6,901 research outputs found
Light Trapping Textures Designed by Electromagnetic Optimization for Sub-Wavelength Thick Solar Cells
Light trapping in solar cells allows for increased current and voltage, as
well as reduced materials cost. It is known that in geometrical optics, a
maximum 4n^2 absorption enhancement factor can be achieved by randomly
texturing the surface of the solar cell, where n is the material refractive
index. This ray-optics absorption enhancement limit only holds when the
thickness of the solar cell is much greater than the optical wavelength. In
sub-wavelength thin films, the fundamental questions remain unanswered: (1)
what is the sub-wavelength absorption enhancement limit and (2) what surface
texture realizes this optimal absorption enhancement? We turn to computational
electromagnetic optimization in order to design nanoscale textures for light
trapping in sub-wavelength thin films. For high-index thin films, in the weakly
absorbing limit, our optimized surface textures yield an angle- and
frequency-averaged enhancement factor ~39. They perform roughly 30% better than
randomly textured structures, but they fall short of the ray optics enhancement
limit of 4n^2 ~ 50
Photonic Design: From Fundamental Solar Cell Physics to Computational Inverse Design
Photonic innovation is becoming ever more important in the modern world.
Optical systems are dominating shorter and shorter communications distances,
LED's are rapidly emerging for a variety of applications, and solar cells show
potential to be a mainstream technology in the energy space. The need for
novel, energy-efficient photonic and optoelectronic devices will only increase.
This work unites fundamental physics and a novel computational inverse design
approach towards such innovation.
The first half of the dissertation is devoted to the physics of
high-efficiency solar cells. As solar cells approach fundamental efficiency
limits, their internal physics transforms. Photonic considerations, instead of
electronic ones, are the key to reaching the highest voltages and efficiencies.
Proper photon management led to Alta Device's recent dramatic increase of the
solar cell efficiency record to 28.3%. Moreover, approaching the
Shockley-Queisser limit for any solar cell technology will require light
extraction to become a part of all future designs.
The second half of the dissertation introduces inverse design as a new
computational paradigm in photonics. An assortment of techniques (FDTD, FEM,
etc.) have enabled quick and accurate simulation of the "forward problem" of
finding fields for a given geometry. However, scientists and engineers are
typically more interested in the inverse problem: for a desired functionality,
what geometry is needed? Answering this question breaks from the emphasis on
the forward problem and forges a new path in computational photonics. The
framework of shape calculus enables one to quickly find superior, non-intuitive
designs. Novel designs for optical cloaking and sub-wavelength solar cell
applications are presented.Comment: 137 pages, 55 figures. PhD thesis, Electrical Engineering, Berkeley
(2012
The effectiveness of thin films in lieu of hyperbolic metamaterials in the near field
We show that the near-field functionality of hyperbolic metamaterials (HMM),
typically proposed for increasing the photonic local density of states (LDOS),
can be achieved with thin metal films. Although HMMs have an infinite density
of internally-propagating plane-wave states, the external coupling to nearby
emitters is severely restricted. We show analytically that properly designed
thin films, of thicknesses comparable to the metal size of a hyperbolic
metamaterial, yield a LDOS as high as (if not higher than) that of HMMs. We
illustrate these ideas by performing exact numerical computations of the LDOS
of multilayer HMMs, along with their application to the problem of maximizing
near-field heat transfer, to show that thin films are suitable replacements in
both cases.Comment: 5 pages, 3 figure
Optimization of sharp and viewing-angle-independent structural color
Structural coloration produces some of the most brilliant colors in nature
and has many applications. However, the two competing properties of narrow
bandwidth and broad viewing angle have not been achieved simultaneously in
previous studies. Here, we use numerical optimization to discover geometries
where a sharp 7% bandwidth in scattering is achieved, yet the peak wavelength
varies less than 1%, and the peak height and peak width vary less than 6% over
broad viewing angles (0--90) under a directional illumination. Our
model system consists of dipole scatterers arranged into several rings;
interference among the scattered waves is optimized to yield the
wavelength-selective and angle-insensitive response. Such designs can be useful
for the recently proposed transparent displays that are based on
wavelength-selective scattering
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