Date of Award

Spring 2013

Project Type

Dissertation

Program or Major

Physics

Degree Name

Doctor of Philosophy

First Advisor

Karsten Pohl

Abstract

Enormous efforts have been made in seeking alternative pathways to more effectively use solar energy. Organic solar cells, composed essentially of carbon-based organic molecular materials, have attracted considerable scientific and industrial attentions because of their economic and environmental benefits. It is advantageous to have the comprehension of the molecular structures and interfacial morphologies for these active molecular materials on substrates in the nanoscale regime.

Controlling the heterostructures of the molecular donor and acceptor materials is essential to overcoming the efficiency bottleneck in organic photovoltaics. Molecular self-assembly on patterned substrates provides a bottom-up approach to create well-controlled molecular heterojunctions and a pathway to engineer the donor-acceptor interface at the molecular level. I present a study of the self-assembly of functionalized pentacenes (6,13-dichloropentacene) and fullerenes (C60) into a layer-by-layer heterojunction on a stepped gold vicinal surface Au(788) by scanning tunneling mi- croscopy (STM) characterizations and density functional theory (DFT) calculations. Au(788) is a vicinal surface of Au(111) and exhibits complex reconstruction patterns that provide a natural template for organic molecular self-assembly. The 6,13- dichloropentacene (DCP) molecules, optimized for photovoltaic applications, form a striking long-range ordered self-assembled monolayer (SAM) on a stepped Au(788) vicinal surface. The SAM resembles a perfect brick-wall pattern with the long-axis parallel to the step edges. This DCP SAM serves as the electron-donor layer. Subsequently deposited C60 molecules form long parallel chains with a rectangular arrangement atop the intact DCP SAM on Au(788). The C60 molecular chain is commensurate (3:2) with the DCP lattice along the long axis: three fullerenes line up with two DCP molecules. The C60 adlayer serves as the electron-acceptor layer. The novel organic-metal and organic donor-acceptor interfacial interactions, as well as the adsorption geometry, have been explored by DFT. The interaction between the C60 molecules and the gold substrate is well screened by the DCP monolayer. The perfectly ordered DCP SAM is unaffected by the C 60 chain formation. The charge transfer and dipole moment between the C60 and DCP monolayer interface have been calculated by DFT. Superior electronic transport properties are expected for these well-ordered bilayer heterojunctions for a potential improvement of the photovoltaic efficiency.

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