Solar Cell Light Trapping beyond the Ray Optic Limit

In 1982, Yablonovitch proposed a thermodynamic limit on light trapping within homogeneous semiconductor slabs, which implied a minimum thickness needed to fully absorb the solar spectrum. However, this limit is valid for geometrical optics but not for a new generation of subwavelength solar absorbers such as ultrathin or inhomogeneously structured cells, wire-based cells, photonic crystal-based cells, and plasmonic cells. Here we show that the key to exceeding the conventional ray optic or so-called ergodic light trapping limit is in designing an elevated local density of optical states (LDOS) for the absorber. Moreover, for any semiconductor we show that it is always possible to exceed the ray optic light trapping limit and use these principles to design a number of new solar absorbers with the key feature of having an elevated LDOS within the absorbing region of the device, opening new avenues for solar cell design and cost reduction

Light trapping beyond the 4n^2 limit in thin waveguides

We describe a method for determining the maximum absorption enhancement in thin film waveguides based on optical dispersion relations. For thin film structures that support one, well-confined guided mode, we find that the absorption enhancement can surpass the traditional limit of 4n^2 when the propagation constant is large and/or the modal group velocity is small compared to the bulk value. We use this relationship as a guide to predicting structures that can exceed the 4n^2 light trapping limit, such as plasmonic and slot waveguides. Finally, we calculate the overall absorption for both single and multimode waveguides, and show examples of absorption enhancements in excess of 4n^2 for both cases

Simulations of solar cell absorption enhancement using resonant modes of a nanosphere array

We propose an approach for enhancing the absorption of thin-film amorphous silicon solar cells using periodic arrangements of resonant dielectric nanospheres deposited as a continuous film on top of a thin planar cell. We numerically demonstrate this enhancement using 3D full field finite difference time domain simulations and 3D finite element device physics simulations of a nanosphere array above a thin-film amorphous silicon solar cell structure featuring back reflector and anti-reflection coating. In addition, we use the full field finite difference time domain results as input to finite element device physics simulations to demonstrate that the enhanced absorption contributes to the current extracted from the device. We study the influence of a multi-sized array of spheres, compare spheres and domes and propose an analytical model based on the temporal coupled mode theory

Silicon Solar Cell Light-Trapping Using Defect Mode Photonic Crystals

Nanostructured active or absorbing layers of solar cells, including photonic crystals and wire arrays, have been increasingly explored as potential options to enhance performance of thin film solar cells because of their unique ability to control light. We show that 2D photonic crystals can improve light trapping by an enhanced density of optical states and improved incoupling, and demonstrate, using FDTD simulation, absorption enhancements in 200nm thick crystalline silicon solar cells of up to 205% from λ = 300nm to 1100nm compared to a planar cell with an optimized two-layer antireflection coating. We report here a method to further enhance absorption by introducing a lattice of coupled defect modes into the photonic crystal, which modify the available optical states in the absorber. Our results show that 2D photonic crystals are a viable and rich research option for light trapping in thin film photovoltaics

Light trapping in ultrathin silicon photonic crystal superlattices with randomly-textured dielectric incouplers

We report here several different superlattice photonic crystal based designs for 200nm thick c-Si solar cells, demonstrating that these structures have the ability to increase broadband absorption from λ = 300nm to 1100nm by more than 100% compared to a planar cell with an optimized anti-reflection coating. We show that adding superlattices into photonic crystals introduces new optical modes that contribute to enhanced absorption. The greatest improvements are obtained when combining a superlattice photonic crystal with a randomly textured dielectric coating that improves incoupling into the modes of the absorbing region. Finally, we show that our design methodology is also applicable to layers 1 to 4 microns in thickness, where absorbed currents competitive with conventional thick Si solar cells may be achieved

Extremely broadband ultralight thermally emissive metasurfaces

We report the design, fabrication and characterization of ultralight highly emissive metaphotonic structures with record-low mass/area that emit thermal radiation efficiently over a broad spectral (2 to 35 microns) and angular (0-60 degrees) range. The structures comprise one to three pairs of alternating nanometer-scale metallic and dielectric layers, and have measured effective 300 K hemispherical emissivities of 0.7 to 0.9. To our knowledge, these structures, which are all subwavelength in thickness are the lightest reported metasurfaces with comparable infrared emissivity. The superior optical properties, together with their mechanical flexibility, low outgassing, and low areal mass, suggest that these metasurfaces are candidates for thermal management in applications demanding of ultralight flexible structures, including aerospace applications, ultralight photovoltaics, lightweight flexible electronics, and textiles for thermal insulation

Stabilization of High-Valent Metals by Corroles: Oxo[tris(pentafluorophenyl)corrolato]chromium(V)

The aerobic reaction of Cr(CO)_6 with tris(pentafluorophenyl)corrole (H_3(TpFPC)) in toluene gives the dark red oxochromium(V) compound (TpFPC)Cr(O), which has been characterized by X-ray crystallography, electrochemistry, and EPR spectroscopy. Short Cr−N (1.927−1.943 Å) bonds as well as relatively large ^(14)N and small ^(53)Cr coupling constants suggest that σ(N → Cr) donation is responsible for the unusual stability of chromium(V) in this complex. The Cr^(V/IV) reduction potential (0.11 V vs Ag/AgCl) is 0.65 V below that of oxo(tetramesitylporphinato)chromium(V)

Nanophotonic design principles for ultrahigh efficiency photovoltaics

To date, solar-cell efficiencies have remained well below the thermodynamic limits. However new nanophotonic and microphotonic approaches to light management that systematically minimize thermodynamic losses can enable ultrahigh efficiencies previously considered to be out of reach