Browsing by Author "Atwater, Harry A."

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Mimicking surface polaritons for unpolarized light with high-permittivity materials
(2019-01-10)
Papadakis, Georgia T.
;
Davoyan, Artur
;
Yeh, Pochi
;
Atwater, Harry A.
;
光電工程學系
Tailoring near-field optical phenomena often requires excitation of surface plasmon polaritons (SPPs) or surface phonon polaritons (SPhPs), surface waves at the interface between media with electric permittivities of opposite sign. Despite their unprecedented field confinement, surface polaritons are limited by polarization: only transverse magnetic fields enable their excitation, leaving transverse electric fields unexploited. By contrast, guided modes in positive permittivity materials occur for both linear polarizations, however, they typically cannot compete with SPPs and SPhPs in terms of confinement. Here we show that omnipolarization guided modes in materials with high-permittivity resonances can reach confinement factors similar to SPPs and SPhPs, while surpassing them in terms of propagation distance. We explore the cases of silicon carbide and transition-metal dichalcogenides near their permittivity resonances, and compare with SPhPs in silicon carbide and SPPs in silver, at infrared and visible frequencies, respectively.
Suppression of surface recombination in CuInSe2 (CIS) thin films via Trioctylphosphine Sulfide (TOP:S) surface passivation
(2016-03)
Luo, Shi
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Eisler, Carissa
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Wong, Tsun-Hsin
;
Xiao, Hai
;
Lin, Chuan-En
CuInSe2 (CIS) solar cells are promising candidates for thin film photovoltaic applications, one key limitation in their performance is surface recombination in these thin films. We demonstrate that passivating CIS films with Trioctylphosphine Sulfide (TOP:S) solution increases photoluminescence (PL) intensity by a factor of similar to 30, which suggests that this passivation significantly reduces surface recombination. X-ray photoelectron spectroscopy (XPS) reveals that TOP:S forms both -S and -P bonds on the CIS film surface, which leads to a similar to 4-fold increase in the surface Na peak intensity. This value is significantly higher than what would be expected from high temperature annealing alone, which has been linked to improvements in surface morphology and device efficiency in CIGS solar cells. We use Energy Dispersive X-ray Spectroscopy (EDS) to measure the solid-state transport of Na within CIS films with and without passivation. EDS spectra on CIS film cross-sections reveals a saddle-shaped Na profile in the as fabricated films and a concentration gradient towards the film surface in the passivated films, with 20% higher surface Na content compared with the unpassivated films. We employ Hybrid (B3PW91) Density Functional Theory (DFT) to gain insight into energetics of Na defects, which demonstrate a driving force for Na diffusion from bulk towards the surface. DFT Calculations with TOP:S-like molecules on the same surfaces reveal a similar to 1eV lower formation energy for the Na-cu defect. The experiments and computations in this work suggest that TOP:S passivation promotes Na diffusion towards CIS film surfaces and stabilizes surface Na defects, which leads to the observed substantial decrease in surface recombination. (C) 2016 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.