The Order of the Dot

Quantum dots (QDs) are semiconductor single crystals of nanoscale dimensions. These nanomaterials can be solution-synthesized in a low-cost manner. QDs are home to a wide array of tantalizing optical and electronic properties, such as strong and tunable optical absorption and fast charge transport, absent from their corresponding bulk forms. These surprising characteristics are a virtue of a quantum mechanical phenomenon, the ‘quantum confinement effect’, which comes into play when materials are reduced in dimensions below a certain size.

This size limit is defined by the material’s properties. When below this limit, electrons (and holes) start experiencing the physical boundaries of the confined matter. The charges stop behaving as free particles giving rise to exotic properties, the most prominent among which is the size-tunable band gap: changing the size of the particle changes the color it absorbs and emits. Another related property is the sharp emission linewidths. It is primarily for these two properties that QDs have been widely pursued with intense vigor over the past two decades for applications in third generation photovoltaics and as a low-cost flexible display technology. In particular, tunability of light absorption can help absorb and manage sunlight in a better way by also absorbing the infrared radiation that makes up 50% of the spectrum and cannot be absorbed by the current silicon solar cell technology, due to the latter’s fixed band gap.

Figure 1: Quantum confinement effect. Tuning the QD size tunes their optical bandgaps, without the need to change their chemical composition or synthesis protocol. (Credit: https://area-info.net/quantum-dots-market-2016-2023-research-report-by-decisiondatabases/)

QDs have a large surface-to-volume ratio. Comprising of a few-hundred to a few-thousand atoms, they are usually capped with organic surfactants. Chiefly, the surfactants keep the QDs from aggregating, render them soluble in organic solvents, and protect their pristine surfaces from the environment. There is another very exciting thing these surfactants do – they allow the QDs to self-assemble. When the QD ink is cast into a thin film, for example, the interaction of these surfactants with the drying solvent defines the long-range order that the QD ensemble will achieve. This long-range order, however, has thus far been of limited application for researchers studying charge transport as the long-range ordered ensemble contains these organic, long-chain surfactants that impede the flow of charges and the ensemble behaves as an insulator. Enter the shorter-chain molecules…

If this QD film with insulating surfactants is soaked briefly in a solution containing shorter-chain molecules (ligands), an exchange reaction occurs on the QD surfaces. The shorter ligands clip-off the surfactants and replace them, bringing the QDs closer for improved charge transport. Electrical conductivity increases by several orders of magnitude, however, at the cost of the long-range order. What remains is a glassy, amorphous QD film – with tremendous charge transport properties (compared to the initial insulating film). The resulting disorder is expected since the solid film undergoes a sudden volume reduction due to the exchange reaction.

Figure 2: Effect of ligand exchange. a) Cubic perovskite quantum dots initially capped with long-chain organic surfactants are made to undergo ligand exchange. The initial long-range order is lost. b) These exchanged QD films are used as absorbers in the latest high-performing QD solar cells.

The exchange process described above is called the solid-state exchange, since it is performed on a solid film. There is another chemical route to achieving this, the solution-phase exchange. To perform the latter, the QD ink is mixed with the shorter ligand solution leading to exchange in solution-phase. Since this is carried out in the solution-phase, it is expected to preserve the initial long-range order, at least, to some extent. Against expectation, however, these QDs also have failed to show any long-range order to date.

This brings us to the question: of what use is the long-range order? Order in the final conductive film has thus far remained elusive. Quantum mechanics tells us that having order in the final, exchanged QD film can have interesting implications for charge transport. Carriers can flow faster allowing ‘band-like transport’ – the charge transport mechanism common in atomic solids with excellent charge mobilities. New research is beginning to reveal the fruitful consequences of preserving order in QD films. Balazs et al (https://onlinelibrary.wiley.com/doi/full/10.1002/adma.201802265) recently reported electron mobilities of 24 cm²V^-1s^-1 from lead chalcogenide QD films exhibiting substantial order  (lead chalcogenide QDs are a family of QDs that have garnered the most attention due to their superior electronic properties). Mobilities in these QDs had not been reported to exceed 5 cm²V^-1s^-1 prior to this report, highlighting the importance of long-range order.

Disorder in QD films impedes the flow of electrons and holes in these materials. This means that charge carriers generated inside QD solar cells ‘die’ before they could be extracted in the form of useful electrical power. This is one of the major reasons that threatens further progress toward QD solar cell technology. Further improvements in QD optoelectronics are on offer if a method is found to suppress disorder, and Balazs’ results are a step in the right direction. It is time to search for the order of the dot!