Emily C. Warmann

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.

Spectrum-splitting photovoltaics: Holographic spectrum splitting in eight-junction, ultra-high efficiency module

To achieve photovoltaic energy conversion with ultra-high module efficiency, the number of junctions can be increased beyond the 3-5 used in conventional lattice-matched multi-junction photovoltaics. We demonstrate a photovoltaic design that incorporates eight III-V semiconductor junctions arranged laterally as four dual-junction subcells operating electrically and optically in an independent manner. An integrated holographic optical element is used to split the incoming solar spectrum into four different spectral bands, each diffracted onto the appropriate subcell. In addition, two-axis concentration is used to achieve total concentration of 672 suns. A preliminary design for this holographic spectrum splitter composed of twelve simple, commercially available holograms predicts an achievable 76.1% optical efficiency and 37.1% two-terminal module efficiency; opportunities for improved efficiency are discussed.

Spectrum splitting photovoltaics: light trapping filtered concentrator for ultrahigh photovoltaic efficiency

While monolithic multijunction solar cell approaches have been quite successful, current and lattice matching requirements limit the maximum possible achievable efficiencies. Spectrum splitting, where light is optically distributed among subcells with differing bandgaps, avoids these constraints and offers a route to achieving higher efficiencies (<50%). We investigate a spectrum splitting approach where concentrated sunlight is trapped in a textured dielectric slab and then selectively coupled into underlying solar cells of different bandgaps through omnidirectional filters. We develop a multipass optical model to find regimes of high optical efficiency based on parameters such as slab refractive index, number of subcells, and angle restriction of light escape from the slab. Based on these results and filter design considerations, we describe a specific design featuring a textured slab of SiO2 coated with angle restricting incoupling elements based on compound parabolic concentrators and three underlying multijunction junction solar cells, for a total of eight junctions with bandgaps ranging from 2.2eV to 0.7. Using the multipass model in conjunction with modified detailed balance calculations, we find module efficiencies exceeding 50% are possible with an acceptance angle restricted to 20° or less and concentrations of a few hundred suns with ideal omnidirectional filters. Finally as proof of concept, we design a full set of omnidirectional filters for this design. Based on alternating layers of TiO2 and SiO2, we achieve angle averaged reflectivity greater than 90% within the reflection band and angle averaged transmission of approximately 90% within the transmission band for the long pass filter, for nearly 48% receiver efficiency.

Spectrum splitting photovoltaics: Polyhedral specular reflector design for ultra-high efficiency modules

A design for ultra-high efficiency solar modules (>50%) using spectrum splitting is proposed. In the polyhedral specular reflector design, seven subcells are arranged around a solid parallelepiped. Incident light enters the parallelepiped and is directed via specular reflection onto each subcell in order from highest to lowest bandgap. We analyze optical losses due to external concentration and parasitic absorption and optimize the design for >50% module efficiency. We find that moderate concentration designs (90-170x) with a high index parallelepiped and perfect shortpass filters meet target efficiencies and demonstrate an initial design.

Spectrum splitting photovoltaics: Materials and device parameters to achieve ultrahigh system efficiency

We describe an approach for spectrum splitting solar module design using 8 independently connected subcells with optimized band gaps. Modified detailed balance calculations using parameters to account for nonideal absorption and recombination behavior are used to identify the efficiency of spectrum splitting modules with 2 to 20 subcells. Potential optical designs for solar spectrum splitting among subcells are briefly described. Spectrum splitting improves use of the power in the solar spectrum by decreasing thermalization losses. A multijunction design utilizing independently connected subcells employed in a concentrator photovoltaic receiver allows flexibility in subcell selection, fabrication and operating power point optimization when a large number of subcells are employed.

Polyhedral specular reflector design for ultra high spectrum splitting solar module efficiencies (>50%)

One pathway to achieving ultra-high solar efficiencies (<50%) is employing a spectrum splitting optical element with at least 6 subcells and significant concentration (100-500 suns). We propose a design to meet these criteria, employing specular reflection to split and divide the light onto appropriate subcells. The polyhedral specular reflector incorporates a high index parallelepiped with seven subcells. The subcells are placed around the parallelepiped such that light entering at normal incidence encounters the subcells in order from highest to lowest bandgap, with the ray path reflecting at a 90° angle until the light is fully absorbed. Previous studies of the design have shown that concentration and filters are necessary to achieve high efficiencies and thus the current iteration of the design employs shortpass filters and two stages of concentration. Ray tracing of the current iteration shows exceeding 50% efficiency is possible for current subcell qualities with perfect shortpass filters while 50% module efficiencies are only possible for very high quality (<6% ERE) subcells with commercially available shortpass filters. However, even with commercially available filters and achievable subcell quality, ray tracing results show very high (<43%) module efficiency.

Optoelectronic design of multijunction wire-array solar cells

Microwire solar cells have demonstrated promising optical and photovoltaic performance in arrays of single junction Si wires. Seeking higher efficiencies, we have numerically investigated III-V on Si1−xGex architectures as candidates for tandem microwire photovoltaics via optical and electronic transport modeling. Optical modeling indicates that light trapping is an important design criterion. Absorption is more than doubled by the presence of Al2O3 scattering particles around the wires, leading to high overall light collection despite low wire packing fraction. Texturing of the microwire outer surface, which was found to occur experimentally for GaP/Si microwires, is also shown to enhance absorption by over 50% relative to wires with smooth surfaces, allowing for the use of thinner layers. Finally, full optoelectronic simulations of GaAs on Ge structures revealed that current matching is attainable in these structures and that wire device efficiencies can approach those of planar cells.

Design improvements for the polyhedral specular reflector spectrum-splitting module for ultra-high efficiency (>50%)

A spectrum-splitting module design, the polyhedral specular reflector (PSR), is proposed for ultra-high photovoltaic efficiency (>50%). Incident light is mildly concentrated (≤16 suns) and subsequently split seven ways by a series of multilayer dielectric filters. The split spectrum is directed into compound parabolic concentrators (CPCs) and each concentrates a given slice of the spectrum onto one of seven subcells for conversion. We have recently made significant improvements to the design, such as vertically stacking each submodule and rearranging the subcell order to increase the optical efficiency of the design. We optimize the concentration and composition of the parallelepiped prism (hollow vs. solid) and model designs with >50% module efficiencies including optical and cell nonidealities.

Spectrum splitting photovoltaics: Light trapping filtered concentrator for ultrahigh photovoltaic efficiency

We investigate a spectrum splitting approach where light is trapped in a textured dielectric slab and coupled into underlying solar cells of different bandgaps through filters. We describe a specific design featuring an SiO2 textured slab coated with angle restricting incoupling elements and four underlying dual junction subcells, based on light trapping and efficiency considerations. We then design a full set of four omnidirectional filters based on aperiodic dielectric multilayer films to split the light entering each subcell into spectral bands tuned to the cell bandgaps. Based on the calculated filter performance, we estimate a receiver efficiency of 45% with this initial design.

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.