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Title:Experimental and computational approach to investigating charge transfer across semiconductor surfaces
Author(s):Kearney, Kara L
Director of Research:Rockett, Angus
Doctoral Committee Member(s):Dillon, Shen; Ertekin, Elif; Schleife, André
Department / Program:Materials Science & Engineerng
Discipline:Materials Science & Engr
Degree Granting Institution:University of Illinois at Urbana-Champaign
Subject(s):Photoelectrochemical cells
Density functional theory
Device modeling
Charge transfer mechanisms
Functionalized semiconductors
Abstract:A new generation of photoelectrochemical cells showing strikingly high conversion efficiencies have emerged. A promising photoelectrochemical process is solar-driven water-splitting in which hydrogen and oxygen gas are produced from liquid water. To obtain high conversion efficiencies in a photoelectrochemical water-splitting cell, a semiconductor electrode with a small enough band gap to efficiently absorb sunlight is required; however, most low-gap semiconductors are chemically unstable. Efforts are now focused on developing methods to stabilize low-gap semiconducting materials so that stable and efficient photoelectrochemical solar-driven water-splitting is achieved. Two well-developed techniques used to stabilize low-gap semiconductors are the attachment of organic molecules and/or the deposition of thin film inorganic passivation layers to the semiconductor surface. In both cases, the technique is effective at stabilizing the surface, but the functionalization is often accompanied by a reduction in the performance of the photoelectrode. An understanding of the limitations behind the observed reduction in performance associated with surface functionalization is still lacking, creating a roadblock for further improvements in photoelectrochemical solar-driven water-splitting devices. In this thesis, a multiscale device modeling approach for elucidating charge transport mechanisms across functionalized semiconductor photoelectrodes and subsequently predicting strategies for improving performance is described. The multiscale device model consists of a combination of finite-element charge transfer modeling and first-principles density functional theory. The applicability of the device model was demonstrated by elucidating the charge transfer limitations due to organic functionalization and the deposition of inorganic passivation layers. It was found that the predominant effect due to organic functionalization is the modification of the dipole on the semiconductor surface and a method for predicting the surface dipole and the resulting performance for an arbitrary organic moiety is described. It was found that thin film inorganic passivation layers may function as either "leaky" or "non-leaky" dielectrics. In the case of leaky dielectrics, charge transport is limited by field-assisted thermionic emission at the semiconductor|dielectric interface which can be improved by increasing the electric field across the dielectric. In the case of non-leaky dielectrics charge is limited by absorption in the inorganic film which can be avoided using wavelength filters on the light illuminating the semiconductor surface.
Issue Date:2017-06-26
Rights Information:Copyright 2017 Kara Kearney
Date Available in IDEALS:2017-09-29
Date Deposited:2017-08

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