Date of Award

Spring 1-1-2015

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Chemical & Biochemical Engineering

First Advisor

Arthi Jayaraman

Second Advisor

Garry Rumbles

Third Advisor

Charles Musgrave

Fourth Advisor

Andrew Goodwin

Fifth Advisor

Matthew Glaser

Abstract

Organic photovoltaic device efficiency is strongly influenced by the morphology of the active layer composed of conjugated polythiophenes (electron donors) and fullerene derivatives (electron acceptors). The goal of this thesis is to understand how molecular features affect the morphology and charge carrier dynamics in neat polythiophene and polythiophene-acceptor blends, using simulations and experiments.

We use molecular dynamics simulations with our newly developed intermediate resolution model to study how oligothiophene (eg. poly(3-hexylthiophene), P3HT) and acceptor (eg. [6,6]-phenyl-C61-butyric acid methyl ester, PCBM) architecture and chemistry affect neat and blend morphology. Our coarse-grained model enables observation of molecular-level packing, e.g. the experimentally observed intercalation of acceptor molecules between oligomer side chains, as well as the transition from a disordered initial state to experimentally observed ordered morphologies. With this validated model, we study the impact of alkyl side-chain length and placement along generic oligothiophenes on the order-disorder transition temperature and molecular packing. We also predict the appropriate backbone-acceptor attraction that leads to blend morphologies with increased connectivity and interfacial area, characteristics beneficial for exciton dissociation and charge transport. We also predict additive design that provides a desired degree of acceptor macrophase separation, and acceptor intercalation. Lastly, in collaboration with Briseño et al. at University of Massachusetts, we study monomers and dimers of 2,5-bis(3-alkylthiophen-2-yl)thieno[3,2-b]thiophene (BTTT) mixed with PCBM, and find, in agreement with experimental structural characterization, that the BTTT2-PCBM blends have higher crystallinity than neat BTTT2, and that in blends, PCBMs form rows between alkyl side chains of BTTT2.

To complement the morphological simulation studies, we use Time-Resolved Microwave Conductivity at the National Renewable Energy Laboratory to elucidate how polythiophene crystalline domain sizes, determined using X-Ray Diffraction, affect the yield and lifetime of photogenerated charge carriers in P3HT and poly(2,2':5',2"-3,3"-dihexyl-terthiophene) (PTTT) films. We show that polythiophene crystallite size is tuned with casting temperature and that films with larger crystalline domains have longer charge carrier lifetimes in neat films and films with phthalocyanine acceptor molecules. Thus, charge carrier lifetime is modulated by crystallite size both in films with low charge carrier concentrations and in films with higher charge carrier concentrations approaching those in organic electronic devices.

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