No description available.

No description available.

Organic semiconductors (OSCs), compared to the more traditional silicon, are enticing materials for the fabrication of optoelectronic devices (e.g., transistors, photovoltaic cells, light-emitting diodes) due notably to the lower cost associated with their preparation and purification, as well as their increased solubility in solvents which can accommodate large-scale fabrication. However, a higher degree of molecular disorder typically results in lower performance than silicon and remains an issue to be adressed. As the structure of an OSC is crucial to its performance, understanding the nature of this structure-property relationship is key to further the field of OSCs. In this regard, this thesis explores the optoelectronic properties of different π-conjugated organic frameworks which incorporate sulphur and nitrogen atoms along the rigid conjugated backbone for their desirable impacts on charge mobilities and stability. After a brief review of both small-molecule OSCs as well as key experimental techniques employed in the course of this work, chapter three covers the synthesis and characterization of dithiatetrazocines (DTTA), electron-deficient sulphur-nitrogen heterocycles, which were functionalized with various (oligo)thienyls pendants. The impact of both the substitution patterns and the degree of conjugation on the optoelectronic and solid-state properties of the ring system was investigated. The fourth chapter expands on previous work from the Brusso group that focused on extending the 2D conjugation of tetrathienoanthracene. While oligothienyls were previously shown to effectively increase the degree of conjugation, little to no change in device performance were observed, which was ascribed to disorder of the rotatable pendants. Here, rigid thieno[3,2-b]thiophene was used instead to increase both the degree of conjugation while maintaining structural rigidity, as assessed by optical, electrochemical and theoretical studies. The fifth chapter introduces preliminary work toward expanding the electron-deficient hexaazatrinaphtylene core with thiophene rings. The resulting concentric donor-acceptor structure promotes luminescent behavior with pronounced emission solvatochromism. Optical measurements were performed before and after intramolecular cyclization of the thiophene rings, to study the impact of aromatization on the optoelectronic properties of the system.

Spintronics, also known as spin electronics, or magnetoelectronics, refers to the study of the role that electron and (less frequently) nuclear spins play in solid state physics, and a group of devices that specifically exploit both the intrinsic spin of the electron and its associated magnetic moment, in addition to its fundamental electronic charge. As a principal type of spintronic device, a spin-valve is a device that uses ferromagnetic electrodes to polarize and analyze the electronic spins. The electrical resistance of the device depends sensitively on the relative magnetization of its two ferromagnetic electrodes, a phenomenon referred to as Giant Magnetoresistance (GMR). Having been successfully applied in the field of data storage, GMR also shows potential for future logic devices. Organic semiconductors possess many advantages in electronic device applications. Therefore, using organic semiconductors in spintronics is very interesting and promising, in part, because of their exceptionally long spin-decoherence times. This thesis concerns itself with the scientific study of magnetic field and spin effects in organic spin valves (OSV) and organic light emitting diodes (OLED). Three projects were finished, achieving a better understanding of the transportation of charge and spin carriers inside organic films, and paving the way to enhancing the spin diffusion length and the organic magnetoresistance (OMAR) effect. Firstly, C60 films were used as the spin-transport layer of OSV devices, because of its low hyperfine coupling and high mobility, which prior work suggested to be beneficial. Subsequently we studied the spin injection and transport properties by measuring the devices' magnetoresistance (MR) response at various biasing voltages, V, temperatures, T and different C60 film thickness. But we do not observe a significantly increased spin-diffusion length compared to OSV devices based on other organic semiconductors. We propose conductivity mismatch as a likely cause of the loss of spin-valve signal with increasing C60 layer thickness. There exists some disagreement in the scientific literature regarding whether OSV operate in the so-called tunneling regime or the so-called injection regime. To shed light on this question, we fabricated spin-valve devices made of organic semiconductor thin films of rubrene sandwiched between ferromagnetic cobalt and iron electrodes. Current-voltage (I-V) characteristics in Co/AlOx/rubrene/Fe junctions with a rubrene layer thickness, d, ranging from 5-50 nm, were measured, and we found two different modes of conductivity. The first mode, tunneling, occurs in relatively thin junctions, d < 15 nm, and decays exponentially with increasing rubrene thickness. We determined the tunneling decay length to be 1 nm. The tunneling mode is also characterized by a weak temperature dependence and a nearly parabolic differential conductance. The second mode, injection followed by hopping, occurs in relatively thick devices, d ≥ 15 nm, and can be identified by strongly temperature dependent, highly non-linear I-V traces that are similar to those commonly measured in organic injection devices such as OLEDs. We observed MR in devices with a rubrene thickness of 5 nm and 10 nm. Those devices are clearly in the tunneling regime. For the 15 nm device, for which the tunneling current is just barely measurable we could not observe MR. In the third project, we show that the performance of both OMAR and OSV devices very sensitively depends on whether the metallic layers are deposited by thermal evaporation or electron-beam evaporation. A strongly reduced spin diffusion length and an enhanced OMAR response can be achieved in devices fabricated by electron-beam evaporation. Then we showed that the difference must be attributed to the generation of traps resulting from the exposure of the organic layer to X-ray bremsstrahlung that is generated during the e-beam evaporation process. We also used the thermally stimulated current technique (TSC) to characterize these traps.

Abstract: With the worldwide dependence on non-renewable fossil fuels and increasing concerns over their impact on our planet through greenhouse gas emissions finding an alternative source of clean energy is a global imperative. The solar energy is one source of renewable energy resources, and It has the highest potential to contribute substantially to the future of carbon-free power needs. Solar to hydrogen has attracted much attention in the past decade due to its abundance and the spotlessness of hydrogen as fuel for energy usage. However, practically the requirements to convert solar energy to hydrogen, require a stable photocatalyst that’s able to operate efficiently over a wide range of the UV-VIS spectrum. Organic semiconductors have been widely used in hydrogen evolution due to their earth abundance, aqueous stability, and optical absorption that can be tuned to the UV-VIS spectrum. In chapter 3, The effect of different sacrificial regents on hydrogen evolution activity was systemically investigated by using poly(9,9-dioctylfluorene-alt-benzothiadiazole) (F8BT) nanoparticles dispersion large and small diameter with Sodium dodecyl sulfate (SDS) as stabilizer. Ascorbic acid (AA), diethylamine (DEA), triethanolamine (TEOA), and triethylamine mixed with methanol (TEA/MeOH) were chosen as sacrificial reagents. The results indicate that the large diameter give improved efficiency with ascorbic acid, and the small diameter improved activity in the presence of diethylamine. The results indicated that the comparison between different sacrificial reagents is difficult because, the conditions of every experiment is different to another, depending on (the type of photocatalyst used, solubility, activity..) so to date, there is no clear concurrence in which sacrificial reagent is better than others. Photocatalysts formed from a single organic semiconductor typically suffer from inefficient intrinsic charge generation, which leads to low photocatalytic activities. In chapter 4, To overcome this limitation, we have used BTR, O-IDTBR, and PC71BM in binary and tertiary heterojunction nanoparticles between non fullerene donors’ small molecules and fullerene acceptor. The resulting photocatalyst display unprecedentedly a high hydrogen evolution rate over 12000 μmolh-1g-1 under AM 1.5g illumination.

A thin film organic/ inorganic hybrid solar cell was fabricated by incorporating ZnO nanowires, n- and p-type organic semiconductors and inorganic quantum dots. The basic cell design involved the electrodeposition of ZnO nanowires grown on a substrate coated with a transparent conductive oxide. The ZnO nanowires were coated with a thin layer of an organic n-type material, followed by a deposition of inorganic quantum dots. A p-type polymer layer was subsequently deposited and the sample was then contacted with gold to form a quantum dot layer sandwiched between a p-n junction of organic conductive materials. Various materials and processing methods were adjusted, using I-V characteristics, photovoltage and/ or photocurrent measurements to determine the performance of the cell. Each constituent material in the basic device design was evaluated in terms of its contribution to the sample characteristics. A variety of deposition techniques were investigated to obtain homogeneous layers. Different annealing procedures were explored with the intent of balancing the time and temperatures required for electrical activation with material constraints such as tendency towards oxidation and low melting points. The effect of time on the sample characteristics was also observed. The evaluation primarily includes data for samples that led to design modifications aimed at improving both electrical properties and quantum efficiencies. This research led to the development of a hybrid solar cell sensitized by the addition of quantum dots. The organic semiconductors were used to form a p-n junction, and the p-type polymer also served as an active absorber layer. The quantum dots were used as the inorganic absorber fayer, and the results show that the range of optical absorption in the cell can be modified by adjusting particle size. In addition, the ZnO nanowires appear to improve charge transfer, when used with materials that have favorable band offsets.

No description available.

No description available.

<p dir="ltr">Efficiently identifying commercial and improvised explosives is a crucial prerequisite for disarming and disposing of these dangerous materials. In conjunction with traditional techniques (e.g., ion mobility spectrometry and mass spectrometry), electrochemical sensors can function as low-form factor and inexpensive options to quickly identify chemical threats. In particular, organic electrochemical transistors (OECTs) have many attractive properties, and they have become viable options for biosensing. OECTs employ a simple geometry consisting of a conducting polymer active layer and an electrolyte controlled by a gate electrode. In turn, this provides a means for the solution-phase detection in short times. Here, the OECT architecture was extended to the challenge of explosive trace detection. These sensors were adapted to detect several families of explosives (i.e., acid-salts, nitroaromatics, nitroamines, nitrate-esters, and peroxides). Many of these sensors incorporated molecularly imprinted polymers (MIPs) to improve chemical selectivity. These MIPs were shown to introduce size exclusive properties to the OECTs, which can be leveraged to detect acid salts explosives. MIPs that were complementary to nitrated explosives (nitroaromatics, nitroamines, and nitrate-esters) also were prepared. These MIPs can adsorb their respective explosive decreasing their polymer porosity and ion-transport. Finally, a stand-alone OECT design was applied to detect peroxide-based explosives. These explosives were decomposed to hydrogen peroxide intermediates. The evolved hydrogen peroxide was then identified as it was electro-oxidized at the gate electrode. After establishing the viability of the discussed techniques, two new directions for designing OECT sensors were proposed. Finally, these two outlooks were combined as a potential strategy for directly detecting peroxide-based explosives. While only a small subset of explosives was considered, the strategies applied were not unique to these specific targets. Indeed, these OECTs detecting principles could be applied to a broader scope of explosives detection as well as novel chemical sensing horizons.</p>

This thesis presents studies of exciton diffusion in organic semiconductors measured using exciton-exciton annihilation and the measurements were performed on materials important for organic solar cells. In the conjugated polymer poly(3-hexylthiophene) (P3HT), the effect of molecular weight (4-76 kgmol⁻¹) was explored. Using exciton-exciton annihilation measurements, the highest diffusion coefficient was observed in the intermediate molecular weight region and was correlated with long conjugation lengths, higher fraction of aggregated states and more delocalised excitons within the aggregate. The results demonstrated that the molecular weight dependence is due to a complex relationship between intermolecular interactions, aggregate size and Boltzmann statistics. This thesis also includes an investigation of exciton diffusion in diketopyrrolopyrrole(DPP)-based small molecules as a function of molecular structure. Significant changes in photophysical and exciton diffusion properties were observed due to minor changes in molecular structure. Long conjugation lengths, bulky side chains or reduced steric hindrance due to absence of end alkyl chains correlated with reduced film crystallinity and reduced diffusion coefficients. The increase in disorder observed due to large conformational torsions resulted in inhomogeneous broadening of density of states and as a result exciton diffusion becomes dispersive. In this case, a slowdown of exciton diffusion is observed. This study demonstrates that enhanced exciton diffusion can be achieved by designing more rigid and planar conjugated backbones with smaller conjugation lengths. Finally, exciton diffusion measurements were used to rationalise the performance of T3 truxene oligomers as explosive sensors. Side chain lengths were found to have a subtle influence on exciton diffusion. Time-resolved PL quenching measurements were used to estimate the quencher concentration. Differences in quencher concentration were observed suggesting different interaction strengths of the quencher with the truxene oligomer which help explain the explosive sensing performance.