Photocatalytic Template Removal by Non-Ozone-Generating UV Irradiation for the Fabrication of Well-Defined Mesoporous Inorganic Coatings

The processing of mesoporous inorganic coatings typically requires a high-temperature calcination step to remove organic precursors that are essential during the material assembly. Lowering the fabrication energy costs and cutting back on the necessary resources would provide a greater scope for the deployment in applications such as architectural glass, optical components, photovoltaic cells, and energy storage, as well as further compatibilize substrates with low temperature stability. Organic removal methods based on UV-ozone treatment are increasing in popularity, but concerns remain regarding large-scale ozone generation and usage of mercury-containing UV lamps. To this end, we present a method that relies on non-ozone-generating UV radiation at 254 nm (UV₂₅₄) and incorporation of small amounts of photocatalytic material in the formulation, here demonstrated with TiO₂ nanocrystals. At concentrations as low as 5 wt % relative to the main inorganic aluminosilicate material, the TiO₂ nanocrystals catalyze a "cold combustion" of the organic components under UV₂₅₄ irradiation to reveal a porous inorganic network. Using block copolymer-based co-assembly in conjunction with photocatalytic template removal, we produce well-defined mesoporous inorganic thin films with controlled porosity and refractive index values, where the required processing time is governed by the amount of TiO₂ loading. This approach provides an inexpensive, flexible, and environmentally friendly alternative to traditional organic removal techniques, such as UV-ozone degradation and thermal calcination.

Single-step fabrication process of 1-D photonic crystals coupled to nanocolumnar TiO2 layers to improve DSC efficiency

The present work proposes the use of a TiO₂ electrode coupled to a one-dimensional photonic crystal (1DPC), all formed by the sequential deposition of nanocolumnar thin films by physical vapor oblique angle deposition (PV-OAD), to enhance the optical and electrical performance of DSCs while transparency is preserved. We demonstrate that this approach allows building an architecture combining a non-dispersive 3 µm of TiO₂ electrode and 1 µm TiO₂-SiO₂ 1DPC, both columnar, in a single-step process. The incorporation of the photonic structure is responsible for a rise of 30% in photovoltaic efficiency, as compared with a transparent cell with a single TiO₂ electrode. Detailed analysis of the spectral dependence of the photocurrent demonstrates that the 1DPC improves light harvesting efficiency by both back reflection and optical cavity modes confinement within the TiO₂ films, thus increasing the overall performance of the cell.

Block copolymer self-assembly for nanophotonics

The ability to control and modulate the interaction of light with matter is crucial to achieve desired optical properties including reflection, transmission, and selective polarization. Photonic materials rely upon precise control over the composition and morphology to establish periodic interactions with light on the wavelength and sub-wavelength length scales. Supramolecular assembly provides a natural solution allowing the encoding of a desired 3D architecture into the chemical building blocks and assembly conditions. The compatibility with solution processing and low-overhead manufacturing is a significant advantage over more complex approaches such as lithography or colloidal assembly. Here we review recent advances on photonic architectures derived from block copolymers and highlight the influence and complexity of processing pathways. Notable examples that have emerged from this unique synthesis platform include Bragg reflectors, antireflective coatings, and chiral metamaterials. We further predict expanded photonic capabilities and limits of these approaches in light of future developments of the field.

Panchromatic porous specular back reflectors for efficient transparent dye solar cells

A panchromatic specular reflector that mimics the reflection properties of a standard diffuse scattering layer in a broad spectral range within the visible is coupled to a dye sensitized nanocrystalline titania electrode to attain a solar cell of largely enhanced efficiency that, at the same time, preserves its transparency.

A panchromatic specular reflector based dye solar cell is presented herein. 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 in a complementary spectral range, and are designed to leave transmittance unaltered in a narrow red-frequency range in which the sensitized electrode shows negligible absorption, thus allowing us to see through the cell. The reflectance bandwidth achieved is three times as broad as the largest bandwidth previously achieved using any photonic structure integrated into a dye solar cell.

Enhanced light harvesting in dye-sensitized solar cells coupled with titania nanotube photonic crystals: a theoretical study

Herein we present a theoretical analysis on the optical properties and the photocurrent enhancement of nanotube-based dye-sensitized solar cells (DSSCs) coupled with a TiO2 nanotube (NT) photonic crystal (PC). It is found that the introduction of a TiO2 nanotube PC produces both Bragg mirror effect and Fabry-Perot cavity behavior, leading to a significant enhancement of light harvesting for photons in the photonic bandgap and at the two band edges. In addition, an increased amount of surface-anchored dye due to the larger surface area in the NT PC layer also causes absorption enhancement in the whole visible spectrum. The effects of structural parameters of the PC, such as the thickness of the PC layer, the axial lattice constant, the diameter of the nanotube, and light incident angle, on the optical properties and photocurrent magnification are thoroughly studied. The optimum structural parameters are proposed, which not only provide guidance but also offer further opportunities in the design and applications of TiO2 nanotube photonic crystals.

See-through dye-sensitized solar cells: photonic reflectors for tandem and building integrated photovoltaics

See-through dye-sensitized solar cells with 1D photonic crystal Bragg reflector photoanodes show an increase in peak external quantum efficiency of 47% while still maintaining high fill factors, resulting in an almost 40% increase in power conversion efficiency. These photoanodes are ideally suited for tandem and building integrated photovoltaics.

Angular response of photonic crystal based dye sensitized solar cells

Herein we report an experimental analysis of the performance of photonic crystal based dye solar cells (PC-DSCs) as the incident light angle moves away from the normal with respect to the cell surface. Nanoparticle multilayers operating at different wavelength ranges were coupled to the working electrode of a dye solar cell for this study. The interplay between optical and photovoltaic properties with the incident light angle is discussed. We demonstrate that an efficiency enhancement is attained for PC-DSCs at all angles measured, and that rational design of the photonic crystal back mirror leads to a reduction of the photocurrent losses related to the tilt angle of the cell, usually labeled as cosine losses. Angular variations of the cell transparency are also reported and discussed. These angular properties are relevant to the application of these solar devices in building integrated photovoltaics as potential window modules.

Self-cleaning antireflective optical coatings

Low-cost antireflection coatings (ARCs) on large optical surfaces are an ingredient-technology for high-performance solar cells. While nanoporous thin films that meet the zero-reflectance conditions on transparent substrates can be cheaply manufactured, their suitability for outdoor applications is limited by the lack of robustness and cleanability. Here, we present a simple method for the manufacture of robust self-cleaning ARCs. Our strategy relies on the self-assembly of a block-copolymer in combination with silica-based sol-gel chemistry and preformed TiO2 nanocrystals. The spontaneous dense packing of copolymer micelles followed by a condensation reaction results in an inverse opal-type silica morphology that is loaded with TiO2 photocatalytic hot-spots. The very low volume fraction of the inorganic network allows the optimization of the antireflecting properties of the porous ARC despite the high refractive index of the embedded photocatalytic TiO2 nanocrystals. The resulting ARCs combine high optical and self-cleaning performance and can be deposited onto flexible plastic substrates.

Nanomorphology tuning of the thermal response of TiO2/SiO2 Bragg stacks

Herein, we present a comparative study of thermo- and environmentally responsive TiO2/SiO2 one-dimensional photonic crystals (Bragg stacks) fabricated by different deposition methods and fabrication schemes, featuring various multilayer nanomorphologies. These include dense multilayer systems processed by physical vapor deposition and wet-chemistry protocols, as well as porous systems, namely, nanoparticle-based optical filters exhibiting textural porosity, and evaporation-induced self-assembled mesoporous Bragg stacks exhibiting predominantly structural porosity, as well as hybrid structures comprising both dense and porous layers. We investigate the spectral shift of the photonic stop band for the different Bragg stack nanomorphologies induced by the humidity-enhanced thermo-optic effect in a temperature range from 15 to 60 °C. We also demonstrate the response and recovery kinetics of the multilayer systems during external changes in ambient humidity. Notably, the choice of fabrication method plays a significant role in the thermal and humidity response of the system. Taking advantage of different material nanomorphologies we can tune the thermal shift of the photonic stop band in the range 0.2–32.9 nm for the Bragg stacks at ambient relative humidity. In addition, we can design dense multilayer systems nonresponsive to humidity and achieve time responses of the porous systems to external changes in humidity ranging from about 1 to 3 s.

Avoiding cracks in nanoparticle films

A new method utilizing subsequent depositions of thin crack-free nanoparticle layers is demonstrated to avoid the formation of cracks within silica nanoparticle films. Using this method, films can be assembled with thicknesses exceeding the critical cracking values. Explanation of this observed phenomenon is hypothesized to mainly arise from chemical bond formation between neighboring silica nanoparticles. Application of this method for fabricating crack-free functional structures is demonstrated by producing crack-free Bragg reflectors that exhibit structural color.