Integration of Photonic Crystals into Flexible Dye Solar Cells: A Route toward Bendable and Adaptable Optoelectronic Devices Displaying Structural Color and Enhanced Efficiency

Herein is presented what is believed to be the fi rst example of integration of photonic structures in a fl exible optoelectronic device. The resulting devices may be designed to display any color in the visible range and, simultaneously, present enhanced power conversion effi ciency as a consequence of the increased light harvesting caused by the colored back refl ection. The achievement results from the incorporation of nanoparticle-based multilayers with photonic crystal properties that are modifi ed to be compatible with the chemical and physical processing of fl exible nanocrystalline titania electrodes of dye solar cells. The photovoltaic performance of these colored fl exible cells remains unaltered after one hundred bending cycles, thus showing the high-mechanical stability of the ensemble. These devices reunite most characteristics required for building integration or for the construction of solar window panes, such as light weight, stability upon bending, adaptability, and color. This work may trigger promising applications of these highly adaptable and versatile photonic crystals in other fl exible devicesEuropean Union 307081, 622533Ministerio de Economía y Competitividad MAT2014-54852-

Molding with nanoparticle-based one-dimensional photonic crystals: A route to flexible and transferable Bragg mirrors of high dielectric contrast

Self-standing, flexible Bragg mirror films of high refractive index contrast and showing intense and wide Bragg peaks are herein presented. Nanoparticle-based one-dimensional photonic crystals are used as templates to infiltrate a polymer, which provides the multilayer with mechanical stability while preserving the dielectric contrast existing in the mold. Such films can be lifted off the substrate and used to coat another surface of arbitrary shapeMinisterio de Ciencia y Educación MAT2008-02166Junta de Andalucía FQM-357

Solution processed high refractive index contrast distributed Bragg reflectors

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.España, Ministerio de Economía y Competitividad MAT2014 54852-REspaña, European Union’s Seventh Framework Programme FP7/2007–2013)/ER

Environmental Effects on the Photophysics of Organic–Inorganic Halide Perovskites

The photophysical properties of films of organic–inorganic lead halide perovskites under different ambient conditions are herein reported. We demonstrate that their luminescent properties are determined by the interplay between photoinduced activation and darkening processes, which strongly depend on the atmosphere surrounding the samples. We have isolated oxygen and moisture as the key elements in each process, activation and darkening, both of which involve the interaction with photogenerated carriers. These findings show that environmental factors play a key role in the performance of lead halide perovskites as efficient luminescent materials.Unión Europea FP7/2007-2013Ministerio de Economía y Competitividad MAT2014-54852-

Optical design of all-perovskite tandem solar cells

We present a thorough analysis of the optical properties of CH 3NH 3Sn xPb1-x I 3 films, providing the field with definitive insights about the possibilities of these materials for perovskite solar cells of superior efficiency

Nanometer-Scale Precision Tuning of 3D Photonic Crystals Made Possible Using Polyelectrolytes with Controlled Short Chain Length and Narrow Polydispersity

Nanometer‐scale tuning of the optical properties of prefabricated photonic crystals is achieved via layer‐by‐layer assembly of polyelectrolytes in the interstitial spaces of the photonic lattice. The key to the approach is using polyelectrolytes with controlled short chain lengths. This ensures they do not block the air voids, thereby maintaining uniform coating and thus precise and reproducible optical tuning.Ministerio de Economía y Competitividad MAT2011-23593, CSD2007-00007Junta de Andalucía FQM3579, FQM5247European Union 30708

Photophysical Analysis of the Formation of Organic–Inorganic Trihalide Perovskite Films: Identification and Characterization of Crystal Nucleation and Growth

In this work we demonstrate that the different processes occurring during hybrid organic–inorganic lead iodide perovskite film formation can be identified and analyzed by a combined in situ analysis of their photophysical and structural properties. Our observations indicate that this approach permits unambiguously identifying the crystal nucleation and growth regimes that lead to the final material having a cubic crystallographic phase, which stabilizes to the well-known tetragonal phase upon cooling to room temperature. Strong correlation between the dynamic and static photoemission results and the temperature-dependent X-ray diffraction data allows us to provide a description and to establish an approximate time scale for each one of the stages and their evolution. The combined characterization approach herein explored yields key information about the kinetics of the process, such as the link between the evolution of the defect density during film formation, revealed by a fluctuating photoluminescence quantum yield, and the gradual changes observed in the PbI2-related precursor structure.Unión Europea Seventh Framework Programme (FP7/2007-2013)/ERC Grant Agreement No. 307081 (POLIGHT)España, Ministerio de Economía y Competitividad Grants MAT2014- 54852-R and MAT2012-31659España, Comunidad de Madrid programme S2013/MIT-274

Angular emission properties of a layer of rare-earth based nanophosphors embedded in one-dimensional photonic crystal coatings

The angular properties of light emitted from rare-earth based nanophosphors embedded in optical resonators built in one-dimensional photonic crystal coatings are herein investigated. Strong directional dependence of the photoluminescence spectra is found. Abrupt angular variations of the enhancement caused by the photonic structure and the extraction power are observed, in good agreement with calculated polar emission patterns. Our results confirm that the optical cavity favors the extraction of different wavelengths at different angles and that integration of nanophosphors within photonic crystals provides control over the directional emission properties that could be put into practice in phosphorescent displays.Ministerio de Ciencia e Innovación MAT2008- 02166, CSD2007-00007Junta de Andalucía FQM3579, FQM524

Environmentally responsive nanoparticle-based luminescent optical resonators

In this work, we demonstrate that optical resonators built using all-nanoparticle-based porous building blocks provide a responsive multifunctional matrix, totally different emission spectra being attained from the same embedded luminescent nanophosphors under varying environmental conditions. We show a clear correlation between modifications in the ambient surroundings, the induced changes of the resonant modes, and the resulting variations in the emission response. The method is versatile and allows nanophosphors of arbitrary shape to be integrated in the cavity. By precise control of the spectral features of the optical resonances, luminescence is strongly modulated in selected and tuneable wavelength ranges. Applications in the fields of sensing and detection are foreseen for these materialsEspaña Ministerio de Ciencia e Innovación MAT2008-02166 CONSOLIDER HOPE CSD2007-00007Junta de Andalucía FQM357

Multidirectional Light-Harvesting Enhancement in Dye Solar Cells by Surface Patterning

One dimensional gratings patterned on the surface of nanocrystalline titania electrodes are used as a light harvesting strategy to improve the overall performance of dye solar cells under both frontal and rear illumination conditions. A soft-lithography-based micromoulding approach is employed to replicate a periodic surface relief pattern onto the surface of the electrode, which is later sensitized with a dye. As the patterned surface acts as an optical grating both in reflection and transmission modes, its effect is to increase the light path of diffracted beams within the absorbing layer when it is irradiated either from the electrode or the counter electrode for a broad range of angles of incidence on each surface. Full optical and photovoltaic characterization demonstrates not only the optical quality of the patterned surfaces but also the multidirectional character of the enhancement of light harvesting and conversion efficiency. The approach herein presented thus permits to preserve the operation of the cell when irradiated from its two faces while increasing its overall power conversion efficiency. This feature is a key advantage over other light harvesting efficiency enhancing methods, such as the deposition of a back diffuse scattering layer, in which the performance of the cell under illumination from one of its sides is enlarged at the expense of reducing the output under reverse irradiation conditions.European Union’s Seventh Framework Programme 307081Ministerio de Economía y Competitividad MAT2011–23593, CSD2007–00007Junta de Andalucía FQM524