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Perovskite quantum dots (PeQDs) are a promising material option. Despite their relative youth, they have made tremendous progress. Today, the green inorganic PeQDs are inching towards sufficient stability for use in enhancement mode films. Many demonstrators now exist. Some are even developing in-situ polymerization of fluoropolymer (PVDF) films together with PeQDs to drive down cost. The green PeQDs however are not as stable as alternative and still require high-performance encapsulation (1e-3 to 1e-4 g/day/sqm), a performance level that was required for other QD material systems years ago.
The key proposition of PeQDs is that the material is intrinsically more tolerant of defects as many defect’s energy levels reside outside the bandgap, thus no corrupting the optical properties. This means that PeQDs, formed at low temperature and even without shells, can achieve 18-20nm FWHMs and excellent quantum yields (QYs), even in some cases beating best-in-class Cd-based QDs. The reds, however, are still lacking in stability and are not ready even for sampling. Here, the debate about the origin of the instability and the remedying procedures are actively underway in the scientific community.
Given the instability of the red, green PeQD films are proposed as hybrids, used in conjunction with (a) narrowband KSF red phosphors, (b) red and blue LEDs, or (c) other QD material systems. Option (a) can suffer from long decays, thus potentially limiting display response time; option (b) adds extra complexity, especially in terms of drive electronics and the management of differential aging; while option (c) plays to the strength of each QD material system but might require special resin formulations.
Green PeQDs also offer high blue absorbance. This is a feature on which green InP QDs still fall short. This characteristic is critical for ensuring color purity in QDCF implementation, be in an OLED or an LCD display. Preliminary results suggest that PeQDs can be used in CFs although ensuring that PeQDs survive the patterning process, be it inkjet or photolithography, is a challenge. It is not unreasonable to assume that in time efforts will overcome these challenges, however. What might be riskier though is the presence of lead. In enhancement film, the lead concentration likely falls below the limit. In color filters, however, it might exceed the threshold. Lead-free alternatives do not perform nearly as well despite efforts even to leverage AI to find optimal alternatives. In any case, display makers still invest in development efforts given the potential and are considering workarounds such as the use of hybrid arrangements. To learn more about PeQDs and other QD technologies, development trends, applications, players and market forecasts please visit “Quantum Dot Materials and Technologies 2019-2029: Trends, Markets, Players”.
Lead sulphide QDs are emerging as a popular choice for sensing and some color conversion applications. In sensors, the proposition is that lead sulphide QDs allow accessing a wide range of the IR spectrum beyond what silicon sensors can access. Furthermore, they can be spin coated onto silicon read-out circuits (ROICs). As such, they can enable high-resolution monolithically-integrated silicon-based IR or SWIR (short wave IR) sensors. The applications for SWIR are numerous, ranging from silicon wafer inspection to AR/VR glass to night vision or lidar photodetector in autonomous mobility.
Major consumer electronic firms have spent years building up their technology access and value chain on QD-Si hybrid Si image sensors. Indeed, until very recently, it was believed that a major US consumer electronics firm was using a UK QD company as essentially a contract manufacturer to develop IR sensors with the chip to be supplied by a French-Italian company. The recent rumour is that this company has pulled the plug on the QD contract manufacture.
Despite this setback, IDTechEx Research think the QD-Si UK has long-term potential. Some firms are already offering such sensors on the market. The development challenges are however still numerous. Stability is a key concern and different methods of device-level and QD-level encapsulation are being pursued. Photostability is also another concern and thus far devices are constrained to low-level indoor light and are far from outdoor automotive-grade stability. Further challenges remain over the ability to achieve defect-free and complete solution casting of QDs on large-area silicon dies with appropriate ligands and curing to ensure close and uniform packing and high inter-QD conductivity. In some cases, even the ability to deliver QDs with an arbitrary absorption characteristic in high volumes and with high batch-to-batch consistency is questioned. These challenges represent material development opportunities for those skilled or interested in the art. To learn more about non-display applications of QDs, including sensors and lighting, please visit “Quantum Dot Materials and Technologies 2019-2029: Trends, Markets, Players.”
There are many other ongoing opportunities. Companies are working on CuInS2/ZnS QDs. These offer broad emission (180nm or so) even though individual QDs can be narrow emission (20nm). These were targeted at a host of applications in solar cells, security tagging, luminescent solar concentrator, but now focus is on spectrum conversion films used in agricultural greenhouses to boost plant growth. Researchers are working on ZnTeSe QDs. This would be totally free of toxic and potentially cariogenic ingredients. This is early stage research but may offer a route for high efficiency blue at the right emission wavelength. Thermal and photostability data is still lacking, further pointing towards technology immaturity. Companies are launching commercial products based on graphene and carbon QDs. These give broad emission but might allow achieving sufficient low costs and solvent compatibility to be used as a liquid security (or ID) taggant in, say, petroleum products. Some are working on InSeCuAl to achieve toxicant free QDs with Al shelling. Today, the FWHM is young but is fast narrowing. Yet others are developing CIS QDs. These exhibit wide FWHM despite the individual QDs showing narrow emission. This is due to random distribution of mid-gap defect states (Cu related) within the QDs. These materials are being targeted at luminescent solar concentrator and agricultural color conversion films.