“Our research group specializes in the development and spectroscopic study of nanoscale semiconductor particles, known as nanocrystals or quantum dots,” says team leader Victor Klimov of Los Alamos. “Based on our previous studies of these nanoscale materials, we found that their composition often plays a less important role than their size or shape. Quantum dots, for example, can exhibit certain size-dependent trends that are almost insensitive (or only weakly sensitive) to their composition.

“Our new work on perovskite quantum dots confirms this general notion.”

Organometal trihalide perovskite semiconductors have the formula (CH3NH3)PbX3 (where Pb is lead and X can be iodine, bromine or chlorine). The power conversion efficiencies of solar cells made of these materials has increased from below 4% to more than 20% in the last five years, and in addition to photovoltaics, they are promising for a host of other photonic-related applications.

Room-temperature single-photon emission

In many ways, quantum dots made from perovskites behave in the same way as dots made from other materials, says Klimov. “Indeed, we have observed that they can operate as single-photon sources, or quantum emitters, that emit photons one by one, with the lag time between successive photon emission events being defined by how long the emitter’s excited state lasts. As has been seen for other types of colloidal quantum dots, we have found this behaviour at room temperature – something that eliminates the need for cryogenic cooling, often required for other types of single-photon emitters.”

Klimov’s team fabricated perovskite quantum dots via a colloidal route developed by Maksym Kovalenko’s group at ETH Zurich earlier this year (Nano Lett. DOI: 10.1021/nl5048779). These dots are cubic-shaped nanocrystals with a side length of around 10 nm (or 100 Angstroms). “As well as tuning their size, we can adjust the colour they emit by varying the composition of the anion (halide) component,” says Klimov. “For example, bromide-based particles (CsPbBr3) emit light at around 510 nm (cyan), while iodide-based nanocrystals emit at around 680 nm (red). Using a mixture of iodide and bromide (CsPbIxBr3-x), we can thus obtain all the colours in between.”

Another interesting property of these quantum dots is that we can easily control the composition of the anion via straightforward ion-exchange reactions, providing a simple and practical tool for controlling emission colour, he tells nanotechweb.org. “This is considerably more difficult to do in traditional quantum dots, based on say II-VI semiconductors such as CdSe and CdTe.”

Applications in colour-converting phosphors

According to the researchers, who report their work in ACS Nano DOI: 10.1021/acsnano.5b04584, these quantum dots might find use in light-emitting diodes with obvious practical applications in displays – first as colour-converting phosphors, and then as active elements in schemes that rely on direct electrical injection.

“They might be used in lasers too, but immediate applications here are less likely,” says Klimov. “As in other types of quantum dots, a big problem here is ‘nonradiative Auger recombination’, which leads to extremely fast depletion of optical gain (on timescales as short as 10–100 picoseconds).”

Improving the photostability of the dots is also a serious challenge. “In our studies we observed that perovskite quantum dots have a tendency to degrade, and this problem appears in both the bulk and nanostructured form of these materials,” adds Klimov. “This is where lessons learned from previous work on colloidal nanostructures will be helpful for us. Researchers working in this field have successfully solved the problem of instability for many types of extremely sensitive materials, including lead-based structures. We will hopefully be able to apply some of the advanced methods they used to perovskite nanostructures, and so help to advance these materials so that they can be exploited in real-world devices.”

For more on the latest developments on perovskite solar cells, visit the Nanotechnology collection Focus on Perovskite Solar Cells.