Sandra F. H. Correia

Luminescent solar concentrators: challenges for lanthanide-based organic–inorganic hybrid materials

Luminescent solar concentrators (LSCs) are devices comprising a transparent matrix embedding optically active centres that absorb the incident radiation, which is re-emitted at a specific wavelength and transferred by total internal reflection to photovoltaic (PV) cells located at the edges of the matrix. Organic–inorganic hybrids incarcerating trivalent lanthanide ions (Ln3+) are a very promising class of materials for addressing the required challenges in the LSC design to improve solar energy harvesting and, then, PV energy conversion. This feature article offers a general overview of the potential of down-shifting-based Ln3+-containing organic–inorganic hybrids for the development of the area with special focus on (i) optically active layer design, (ii) energy conversion mechanisms, (iii) performance and geometry and (iv) figures of merit in PV cell enhancement. Finally, a prospective outlook on future progress, e.g. optically active centre alignment, geometry optimization and building integration, is provided. The use of Ln3+-containing hybrids in LSCs is at an infant initial research step and considerable basic knowledge is still needed to enable prototypes to become a commercial reality.

Optical fiber relative humidity sensor based on a FBG with a di-ureasil coating.

In this work we proposed a relative humidity (RH) sensor based on a Bragg grating written in an optical fiber, associated with a coating of organo-silica hybrid material prepared by the sol-gel method. The organo-silica-based coating has a strong adhesion to the optical fiber and its expansion is reversibly affected by the change in the RH values (15.0–95.0%) of the surrounding environment, allowing an increased sensitivity (22.2 pm/%RH) and durability due to the presence of a siliceous-based inorganic component. The developed sensor was tested in a real structure health monitoring essay, in which the RH inside two concrete blocks with different porosity values was measured over 1 year. The results demonstrated the potential of the proposed optical sensor in the monitoring of civil engineering structures.

High-Performance Near-Infrared Luminescent Solar Concentrators

Luminescent solar concentrators (LSCs) appear as candidates to enhance the performance of photovoltaic (PV) cells and contribute to reduce the size of PV systems, decreasing, therefore, the amount of material needed and thus the cost associated with energy conversion. One way to maximize the device performance is to explore near-infrared (NIR)-emitting centers, resonant with the maximum optical response of the most common Si-based PV cells. Nevertheless, very few examples in the literature demonstrate the feasibility of fabricating LSCs emitting in the NIR region. In this work, NIR-emitting LSCs are reported using silicon 2,3-naphthalocyanine bis(trihexylsilyloxide) (SiNc or NIR775) immobilized in an organic-inorganic tri-ureasil matrix (t-U(5000)). The photophysical properties of the SiNc dye incorporated into the tri-ureasil host closely resembled those of SiNc in tetrahydrofuran solution (an absolute emission quantum yield of ∼0.17 and a fluorescence lifetime of ∼3.6 ns). The LSC coupled to a Si-based PV device revealed an optical conversion efficiency of ∼1.5%, which is among the largest values known in the literature for NIR-emitting LSCs. The LSCs were posteriorly coupled to a Si-based commercial PV cell, and the synergy between the t-U(5000) and SiNc molecules enabled an effective increase in the external quantum efficiency of PV cells, exceeding 20% in the SiNc absorption region.

Sustainable luminescent solar concentrators based on organic–inorganic hybrids modified with chlorophyll

Luminescent solar concentrators (LSCs) are luminescent waveguide layers that convert sunlight into specific wavelengths which are then guided by total internal reflection to a PV device located at the edges of the LSC. Their ability to concentrate sunlight onto small areas makes LSCs a useful complement to silicon-based PVs in a series of applications, such as urban integration and flexible fabrics towards mobile solar-energy. Challenges for the luminescent layer include the use of low-cost and sustainable nature-based organic molecules. We report novel chlorophyll-based LSCs with emission properties in the red/NIR spectral region. Here, chlorophyll molecules extracted from Spirulina maxima, an abundant cyanobacterium and an attractive natural source, are immobilized in organic–inorganic di- and tri-ureasil matrices enabling the production of sustainable LSCs. At low chlorophyll concentrations (<3 × 1017 molecules per cm3), the photophysical properties of the chlorophyll molecules after incorporation into the hybrids closely resemble those in ethanolic solution (with an absolute emission quantum yield of ∼0.16 and a fluorescence lifetime of ∼8 ns). The LSCs were coupled to a Si-based commercial PV device revealing optical conversion efficiency and power conversion efficiency values of ∼3.70% and 0.10%, respectively, illustrating the potential of this approach for the development of nature-based LSCs meeting the requirements of reliable, sustainable and competitive energy systems.

Thermo-optic variable attenuator/waveplate based on waveguides patterned on organic-inorganic hybrids

This work reports a variable attenuator/waveplate based on thermo-optic (TO) effect induced on a waveguide patterned by direct UV-laser writing on films of organic-inorganic di-ureasil hybrids. The waveguide temperature was tuned inducing phase retardation between the transverse electric (TE) and transverse magnetic (TM) modes, resulting in a controllable waveplate. Furthermore, the waveguide TO actuation allows obtaining a variable optical attenuator. The relevant properties, such as attenuation, polarization dependence of the thermal actuation and power consumption will be presented in the NIR (1550 nm). The required electrical power and temperature variation to attain the optical signal extinction and the retardation phase of π/2 were estimated.

Highly Efficient Luminescent Polycarboxylate Lanthanide Complexes Incorporated into Di-Ureasils by an In-Situ Sol—Gel Process

In order to prepare efficient luminescent organic–inorganic hybrid materials embedded with a lanthanide (Ln3+) complex with polycarboxylate ligands, Ln3+-doped di-ureasils with 4,4-oxybis(benzoic acid) and 1,10-phenanthroline ligands were synthesized via an in-situ sol–gel route. The resulting hybrids were structurally, thermally, and optically characterized. The energy levels of the ligands and the host-to-ion and ligand-to-ion energy transfer mechanisms were investigated (including DFT/TD–DFT calculations). The results show that these Ln3+-based di-ureasil hybrids exhibit promising luminescent features, e.g., Eu3+-based materials are bright red emitters displaying quantum yields up to 0.50 ± 0.05. The luminescent color can be fine-tuned either by selection of adequate Ln3+ ions or by variation of the excitation wavelength. Accordingly, white light emission with CIE coordinates of (0.33, 0.35) under 310 nm irradiation was obtained.