with hydrophobic octadecylamine ligands (QD coordinators). They then transfer this layer to an Al-doped ZnO (AZO) glass that acts at the bottom electrode of an LED. The insulator “egg carton” layer is made of Aluminum Oxide (Al2O3). To deposit this thin layer of Al2O3 over the QD’s, alternating sprays of water and an organometallic compound containing aluminum are applied at 150 degrees Celsius. Since the QD’s are positioned using hydrophobic ligands, only the oxygen adheres to create the layer of Al2O3. All of this is finally covered with an AZO top electrode.
Edward Likovich, author of the paper, explained, "The process provides a mechanical and energetic barrier between adjacent quantum dots, so the current tends to flow perpendicularly as opposed to being dissipated among dots in the layer... Also, because we have this mechanical barrier between the dots, we can do post-processing to remove the ligands while holding the dots in place, preventing agglomeration.”
A simple test using LEDs shows that the egg-carton does funnel electrons through the QD’s far better than untreated QD’s. Researchers believe the efficiency of these circuits can be improved by changing the ligands used to coordinate the position of the dots. Since the team assumes ligands become charged themselves and divert electrons from passing in between the electrodes.
Along with the solar cell based on QDs from NREL, perhaps Harvard's new QD system will usher in a new era of solar energy collection.
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