V. Bermúdez

Lanthanide-Containing Light-Emitting Organic-Inorganic Hybrids: A Bet on the Future

Interest in lanthanide-containing organic-inorganic hybrids has grown considerably during the last decade, with the concomitant fabrication of materials with tunable attributes offering modulated properties. The potential of these materials relies on exploiting the synergy between the intrinsic characteristics of sol-gel derived hosts (highly controlled purity, versatile shaping and patterning, excellent optical quality, easy control of the refractive index, photosensitivity, encapsulation of large amounts of isolated emitting centers protected by the host) and the luminescence features of trivalent lanthanide ions (high luminescence quantum yield, narrow bandwidth, long-lived emission, large Stokes shifts, ligand-dependent luminescence sensitization). Promising applications may be envisaged, such as light-emitting devices, active waveguides in the visible and near-IR spectral regions, active coatings, and bio-medical actuators and sensors, opening up exciting directions in materials science and related technologies with significant implications in the integration, miniaturization, and multifunctionalization of devices. This review provides an overview of the latest advances in Ln(3+)-containing siloxane-based hybrids, with emphasis on the different possible synthetic strategies, photoluminescence features, empirical determination.

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.

Sol-gel processing and structural study of europium-doped hybrid materials

The thermal, morphological and conducting properties of macromolecular organic-inorganic networks synthesized by the sol-gel process and containing a wide concentration range of europium trifluoromethanesulfonate, Eu(CF 3 SO 3 ) 3 , have been investigated. The hybrid matrix, U(2000), of these materials, classed as ureasils, includes a siliceous backbone and short polyether-based segments. The link between these components is made by urea bridges. The behaviour of the doped ureasils is significantly better than that of previously characterized electrolytes based on poly(ethylene oxide), PEO. The essentially amorphous character of the hybrid host contributes to an improved transparency and moderate conductivities of certain electrolytes at room temperature (ca. 5×10 –6 Ω –1 cm –1 ). DSC, XRD and FTIR measurements show that the formation of a high-melting crystalline complex results from the coordination of the Eu 3+ ions by the ether oxygens of the organic moieties at composition n=5 [where n represents the molar ratio of (OCH 2 CH 2 ) units per Eu 3+ ion]. The results obtained for samples with n≥20 suggest that in this range of composition the principal effect of the polyether chains is to exert a plasticizing influence, as the coordination of the cations by the urea linkages is favored.

Self-frequency doubling in Yb3+ doped periodically poled LiNbO3:MgO bulk crystal

Continuous-wave laser action from an Yb3+ doped periodically poled LiNbO3:MgO bulk crystal at 1.06 μm is reported. In addition, efficient and stable self-frequency-doubled laser action at 531 nm was obtained by quasiphase matching. Up to 10.5 mW of green output power is obtained from a total laser output power of 58 mW. The experiments were carried out by end pumping with a Ti:sapphire laser, as a surrogate source for a diode laser, at 980 nm. Laser operation was stable at room temperature. The results are compared with those corresponding to single-domain Yb-doped crystals.

Bulk periodic poled lithium niobate crystals doped with Er and Yb

Periodic poled lithium niobate crystals have been prepared by the Czochralski technique using a non-symmetrical temperature field. Er and Yb ions have been added to the melt in different concentrations and its effect on the formation of the periodical structures has been studied. The periodical domain structures are improved for dopant concentrations lower than the solubility limit. The formation of the periodical domain structures is related with the dopant segregation coefficient value and with the temperature gradient in the interface.

Photoluminescence and photoconductivity in CdTe crystals doped with Bi

Defect levels in CdTe doped with Bi are studied by low temperature photoluminescence, photoinduced current transient spectroscopy, photoconductivity measurements, and optical absorption. Two centers associated with the doping with Bi are reported. The first one, a deep level located at Ev+0.71eV, only present at low dopant concentrations, has donor character and hole-trap properties, and is mainly responsible for the high resistivity and very high photoconductivity of the samples. The second one, an acceptor center located at Ev+0.30eV, assigned to BiTe species, is only present at high dopant concentrations and is mainly responsible for the low resistivity and poor photoconductivity of these samples.

Raman scattering analysis of electrodeposited Cu(In,Ga)Se2 solar cells: Impact of ordered vacancy compounds on cell efficiency

This work reports the detailed Raman scattering analysis of Cu-poor Cu(In,Ga)Se2 (CIGS) electrodeposited solar cells using different excitation wavelengths. The systematic assessment of cells fabricated with Cu-poor absorbers that were synthesized with different Cu contents has allowed identifying the existence of a quasi-resonant excitation of a Raman peak characteristic of an Ordered Vacancy Compound (OVC) secondary phase when using a 785 nm excitation wavelength. The enhanced sensitivity of the spectra measured with these conditions to the presence of the OVC phase provides with a suitable tool for the non destructive assessment on the occurrence of this Cu-poor secondary phase in the surface region of the CIGS absorbers from measurements performed on finished cells. The correlation between the Raman scattering data and the optoelectronic parameters of the devices shows the existence of an optimum OVC content range leading to devices with highest open circuit voltage and efficiency. These data provide ...

Process monitoring of chalcopyrite photovoltaic technologies by Raman spectroscopy: an application to low cost electrodeposition based processes

This work describes the use and capabilities of Raman scattering for process monitoring and quality control applications in thin film chalcopyrite photovoltaic technologies. Main vibrational modes in the Raman spectra from the chalcopyrite layers are very sensitive to features related to their crystalline quality, chemical composition and the presence of secondary phases that are relevant for the optoelectronic properties of the absorbers and the efficiency of the solar cells and modules. Measurements performed at different process steps allow for the monitoring of the synthesis process of the chalcopyrite layers, giving information directly related to their processing conditions. These techniques have been successfully applied for the monitoring at on-line and in situ (real time) levels of the electrodeposition processes involved in the fabrication of low cost electrochemical based chalcopyrite solar cells. The results obtained corroborate the strong potential of Raman scattering for these applications, and open interesting perspectives on the development of new real time process control strategies.

Dual role of a di-urethanesil hybrid doped with europium β-diketonate complexes containing either water ligands or a bulky chelating ligand

In the present work we report the unusual role played by a sol-gel derived di-urethane cross-linked poly(oxyethylene) (POE)/siloxane (di-urethanesil, d-Ut(600)) hybrid matrix in the immobilization of the β-diketonate aquocomplex Eu(btfac)3(H2O)2 (btfac− is 4,4,4-trifluoro-1-phenyl-2,4-butanedionate) and in the quasi preservation in the hybrid of the 5D0 quantum efficiency q(5D0) value displayed by this complex in the isolated state (0.12 versus 0.18, respectively). We demonstrate that the d-Ut(600) framework acts as an inert (although optically active) support towards Eu(btfac)3(H2O)2, enabling Eu3+ sensitization by the btfac− ligands and energy transfer between the hybrid host excited states and the lanthanide intra-4f6 levels, while the number of Eu3+-coordinated water molecules remains constant. We also provide evidence that the incorporation of the Eu(btfac)3phen (phen is 1,10-phenantroline) complex into the same hybrid matrix is disadvantageous from the 5D0 quantum efficiency standpoint, because of the severe steric hindrance that emerges between the POE chains of the host structure and the bulky phen molecules, leading to the expulsion of these ligands from the Eu3+ first coordination sphere and to their replacement by two water molecules. As a consequence, embedding of Eu(btfac)3phen in d-Ut(600) results in a significant reduction of the q(5D0) value of the isolated complex (from 0.47 to 0.16). The behaviour observed in the presence of this bidentate chelating ligand correlates well with the practically identical q(5D0) values and number of Eu3+-coordinated water molecules (ca. 2) estimated for the two hybrids.

Impact of electronic defects on the Raman spectra from electrodeposited Cu(In,Ga)Se2 solar cells: Application for non-destructive defect assessment

This work reports on the electrical and Raman scattering analysis of Cu(In,Ga)Se2 cells synthesised with different densities of Se and Cu related point defects. The analysis of the Raman spectra from the surface region of the absorbers shows a direct correlation between the spectral features of the main Raman peak and the density of Se vacancies detected by admittance spectroscopy, being sensitive to the presence of vacancy densities higher than 1015 cm−3. These results corroborate the potential of Raman scattering for the non-destructive detection of electronic defects with potential impact on the characteristics of the solar cells.