From Energy Generation to Storage: The Exploration of Unusual Architectures and Functions of Conjugated Organic Molecules
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The advent of conjugated polymers spurred the field of organic electronics. Since the inception of the field, many have lauded the potential benefits of organic over inorganic platforms, including their light weight, flexibility, and solution-processability, the latter of which makes them amenable to large-scale mass production. Unfortunately, the performance of organic electronics generally pales in comparison to their inorganic analogues. However, synthetic chemistry allows the unprecedented control over the structure and function of conjugated materials at the molecular levels, which is difficult to achieve in bulk inorganic crystals. Currently, donor-acceptor (D-A) polymers and small molecules are the benchmark for light-harvesting materials in organic photovoltaics (OPVs). Using careful design criteria, manipulation of the structure and components of D-A materials can yield organic materials with unique electronic and photonic properties. This can provide increasingly nuanced methods with which to construct novel materials and modify their properties. Such precise and predictable structure-property control is crucial to realizing technological advances in organic electronics. This body of work explores two non-traditional architectures for conjugated polymers and dual-purpose electrochromic light-harvesting molecules. In the first case, a thiophene-triphenylamine polymer with aldehyde-functionalized indacenodithiophene (IDT) side chains was synthesized. This parent polymer was modified by condensing electron-deficient units on the IDT side chain to achieve a series of polymers with differing absorption and energy levels. The effect of acceptor identity on the polymer properties was probed optically and electrochemically using UV-visible spectroscopy and cyclic voltammetry, respectively. The polymers were tested in organic field effect transistors (OFETs) and OPV devices. It was found that the more electron-deficient molecules led to a deeper lowest unoccupied molecular orbital (LUMO) while leaving the highest occupied molecular orbital (HOMO) unchanged, which thereby caused a redshift of the internal charge transfer (ICT) absorption peak. This was corroborated using Density Functional Theory (DFT), which revealed that the LUMO of the polymer was localized at the terminus of the side chain, while the polymer HOMO was delocalized along the polymer backbone. Overall, the polymer with a thiobarbituric acid acceptor showed the best performance, with a hole mobility (μh) of ~1 × 10-3 cm2/Vs and a power conversion efficiency (PCE) of 2.5%. In the second case, a comb copolymer with poly(3-hexyl)thiophene (P3HT) side chains and a carbazole-diketopyrrolopyrrole backbone was synthesized via a graft through approach. This required the development of an unexplored synthetic methodology, which involved the growth of P3HT from a boronic ester-functionalized carbazole to yield a carbazole-P3HT macromonomer, which was then coupled to the diketopyrrolopyrrole monomer through a Suzuki polymerization. Macromonomers with four different P3HT lengths were synthesized and characterized with UV-visible spectroscopy, cyclic voltammetry, gel permeation chromatography and matrix-assisted laser desorption ionization time-of-flight (MALDI-TOF). These macromonomers were then used to make the final comb copolymers, which were subsequently characterized with the same techniques, as well as differential scanning calorimetry (DSC). It was found that the optical and electronic properties of the P3HT chains and the D-A backbone were largely independent. Moreover, the relative expression of the P3HT and D-A properties were proportional to the P3HT length; longer P3HT chains suppressed the D-A characteristics and vice versa. These final comb copolymers were further utilized in OFETs and their films were characterized by atomic force microscopy (AFM). As the active material in OFETs, these comb copolymers behave most like their largest constituent component. The comb copolymer with the longest P3HT chains showed p-type behavior with similar μh values (~6 × 10-4 cm2/Vs) compared to neat P3HT. Meanwhile, the comb copolymers with the shortest P3HT chains showed ambipolar charge transport and similar mobilities to the control D-A polymer. Additionally, a number of molecules have been designed, synthesized and characterized for use in dual-purpose electrochromic light-harvesting windows with the architecture of a dye-sensitized solar cell (DSSC). The dyes were intended to function as the TiO2 sensitizer in DSSC function; upon applying a reverse bias to the DSSC, these dyes would ideally transition from a colored to a transparent state. Thus, this DSSC could harvest sunlight on a bright day, but, by reversing current flow, could also allow passive solar heating/lighting on a cloudy day. The design and synthesis of a menagerie of dyes is discussed, including dyes based on perylene diimide (PDI), Methylene Blue, and phthalocyanines. To date, one phthalocyanine dye has shown electrochromic behavior, but it transitions between a green (neutral state) and magenta (reduced state) color. Finally, hyperbranched ambipolar triphenylamine-based polymers were designed for use as the electrodes of symmetric supercapacitors. Ideally, these polymers would be able to retain more charge than activated carbon (the current benchmark for electrode materials) due to their predilection to engage in redox processes. A series of three triphenylamine-napthalene diimide polymers with a differing number of thiophene spacers were generated through solution polymerization. Initially, a symmetric supercapacitor was made from the first iteration, but yielded poor performance. To further investigate the properties of these materials, the polymers were used to make asymmetric supercapacitors. These devices revealed that the polymers had much better n-type properties, making them useful as the negative electrode of the supercapacitor. However, they did not readily engage in oxidation, making their utility as a positive electrode limited. Overall, the best of the asymmetric supercapacitors achieved a reasonable capacitance of ~22 F/g, which is roughly one order of magnitude lower than activated carbon. These studies showcase the versatility of organic materials for electronics applications. The synthetic methodologies discussed herein can be used to exercise complete control the structure of organic molecules, and to tune their resultant electronic, optical and chemical properties. This adaptability is evidenced by the fact that they can be used for light-harvesting, energy storage and numerous other applications in the realm of organic electronics. Overall, this body of research provides a glimpse into the vast number of unexplored structures and applications of conjugated organic materials.
- Chemistry