For thin-fi lm solar cells, light absorption is usually proportional to the fi lm thickness. However, if freely propagating sunlight can be transformed into a guided mode, [ 1 ] the optical path length signifi cantly increases and results in enhanced light absorption within the cell. [ 2 ] We propose here a light absorber based on coupling from a periodic arrangement of resonant dielectric nanospheres. It is shown that whispering gallery modes in the spheres can be coupled into particular modes of the solar cell and signifi cantly enhance its effi ciency. We numerically demonstrate this enhancement using full-fi eld fi nite difference time-domain (FDTD) simulations of a nanosphere array above a typical thin-fi lm amorphous silicon (a-Si) solar cell structure. The in-coupling element in this design is advantageous over other schemes as it is composed of a lossless material, and its spherical symmetry naturally accepts large angles of incidence. Also, the array can be fabricated using simple, well-developed methods of self assembly and is easily scalable without the need for lithography or patterning. This concept can be easily extended to many other thin-fi lm solar cell materials to enhance photocurrent and angular sensitivity.Thin-fi lm photovoltaics offer the potential for a signifi cant cost reduction [ 3 ] compared to traditional, or fi rst generation, photovoltaics usually at the expense of high effi ciency. This is achieved mainly by the use of amorphous or polycrystalline optoelectronic materials for the active region of the device, for example, a-Si. The resulting carrier collection effi ciencies, operating voltages, and fi ll factors are typically lower than those for single-crystal cells, which reduce the overall cell effi ciency. There is thus great interest in using thinner active layers combined with advanced light trapping schemes to minimize these problems and maximize efficiency. A number of light trapping schemes have been proposed and demonstrated including the use of plasmonic gratings, [ 4 , 5 ] arrays of metal cavities that support void plasmons, [ 6 ] photonic crystals, [ 7 ] nano-and microwires, [ 8 , 9 ] nanodomes, [ 10 ] aggregates of nanocrystallites, [ 11 ] and dielectric diffractive structures. [ 12 ] A previous report [ 12 ] described the use of sub-wavelength dielectric spheres and other scattering mechanisms for enhancing light scattering and absorption. Here, we propose a new concept for light trapping in thin-fi lm solar cells through the use of wavelength-scale resonant dielectric nanospheres that support whispering gallery modes to enhance absorption and photocurrent.Wavelength-scale dielectric spheres [ 13 ] are interesting photonic elements because they can diffractively couple light from free space and also support confi ned resonant modes. Moreover, the periodic arrangement of nanospheres can lead to coupling between the spheres, resulting in mode splitting and rich bandstructure. [ 14 , 15 ] The coupling originates from whispering gallery modes (WGM) inside the spheres...

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.

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Silica nanosphere functionalizationSilica spheres of 700 nm diameter were obtained from Polysciences Inc. as a 10% (by weight) suspension in water. This suspension was filtered on a fine filtration frit, rinsed with tetrahydrofuran and acetone. The powder of spheres was washed with 10 mL of 1:1 methanol/HCl, and rinsed again with acetone. The mostly dried powder was then heated in an oven for 5 minutes at 110 °C and dried under vacuum overnight. To 25 mL toluene in a 50 mL round-bottomed flask, 786 mg of dry silica spheres were suspended and stirred. To this suspension was added 1 mL 3-aminopropyl(diethoxy)methyl silane. The suspension was stirred 72 hours, filtered on a fine frit, rinsed with toluene and dried in vacuo to yield 756 mg dry, amine-functionalized silica spheres. Langmuir-Blodgett depositionA ~1% (by weight) suspension for Langmuir-Blodgett deposition was prepared by suspending 235 mg of functionalized silica spheres in a solution of 4 mL ethanol and 17 mL methylene chloride. We first perform an isotherm measurement where we record the surface pressure of the water as a function of the surface area, which is reduced using the compression barriers of the LB trough. When the area of the trough is large, the surface pressure of the water is around 4 mN/m. The spheres are freely spread on the surface of the water. This is the so-called "gaseous" state. While the LB trough's barriers compress the spheres and reduce the area where the spheres stand on, the surface pressure slowly increases until 5 mN/m. The slope abruptly increases until 10 mN/m. This is the "liquid" state corresponding to a dense and condensed monolayer of hexagonally close packed spheres at the surface of the water. Upon further compression, the slope of the curve decreases and the monolayer collapses into multilayer structures. For our purpose, the optimal point is at the middle of the "liquid" condensed state where the spheres are well close packed and still form a monolayer. This point is reached when the surface pressure is around 7.5 mN/m. In a second step, knowing the optimal surface pressure for the deposition, we perform a dipping experiment. While the spheres are on the surface of the water in the "gaseous" state, we immerse the substrate into the LB trough. We then close the LB's barriers until the surface pressure reaches 7.5 mN/m. From that point, we slowly pull up the substrate at a rate of 1 mm/min while simultaneously keeping the surface pressure constant with a computer controlled feedback system between the electrobalance measuring of the surface pressure and the barrier moving mechanism. Consequently, the floating hexagonally close packed monolayer is adsorbed on the ITO surface. When the structure is totally removed from the water, the part that was initially immersed in the water is coated by a large area of nanoscale dielectric nanospheres on its entire surface. Transfer printing preparationPoly(vinyl alcohol) (avg. MW = 10,000 g/mol, 88% hydrolyzed, Sigma Aldrich) was spin cast from an aqueous solution containing ...

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