Chemistry and Chemical Biology ETDs

Jian Gao

Publication Date

9-12-2014

Abstract

This dissertation presents the results of investigations into organic semiconducting systems at the nano- to mesoscales through spectroscopic and electrical imaging techniques. Particular emphasis was put on capturing the role of conformation and morphology on electronic properties and processes. The combination of experimental methods and data analysis techniques used in this work reveals useful insights into the correlations between polymer structures and their photovoltaic device performance, and provide clues to developing better performing organic solar cells. The main approach of this work is using prototypical conjugated polymer systems, either ensemble or in their nanostructures, to study the solar cell active layer morphologies and chemical doping interactions. The first half of this work focuses on studying the active layer morphologies by self-assembled nanostructures, such as nanoparticles, nanofibers and intercalated bimolecular crystals, and their impact on electronic processes and solar cell device efficiencies. Firstly, nanoparticles of poly(9,9-dioctylfluorene-co-bis-N,N-(4-butylphenyl)-bis-N,N-phenyl-1,4-phenylenediamine) (PFB) and poly(9,9-dioctylfluorene-co-benzothiadiazole) (F8BT) were fabricated and used to study the interface characteristics of donor/acceptor. Efficient energy transfer from PFB to F8BT was observed instead of charge transfer states typically seen in well mixed F8BT/PFB films. Poly(3-hexylthiophene) (P3HT) nanofibers (NFs) were next fabricated to study the pure polymer aggregates and examine the delicate interplay between intra- and inter-chain exciton couplings in \u03c0-stacked polymer aggregates. J-type aggregate features of P3HT NFs were initially present and preserved after encapsulation. A time-dependent Raman intensity analysis was used to quantitatively estimate vibrational mode-specific excited state structural displacements. Furthermore, poly(2,5-bis(3-tetradecylthiophen-2-yl)thieno[3,2-b]thiophene) (PBTTT) and [6,6]-phenyl C61 butyric acid methyl ester (PCBM) intercalated systems were used to study the polymer/fullerene intermixed regions. Different PBTTT conformers are identified and their contributions to photocurrent production are also mapped in solar cell devices. The ordered PBTTT chains are most concentrated in PCBM-rich crystallites that exhibit high non-geminate recombination rates and low photocurrent. The later part of this work focuses on the chemical doping interactions between P3HT and the small molecule electron acceptor, 2,3,5,6-tetra\ufb02uoro-7,7,8,8-tetracyanoquinodimethane (F4-TCNQ). Firstly, we examine the changes in polymer conformation when P3HT comes into contact with F4-TCNQ. Doping P3HT with F4-TCNQ appears to planarize the polymer backbone and decrease pristine aggregate amounts, and increasing F4-TCNQ loading results in larger P3HT/F4-TCNQ fibril-like domains. Next we investigate the effects of P3HT conformation and packing on charge transfer interactions between P3HT and F4-TCNQ using regioregular (r-Re) and regiorandom (r-Ra) P3HT. R-Re P3HT is seen to produce significantly larger numbers of free charge carriers compared to r-Ra P3HT with similar F4-TCNQ loadings. This suggests that aggregates are necessary for hole delocalization and efficient doping.

Language

English

Keywords

polymer, single molecule, Raman, energy transfer, microscopy, optoelectronic materials, nanoscience, electronic device, charge transfer, photophysics, optical spectroscopy, laser spectroscopy, nanomaterials

Document Type

Dissertation

Degree Name

Chemistry

Level of Degree

Doctoral

Department Name

Department of Chemistry and Chemical Biology

First Committee Member (Chair)

Yang, Qin

Second Committee Member

Habteyes, Terefe

Third Committee Member

Dunlap, David

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