Date of Award

2015-01-01

Degree Name

Doctor of Philosophy

Department

Electrical Engineering

Advisor(s)

David Zubia

Second Advisor

Xiaowang Zhou

Abstract

Cadmium Telluride is a material widely used in terrestrial thin film photovoltaic applications due to its nearly ideal band gap (~1.5 eV) and high absorption coefficient. Due to its low manufacturing cost, this technology has the potential to become a significant energy resource if higher energy conversion efficiencies are achieved. However, the module efficiencies (~14%) are still far from the theoretical maximum (~30%) for this material in a single junction configuration. The reason behind this low performance is attributed to the high number of defects that are present within the device materials. The physics behind the formation mechanisms of these imperfections and their effect in the electrical the operation of a device is not well understood, and the topic is a focus of a wide variety of studies.

In this work we perform molecular dynamics simulations of epitaxial growth of CdTe on CdS (0001) single crystal substrates in order to exploit the powerful structure prediction capability of high fidelity modelling to capture defect formation mechanisms. In addition, advanced visualization and defect detection software tools are employed to characterize 3-D simulation data and identify, quantify, and index defects within the simulated epilayer with atomic resolution. The combination of accurate modelling and advance characterization software tools creates an analysis capability that provides defect insight beyond the limits offered achievable by means of the experimental characterization equipment in existence. The analysis capability was used to evaluate three CdTe film defect reduction strategies. These approaches were selective area growth, the incorporation of Zn alloying in a buffer layer, and a nano-structure that combined the two previously mentioned methods.

The capability of the defect analysis software was expanded by adding code that enables it to detect defects inside a hexagonal lattice. Previously, the software was only able to detect defects within a diamond lattice. The simulated dislocation density trends observed are in good agreement with experimental data reported in literature and validated the simulation approach. The analysis results showed that selective area growth is an effective strategy in reducing epilayer defects. The incorporation of the Zn buffer layer was only beneficial at low concentrations (<4%), and detrimental otherwise. The combination of both strategies produced the lowest defect density films in this study.

Language

en

Provenance

Received from ProQuest

File Size

128 pages

File Format

application/pdf

Rights Holder

Jose Juan Chavez

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