Francesco Meinardi

Low power, non-coherent sensitized photon up-conversion: modelling and perspectives

In the last few years, non-coherent sensitized photon up-conversion (SUC) in multi-component systems has been developed to achieve significantly high quantum yields for various chromophore combinations at low excitation powers, spanning from the ultraviolet (UV) to near infrared (NIR) spectrum. This promising photon energy management technique became indeed suitable for wide applications in lighting technology and especially in photovoltaics, being able to recover the sub-bandgap photons lost by current devices. A full and general description of the SUC photophysics will be presented, with the analysis of the parameter affecting the photon conversion quantum yield and the quantities which define the optimal working range of any SUC system, namely the threshold and saturation excitation intensity. It will be shown how these quantities depend on intrinsic photophysical properties of the moieties involved and on the SUC solid host matrix. The model proposed represents a powerful tool for evaluation of a newly proposed system, and its reliability will be discussed in respect to an optimized system with SUC yield of 0.26 ± 0.02. The results obtained will outline the research guidelines which must be pursued to optimize the SUC efficiency for its perspective technological applications.

Highly efficient large-area colourless luminescent solar concentrators using heavy-metal-free colloidal quantum dots

Luminescent solar concentrators serving as semitransparent photovoltaic windows could become an important element in net zero energy consumption buildings of the future. Colloidal quantum dots are promising materials for luminescent solar concentrators as they can be engineered to provide the large Stokes shift necessary for suppressing reabsorption losses in large-area devices. Existing Stokes-shift-engineered quantum dots allow for only partial coverage of the solar spectrum, which limits their light-harvesting ability and leads to colouring of the luminescent solar concentrators, complicating their use in architecture. Here, we use quantum dots of ternary I-III-VI2 semiconductors to realize the first large-area quantum dot-luminescent solar concentrators free of toxic elements, with reduced reabsorption and extended coverage of the solar spectrum. By incorporating CuInSexS2-x quantum dots into photo-polymerized poly(lauryl methacrylate), we obtain freestanding, colourless slabs that introduce no distortion to perceived colours and are thus well suited for the realization of photovoltaic windows. Thanks to the suppressed reabsorption and high emission efficiencies of the quantum dots, we achieve an optical power efficiency of 3.2%. Ultrafast spectroscopy studies suggest that the Stokes-shifted emission involves a conduction-band electron and a hole residing in an intragap state associated with a native defect.

Multicomponent polymeric film for red to green low power sensitized up-conversion.

The realization of red to green photon energy up-conversion in a multicomponent polymeric organic solid film with good photochemical stability is presented. Up-converted light is obtained by using an ultralow excitation power density in the range of 1 mW cm(-2), suitable to recover the low energy tail of the solar emission spectrum.

Sensitized NIR Erbium(III) Emission in Confined Geometries: A New Strategy for Light Emitters in Telecom Applications

A new hybrid material, based on Er(3+) exchanged zeolite L loaded with DFB molecules, is proposed as an efficient emitter in the third telecommunication window. The close proximity between the Er(3+) ions and perfluorinated dyes, induced by the restricted geometry of the zeolite nanochannels, allows sensitized emission at 1.5 mum, with a lifetime >2 orders of magnitude longer than that for classic erbium organic complexes using nonfluorinated ligands. This approach, circumventing the requirement of the creation of real chemical bonds between the organic species and the metal ion, opens the way to using as an efficient antenna, the organic molecules for which the complexation to the metal ions cannot be realized.

High Stokes shift perylene dyes for luminescent solar concentrators

Highly efficient plastic based single layer Luminescent Solar Concentrators (LSCs) require the design of luminophores having complete spectral separation between absorption and emission spectra (large Stokes shift). We describe the design, synthesis and characterization of a new perylene dye possessing Stokes shift as high as 300 meV, fluorescent quantum yield in the LSC slab of 70% and high chemical and photochemical stability.

Ultraviolet photoluminescence of porous silica

Excitation pattern and decay kinetics of ultraviolet photoluminescence of porous silica are investigated between 4.5 and 10 eV by means of synchrotron radiation. Spectra are dominated by a 3.7 eV emission similar to the recently observed ultraviolet emission of oxidized porous Si and Si nanostructures. Emission intensity is found to be controlled by the material specific surface. Other emissions are observed at 2.9, 3.8, and 4.2 eV. All emissions show lifetimes of a few nanoseconds. Spectral and kinetic features are sensibly different than in glassy SiO2, suggesting a revision of previous assignments of ultraviolet emissions in oxidized porous Si and Si nanostructures.

Dynamic Hole Blockade Yields Two-Color Quantum and Classical Light from Dot-in-Bulk Nanocrystals

Semiconductor nanocrystals (NCs) are an emerging class of color-tunable, solution-processable, room-temperature single-photon sources. Photon antibunching in NCs arises from suppression of multiphoton emission by nonradiative Auger recombination. Here, we demonstrate a new antibunching mechanism-dynamic Coulomb blockade-which allows for generating both quantum and classical light from the same NC without detrimental effects of Auger decay. This mechanism is realized in novel dot-in-bulk (DiB) nanostructures comprising a quantum-confined CdSe core overcoated with a thick, bulk-like CdS shell. The presence of one hole in the core suppresses the capture of the second hole forcing it to recombine in the shell region. Under weak excitation, these NCs emit red antibunched light (core emission). At higher pump levels they exhibit an additional green band (shell emission) with bulk-like, Poissonian photon statistics. The unusual versatility of these novel nanoscale light sources, that combine mutually correlated channels for quantum and classical emission and additionally allow for facile tunability of effective color, opens new interesting opportunities for a range of applications from quantum optics to sensing and nanoscale imaging.

Influence of doping on the emission and scintillation characteristics of PbWO4 single crystals

X-ray excited luminescence spectra, wavelength-resolved thermoluminescence glow curves, ultraviolet and 22Na excited emission decays, and 60Co excited light yield were measured on a set of high purity (99.9999%) PbWO4:X (X=Lu3+, Y3+, Gd3+, Sc3+, Nb5+, Pb2+) samples grown in an equivalent way. Positive influence of trivalent doping (Lu, Gd, Y) was noted, which consists in suppressing the deeper trapping states in the PbWO4 structure. Such states are related to the radiative recombination processes in the green and red part of the spectra. The presence of the green emission centers also results in the increase of the slow recombination decay components in the microsecond time scale. High concentration of the dopant ions (670 atomic ppm for PbWO4:Nb) results in the creation of new nonradiative recombination sites, which suppress the recombination decay components and the light yield as well.

Sensitized near infrared emission from lanthanide-exchanged zeolites

In this work, we present an alternative approach to sensitize the near infrared emission of Er3+ ions (used in telecom applications) by exploiting the geometrical confinement occurring in porous zeolites structures. The sensitization of the Ln ion is obtained by energy transfer between a suitable organic molecule acting as an antenna and the emitting ion arranged in closed proximity, thus, avoiding the limits imposed by the coordination chemistry.

Broadband Up-Conversion at Subsolar Irradiance: Triplet−Triplet Annihilation Boosted by Fluorescent Semiconductor Nanocrystals

Conventional solar cells exhibit limited efficiencies in part due to their inability to absorb the entire solar spectrum. Sub-band-gap photons are typically lost but could be captured if a material that performs up-conversion, which shifts photon energies higher, is coupled to the device. Recently, molecular chromophores that undergo triplet-triplet annihilation (TTA) have shown promise for efficient up-conversion at low irradiance, suitable for some types of solar cells. However, the molecular systems that have shown the highest up-conversion efficiency to date are ill suited to broadband light harvesting, reducing their applicability. Here we overcome this limitation by combining an organic TTA system with highly fluorescent CdSe semiconductor nanocrystals. Because of their broadband absorption and spectrally narrow, size-tunable fluorescence, the nanocrystals absorb the radiation lost by the TTA chromophores, returning this energy to the up-converter. The resulting nanocrystal-boosted system shows a doubled light-harvesting ability, which allows a green-to-blue conversion efficiency of ∼12.5% under 0.5 suns of incoherent excitation. This record efficiency at subsolar irradiance demonstrates that boosting the TTA by light-emitting nanocrystals can potentially provide a general route for up-conversion for different photovoltaic and photocatalytic applications.