Quasiperiodic plasmonic concentrators for enhanced light absorption in ultra-thin film solar cells
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Quasiperiodic plasmonic concentrators for enhanced light absorption in ultra-thin film solar cells Patrick W. Flanigan1, Aminy E. Ostfeld1, Zhen Ye1, Natalie G. Serrino1, and Domenico Pacifici1 1 School of Engineering, Brown University, 184 Hope Street, Providence, RI 02912, U.S.A. ABSTRACT This report will demonstrate broadband, wide-angle, and polarization-insensitive absorption enhancement in ultra-thin films resting on metal substrates that have been etched with arrays of shallow sub-wavelength cylindrical holes. Absorption enhancement will be studied as a function of array geometry, with particular emphasis given to quasiperiodic arrays (a class of deterministic aperiodic arrays that were originally developed to tessellate 2-D planes with regular polygons). Through simulations and experimental data, it was found that absorption enhancement is heavily dependent on the rotational symmetry of the pattern of holes, as well as the inter-hole distance. INTRODUCTION With the recent advancements in the field of thin film solar cells, numerous light-trapping paradigms have been developed to help mitigate the weak absorption properties of thin photovoltaic active layers [1]. This report presents experimental and simulated data on a particular type of plasmonic concentrator designed for photovoltaic applications, known as a nanohole array (NHA). When employed in a solar cell, thousands of shallow sub-wavelengths holes are milled on the surface of the back metal contact. These holes serve to collect the incident light and redirect it along the dielectric / metal interface in the form of surface plasmon polaritons (SPPs). Absorptance in the dielectric material can then be enhanced through two distinct mechanisms: (i) diffractive scattering at each hole location generates SPP modes that propagate parallel to the interface, thus increasing the scale of light-matter interaction compared to single-pass absorption in a thin film; (ii) constructive interference can occur between SPP modes excited by neighboring holes, potentially leading to increased local electromagnetic fields on the plane. Despite preliminary research on nanohole arrays for photovoltaic applications [2], a systematic understanding of the role of structural geometry and SPP interference effects in plasmonically-enhanced absorption paradigms is still lacking. To this end, a certain class of twodimensional array will be investigated: quasiperiodic (QP) [3,4]. QP arrays lack the long-range translational symmetry typical of periodic arrays, but can have very high degrees of local order and rotational symmetry, which is completely absent in random arrays. Additionally, QP arrays can pack many more holes into the plane than periodic arrays (square, triangular, etc.) with analogous lattice parameters. The organic bulk heterojunction comprised of the polymer poly(3-hexylthiophene) (P3HT) and the fullerene (6,6)-phenyl C61 butyric acid methyl ester (PCBM) was chosen as a proof of concept, but this structure should work in any thin film solar cell material.
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