Ali Dabirian

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.

Dielectric core–shells with enhanced scattering efficiency as back-reflectors in dye sensitized solar cells

Particulate back-reflector films are conventionally used for the improvement of light harvesting in dye solar cells (DSC). The back-reflection of the films is directly related to the scattering efficiency of the individual particles. Inspired by the idea of multilayer optical thin films, here it is demonstrated theoretically and experimentally that putting a SiO2 shell around spherical rutile-TiO2 particles leads to improved light scattering by the particles. These dielectric core–shells not only enhance the overall diffuse reflection of the films, but they also cause a relative improvement in the red and near infrared regions. Back-reflector films of these core–shell particles employed in DSCs result in an enhanced device efficiency of up to 9.52%. Contrary to the general notion, the conventional DSC back-reflector films are far from ideal, i.e. they show less than a 50% reflection in real device conditions. This is due to the limited thickness of the films (a few micron), as well as the low “wet” reflection of the back-reflector films, i.e. the reflection of the scattering particles embedded in a liquid electrolyte. This fact reveals the need for more optimized scattering particles. We suggest dielectric core–shell particles as an alternative.

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.

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.

Resonant-size spherical bottom scatterers for dye-sensitized solar cells

We numerically evaluate the effect of a monolayer of resonant-size TiO2 spheres on the performance of dye-sensitized solar cells (DSCs). This scattering layer is placed between the transparent conducting layer and the dye-sensitized mesoporous TiO2 layer. We carried out our numerical calculations by solving full-wave Maxwell equations in the entire DSC structure using the rigorous coupled-waves approach (RCWA). The layer of TiO2 spheres functions as a strong reflector, leading to strong confinement of the incident light within the absorbing layer of the DSC. The reflectance from this layer originates from coupling of light to the optical resonance modes of the TiO2 spheres. Comparing maximum achievable photocurrent densities of the DSC with TiO2 spheres relative to the standard planar DSC shows that this scattering layer enhances the cell performance as long as the thickness of the dye-sensitized layer stays below 6 μm. Therefore this photon management strategy is useful for solid-state DSCs with thin absorbing layers of up to 3 μm thick. This scattering layer can be used in combination with the widely applied top-scattering layers, leading to an upper limit of 150% and 60% enhancements in DSCs having a 1 μm and 3 μm thick sensitized layers if 600 nm TiO2 spheres are used.

Photonic design of embedded dielectric scatterers for dye sensitized solar cells

Embedded dielectric scatterers comprise an important approach for light trapping in dye-sensitized solar cells (DSCs) due to their simple fabrication process. The challenge in applying these scatterers lies in finding the optimal dimensions and concentration of the scatterers. This requires many experiments and it is often quite difficult to have a starting point for optimizing the concentration. Based on theories of light propagation in random media, we propose a simple model for DSCs with embedded silica spherical particles. Then, by full-wave optical calculations, we determine a narrow range for the concentration of silica particles that leads to the largest optical absorption in the cell. The simulation results were verified by realizing DSCs with different concentrations of silica particles. A power conversion efficiency of 8.08% in an 11 μm-thick N719-sensitized DSC was achieved with 6 vol% embedded silica, which further increased to 9.30% by applying a white scattering layer on the rear-side of the counter electrode. The design approach, presented here, is a general approach that can be applied for other types of solar light harvesting structures with low optical absorption coefficient.

Propagation of light in photonic crystal fibre devices

We describe a semi-analytical approach for the three-dimensional analysis of photonic crystal fibre devices. The approach relies on modal transmission-line theory. We offer two examples illustrating the utilization of this approach in photonic crystal fibres: the verification of the coupling action in a photonic crystal fibre coupler and the modal reflectivity in a photonic crystal fibre distributed Bragg reflector.

Influence of Au thickness on the performance of plasmonic enhanced hematite photoanodes

The surface plasmon effect of Au nanostructures placed on the surface of hematite has recently been used to enhance light absorption within its carrier collection distance of 10 nm from the water–hematite interface. Despite significant narrow band absorption enhancements in the visible region, the reported enhancements in the overall performance of the cell under standard AM1.5 sunlight illumination are not significant. We numerically design an array of Au stripes on the surface of an extremely thin layer of hematite to maximize the number of charge carriers that can reach the hematite–water interface and explore the optical processes involved. The lateral dimensions and in particular the thickness of the Au stripes turn out to be influential parameters for the overall enhancement that Au stripes provide. In general, the thickness of Au stripes for the optimized structure needs to be larger than 100 nm to obtain overall enhancements higher than 25% at incidence angles of up to 40°. The optimized structure is robust against edge bluntness, which is usually an artefact of the fabrication process. However, the presence of a 5 nm SiO2 insulating layer decreases the enhancement of the optimized configuration to 16%. These insights give important clues to design efficient plasmonic enhanced light harvesting structures for extremely thin layers of hematite.

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.