(Opto)electronic Properties of Xylindein and Organic (Opto)electronic Devices

Organic semiconductors are of interest for (opto)electronic applications due to their low cost, solution processability, and tunable properties. Natural product-derived organic pigments have attracted attention due to their extraordinary environmental stability and unexpectedly good optoelectronic performance, in spite of only partially conjugated molecular structures. Fungi-derived pigments are a naturally sourced, sustainable class of materials that are largely unexplored as organic semiconductor materials. My research has focused largely on the optical and electronic properties of a fungi-derived pigment xylindein, which is secreted by the wood-staining fungi Chlorociboria (C.) aeruginosa. Device applications for xylindein have been explored to utilize its unique properties. A second project in my research focuses on polaritons in acenes and thiophenes, with a goal of harnessing polaritons to enhance physical and chemical properties relevant for electronic applications such as conductivity and stability. In particular, I have succeeded in incorporating an optical cavity into an anthradithiophene field effect transistor structure and explored the effects of the polaritons on device performance. Optical and electronic properties of xylindein were explored initially, and a strategy to improve the innately poor processability was developed. Optical absorption spectra in solutions of various concentrations and in film were compared and are consistent with aggregate formation in concentrated solutions and films. An amorphous polymer PMMA was introduced to xylindein to form xylindein:PMMA blends in order to improve film morphology. Current-voltage characteristics and hole mobilities extracted from space-charge limited currents were found to be comparable between pristine xylindein and xylindein:PMMA films. Side by side comparison of the photoresponse of pristine xylindein and xylindein:PMMA films at 633 nm revealed an increase in the photosensitivity in xylindein:PMMA films due to the improved morphology favoring enhanced charge generation. After seeing large variations in data from different batches of xylindein, a simple purification step (“ethanol wash”) was developed, and the impact of contaminants was investigated. The optical and electronic properties of solutions and films were studied with and without this processing procedure. The “post-wash” xylindein solutions exhibited considerably lower absorption in the ultraviolet spectral range and dramatically reduced photoluminescence below 600 nm, due to removal of contaminants most likely to be fungal secondary metabolites. The “post-wash” xylindein-based films were characterized by two orders of magnitude higher charge carrier mobilities as compared to “pre-wash” samples. This underlines the importance of minimizing contaminants that disrupt the conductive xylindein network in xylindein-based electronic devices. With a simple and effective purification protocol in place, the optical and electronic properties of xylindein were studied in more depth, along with its blends with poly(methyl methacrylate) (PMMA) and crystalline nanocellulose (CNC). Optical absorption spectra of xylindein revealed the presence of two tautomers whose structures and properties were established using density functional theory. Pronounced pigment aggregation in polar solvents and in films, driven by intermolecular hydrogen bonding, was also observed. The pigment exhibited high photostability, electron mobility up to 0.4 cm2/(Vs) in amorphous films, and thermally activated charge transport and photoresponse with activation energies of ~0.3 eV and 0.2 eV, respectively. The dark and photocurrents in xylindein:PMMA blends were comparable to those in pristine xylindein film, whereas blends with CNC exhibited lower currents due to inhomogeneous distribution of xylindein in the CNC. One of reasons for studying xylindein is its remarkable stability, which is often an issue for organic semiconductors. Unlike most conventional organic semiconductors, xylindein has hydroxyl (OH) groups in its molecular structure. We sought to determine what role those OH groups and the hydrogen bonding they enable play in the stability and (opto)electronic properties of xylindein. We determined that the presence of the OH groups is critical for enabling its enhanced stability and relatively high electron mobility. In particular, we synthesized a methylated derivative of xylindein, dimethylxylindein, where the OH groups are replaced with OCH3 groups, and compared photophysics and (opto)electronic properties of dimethylxylindein and xylindein. We revealed the presence of a long-lived excited state in dimethylxylindein, in contrast to xylindein, which has an efficient fast non-radiative pathway to the ground state. This results in significantly reduced photostability of dimethylxylindein as compared to xylindein. The effective electron mobility, obtained from space-charge-limited currents, in amorphous xylindein films was found to be four orders of magnitude higher than that in amorphous and crystalline dimethylxylindein films. In contrast, the photosensitivity of dimethylxylindein is about two orders of magnitude higher than that of xylindein. The mechanism of charge transport in all films was thermally-activated hopping, with the xylindein films characterized by considerably shallower charge traps than dimethylxylindein films, attributed to hydrogen bonding via hydroxyl groups promoting efficient conductive network in xylindein. Potential (opto)electronic device applications were explored for xylindein with the hopes of using it as a stable, environmentally friendly alternative to current device materials. Thin films of various blends of xylindein with other organic semiconductor materials were studied for evidence of charge transfer for potential use in donor-acceptor (D-A) bulk heterojunction (BHJ) solar cells. Fullerenes are the most common high performance acceptor material in organic BHJ solar cells, though they are costly and have degradation issues. Thus, there is much interest in finding stable non-fullerene acceptor materials. The high photostability and decent electron mobility of xylindein made it an attractive candidate in this application. Enhanced photostability was observed in thin films of PTB7-Th (a high-performance donor material) blended with xylindein on Au electrodes with 100V applied. BHJ solar cells were fabricated and cells with xylindein blends were observed to be non-functional. Ternary blend solar cells were fabricated with 0-5% xylindein, and xylindein was observed to have a detrimental effect on performance, even in small quantities. With these observations, we hypothesized that the inhibition of charge separation and extraction is likely caused by the short exciton lifetime of xylindein, which promotes exciton recombination. Organic filed effect transistors (OFETs) were also explored as a potential application. Stable n-type organic semiconductors are rare, and their performance tends to lag behind their p-type counterparts. Complimentary circuits require both n- and p-type transistors to operate, so there is a need for stable, high-mobility, n-type organic semiconductors. The high stability and decent electron mobility make xylindein an attractive candidate for OFETs as well. OFETs were fabricated and tested in a variety of configurations with xylindein. Unfortunately, minimal switching behavior was observed, due to processing limitations. To bypass some of those limitations and take advantage of potential protonic conductivity in xylindein, organic electrochemical transistor (OECT) configurations were tested as well. Functioning transistors were fabricated using deionized water as a gating medium. Xylindein has the benefit not only of being environmentally friendly, but also non-toxic, which is important for biosensing applications, which OECTs and water gated transistors are commonly used for. Other electrolyte gates show redox activity that may point to potential use for xylindein in energy storage applications, which is currently under investigation. A second research project discussed in this thesis focuses on hybrid light matter quasi-particles known as polaritons. Polaritons have been shown to exhibit interesting physical properties, which we hope to utilize in overcoming some of the bottlenecks in organic (opto)electronics today, such as low stability and low charge carrier mobility. OFETs were further explored using 2,8-difluoro-5,11- bis(triethylsilylethynyl)anthradithiophene (diF-TES-ADT or ADT for short) as an active material. ADT has been shown recently to strongly couple to an optical cavity to form exciton polaritons, and is already well known as a p-type material for OFETs. Polariton states have been demonstrated theoretically to increase charge transfer and mobility in an organic semiconductor, thus have the potential to enhance (opto)electronic device performance. Interestingly, coupling can occur in an optical cavity in the absence of incident light, so it is theoretically possible to utilize polaritons in a conventional transistor in the dark, though there may be benefits to the operation of a phototransistor as well. Bottom gate top contact OFETs were fabricated with a fully reflective aluminum gate on the bottom, a top insulating layer on top of the active layer, and a thin aluminum layer on top as a partially reflective mirror to form an optical cavity. Some OFETs were left without the top aluminum layer as a control, and the bottom dielectric layer thickness was varied to give different cavity resonances. Transistor characteristics were measured along with phototransistor and photocurrent measurements to probe the effects of the optical cavity and polariton states induced in the devices

Molecular Packing-Dependent Exciton and Polari on Dynamics in Anthradithiophene Organic Crystals

Polarization-dependent absorption spectra of two functionalized derivatives of fluorinated anthradithiophene, diF TES-ADT and diF TDMS-ADT, were studied in the crystal phase using a Holstein-like Hamiltonian. For both molecules, the primary contribution to the lowest energy absorption was found to be the S-0-S-1 excitonic transition perturbed by an intermolecular coupling of 15 meV for both TES and TDMS. A secondary contribution, consistent with that from charge-transfer states, was also found. Additionally, absorption spectra were analysed when crystals were placed inside of optical microcavities formed by two metal mirrors. Cavities exhibited a primary absorption peak determined to be an enhanced absorption from the lowest-energy S-0-S-1 transition

Optimizing Xylindein from Chlorociboria spp. for (Opto)electronic Applications

Xylindein, a stable quinonic blue-green fungal pigment, has shown potential for use not only as a colorant but also as an (opto)electronic material. As no method presently exists to synthesize the pigment, organic production by slow-growing fungi from the genus Chlorociboria is the only method to obtain it. This has resulted in limited quantities of impure xylindein, hampering research. In order to improve quantity and quality of pigment for optoelectronic applications, speed of xylindein production by Chlorociboria aeruginosa and its relative purity were compared across liquid and solid-state fermentation conditions on selected nutrient sources. Liquid 2% malt shaking cultures produced the same amount of pigment in 5 weeks that previous testing produced in 2 months. Xylindein generation speed, purity, and conductive properties of produced pigment for (opto)electronics was then compared between two Chlorociboria species native to North America, Chlorociboria aeruginosa and Chlorociboria aeruginascens. Differences were seen in the conductivity of extracted pigment between species and strains, with xylindein from C. aeruginascens strain UAMH 7614 producing films with the highest effective electron mobility. The identification of the most effective growth conditions and the strain with highest purity xylindein production should support further development of sustainable organic (opto)electronics. Future work identifying new strains with reduced production of interfering metabolites and new extraction methodologies will help to produce very low cost xylindein, supporting sustainable technologies based on the pigment

Resonance properties of quartz crystal microbalance immersed in high solid content suspensions

The resonance properties, frequency and half-band-half-width, of a quartz crystal microbalance (QCM) immersed in concentrated suspensions of 16.2 vol% TiO2 are shown to be a function of pH. The overall QCM response is dependent on the complex interactions between the QCM sensor and overlying particle suspension. Atomic force microscopy confirms pH dependent interaction forces between the QCM sensor (gold-coated) and a TiO2 particle: a strong attraction is measured between pH 4–4.5, and the interaction becomes increasingly repulsive at all pH > 6.5. Yield stress measurements of the concentrated TiO2 suspensions also confirm the changing particle-particle interaction strength as the pH is adjusted from acidic to basic conditions. For the chosen system, the total potential energy of interaction (VT) between the sensor-suspension (Au-TiO2) is comparatively stronger than the particle-particle (TiO2-TiO2) interaction; hence the QCM responds to changes in VT sensor-suspension, as verified by the calculated interaction energy between two dissimilar surfaces (Hogg-Healy-Fuerstenau (HHF) theory), and not the suspension yield stress. Slight deviation between the measured QCM responses and the theoretical sphere-plate interaction strength is shown over a narrow pH range and likely corresponds to strengthening particle-particle interactions. Although the suspensions exhibit significant yield strengths, the QCM response can be suitably described by the sensor-suspension contact mechanics of inertial loading. Combined with our previous study [1], the current study confirms the suspension yield strength can only be measured when VT sensor-suspension is attractive and comparatively weaker than VT particle-particle