Energy production advantage of independent subcell connection for multijunction photovoltaics

Increasing the number of subcells in a multijunction or “spectrum splitting” photovoltaic improves efficiency under the standard AM1.5D design spectrum, but it can lower efficiency under spectra that differ from the standard if the subcells are connected electrically in series. Using atmospheric data and the SMARTS multiple scattering and absorption model, we simulated sunny day spectra over 1 year for five locations in the United States and determined the annual energy production of spectrum splitting ensembles with 2–20 subcells connected electrically in series or independently. While electrically independent subcells have a small efficiency advantage over series-connected ensembles under the AM1.5D design spectrum, they have a pronounced energy production advantage under realistic spectra over 1 year. Simulated energy production increased with subcell number for the electrically independent ensembles, but it peaked at 8–10 subcells for those connected in series. Electrically independent ensembles with 20 subcells produce up to 27% more energy annually than the series-connected 20-subcell ensemble. This energy production advantage persists when clouds are accounted for

Holographic spectrum splitter for ultra-high efficiency photovoltaics

To move beyond the efficiency limits of single-junction solar cells, junctions of different bandgaps must be used to avoid losses from lack of absorption of low energy photons and energy lost as excited carriers thermalize to the semiconductor band edge. Traditional tandem multijunction solar cells are limited, however, by lattice-matching and current-matching constraints. As an alternative we propose a lateral multijunction design in which a compound holographic optic splits the solar spectrum into four frequency bands each incident on a dual-junction, III-V tandem cell with bandgaps matched to the spectral band. The compound splitting element is composed of four stacks of three volume phase holographic diffraction gratings. Each stack of three diffracts three bands and allows a fourth to pass straight through to a cell placed beneath the stack, with each of the three gratings in the stack responsible for diffracting one frequency band. Generalized coupled wave analysis is used to model the holographic splitting. Concentration is achieved using compound parabolic trough concentrators. An iterative design process includes updating the ideal bandgaps of the four dual-junction cells to account for photon misallocation after design of the optic. Simulation predicts a two-terminal efficiency of 36.14% with 380x concentration including realistic losses

Energy production advantage of independent subcell connection for multijunction photovoltaics

Increasing the number of subcells in a multijunction or “spectrum splitting” photovoltaic improves efficiency under the standard AM1.5D design spectrum, but it can lower efficiency under spectra that differ from the standard if the subcells are connected electrically in series. Using atmospheric data and the SMARTS multiple scattering and absorption model, we simulated sunny day spectra over 1 year for five locations in the United States and determined the annual energy production of spectrum splitting ensembles with 2–20 subcells connected electrically in series or independently. While electrically independent subcells have a small efficiency advantage over series-connected ensembles under the AM1.5D design spectrum, they have a pronounced energy production advantage under realistic spectra over 1 year. Simulated energy production increased with subcell number for the electrically independent ensembles, but it peaked at 8–10 subcells for those connected in series. Electrically independent ensembles with 20 subcells produce up to 27% more energy annually than the series-connected 20-subcell ensemble. This energy production advantage persists when clouds are accounted for

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

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

Lightweight Carbon Fiber Mirrors for Solar Concentrator Applications

Lightweight parabolic mirrors for solar concentrators have been fabricated using carbon fiber reinforced polymer (CFRP) and a nanometer scale optical surface smoothing technique. The smoothing technique improved the surface roughness of the CFRP surface from ~3 {\mu}m root mean square (RMS) for as-cast to ~5 nm RMS after smoothing. The surfaces were then coated with metal, which retained the sub-wavelength surface roughness, to produce a high-quality specular reflector. The mirrors were tested in an 11x geometrical concentrator configuration and achieved an optical efficiency of 78% under an AM0 solar simulator. With further development, lightweight CFRP mirrors will enable dramatic improvements in the specific power, power per unit mass, achievable for concentrated photovoltaics in space.Comment: IEEE Photovoltaic Specialist Conference (PVSC), DC, USA, 201

Design of photovoltaics for modules with 50% efficiency

We describe a spectrum splitting solar module design approach using ensembles of 2–20 subcells with bandgaps optimized for the AM1.5D spectrum. Device physics calculations and experimental data determine radiative efficiency parameters for III-V compound semiconductor subcells and enable modification of conventional detailed balance calculations to predict module efficiency while retaining computational speed for a wide search of the design space. Accounting for nonideal absorption and recombination rates due to realistic material imperfections allows us to identify the minimum subcell quantity, quality, electrical connection configuration, and concentration required for 50% module efficiency with realistic optical losses and modeled contact resistance losses. We predict a module efficiency of 50% or greater will be possible with 7–10 electrically independent subcells in a spectral splitting optic at 300–500 suns concentration, assuming a 90% optical efficiency and 98% electrical efficiency, provided the subcells can achieve an average external radiative efficiency of 3–5% and a short circuit current that is at least 90% of the ideal. In examining spectrum splitting solar cells with both series-connected and electrically independent subcells, we identify a new design trade-off independent of the challenges of fabricating optimal bandgap combinations. Series-connected ensembles, having a single set of electrical contacts, are less sensitive to lumped series resistance losses than ensembles where each subcells are contacted independently. By contrast, ensembles with electrically independent subcells can achieve lower radiative losses when the subcells are designed for good optical confinement. Distributing electrically independent subcells in a concentrating receiver module allows flexibility in subcell selection and fabrication, and can achieve ultra-high efficiency with conventional III-V cell technology

Simulation and partial prototyping of an eight‐junction holographic spectrum-splitting photovoltaic module

Spectrum‐splitting photovoltaics incorporate optical elements to separate sunlight into frequency bands, which can be targeted at solar cells with bandgaps optimized for each sub‐band. Here, we present the design of a holographic diffraction grating‐based spectrum‐splitting photovoltaic module integrating eight III‐V compound semiconductor cells as four dual‐junction tandems. Four stacks of simple sinusoidal volume phase holographic diffraction gratings each simultaneously split and concentrate sunlight onto cells with bandgaps spanning the solar spectrum. The high‐efficiency cells get an additional performance boost from concentration incorporated using a single or a compound trough concentrator, providing up to 380X total concentration. Cell bandgap optimization incorporated an experimentally derived bandgap‐dependent external radiative efficiency function. Simulations show 33.2% module conversion efficiency is achievable. One grating stack is experimentally fabricated and characterized

Holographic spectrum splitter for ultra-high efficiency photovoltaics

To move beyond the efficiency limits of single-junction solar cells, junctions of different bandgaps must be used to avoid losses from lack of absorption of low energy photons and energy lost as excited carriers thermalize to the semiconductor band edge. Traditional tandem multijunction solar cells are limited, however, by lattice-matching and current-matching constraints. As an alternative we propose a lateral multijunction design in which a compound holographic optic splits the solar spectrum into four frequency bands each incident on a dual-junction, III-V tandem cell with bandgaps matched to the spectral band. The compound splitting element is composed of four stacks of three volume phase holographic diffraction gratings. Each stack of three diffracts three bands and allows a fourth to pass straight through to a cell placed beneath the stack, with each of the three gratings in the stack responsible for diffracting one frequency band. Generalized coupled wave analysis is used to model the holographic splitting. Concentration is achieved using compound parabolic trough concentrators. An iterative design process includes updating the ideal bandgaps of the four dual-junction cells to account for photon misallocation after design of the optic. Simulation predicts a two-terminal efficiency of 36.14% with 380x concentration including realistic losses

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