Unassisted Water Splitting Using Standard Silicon Solar Cells Stabilized with Copper and Bifunctional NiFe Electrocatalysts.

Silicon photovoltaic cells functionalized with water-splitting electrocatalysts are promising candidates for unassisted water splitting. In these devices, the total surface of silicon solar cells is covered with electrocatalysts, causing issues with (i) stabilizing silicon solar cells in water and (ii) device efficiency due to parasitic optical absorption in electrocatalysts. We describe and validate a water-splitting device concept using a crystalline silicon solar cell where the front side is covered with an insulating Si3N5 antireflection coating. The Ag contacts, fired through the antireflection coating, are removed and subsequently substituted with NiFe layered double hydroxide (LDH) or Cu/NiFe-LDH electrocatalysts. In this device, only the site of Ag contacts, nearly 2% of the total device area, is covered by the electrocatalyst. We found that this small area of the catalyst does not limit device performance and the addition of a Cu interlayer between Si and NiFe-LDH improves device performance and stability. The unassisted water-splitting efficiency of 11.31%, measured without separating the evolved gases, is achieved using a device composed of three series-connected silicon solar cells and an NiFe-LDH/Cu/Ni-foam counter electrode in a highly alkaline electrolyte.

Broadband and Low-Loss Plasmonic Light Trapping in Dye-Sensitized Solar Cells Using Micrometer-Scale Rodlike and Spherical Core-Shell Plasmonic Particles.

Dielectric scattering particles have widely been used as embedded scattering elements in dye-sensitized solar cells (DSCs) to improve the optical absorption of the device. Here we systematically study rodlike and spherical core-shell silica@Ag particles as more effective alternatives to the dielectric scattering particles. The wavelength-scale silica@Ag particles with sufficiently thin Ag shell support hybrid plasmonic-photonic resonance modes that have low parasitic absorption losses and a broadband optical response. Both of these features lead to their successful deployment in light trapping in high-efficiency DSCs. Optimized rodlike silica@Ag@silica particles improve the power conversion efficiency of a DSC from 6.33 to 8.91%. The dimension, surface morphology, and concentration of these particles are optimized to achieve maximal efficiency enhancement. The rodlike silica particles are prepared in a simple one-pot synthesis process and then are coated with Ag in a liquid-phase deposition process by reducing an Ag salt. The aspect ratio of silica rods is tuned by adjusting the temperature and duration of the growth process, whereas the morphology of Ag shell is tailored by controlling the reduction rate of Ag salt, where slower reduction in a polyol process gives a smoother Ag shell. Using optical calculations, the superior performance of the plasmonic core-shell particles is related to the large number of hybrid photonic-plasmonic resonance modes that they support.

Parasitic Absorption Reduction in Metal Oxide-Based Transparent Electrodes: Application in Perovskite Solar Cells.

Transition metal oxides (TMOs) are commonly used in a wide spectrum of device applications, thanks to their interesting electronic, photochromic, and electrochromic properties. Their environmental sensitivity, exploited for gas and chemical sensors, is however undesirable for application in optoelectronic devices, where TMOs are used as charge injection or extraction layers. In this work, we first study the coloration of molybdenum and tungsten oxide layers, induced by thermal annealing, Ar plasma exposure, or transparent conducting oxide overlayer deposition, typically used in solar cell fabrication. We then propose a discoloration method based on an oxidizing CO2 plasma treatment, which allows for a complete bleaching of colored TMO films and prevents any subsequent recoloration during following cell processing steps. Then, we show that tungsten oxide is intrinsically more resilient to damage induced by Ar plasma exposure as compared to the commonly used molybdenum oxide. Finally, we show that parasitic absorption in TMO-based transparent electrodes, as used for semitransparent perovskite solar cells, silicon heterojunction solar cells, or perovskite/silicon tandem solar cells, can be drastically reduced by replacing molybdenum oxide with tungsten oxide and by applying a CO2 plasma pretreatment prior to the transparent conductive oxide overlayer deposition.

Self-Assembled Monolayer of Wavelength-Scale Core-Shell Particles for Low-Loss Plasmonic and Broadband Light Trapping in Solar Cells.

Scattering particles constitute a key light trapping solution for thin film photovoltaics where either the particles are embedded in the light absorbing layer or a thick layer of them is used as a reflector. Here we introduce a monolayer of wavelength-scale core-shell silica@Ag particles as a novel light trapping strategy for thin film photovoltaics. These particles show hybrid photonic-plasmonic resonance modes that scatter light strongly and with small parasitic absorption losses in Ag (<1.5%). In addition, their scattering efficiency does not vary significantly with the refractive index of the surrounding medium. A monolayer of these particles is applied as the top-scattering layers in a dye-sensitized solar cells and it improves the short-circuit current density of a cell with 7 µm-thick dye-sensitized layer by 38%. Optical measurements of the scattering properties of these particles confirm that the strong scattering and low-parasitic absorption losses constitute the main reason for this efficient light trapping.

Theoretical Study of Light Trapping in Nanostructured Thin Film Solar Cells Using Wavelength-Scale Silver Particles.

We propose and theoretically evaluate a plasmonic light trapping solution for thin film photovoltaic devices that comprises a monolayer or a submonolayer of wavelength-scale silver particles. We systematically study the effect of silver particle size using full-wave electromagnetic simulations. We find that light trapping is significantly enhanced when wavelength-scale silver particles rather than the ones with subwavelength dimensions are used. We demonstrate that a densely packed monolayer of spherical 700 nm silver particles enhances integrated optical absorption under standard air mass 1.5 global (AM1.5G) in a 7 µm-thick N719-sensitized solar cell by 40% whereas enhancement is smaller than 2% when 100 nm ones are used. Superior performance of wavelength-scale silver particles is attributed to high-order whispering gallery modes that they support. These modes scatter the light over a wider angular range, hence increasing the density of both waveguide and resonance modes within the dye-sensitized layer.

Plasmonic enhancement of the optical absorption and catalytic efficiency of BiVO4 photoanodes decorated with Ag@SiO2 core-shell nanoparticles.

Recent progress in the development of bismuth vanadate (BiVO4) photoanodes has firmly established it as a promising material for solar water splitting applications. Performance limitations due to intrinsically poor catalytic activity and slow electron transport have been successfully addressed through the application of water oxidation co-catalysts and novel doping strategies. The next bottleneck to tackle is the modest optical absorption in BiVO4, particularly close to its absorption edge of 2.4 eV. Here, we explore the modification of the BiVO4 surface with Ag@SiO2 core-shell plasmonic nanoparticles. A photocurrent enhancement by a factor of ~2.5 is found under 1 sun illumination (AM1.5). We show that this enhancement consists of two contributions: optical absorption and catalysis. The optical absorption enhancement is induced by the excitation of localized surface plasmon resonances in the Ag nanoparticles, and agrees well with our full-field electromagnetic simulations. Far-field effects (scattering) are found to be dominant, with a smaller contribution from near-field plasmonic enhancement. In addition, a significant catalytic enhancement is observed, which is tentatively attributed to the electrocatalytic activity of the Ag@SiO2 nanoparticles.

Determination of local refractive index variations in thin films by heterodyne interferometric scanning near-field optical microscopy.

We report on a heterodyne interferometric scanning near-field optical microscope developed for characterizing, at the nanometric scale, refractive index variations in thin films. An optical lateral resolution of 80 nm (lambda/19) and a precision smaller than 10(-4) on the refractive index difference have been achieved. This setup is suitable for a wide set of thin films, ranging from periodic to heterogeneous samples, and turns out to be a very promising tool for determining the optical homogeneity of thin films developed for nanophotonics applications.

The radiated fields of the fundamental mode of photonic crystal fibers.

The six-fold rotational symmetry of photonic crystal fibers has important manifestations in the radiated fields in terms of i) a focusing phenomena at a finite distance from the end-facet and ii) the formation of low-intensity satellite peaks in the asymptotic far field. For our study, we employ a surface equivalence principle which allows us to rigorously calculate radiated fields starting from fully-vectorial simulations of the near field. Our simulations show that the focusing is maximal at a characteristic distance from the end-facet. For large-mode area fibers the typical distance is of the order 10xLambda with Lambda being the pitch of the triangular air-hole lattice of the photonic crystal fiber.