Dennis M. Callahan

Light Absorption Enhancement in Thin‐Film Solar Cells Using Whispering Gallery Modes in Dielectric Nanospheres

Freely propagating sunlight can be diffractively coupled and transformed into several guided whispering gallery modes within an array of wavelength scale dielectric spheres. Incident optical power is then transferred to the thin-film cell by leaky mode coupling into a thin solar cell absorber layer and significantly enhances its efficiency by increasing the fraction of incident light absorbed.

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.

Gallium Arsenide Solar Cell Absorption Enhancement Using Whispering Gallery Modes of Dielectric Nanospheres

Based on a perfectly flat gallium arsenide solar cell, we show that it is possible to modify the flow of light and enhance the absorption without modifying the active material structure or degrading its electrical properties. The sunlight couples into confined resonant modes formed by a periodic arrangement of dielectric nanospheres above the solar cell. The in coupling element is lossless and, thus, has the advantage that no energy is lost within the dielectric nanospheres. This stored energy is absorbed by the underlying active material which directly contributes to the photocurrent enhancement of the solar cell.

Light trapping beyond the 4n2 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 three dimensional (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.

Wafer-Scale Strain Engineering of Ultrathin Semiconductor Crystalline Layers

The fabrication of a wafer-scale dislocation-free, fully relaxed single crystalline template for epitaxial growth is demonstrated. Transferring biaxially-strained Inx Ga1-x As ultrathin films from InP substrates to a handle support results in full strain relaxation and the Inx Ga1-x As unit cell assumes its bulk value. Our realization demonstrates the ability to control the lattice parameter and energy band structure of single layer crystalline compound semiconductors in an unprecedented way.

Metal–Polymer–Metal Split‐Dipole Nanoantennas

The conjugated polymer semiconductor poly(3-hexylthiophene), (P3HT), is integrated directly into the slot region of resonant plasmonic split-dipole nanoantennas. The P3HT radiative emission rate is enhanced by a factor of up to 29, in experiment, and 550 for the ideal case, due to the large local density of optical states in the nanoantenna slot region. Additionally, the theoretical modified luminescence quantum efficiency is shown to increase from 1% to 45% for optimized nanoantenna parameters.

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.

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.

Three efficiency benefits from thin film plasmonic solar cells

In a race to reduce the cost per Watt of solar generated power, there is generally a tradeoff between high efficiency and low cost. By going to thinner devices, less material can be used; however, clever light management designs must be utilized to avoid the loss in current caused by reduced absorption in a thin active layer. Here we discuss such design schemes incorporating either dielectric or metallic structures to approach the bulk absorption limit in optically thin layers. As a specific example, a plasmonic back grating can result in absorption of 80% of the incident above bandgap light in a GaAs layer of only 200 nm. If the reduction in current upon thinning of the cell is limited, an improvement in the open circuit voltage can be obtained through a reduction of the bulk recombination current. Under the condition that the open circuit voltage increases more rapidly than the short circuit current decreases, thinner layers will produce more efficient cells. Finally, the incorporation of metallic scatterers can potentially improve the fill factor by reducing the sheet resistance of a top surface-passivating layer. We show experiments that suggest that the sheet resistance decreases for a metal particle decorated GaAs structure, which can be modeled using a simple circuit diagram. By combining all three effects, we consider the possibility of high efficiency solar cells that are an order of magnitude thinner than their bulk counterparts and consider their role for future photovoltaic device architectures.