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6 páginas, 4 figuras, 1 tabla ; A panchromatic specular reflector based dye solar cell is herein presented. Photovoltaic performance of this novel design is compared to that of cells in which standard diffuse scattering layers are integrated. The capability of the proposed multilayer structures to both emulate the broad band reflection of diffuse scattering layers of standard thickness (around 5 microns) and give rise to similarly high light harvesting and power conversion efficiencies, yet preserving the transparency of the device, is demonstrated. Such white light reflectors are comprised of stacks of different porous optical multilayers, each one displaying a strong reflection at a complementary spectral range, and are designed to leave transmittance unaltered in a narrow red-frequency range at which the sensitized electrode shows negligible absorption, thus allowing to see through the cell. The reflectance bandwidth achieved is three times as broad as the largest previously achieved with any photonic structure integrated in a dye solar cell. ; The research leading to these results has received funding from 55 the European Research Council under the European Union's Seventh Framework Programme (FP7/2007-2013)/ERC grant agreement n° 307081 (POLIGHT), the Spanish Ministry of Economy and Competitiveness under grants MAT2011-23593 and CONSOLIDER HOPE CSD2007-00007, and the Junta de 60 Andalucía under grants FQM3579 and FQM5247. ; The authors also thank the Unit of Information Resources for Research of the Spanish National Research Council (URICI-CSIC) for the co-financing of this publication in Open Access. ; Peer reviewed

We have developed a method to alternate porous and dense dielectric films in order to build high refractive index contrast distributed Bragg reflectors (DBRs) capable of reflecting very efficiently in a targeted spectral range employing a small number of layers in the stack. Porous layers made of SiO2 nanoparticles and compact sol–gel processed TiO2 layers are sequentially deposited. The key to the preservation of porosity of every other layer during the deposition process is the use of a sacrificial layer of polystyrene that prevents the infiltration of the interstitial voids between nanoparticles with the homogeneous solution of TiO2 precursors. Our approach allows preparing a series of DBRs operating along the whole visible spectral range. Reflectance values as high as 90% are achieved from only seven layers. The particular distribution of porosity along one direction gives rise to an interesting interplay between the optical properties of the system and the vapor pressure in the surrounding atmosphere, which we foresee could be put into practice in gas sensing devices ; The research leading to these results received funding from the European Research Council under the European Union's Seventh Framework Programme (FP7/2007–2013)/ERC grant agreement no. 307081 (POLIGHT) and the Spanish Ministry of Economy and Competitiveness under grant MAT2014 54852-R. MA is grateful to "La Caixa" Foundation for its financial support. FESEM characterization was performed at CITIUS, and we are grateful for its support. ; We acknowledge support by the CSIC Open Access Publication Initiative through its Unit of Information Resources for Research (URICI). ; Peer reviewed

The combination of light‐emitting diodes (LEDs) and rare earth (RE) phosphors as color‐converting layers comprises the basis of solid‐state lighting. Indeed, most LED lamps include a photoluminescent coating made of phosphor material, i.e., crystalline matrix suitably doped with RE elements, to produce white light from a blue or ultraviolet LED chip. Transparent phosphor‐based films constitute starting materials for new refined emitters that allow different photonic designs to be implemented. Among the different photonic strategies typically employed to tune or enhance emission, surface texturing has proved its versatility and feasibility in a wide range of materials and devices. However, most of the nanofabrication techniques cannot be applied to RE phosphors directly because of their chemical stability or because of their cost. The first monolithic patterned structure of down‐shifting nanophosphors with square arrays of nanoholes with different lattice parameters is reported in this study. It is shown that a low‐cost soft‐nanolithography procedure can be applied to red‐emitting nanophosphors (GdVO4:Eu3+ nanocrystals) to tune their emission properties, attaining a twofold directional enhancement of the emitted light at predesigned emission wavelengths in specific directions. ; This project received funding from the Spanish Ministry of Economy and Competitiveness under grant MAT2017‐88584‐R (AEI/FEDER, UE) and PID2019‐106860GB‐I00 (AEI/FEDER, UE), the excellence program SEV‐2015‐0496, and the European Research Council (ERC) under the European Union's Horizon 2020 Research and Innovation Programme (NANOPHOM, grant agreement no. 715832 and ENLIGHTMENT, grant no. 637116). P.M acknowledges financial support from an FPI contract (2017) of the MICINN (Spain) cofounded by the ESF and the UAB. E.C.O. acknowledges the Spanish Ministry of Universities for the funding through an FPU program (FPU19/00346). ; Peer reviewed

20 páginas, 5 figuras ; In order to enhance optical absorption, light trapping by multiple scattering is commonly achieved in dye sensitized solar cells by adding particles of a different sort. Herein we propose a theoretical method to find the structural parameters (particle number density and size) that optimize conversion efficiency for electrodes of different thickness containing spherical inclusions of diverse composition. Our work provides a theoretical framework in which the response of solar cells containing diffuse scattering particles can be rationalized. Optical simulations are performed by combining a Monte Carlo approach with Mie theory, in which the angular distribution of scattered light is accounted for. Several types of scattering centers, such as anatase, gold and silver particles, as well as cavities, are considered and their effect compared. Estimates of photovoltaic performance, insight into the physical mechanisms responsible for the observed enhancements, and guidelines to improve the cell design are provided. We discuss the results in terms of light transport in weakly disordered optical media and find that the observed variations between the optimum scattering configurations attained for different electrode thickness can be understood as the result of the randomization of light propagation direction at different depths within the active layer. A primary conclusion of our study is that photovoltaic performance is optimised when the scattering properties of the film are adjusted so that the distance over which incident photons are randomized is comparible to the thickness of the film. This simple relation could also be used as a design rule to attain the optimum optical design in other photovoltaic materials. ; The research leading to these results has received funding from the European Research Council under the European Union's Seventh Framework Programme (FP7/2007-2013)/ERC grant agreement n° 307081 (POLIGHT), the Spanish Ministry of Economy and Competitiveness under grants MAT2011-23593 and CONSOLIDER HOPE CSD2007-00007, and the Junta de Andalucía under grants FQM3579 and FQM5247. PB is grateful to funding from the EPSRC APEX project (EP/H040218/1) and fellowship (EP/J002305/1. The authors also thank the Unit of Information Resources for Research (URICI-CSIC) for the co-financing of this publication in Open Access. ; Peer reviewed

4 páginas, 5 figuras.- Made freely available through Open Access (Author Choice program) ; Herein we show experimental evidence of resonant photocurrent generation in dye-sensitized periodically nanostructured photoconductors, which is achieved by spectral matching of the sensitizer absorption band to different types of localized photon modes present in either periodic or broken symmetry structures. Results are explained in terms of the calculated spatial distribution of the electric field intensity within the configurations under analysis. ; The research leading to these results has received funding from the European Research Council under the European Union's Seventh Framework Programme (FP7/2007−2013)/ERC grant agreement n° 307081 (POLIGHT), the Spanish Ministry of Economy and Competitiveness under grants MAT2011−23593 and CONSOLIDER HOPE CSD2007−00007, and the Junta de Andalucía under grants FQM3579 and FQM5247 ; Peer reviewed

Among the different approaches to humidity sensing available, those based on fluorescent signals are gathering a great deal of attention due to their fast response and versatility of detection and design. So far, all proposals have focused on the use of luminescent probes whose emission is either triggered or inhibited by the presence of water that reacts or alters their chemical environment, hence inducing the signal change. Here, a novel concept in fluorescent humidity sensing based on combining stimuli-responsive photonic resonators with molecular fluorescent probes is introduced. The resonator is assembled from humidity-swellable antimony phosphate nanosheets embedding a planar light-emitting probe, whose emission is dramatically modified by the changes that ambient humidity causes in its photonic environment. Guided by "in silico" optical design of the resonator architecture and subsequent experimental realization, two embodiments of fluorescent photonic humidity sensors featuring turn-on and turn-off detection schemes are presented. The interplay between the luminescent properties of an emitter and its photonic environment implies a fundamental advantage as the emitters are not chemically altered during the detection process. At the same time, it paves the way toward a new generation of photonic humidity sensors which can conveniently be interfaced with common fluorescence detection schemes. ; Financial support granted by the Max Planck Society, the University of Munich (LMU), the Center for NanoScience (CeNS), the Deutsche Forschungsgemeinschaft (DFG) through the Cluster of Excellence Nanosystems Initiative Munich (NIM), the European Research Council under the European Union's Seventh Framework Programme (FP7/2007‐2013)/ERC Grant Agreement No. 307081 (POLIGHT), and the Spanish Ministry of Economy and Competitiveness under grant MAT2014‐54852‐R is gratefully acknowledged. A.J.‐S. acknowledges the Spanish Ministry of Economy and Competitiveness for funding through an FPI program under the project MAT2011‐23593.

Herein we realize an optical design that optimizes the performance of bifacial solar cells without modifying any of the usually employed components. In order to do so, dielectric scatterers of controlled size and shape have been successfully integrated in the working electrodes of dye-sensitized solar cells (DSSCs), resulting in bifacial devices of outstanding performance. Power conversion efficiencies (PCEs) as high as 6.7% and 5.4% have been attained under front and rear illumination, respectively, which represent a 25% and a 33% PCE enhancement with respect to an 8 mm-thick standard solar cell electrode using platinum as the catalytic material. The remarkable bifacial character of our approach is demonstrated by the high rear/front efficiency ratio attained, around 80%, which is among the largest reported for this sort of device. The proposed optimized design is based on a Monte Carlo approach in which the multiple scattering of light within the cell is fully accounted for. We identified that the spherical shape of the scatterers is the key parameter controlling the angular distribution of the scattering, the most efficient devices being those in which the inclusions provide a narrow forward-oriented angular distribution of the scattered light. ; Funding for the development of the research leading to these results was received from the European Research Council under the European Union's Seventh Framework Programme (FP7/2007-2013)/ERC grant agreement no. 307081 (POLIGHT) and the Spanish Ministry of Economy and Competitiveness under grant MAT2014-54852-R. JMMM acknowledges the Spanish Ministry of Education, Culture and Sport for the funding through an FPU program. AJS acknowledges the Spanish Ministry of Economy and Competitiveness for funding through an FPI program under the project MAT2011–23593. YL acknowledges the funding from the People Programme (Marie Curie Actions) of the European Union's Seventh Framework Programme FP7/2007-2013/under REA grant agreement no. 622533. ; We acknowledge support by the CSIC Open Access Publication Initiative through its Unit of Information Resources for Research (URICI) ; Peer reviewed

We demonstrate that a combined optimization approach based on the sequential alternation of theoretical analysis and experimental realization gives rise to plastic supported dye solar cells for which both light harvesting efficiency and electron collection are maximized. Rationalized configurations with optimized light trapping and charge extraction are realized to achieve photoanodes on plastic prepared at low temperature, showing a power conversion efficiency of 8.55% and a short circuit photocurrent of 16.11 mA cm 2, unprecedented for plastic based dye solar cell devices. Furthermore, the corresponding fully flexible designs present stable mechanical properties after several bending cycles, displaying 7.79% power conversion efficiency, an average broadband internal quantum efficiency above 90%, and a short circuit photocurrent of 15.94 mA cm 2, which is the largest reported value for bendable cells of this sort to date ; Financial support from the European Research Council under the European Union's Seventh Framework Programme (FP7/2007–2013)/ERC grant agreement no. 307081 (POLIGHT) and the Spanish Ministry of Economy and Competitiveness under grant MAT2014-54852-R is gratefully acknowledged. YL acknowledges the financial support from the People Programme (Marie Curie Actions) of the European Union's Seventh Framework Programme FP7/2007–2013/ under REA grant agreement no. 622533. AJS thanks Spanish Ministry of Economy and Competitiveness for funding through an FPI program under the project MAT2011–23593. MJK acknowledges funding support from the Global Frontier R&D Program on Center for Multiscale Energy System (2012M3A6A7054856), the Technology Development Program to Solve Climate Changes (2015M1A2A2056824) and 2015 University-Institute cooperation program funded by the National Research Foundation under the Ministry of Science, ICT & Future Planning, Korea. This work was also supported by the KIST institutional programs. ; We acknowledge support by the CSIC Open Access Publication Initiative through its Unit of Information Resources for Research (URICI). ; Peer reviewed

In spite of the impressive efficiencies reported for perovskite solar cells (PSCs), key aspects of their working principles, such as electron injection at the contacts or the suitability of the utilization of a specific scaffold layer, are not yet fully understood. Increasingly complex scaffolds attained by the sequential deposition of TiO2 and SiO2 mesoporous layers onto transparent conducting substrates are used to perform a systematic characterization of both the injection process at the electron selective contact and the scaffold effect in PSCs. By forcing multiple electron injection processes at a controlled sequence of perovskite–TiO2 interfaces before extraction, interfacial injection effects are magnified and hence characterized in detail. An anomalous injection behavior is observed, the fingerprint of which is the presence of significant inductive loops in the impedance spectra with a magnitude that correlates with the number of interfaces in the scaffold. Analysis of the resistive and capacitive behavior of the impedance spectra indicates that the scaffolds could hinder ion migration, with positive consequences such as lowering the recombination rate and implications for the current–potential curve hysteresis. Our results suggest that an appropriate balance between these advantageous effects and the unavoidable charge transport resistive losses introduced by the scaffolds will help in the optimization of PSC performance. ; The research leading to these results received funding from the European Research Council under the European Union's Seventh Framework Program (FP7/2007–2013)/ERC grant agreement no. 307081 (POLIGHT) and the Spanish Ministry of Economy and Competitiveness under grants MAT2014-54852-R and MAT2015-70611-ERC. MA is grateful to "La Caixa" Foundation and VPPI of the University of Seville for their financial support. Structural characterization was performed at CITIUS, and we are grateful for their support. B. C. H. is grateful for the support of the National Council of Technological and Scientific Development (CNPq), Brazil, through the Science without Borders program.307081 ; We acknowledge support by the CSIC Open Access Publication Initiative through its Unit of Information Resources for Research (URICI). ; Peer reviewed

Organic–inorganic perovskite structures in which lead is substituted by tin are exceptional candidates for broadband light absorption. Herein we present a thorough analysis of the optical properties of CH3NH3SnxPb1−xI3 films, providing the field with definitive insights about the possibilities of these materials for perovskite solar cells of superior efficiency. We report a user's guide based on the first set of optical constants obtained for a series of tin/lead perovskite films, which was only possible to measure due to the preparation of optical quality thin layers. According to the Shockley–Queisser theory, CH3NH3SnxPb1−xI3 compounds promise a substantial enhancement of both short circuit photocurrent and power conversion efficiency in single junction solar cells. Moreover, we propose a novel tandem architecture design in which both top and bottom cells are made of perovskite absorbers. Our calculations indicate that such perovskite-on-perovskite tandem devices could reach efficiencies over 35%. Our analysis serves to establish the first roadmap for this type of cells based on actual optical characterization data. We foresee that this study will encourage the research on novel near-infrared perovskite materials for photovoltaic applications, which may have implications in the rapidly emerging field of tandem devices ; The research leading to these results has received funding from the European Research Council under the European Union's Seventh Framework Programme (FP7/2007-2013)/ERC grant agreement no. 307081 (POLIGHT), the Spanish Ministry of Economy and Competitiveness under grant MAT2014-54852-R. MA is grateful to "La Caixa" Foundation for its financial support. AA has received funding from the European Union's Seventh Framework Programme for research, technological development and demonstration under grant agreement no 291771. MS is supported by the co-funded Marie Skłodowska Curie fellowship, H2020 grant agreement no. 665667. FESEM characterization was performed at CITIUS, and we are grateful for its support. ; We acknowledge support by the CSIC Open Access Publication Initiative through its Unit of Information Resources for Research (URICI). ; Peer reviewed

Osmotic shock in a vesicle or cell is the stress build-up and subsequent rupture of the phospholipid membrane that occurs when a relatively high concentration of salt is unable to cross the membrane and instead an inflow of water alleviates the salt concentration gradient. This is a well-known failure mechanism for cells and vesicles (for example, hypotonic shock) and metal alloys (for example, hydrogen embrittlement)1, 2, 3. We propose the concept of collective osmotic shock, whereby a coordinated explosive fracture resulting from multiplexing the singular effects of osmotic shock at discrete sites within an ordered material results in regular bicontinuous structures. The concept is demonstrated here using self-assembled block copolymer micelles, yet it is applicable to organized heterogeneous materials where a minority component can be selectively degraded and solvated whilst ensconced in a matrix capable of plastic deformation. We discuss the application of these self-supported, perforated multilayer materials in photonics, nanofiltration and optoelectronics. ; We would like to acknowledge S. Vignolini, C. Wang (beamline 11.0.1.2, LBNL), S. Alvarez, C. Lopez and Q. Song, and the insightful comments of U. Steiner and E. J. Kramer. This work was funded by the Qatar Foundation (QNRF), the Engineering and Physical Sciences Research Council (EPSRC), the Consejo Nacional de Ciencia y Tecnología (CONACyT), the Spanish Ministerio de Ciencia e Innovación (MICINN, Consolider HOPE) and the Government of Andalucía. ; Peer Reviewed