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Doping of Organic Semiconductors: Effects of Crosslinking and Dopant Substituents

(Georgia Institute of Technology, 2021-12-15) Saeedifard, Farzaneh

Doping is the process of addition of dopants to host semiconductors to improve their conductivity and charge transport behavior. Organic solids are held together by weak van der Waals interactions between the molecules and Coulombic attractions between the charged species. Because of these weaker interactions in organic materials, the molecules themselves have higher mobilities within the host material, and therefore, have a higher tendency to move. In most of the devices, the diffusion of the dopants in device stacks is detrimental and therefore, minimizing dopant diffusion within device interlayers is a very important topic to be consider. Considering the widespread usage of DMBI-H derivatives for doping of organic semiconductors, this work will focus on two aspects of doping; investigation of different approaches to address the diffusion of DMBI-H derivatives and studying the effect of dopant substituents on charge transport behavior. The first and second chapters of this thesis, will focus on crosslinking as new approaches for minimizing the dopant diffusion in the solid state. Chapter 2 will discuss electrostatic crosslinking in which the restriction of dopant ion movement by forming multiple electrostatic sites between the multiply charged ions and ionized host segments. Chapter 3 will discuss chemical crosslinking and chemical bond formation to decrease the diffusion of dopant and the corresponding dopant ions. Chapter 4 will focus on a study in which the effect of a polar side chain on DMBI-H for doping of a donor-acceptor polymer. The final chapter summarizes the findings of the thesis, puts them in a broader perspective, and suggests future directions
Specialized side chain influence on arrangement of conjugated macromolecules in organic electronics

(Georgia Institute of Technology, 2021-12-13) Hicks, Theodore

Specialized side chain influence on arrangement of conjugated macromolecules in organic electronics Theodore Hicks 171 Pages Directed by Dr. Seth R. Marder This dissertation focuses on the design, synthesis, and characterization of pi-conjugated small molecules and 2D polymers of potential interest for organic electronic applications. An investigation of the structure-property relationships that effect two different classes of conjugated macromolecules, covalent organic frameworks (COFs) and small-molecule electron acceptors, is conducted by synthesis of new materials and characterization of the resulting optical, electronic and physical properties. Chapter 2 describes two different approaches toward development of a generalizable method for synthesizing 2D crystalline polymer monolayers. The findings include a remarkable tolerance of interlayer stacking in imine-linked COFs composed of isoindigo as well as the development of air-water polymerization methods to generate monolayer films. The characteristics of the COF materials are demonstrated using a variety of spectroscopic and microscopy techniques. Chapter 3 details the synthesis and characterization of small molecules designed for use in organic photovoltaics. The new compounds are evaluated for electronic and optical suitability before incorporation into test devices. Chapter 4 gives a perspective on the impact of synthetic complexity on organic electronic devices and contains an investigation of new synthetic methods based on ultra-efficient catalysis of precursors required for organic semiconductors.
Investigation of Charge Transport in Conjugated Organic Materials

(Georgia Institute of Technology, 2021-06-21) Al Kurdi, Khaled Al

The demand for energy has increased dramatically with increase in population and industrialization. However, relying on traditional fossil fuels to meet those demands lead to climate change becoming an existential threat to the livelihood of humanity. Thus, to navigate the challenges of meeting the energy demand, researchers need to investigate pathways to pivot to more benign energy sources as well as discovering sustainable materials for a versatile range of energy applications. Conjugated organic materials have been of great interest for optoelectronic applications for the past 50 years to complement and/or substitute their inorganic counterparts. This work aims to design and investigate the charge transport properties of conjugated organic materials. This is achieved by providing a diverse toolbox of structure- property studies to further understand the behavior of doped organic materials and guide future development. In addition, this thesis shows an example of how such polymers can used in solar cells to replace an inorganic oxide. Further, a family of dopants along with investigation into their kinetic behavior is presented to be used in the future developments of polymer: dopant systems.
Development of guidelines towards an understanding of the structure of the cation - structure of the hybrid haloplumbate material relationship

(Georgia Institute of Technology, 2020-07-21) Tremblay, Marie-Helene

Organic-inorganic hybrid haloplumbate materials have attracted ever-increasing interest in the last ten years, initially due to their incorporation in high efficiency solar cells, but increasingly due to their utility in a variety of technological applications (e.g. light emitting diodes, transistors, laser, catalysis). By mixing PbI2 with an organic cation, one can synthesize 3D, 2D, 1D and 0D perovskite or perovskitoid compounds depending on the cation used. This thesis is concerned with identifying structural factors of the organic cation that influence the observed structures of low- dimensional organic lead-iodide perovskites and perovskitoids. This understanding is necessary to develop design strategies that enable one to predict the structure of the perovskite and perovskitoid based on the organic cation used. As previously suggested, it was found that both the interaction between adjacent cations or between the inorganic layer and the organic cation have structure directing roles in perovskites and perovskitoids. In each class of cations examined, the cation- cation and cation-inorganic layer interactions that determine the structures are discussed. An understanding of how the structure of the cation determines the structure of the hybrid material helps modify materials properties such as stability and optical properties. The first chapter surveys the structural types adopted by organic iodoplumbates reported in the literature. Following, several studies are described in which 34 crystal structures are determined for families of compounds including 2D perovskites with substituted phenylethylammonium and benzylammonium cations, a corrugated 2D structure using the 4-nitrophenylethylammonium cation, and 1D perovskitoids. The final chapter summarizes the findings of the thesis, and puts them in a broader perspective, comparing them to related structural relationships among previously reported structures, recapitulates how this thesis has improved our understanding of the cation structure – iodoplumbate structure relationships, and suggests future directions.
Synthesis and Isolation of 2D Covalent Organic Frameworks and an Examination of the Initial Stages of Crystallization

(Georgia Institute of Technology, 2020-05-15) Feriante, Cameron H.

Covalent organic frameworks (COFs) are crystalline network polymers with permanent porosity and a range of potential applications including gas storage and separation, molecular separations, catalysis, charge transport and others. This thesis focuses on two principle aspects of two-dimensional imine-linked COF synthesis; First, the activation process used to isolate the synthesized COF material and remove guest molecules such as solvent, catalyst, or oligomers, and second, the initial stages of the COF synthesis process where crystalline material first forms. A series of three 2D imine-linked COFs are examined after vacuum activation, activation with supercritical carbon dioxide and activation with nitrogen flow and heat. The crystallinity and porosity of the resulting materials is assessed to determine the structural changes induced by each activation method and to provide insight into the best methods for COF activation. Additionally, 2D imine-linked COFs are examined during the initial stages of the COF synthesis through a variety of in situ and ex situ methods to monitor the development of crystallinity during the reaction, and to assess how different structural features of the COF affect this process. The overall ambition of the thesis is to expand the foundational knowledge pertaining to the formation of 2D imine-linked COFs allowing for the synthesis and isolation of higher quality materials.
Redox-active organometallic dimers as dopants and surface modifiers in optoelectronics

(Georgia Institute of Technology, 2019-04-02) Pulvirenti, Federico

This dissertation focuses on the use of the dimers formed by some 19-electron organometallic sandwich compounds to tune the electronic properties of transparent conductive oxides, particularly indium tin oxide (ITO) and fluorinated tin oxide (FTO), and organic electron-transporting materials (ETMs), specifically fullerene and perylene diimide derivatives used in perovskite solar cells (PSCs). The dimers reduce the work function (WF) of electrode materials by transferring electrons to the metal oxides, forming metal-organic monomeric cations electrostatically bound to the reduced electrode, or n-dope ETMs through a similar reaction. This dissertation is an investigation of the relationship between the processing method by which dimers are deposited on metal oxides or along with organic semiconductors, their ability to lower the WF of the electrode and/or to increase the electrical conductivity of ETMs, their chemical state in these doped materials, and the electrical behavior of the modified substrate in simple and complex optoelectronic devices. Moreover, the stability of the WF modification to solvent washing, temperature and processing route of ETM overlayers is explored. Two strategies to prevent washing of monomer cations and their diffusion are proposed: one entailing the combination of phosphonic acid and organometallic dimer modification of metal oxides; one entailing the use of a crosslinker in bulk-doped fullerene derivatives. The effect of the choice of surface-modification layer and ETM on charge collection in PSCs is assessed. The findings of this dissertation will contribute to the development of robust surface modification and doping approaches for the fabrication of efficient and stable optoelectronic devices.
Excited state design of carbazole-oxadiazole compounds for thermally activated delayed fluorescence

(Georgia Institute of Technology, 2018-12-04) Cooper, Matthew William

Materials containing 9H-carbazole and 2,5-diphenyoxadiazole functionalities have been widely exploited in organic electronic applications. Only very recently, however, have they been combined to obtain fluorophores exhibiting thermally activated delayed fluorescence (TADF), a process exploited in efficient, third-generation organic light-emitting diodes. A study of donor-acceptor compounds comprised of 9H-carbazole and 2,5-diphenyl-1,3,4-oxadiazole moieties is presented with an emphasis on developing an understanding of how structure affects properties relevant to TADF. An orbital understanding of the lowest energy singlet and triplet excited states is established wherein the singlet state involves a HOMO to LUMO intramolecular charge transfer state and the triplet state is a locally excited state confined largely on the diphenyloxadiazole moiety. The orbital character of the states is confirmed through sensible structural modifications that preferentially affect the energy one of the two states, reducing the energy between them and allowing for the observation of TADF. OLEDs utilizing these fluorophores as the emitter are shown to exhibit external quantum efficiencies well above what is possible if only singlet excitons are being converted to light, confirming the conversion of triplet excitons to photons in the devices. The findings show the limitations of using these materials as blue-emitters in OLEDs and provides strategies for overcoming these restrictions through rational molecular design. Further optimized structures as well as application of the results to other donor-acceptor systems is discussed.
Spectroscopic characterization of nonlinear optical and biophotonic materials

(Georgia Institute of Technology, 2018-10-29) Allen, Taylor G.

In this dissertation, three studies were presented that involve the spectroscopic characterization of either nonlinear optical or biophotonic materials. The first study (Chapter 3) proposed judicious bulky substitution of cyanine-like, polymethine dyes with large intensity dependent refractive indices to effectively mitigate aggregation between dyes in thin film polymer blends of interest for all-optical signal processing applications in the near infrared (NIR). Linear characterization and Z-scan measurements of these blends at 1550 nm confirmed that bulky groups suppress aggregation in the solid state, thus lowering two-photon absorption (2PA) and improving the nonlinear optical performance of these films. The second study (Chapter 4) proposed a series of planar, fused-ring, organic, quadrupolar chromophores of type A-π-D-π-A with the potential to demonstrate large 2PA cross-sections for optical limiting applications in the NIR. Nondegenerate 2PA measurements revealed that these compounds have large 2PA cross sections and spectral coverage in the NIR, which can be controlled synthetically via structural changes in the core and acceptor groups at the molecule’s periphery. The third study (Chapters 5-6) proposed high refractive index replicas of photonic crystal-like hole patterns harvested from Coscinodiscus wailessii diatom frustules as sustainably-producable, biophotonic elements for lensless light focusing of NIR radiation at the micro-scale. Linear spectrophotometric and imaging measurements confirmed that these replicas concentrate NIR light via diffraction, which can be manipulated with changes in the index of the frustule through solid-gas conversion chemistries with excellent shape preservation.
The synthesis and electrochemical properties of nanoconfined lithium titanate, titanium oxide, iron fluoride and other compounds

(Georgia Institute of Technology, 2018-08-20) Zhao, Enbo

Performance and cost of battery cells are most strongly affected by their electrode and electrolyte materials, which are the basis of battery electrochemistry that enabled electrochemical energy storage applications today. This thesis systematically investigates the nanoconfinement of metal oxides and metal fluorides as electrode materials, from material selection, synthesis, characterization, to variable control, and methodology optimization. First, nanoconfined metal oxides were developed for ultra-high-rate performance applications. Uniform lithium titanate particles within 3 nm confined within porous carbon matrix were reported for the first time and delivered up to 12 times higher gravimetric and volumetric capacities than the state-of-the-art activated carbon electrodes. This technique was used to prepare other nanoconfined metal oxides with similar dimensions, including titanium oxide, nickel oxide, manganese oxide, cuprous oxide, among others. Conversion type cathode materials, widely regarded as the most promising candidates for next-generation lithium-ion batteries (LIBs), were studied for high energy density applications. In particular, I focused on metal fluoride (such as iron (III) fluoride, FeF3) nanoparticles confined in carbon. Iron (III) fluoride offers very high theoretical capacity, and better safety and cost advantage over conventional intercalation-type cathode materials that require the expensive nickel and cobalt. The cyclic capacity retention of the composite produced by electrospinning followed by gas phase fluorination exceeded the state of the art by nearly an order of magnitude in cells. Finally, the shell confinement of iron (III) fluoride cathode by in situ cathode electrolyte interphase (CEI) was studied. The CEI properties could be controlled by electrolyte optimization. Post-mortem analysis after cell cycling revealed insights on the mechanisms of CEI formation.
Work function tuning of electrode materials with small molecule surface modifiers

(Georgia Institute of Technology, 2018-07-31) Kim, Hye Kyung

Energy level alignment at metal/organic interfaces plays a critical role in charge transport across the interfaces within organic and optoelectronic devices. To control the barriers to injection/collection, dipolar surface modifiers can be introduced at the interface to tune the work- function of the electrode material. This results in a shift of the frontier molecular orbitals of the organic semiconductor upon contact with the modified-electrode. An appropriate shift in one direction facilitates electron transport and the opposite direction hole transport. In this work, new small molecule surface modifiers were synthesized and deposited on electrode materials, namely indium tin oxide (ITO) and gold to reduce their work-functions to be potential candidates for low work-function electron selective electrodes. Amines – N-oxide and N-heterocyclic carbenes, in addition to chalcogens – phosphine sulfide phosphines and N-heterocyclic thiones and selones were investigated for their work-function reducing capabilities and surface coverage. All the materials successfully reduced the work-function of the modified electrodes, with the magnitude of the shift dependent, to a degree, on the thickness of the deposited film.