Theoretical framework for charge transport in QD solids
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Theoretical framework for charge transport in QD solids
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olloidal quantum dots (QDs) are nanometer-sized crystalline semiconductors solution-synthesized using low-cost chemical reactions. Chiefly due to their tunable optical and electronic properties, they are attractive for nextgeneration thin-film optoelectronics. Appropriate modification of their size, shape, surface chemistry, and chemical composition can help tailor QD properties. Typically, a thin film assembled from QDs via industrially compatible coating methods such as slot-die coating, spray coating, and inkjet printing, several tens to hundreds of nanometers in thickness, is used as the “active” layer to demonstrate semiconductor device applications. Perovskite QDs and lead chalcogenides (such as lead sulfide) are the two most promising families of this material. The nature of charge transport in QD films has allured researchers. Precise understanding and control over the flow of charge carriers promises improvements in the resulting semiconductor device performance, and is an area of active research. Because of their large surface-to-volume ratio, significant improvements in charge transport have been realized over the years by smart manipulation of the QD surface. The as-synthesized QDs are capped with long-chain organic molecules, such as oleic acid, that aid in their synthesis and help disperse them in organic, green solvents. However, due to their long alkyl chains, the organic surfactants render the QDs electrically insulating when deposited into a thin film. A facile process to improve charge transport has been to perform an exchange of these surfactants, by dipping the initial QD film into a solution of smaller-chain ligands. This enables the QDs to pack closer and improves the flow of charge carriers. However, further understanding on the nature of charge transport is urgently needed to push this promising field forward. A research group from ETH Zürich led by Vanessa Wood has now presented a theoretical platform, backed with experimental evidence, that furthers the fundamental
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understanding of charge transport in QDs. The insights were recently published in Nature Communications (doi:10.1038/ s41467-020-16560-7). The research team started by theoretically simulating the transport of charge carriers in a solid made from QDs. The researchers realized that hopping of charges between neighboring QDs involves deformation of the QD surface. More specifically, the chemical bond between the surface metal atom (lead, Pb) and the surfactant on the PbS QDs deformed when the QD accepted or donated a charge during the transport process. This insight suggests a link between vibrations of the QD surface and charge transport, directly implying that the choice of surfactants and the resulting vibrations can be manipulated to tune charge transport. Introducing doped QDs into the QD film is a commonly used strategy to improve charge transport. However, the simulations suggest that an unintentional oxidation or reduction of these doped QDs can convert them
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