Adele C. Tamboli

Conformal GaP layers on Si wire arrays for solar energy applications

We report conformal, epitaxial growth of GaP layers on arrays of Si microwires. Silicon wires grown using chlorosilane chemical vapor deposition were coated with GaP grown by metal-organic chemical vapor deposition. The crystalline quality of conformal, epitaxial GaP/Si wire arrays was assessed by transmission electron microscopy and x-ray diffraction. Hall measurements and photoluminescence show p- and n-type doping with high electron mobility and bright optical emission. GaP pn homojunction diodes on planar reference samples show photovoltaic response with an open circuit voltage of 660 mV.

Development of ZnSiP

A major technological challenge in photovoltaics is the implementation of a lattice matched optically efficient material to be used in conjunction with silicon for tandem photovoltaics. Detailed balance calculations predict an increase in efficiency of up to 12 percentage points for a tandem cell compared with single junction silicon. Given that the III-V materials currently hold world record efficiencies, both for single and multijunction cells, it would be transformative to develop a material that has similar properties to the III-Vs which is also lattice matched to silicon. The II-IV-V2 chalcopyrites are a promising class of materials that could satisfy these criteria. ZnSiP2 in particular is known to have a bandgap of ~2 eV, a lattice mismatch with silicon of 0.5%, and is earth abundant. Its direct bandgap is symmetry-forbidden. We have grown single crystals of ZnSiP2 by a flux growth technique. Structure and phase purity have been confirmed by X-ray diffraction and transmission electron microscopy. Optical measurements, along with a calculation of the absorption spectrum, confirm the ~2 eV bandgap. Because of its structural similarity to both crystalline silicon and the III-Vs, ZnSiP2 is expected to have good optoelectronic performance.

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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.

for Si-Based Tandem Solar Cells

Transparent conductive adhesives (TCAs) can enable conductivity between two substrates, which is useful for a wide range of electronic devices. Here, we have developed a TCA composed of a polymer-particle blend with ethylene-vinyl acetate as the transparent adhesive and metal-coated flexible poly(methyl methacrylate) microspheres as the conductive particles that can provide conductivity and adhesion regardless of the surface texture. This TCA layer was designed to be nearly transparent, conductive in only the out-of-plane direction, and of practical adhesive strength to hold the substrates together. The series resistance was measured at 0.3 and 0.8 Ω cm2 for 8 and 0.2% particle coverage, respectively, while remaining over 92% was transparent in both cases. For applications in photovoltaic devices, such as mechanically stacked multijunction III-V/Si cells, a TCA with 1% particle coverage will have less than 0.5% power loss due to the resistance and less than 1% shading loss to the bottom cell.

Optoelectronic design of multijunction wire-array solar cells

Silicon microwire arrays have recently demonstrated their potential for low cost, high efficiency photovoltaics. These high aspect ratio, radial junction wire arrays allow for the absorption of nearly all the incident sunlight while enabling efficient carrier extraction in the radial direction. One of the remaining challenges to make this technology commercially viable is scaling up of the microwire array growth. We discuss here a technique we have developed for vapor liquid solid growth of microwire arrays over full six-inch wafers using a cold-wall RF-heated chemical vapor deposition furnace. This geometry allows for fairly uniform growth over large areas, rapid cycle time, and improved run-to-run reproducibility. We have studied these large-area microwire arrays using scanning electron microscopy and confocal microscopy to assess their structural fidelity and uniformity. We have also developed a technique to embed these large-area arrays in polymer and peel them off the substrate, which could enable lightweight, flexible solar cells with efficiencies as high as crystalline Si solar cells. We have tested the energy conversion properties of these microwire array samples grown using a liquid junction contact and a photoelectrochemical cell. Initial efficiencies measured in this way suggest that the material quality of these microwire arrays is similar to earlier small-area wire arrays that we have grown, meaning that this technique is a viable way to scale up microwire array devices.

Transparent Conductive Adhesives for Tandem Solar Cells Using Polymer–Particle Composites

Si wire arrays have recently demonstrated their potential as photovoltaic devices [1–3]. Using these arrays as a base, we consider a next generation, multijunction wire array architecture consisting of Si wire arrays with a conformal GaNxP1−x−yAsy coating. Optical absorption and device physics simulations provide insight into the design of such devices. In particular, the simulations show that much of the solar spectrum can be absorbed as the angle of illumination is varied and that an appropriate choice of coating thickness and composition will lead to current matching conditions and hence provide a realistic path to high efficiencies. We have previously demonstrated high fidelity, high aspect ratio Si wire arrays grown by vapor-liquid-solid techniques, and we have now successfully grown conformal GaP coatings on these wires as a precursor to considering quaternary compound growth. Structural, optical, and electrical characterization of these GaP/Si wire array heterostructures, including x-ray diffraction, Hall measurements, and optical absorption of polymer-embedded wire arrays using an integrating sphere were performed. The GaP epilayers have high structural and electrical quality and the ability to absorb a significant amount of the solar spectrum, making them promising for future multijunction wire array solar cells.