Document Type

Thesis - University Access Only

Award Date

2011

Degree Name

Master of Science (MS)

Department / School

Electrical Engineering and Computer Science

Abstract

Crystalline silicon (c-si) is the most common material used in solar cells. However, the energy produced from c-si solar cells is expensive due to the high cost of pure crystalline silicon and low energy conversion efficiency. Amorphous silicon (a-si) is less expensive than c-si, however, it does not absorb photons with wavelengths greater than 885 nm. Upconversion of low energy photons into high energy photons can increase the efficiency of solar cells. Na YF 4 :3%Er, 17%Yb nanoparticles are the most efficient upconversion material reported so far. However, they are inefficient at low light intensities. The motivation of this thesis is that upconvertors need to be made more efficient. The objective was to increase the efficiency of upconverters using surface plasmons in metallic structures to enhance electric field intensity. Surface plasmons are electromagnetic surface modes coupled with free electrons in metallic surfaces. Surface plasmons were simulated using a finite difference time domain (FDTD) numerical method to study efficiency improvement by enhancing the electric field intensity. Four different types of photonic crystals were studied to maximize the electric field intensity by excitation of surface plasmons. They were cylindrical and rectangular nanopillars in both a square and triangular lattices. A simulation tool, EM Explorer, was used to find the optimum size and spacing of nanopillars. A gold photonic crystal with spacing 620 nm, 310 nm diameter and 70 nm thickness was fabricated using electron-beam lithography. The emission from a 105 nm thick NaYF4:3%Er,17%Yb-PMMA film on this crystal was measured using a confocal microscope. The electric field intensity near the nanopillars due to surface plasmons was 100 times that of the incident electric field intensity. The square lattice photonic crystals had an extra resonance mode for p-polarized light at oblique incident angle due to transverse polarization of nanopillars, resulting in stronger electric field intensity. It was observed that electric field enhancement was associated with the formation of energy loops near the nanopillars which gave a better understanding of electromagnetic energy flow in electric fields enhanced by surface plasmons. The simulation also showed that in order to have significant absorption of the electric field, the imaginary refractive index of the upconversion material should be increased by 1000 times. Experimental results showed that the average emission at 655 nm from NaYF4:3%Er,17%Yb nanoparticles coated over the nanopillars was 37.5 times that of the glass substrate indicating that gold nanopillars can increase the efficiency solar cells if they are used with upconversion materials. However, upconversion efficiency and bandwidth of absorption of upconverters were too low to be cost effective for solar cells even when used with photonic crystals.

Library of Congress Subject Headings

Solar cells

Plasmons (Physics)

Nanoparticles

Format

application/pdf

Number of Pages

135

Publisher

South Dakota State University

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