Multidirectional Polymer Waveguide Lattices for Enhanced Ultrawide-Angle Light Capture in Silicon Solar Cells

Abstract

We report the synthesis and characterization of a polymer thin-film structure consisting of two intersecting broadband optical waveguide lattices, and its performance in wide-angle optical energy collection and conversion in silicon solar cells. The structures are synthetically organized via the concurrent irradiation of photoreactive polymer blends by two arrays of intersecting, microscale optical beams transmitted through the medium. Through optical beam-induced photopolymerization and photopolymerization-induced phase separation, well-organized lattices are produced comprising of cylindrical core–cladding waveguide architectures that intersect one another. The optical waveguide properties of the lattices transform the transmission characteristics of the polymer film so that incident optical energy is collected and transmitted along the waveguide axes, rather than their natural directions dictated by refraction, thereby creating efficient light-collecting capability. The embedded structures collectively impart their wide-angle acceptance ranges to enable the film to efficiently collect and interact with light over a large angular range (±70°). When employed as the encapsulant material for a commercial silicon solar cell, the novel light collection and transmission properties result in greater wide-angle conversion efficiency and electrical current density, compared to a single vertically aligned waveguide array. The sustained and greater conversion of light afforded by the encapsulating optical material promises to increase solar cell performance by enabling ultrawide-angle solar energy conversion.

Fluorescent Waveguide Lattices for Enhanced Light Harvesting and Solar Cell Performance

“…Figure displays the transverse intensity profile of transmitted light through 2WG structures with different dye concentrations (0, 0.1, 0.5 wt %) as a function of the incidence angle. Waveguide structures with no dye content have an angular acceptance window between approximately 30° and 70°, as observed previously . This particular range is delineated by the spotted nature of the transverse intensity profile, with the approximate circular shape of the intensity spots corresponding to the transmission of light from the waveguide cores.…”

“…The benefits of the dye in providing improved contrast are particularly noticeable in the 2WG structures, as we observed improved structure with increased weight fraction of dye. Although the waveguide core diameters in all structures do become divergent and increase in diameter over their length, their consistent diameters at the top of the film for more than ∼1 mm is sufficient to establish the transmission of collected light along the intended direction of the waveguides . It is quite beneficial that the incorporation of a dye-tagged monomer in the formulation, which copolymerizes with the high-index polymer, contributes to the quality of the structure.…”

“…Angle-resolved measurements were performed as described in our previous work. 20 ■ RESULTS AND DISCUSSION Synthesis of Fluorescent Waveguide Lattices. We found that including dye in the photoreactive formulations resulted in waveguide cores with more distinct and clear cross sections.…”

“…Multiwaveguide lattices (MWGLs) have recently gained attention as an effective means of controlling the collection and transmission of light when employed as encapsulants for solar cells. , The waveguide lattices are synthetically organized in photoreactive polymer blends, consisting of a high refractive index acrylate-based free-radical polymerizing resin and a low refractive index cationic polymerizing epoxide-terminated silicone resin . The mixed resin is irradiated by arrays of microscale optical beams that are transmitted through the medium.…”

“…Advantageously, by synthesizing MWGLs comprising even just two lattices each at slanted orientations relative to the surface normal (e.g., ±30°), their acceptance ranges could be angularly rotated to collect more light at more glancing incident angles. Thereby, wide-angle incident radiation is collected and transmitted across the polymer through the waveguides at sharper angles (i.e., closer to the surface normal) than otherwise dictated by Snell’s law. , The combination of multiple (two is in fact sufficient) intersecting waveguide lattices can sweep the entire practical collection window (−70 to 70°) enabling the entire angular range to be efficiently collected and directed toward the solar cell, as compared to both a uniform film and single vertically aligned lattice. Transmitting ultrawide angles of light via more direct paths toward the solar cell mitigates effects that cause losses, such as shading from the solar cell’s front contacts, ,, thereby leading to efficiency enhancements.…”