Lightweight, portable solar blankets, constructed from thin film photovoltaics, are of great interest to
hikers, the military, first responders, and third-world countries lacking infrastructure for transporting
heavy, brittle solar cells. These solar blankets, as large as two square meters in area, come close to
satisfying specifications for commercial and military use, but they still have limited absorption due to
insufficient material efficiency, and therefore are large and too heavy in many cases.
Metasurfaces, consisting of monolayers of periodic and semi-random plasmonic particles patterned in
a scalable manner, are explored to enhance scattering into thin photovoltaic films (currently of
significant commercial and military value), in order to enhance absorption and efficiency of solar
blankets. Without nano-enhancement, absorption is limited by the thickness of the thin photovoltaic
active layer in the long-wavelength region. In this study, lithographically patterned, periodic Al
nanostructure arrays demonstrate experimentally a large absorption enhancement, resulting in a
predicted increase in short-circuit current density of at least 35% and as much as 70% for optimized
arrays atop 200-nm amorphous silicon thin films. Optimized arrays extend thin-film absorption to the
near infrared region. This impressive absorption enhancement and predicted increase in short-circuit
current density may significantly increase the efficiency and reduce the weight of solar blankets,
enabling their use for commercial and military applications.
Nanoparticles and nanostructures with plasmonic resonances are currently being employed to enhance the efficiency of solar cells. <sup>1-3</sup> Ag stripe arrays have been shown theoretically to enhance the short-circuit current of thin silicon layers. <sup>4</sup> Monolayers of Ag nanoparticles with diameter d < 300 nm have shown strong plasmonic resonances when coated in thin polymer layers with thicknesses < d.<sup>5</sup> We study experimentally the diffuse vs. specular scattering from monolayer arrays of Ag nanoparticles (spheres and prisms with diameters in the range 50 – 300 nm) coated onto the front side of thin (100 nm < t < 500 nm) silicon films deposited on glass and flexible polymer substrates, the latter originating in a roll-to-roll manufacturing process. Ag nanoparticles are held in place and aggregation is prevented with a polymer overcoat. We observe interesting wavelength shifts between maxima in specular and diffuse scattering that depend on particle size and shape, indicating that the nanoparticles substantially modify the scattering into the thin silicon film.
Nanoparticles and nanostructures with plasmonic resonances are currently being employed to
enhance the efficiency of solar cells. Ag stripe arrays have been shown theoretically to enhance the
short-circuit current of thin silicon layers. Such Ag stripes are combined with 200 nm long and 60
nm wide “teeth”, which act as nanoantennas, and form vertical rectifying metal-insulator-metal
(MIM) nanostructures on metallic substrates coated with thin oxides, such as Nb/NbO<sub>x</sub> films. We
characterize experimentally and theoretically the visible and near-infrared spectra of these “stripeteeth”
arrays, which act as microantenna arrays for energy harvesting and detection, on silicon
substrates. Modeling the stripe-teeth arrays predicts a substantial net a.c. voltage across the MIM
diode, even when the stripe-teeth microrectenna arrays are illuminated at normal incidence.
Conducting nanoparticles with plasmon resonances create local, nanoscopic field enhancements that boost an analyte
molecule’s surface-averaged Raman scattering cross-section orders of magnitude above the bulk Raman cross-section by an amount known as the enhancement factor (EF). Demonstrations of single-molecule sensitivity with EF ~ 10<sup>13</sup> have been reported from small “hot spots” (e.g., regions of enhanced electromagnetic near fields) on specialized substrates, but realistic chemical sensing requires high average EF over large substrates for practical sampling.<sup>1</sup> By using simple wet chemical methods, NSRDEC scientists have fabricated large-area arrays of novel, highly conducting, anisotropic Ag and Al nanoparticles. The nanoparticles adhere to an ultrathin layer of poly-4(vinyl pyridine), and are anchored by submicron coating of poly-methyl methacrylate on glass and SiO<sub>2</sub>-coated Si substrates. The average interparticle spacing is determined by the dilution of the nanoparticle-water suspension. We present surface-enhanced Raman spectroscopy (SERS), spectrophotometry, and microscopy data from these nanoparticle arrays, model this data and the nanoscopic field enhancement, and determine the SERS EF. We compare the observed absorption resonances and SERS EF with those predicted by finite difference time domain modeling of the nanoscale fields and optical properties, and find good agreement between measured and calculated reflectivity, achieving EF ~ 10<sup>6</sup> for benzenethiol adsorbed onto a monolayer array of 120 nm Ag nanoparticles over an area of ~ 0.5 cm<sup>2</sup>. We discuss a way forward to increase SERS EF to 10<sup>7</sup> with large-area samples assembled using chemical methods, by using spiky Ag “nano-urchins” with very large predicted field enhancements.