Solar cell efficiency enhancement via light trapping in printable resonant dielectric nanosphere arrays

Abstract

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 ...

Light absorption enhancement for ultra-thin Cu(In1−xGax)Se2 solar cells using closely packed 2-D SiO2 nanosphere arrays

“…There are two obvious lobes where the electric field is intensified within the sphere. This particular mode pattern has been attributed to Whispering gallery modes (WGMs) [21,22]. The WGMs act as a cavity for light, trapping it inside the SiO 2 sphere.…”

“…The 2-D dielectric sphere array is a promising structure for preparing nanostructures of other materials as a mask [19,20] and light absorption enhancement in opto-electronic devices [15,[21][22][23]. Confined resonant modes can be supported when the size of spheres is on the scale of the wavelength.…”

“…The energy in the resonant modes can leak into the solar cells if the spheres are placed in close proximity [15]. Additionally, the dielectric sphere arrays have other advantages [15,[19][20][21][22][23][24]: 1) dielectric materials are optically lossless, the concern of parasitic absorption is not necessary; 2) the geometry of the spheres is symmetric, which allows a broader acceptance angle of incident light; 3) the sphere array can be fabricated by the simple and cheap self-assembly method rather than complicated and expensive lithography technologies; 4) the inorganic dielectric materials are thermally stable compared to metallic materials, which makes them compatible to the deposition of the CIGSe layer at high substrate temperatures.…”

“…It has been implied that the 2-D colloidal SiO 2 sphere array on top of a thin-film solar cell can significantly improve the light absorption, which has been demonstrated in amorphous Si solar cells [21,22]. However, since the structure and optical properties of CIGSe solar cells show a pronounced difference to Si solar cells, whether the 2-D colloidal SiO 2 sphere array can serve as an effective light-trapping structure for ultra-thin CIGSe solar cells is unknown.…”

“…The optical constants of each layer were extracted from our own samples via the transfer-matrix method [26]. The refractive index (n) of SiO 2 was set to 1.46 [22]. We should mention here that only the absorption in the CIGSe photoactive layer (Abs CIGSe ) can contribute to the photocurrent in the cell [27].…”