Date of Award

6-10-2015

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Applied Science

First Advisor

Tansel Karabacak

Abstract

Solar energy harvesting has been of great interest for researchers over the past 50 years. Main emphasis has been on developing high quality materials with low defect density and proper band gaps. However, high cost of bulk materials and insufficient light absorption in thin films led to utilization of semiconductor nanostructures in photovoltaics and photonics. Light trapping abilities of nanostructures can provide high optical absorption whereas core/shell nanostructured arrays can allow enhanced charge carrier collection. However, most of the nanofabrication methods that can produce uniform nanostructure geometries are limited in materials, dimensions, and not compatible with industrial production systems. Therefore, it is essential to develop innovative low-cost fabrication approaches that can address these issues. The primary goal of this project is to investigate light trapping and carrier collection properties of glancing angle deposited (GLAD) nanostructured arrays for high-efficiency, low-cost photoconductive and photovoltaic devices using characterization techniques including scanning electron microscopy (SEM), transmission electron microscopy (TEM), ultraviolet-visible-near infrared (UV-vis-NIR) spectroscopy and time resolved photocurrent measurements. Indium sulfide (In2S3) has been chosen as a model material system in this study. GLAD nanostructured arrays of vertical rods, screws, springs, zigzags and tilted rods were fabricated and characterized. A strong dependence of optical absorption on the shapes of nanostructures is observed from UV-vis-NIR spectroscopy. A simulation study using finite difference time domain (FDTD) shows that introducing 3D geometry results in diffuse scattering of light and leads to high optical absorption. Monte Carlo simulations were conducted to determine a simple and scalable fabrication technique for conformal and uniform shell coatings. The results suggest that an atomic flux with angular distribution, which can be easily maintained by sputtering at high working gas pressures, can form conformal and uniform coatings. Finally, a photoconductive device made of In2S3 nanorod arrays coated with conformal silver (Ag) shell was fabricated and characterized to investigate charge carrier collection. Time resolved photocurrent profiles of nanostructured devices show that conformal Ag shell dramatically decreases the time for carriers to reach electrodes and providing fast photo response. This study proves that core/shell GLAD nanorod arrays can serve as major building blocks in low cost photoconductive and photovoltaic devices.

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