Author ORCID Identifier

https://orcid.org/0000-0002-9599-1745

Year of Publication

2020

Degree Name

Doctor of Philosophy (PhD)

Document Type

Doctoral Dissertation

College

Arts and Sciences

Department

Physics and Astronomy

First Advisor

Dr. Joseph Brill

Second Advisor

Dr. Kenneth Graham

Abstract

Applications of organic electronics have increased significantly over the past two decades. Organic semiconductors (OSC) can be used in mechanically flexible devices with potentially lower cost of fabrication than their inorganic counterparts, yet in many cases organic semiconductor-based devices suffer from lower performance and stability. Investigating the doping mechanism, charge transport, and charge transfer in such materials will allow us to address the parameters that limit performance and potentially resolve them. In this dissertation, organic materials are used in three different device structures to investigate charge transport and charge transfer. Chemically doped π-conjugated polymers are promising materials to be used in thermoelectric (TE) devices, yet their application is currently limited by their low performance. Blending two polymers is a simple way to change the TE properties of the film. Here we use an analytical model to calculate the TE properties of polymer blends, which takes into account energetic disorder, energetic offsets between mean energy of states of the two polymers, and localization length. These calculations show that the TE performance of polymer blends can exceed the individual polymers when there is a small (e.g., 0.1-0.2 eV) offset between the mean of the density of states (DOS) distributions of the two polymers, the polymer with the higher energy DOS has a wider DOS distribution and a larger localization length (mobility), and the polymers are homogeneously mixed. We show these improvements are achievable by experimentally testing TE properties of selected polymer blends. These sets of polymers are selected with variations in electrical mobility, ionization energy and degree of crystallinity to cover a range of possibilities explored in the calculations. Further, to investigate the effect of dopant size in polymers, we use organic electrochemical transistors to investigate the effect of anion size on polaron delocalization and the thermoelectric properties of single polymers. This device structure allows us to control the charge carrier concentration with minimizing the effects on the film morphology. Another application of OSC is in organic photovoltaics (OPVs), where they can potentially provide a cheap and flexible source of solar energy, yet they currently suffer from low performance and stability. In OPVs, fluorination of donor molecules is a proven strategy for increasing the performance of OPV donor materials. Herein, we investigate the charge transfer state energy between the electron donor anthradithiophene (ADT) and the electron acceptor C60 upon halogenation of the ADT molecule. Interfacial energetics and charge transfer state energies between donor and acceptor are crucial to the PV performance of these devices. We probe interfacial energetics of donor/acceptor interfaces with ultraviolet photoemission spectroscopy (UPS), charge transfer state energies with sensitive external quantum efficiency (EQE) both in bilayer and bulk heterojunction device structures. These measurements coupled with DFT calculations allow us to explain that in bulk-heterojunction OPVs the halogenated ADT derivatives will likely increase charge recombination due to lower energy CT states present in the mixed phase. Therefore, the less favorable energy landscapes observed upon halogenation suggest that the benefits of fluorination observed in many OPV material systems may be more due to morphological factors.

Digital Object Identifier (DOI)

https://doi.org/10.13023/etd.2020.507

Funding Information

This study was supported by the University of Kentucky startup fund for Dr. Kenneth Graham from 2015 to 2018, National Science Foundation grant no. 1262261 awarded to Dr. Joseph Brill for summer of 2015, The American Chemical Society Petroleum Research Fund, ACS PRF, awarded to Dr. Kenneth Graham for 2018 and 2019, and National Science Foundation,NSF, grant no. 1905734 awarded to Dr. Kenneth Graham for 2020.

Available for download on Wednesday, December 15, 2021

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