Quantum dots (QDs) have extraordinary strong light absorption and size tunable bandgap. However, QD films are typically limited to ~200-300 nm due to their poor charge mobility. This severely limits the quantum efficiency of QD devices for λ <750 nm (infrared). Herein, we report a record 1 μm thick QD film using intercalated graphene layers as transparent current extractors. This overcomes QD poor mobility, ensuring both effective light absorption and charge extraction towards the near-infrared reaching quantum efficiency (EQE) of 90%.
The short diffusion length (LD<200 nm) of QDs limits their useful thickness to ~200-300 nm1–4 , resulting in poor infrared light absorption. To overcome this limitation, we have built a 1 µm thick QD film with intercalated transparent graphene electrodes that keep high charge collection efficiency. As a result, the 1 µm intercalated devices show a superior EQE reaching 90% at λ ~800 nm without the drop of quantum efficiency at λ ~700 nm observed in most QD devices. The EQE of intercalated devices improves over the entire λ~ 600-1100 nm spectrum as the thickness increases from 100 nm to 1 μm, clearly breaking the restriction that the diffusion length of QDs imposes on the film thickness. This improves absorption and charge collection in the infrared.
Colloidal chemistry strategies are mature techniques, now able to provide highly processable nanocrystals (NCs)
soluble in a variety solvents, possessing an adjustable organic interface, for obtaining assembled structures. Indeed the
NCs can be organized in superstructures by means of spontaneous assembly, in order to bridge the gap between nanoand
mesoscale. In self assembly procedures, the organization is driven by the intrinsic information coded into the
building blocks, namely size, shape and surface chemistry. The distinct properties of the nanometer-scale "buildingblocks"
can be thus harnessed in assemblies presenting new collective properties, which can be further engineered by
controlling inter-particle spacing and by material processing. Self assembly approaches of colloidal NCs can effectively
exploit the solvent evaporation to form closely packed superlattices, since collective interaction energy can overcome the
entropy loss due to ordering. The control on the NC characteristics is then crucial for the achievement of well controlled
superstructures, with long range order and stability, being the individual NCs considered as "artificial atoms" in such
superlattice structures. In this perspective the emerging concept of NC based metamaterials, that is a material with
properties occurring from the controlled positioning of the different interacting NCs in an assembly, arise.
In this work the host-guest chemistry of α-cyclodextrin has been investigated in order to mediate the phase transfer from organic solvent to water of blue emitting CdS nanocrystals. Oleic acid capped CdS nanocrystals have been synthesized by using colloidal chemistry routes in non-coordinating solvents and effectively transferred from hexane phase in aqueous solution. The transfer has been mediated by the formation of an inclusion complex between nanocrystal surfactant and α-cyclodextrins. The optical properties of the nanocrystal water solution, the effect of cyclodextrin concentration and the nanocrystal size on the phase transfer efficiency have been investigated. Finally a layer-by-layer assembling procedure of CdS nanocrystals complexed by cyclodextrins has been exploited to set up a supramolecular hierarchical multilayer system with high level of structural complexity.
High quality luminescent CdS and CdSe nanocrystals, with tuneable band edge emission, were synthesized by means of thermal decomposition of precursors in organic solvents, incorporated in polystyrene and poly(methyl methacrylate) and deposited by casting, yielding optically transparent luminescent films.
The obtained nanocomposite films were characterized by spectroscopical (UV-vis absorption and emission) and structural (TEM analysis) techniques. The effect of NC composition, concentration, size, and surface chemistry was evaluated in order to understand the role played by such factors in the nanocomposite optical properties for both the investigated polymers. The presence of organic ligand shell was demonstrated to be critical for the NCs incorporation into the polymer matrix.