Integrating Light Diffusion and Conversion Layers for Highly Efficient Multicolored Fiber-Dye-Sensitized Solar Cells

Fiber solar cells as promising wearable power supplies have attracted increasing attentions recently, while further breakthrough on their power conversion efficiency (PCE) and realization of multicolored appearances remain urgent needs particularly in real-world applications. Here, a fiber-dye-sensitized solar cell (FDSSC) integrated with a light diffusion layer composed of alumina/polyurethane film on the outmost encapsulating tube and a light conversion layer made from phosphors/TiO2/poly(vinylidene fluoride-co-hexafluoropropylene) film on the inner counter electrode is designed. The incident light is diffused to more surfaces of fiber electrodes, then converted on counter electrode and reflected to neighboring photoanode, so the FDSSC efficiently takes advantage of the fiber shape for remarkably enhanced light harvesting, producing a record PCE of 13.11%. These efficient FDSSCs also realize color-tunable appearances, improving their designability and compatibility with textiles. They are further integrated with fiber batteries as power systems, providing a power solution for wearables and emerging smart textiles.

Investigation of thermoluminescence and photoluminescence properties of Tb3+, Eu3+, and Dy3+ doped NaYF4 phosphors for dosimetric applications

Hexagonal phase sodium yttrium fluoride activated with lanthanide ions; Tb³⁺, Eu³⁺ and Dy³⁺ doped NaYF₄ phosphors were synthesized using a simplistic hydrothermal method. The photoluminescence studies demonstrated green, red and blue emission lines corresponding to ⁵D₄ → ⁷F_J (J = 6, 5, 4, 3), ⁵D₀ → ⁷F_J (J = 1, 2 and 4) and ⁴F_9/2 to ⁶H_J (J = 15/2 and 13/2) transitions, which are characteristic of Tb³⁺, Eu³⁺ and Dy³⁺ ions, respectively. The as-synthesized samples were subjected to annealing at varying temperatures from 500 °C to 800 °C primarily for the optimization of the thermoluminescence glow curve. Meanwhile, we studied the influence of thermal annealing treatment on the crystal phase, morphological features, and photoluminescence properties of the phosphors. The spherical-like morphology of NaYF₄:Tb³⁺ phosphor changed to micro block-like structures and the hexagonal phase of NaYF₄ transforms into a cubic phase at a higher annealing temperature of ∼800 °C. The photoluminescence emission intensity also varied at different annealing temperatures. The systematic study using different dopants and annealing temperatures was carried out to accomplish efficient thermoluminescence (TL) properties. The most suitable TL dosimetric glow peak was attained for NaYF₄:0.5%Tb³⁺ phosphor, which was annealed at a temperature of 800 °C, located at 194 °C. The NaYF₄:Tb³⁺ phosphor exhibited TL response fairly linear in the dose range from 1 kGy to 25 kGy of gamma radiation. The phosphor showed TL response at higher doses, which stipulates that the NaYF₄:Tb³⁺ is reasonably well suitable for high dose measurements and respective applications. The phosphor exhibited negligible fading and good reproducibility features. Trap-level analysis and experimental determination of activation energy were performed using the T_m-T_stop technique and initial rise method (IRM). The trapping parameters of the TL glow curve, such as activation energy (E), order of kinetics (b), and frequency factor (s) were estimated by Chen's glow peak shape (PS) method and glow curve deconvolution (GCD) method. The trapping parameters obtained using the IRM, PS and GCD methods are in good accordance with each other. Henceforth, along with efficient photoluminescence properties, the NaYF₄:Tb³⁺ phosphor exhibited favorable thermoluminescence dosimetric properties. Consequently, this study provides new opportunities for utilizing these phosphors in the area of radiation dosimetry applications such as environmental and food monitoring, space dosimetry etc.

Synthesis and optimization of photoluminescence properties in potential reddish orange emitting niobate phosphor for photonic device applications

A series of samarium ions (Sm³⁺ ) activated barium sodium niobate (Ba₂ NaNb₅ O₁₅ ) samples have been successfully synthesized via employing a solid-state reaction technique. Single phase, crystalline tetragonal Ba₂ NaNb₅ O₁₅ were formed and the crystallite size of the prepared sample varied with doping of Sm³⁺ ions. The scanning electron microscopy (SEM) images of Ba₂ NaNb₅ O₁₅ :Sm³⁺ illustrate that the particles possess a non-uniform spherical structure with some agglomeration. The prepared Ba₂ NaNb₅ O₁₅ :Sm³⁺ phosphors were efficiently excited with near-ultraviolet (n-UV) (406 nm) and emit strong visible emission peaks in the range 550-725 nm. The phenomenon of concentration quenching was detected after x = 0.10 mol of Sm³⁺ ions concentration for Ba₂ NaNb₅ O₁₅ , which arises due to non-radiative energy transfer through dipole-dipole interaction among activator ions. Colour coordinates (0.586, 0.412) for the optimized phosphor lies in the visible reddish orange region. A bi-exponential decay behaviour was observed for the photoluminescence decay curve of the optimized phosphor sample with an average decay time in milliseconds. The temperature dependent emission intensity confirms that the Ba_2-x NaNb₅ O₁₅ :xSm³⁺ (x = 0.10 mol) phosphor exhibits adequate thermal stability having high value of activation energy (ΔE = 0.201 eV). The comprehensive study and analysis of the as-prepared samples suggest that the intense reddish orange emitting thermally stable Ba₂ NaNb₅ O₁₅ :Sm³⁺ phosphor can act as a potential luminescent material in phosphor coated lighting, solar cells and other photonic devices.

Directional Emission of Fluorescent Dye-Doped Dielectric Nanogratings for Lighting Applications

By structuring a luminescent dielectric interface as a relief diffraction grating with nanoscale features, it is possible to control the intensity and direction of the emitted light. The composite structure of the grating is based on a fluorescent dye (Lumogen F RED 305) dispersed in a polymeric matrix (poly(methyl methacrylate)). Measurements demonstrate a significant enhancement of the emitted light for specific directions and wavelengths when the grating interface is compared to nonstructured thin films made of the same material. In particular, the maximum enhancement of photoluminescence for a given pump wavelength is obtained at an angle of incidence that is close to the Rayleigh anomaly condition for the first-order diffracted waves. In this condition, the maximum extinction of incident light is observed. Upon excitation with coherent and monochromatic sources, photoluminescence plots show that the Rayleigh anomalies confine the angular interval of the emitted light. Being the anomalies  directly related to the pitch of the diffraction grating, the system can be thus implemented as an optical device whose directional emission can be designed for specific applications. The exploitation of nanoimprinting techniques for the fabrication of the luminescent grating enables production of the device on large areas, paving the way for low-cost lighting and solar applications.

Luminescent Spectral Conversion to Improve the Performance of Dye-Sensitized Solar Cells

Relative to the broadband solar spectrum, a narrow range of spectral absorption of photovoltaic (PV) devices is considered an important determinant that the efficiency of light harvesting of these devices is less than unity. Having the narrowest spectral response to solar radiation among all PV devices, dye-sensitized solar cells (DSSCs) suffer severely from this loss. Luminescent spectral conversion provides a mechanism to manipulate and to adapt the incident solar spectrum by converting, through photoluminescence, the energies of solar photons into those that are more effectively captured by a PV device. This mechanism is particularly helpful for DSSCs because there is much flexibility in both the choice of the light-harvesting materials and the architecture of the DSSC. Here we review and discuss recent advances in the field of luminescent spectral conversion for DSSCs. The focus is on the architectural design of DSSCs, and the complications, advantages and new functionalities offered by each of their configurations are discussed. The loss mechanisms are examined and important parameters governing the spectral conversion mechanism of a DSSC are introduced.