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Thermal diffusivity (D), measuring how fast heat propagates in a medium, is an important quantity in heat conduction. For a medium with great thermal diffusivity, it will reach thermal equilibrium in shorter time. In the field of solid state materials, thermal diffusivity can give information about the quality and morphology of solid, since D is very sensitive to microstructures. However, studies on the thermal diffusion of organic semiconductors are very scarce. In this thesis, the thermal diffusion of different classes of photovoltaic polymers and their blends with molecular electron acceptors were studied by scanning photothermal deflection (SPD) technique. The reliability of the technique was confirmed by the good matching between the SPD derived experimental D values and the nominal D values of different reference materials obtained from literatures. To illustrate that determination of thermal diffusivity is a possible method for studying microscopic properties of organic photovoltaic materials, SPD technique was applied to various films of photovoltaic polymers with different crystallinities. It is observed that photovoltaic polymers always possess small D values in the range of 0.3mm2/s to 2.3mm2/s. It is also discovered that photovoltaic polymers with more planar molecular structure, stronger π-π stacking and higher crystallinity would possess larger D values. When photovoltaic polymers are blended with small molecular acceptors bulk heterojunctions (BHJs), the thermal diffusivity is always reduced due to disrupted polycrystalline structure and increase probability of intermolecular phonon transport. However, for all-polymer BHJs with polymeric acceptor, the reduction in thermal diffusivity can be moderate as the proportion of ultrafast intramolecular phonon transport is SPD technique was also applied to PBDB-T:(ITIC-M+N2200) ternary BHJs with different ITIC-M to N2200 weight ratio. The thermal diffusivity of the ternary blend increases with the weight percentage of N2200 polymeric acceptor. It is observed that PBDB:(ITIC-M+N2200) ternary photovoltaic devices with enhanced thermal diffusion can possess enhanced photostability. Such enhancement in photostability is attributed to the reduced heat trapping at the area being illuminated due to the improved thermal diffusion.

Studies of light-matter interactions in organic semiconductors and in optical tweezer trapping of nanoparticles are presented. In the research related to organic semiconductor materials, a variety of novel materials and their composites have been characterized, and physical mechanisms behind their optoelectronic properties have been established. Three novel functionalized hexacene derivatives were deemed sufficiently stable to enable characterization of these materials in devices. From dark current and photocurrent measurements of the hexacene thin-films, it was determined that all three derivatives are photoconductive in the near-infrared, and space charge limited mobility values were obtained. In addition, physical mechanisms behind charge transfer, charge carrier photogeneration, and charge transport in small-molecule donor/acceptor composite films have been systematically studied. In these studies, it was determined that the charge transfer from the donor to the acceptor molecule can result in either an emissive charge transfer exciton (exciplex) or a non-emissive charge transfer exciton formation, depending on the energy difference between LUMO of the donor and the acceptor. However, the most dramatic trends in photoluminescent and photoconductive properties of the donor/acceptor composites were correlated with the separation between the donor and acceptor molecules at the donor/acceptor interface. In particular, composite films with larger separations exhibited electric field-assisted charge transfer exciton dissociation, which contributed to nanosecond time-scale photocurrents under a 500 ps pulsed photoexciation. Large donor/acceptor separation also resulted in reduced charge carrier recombination, which led to a factor of 5-10 increase in continuous wave photocurrents in certain donor/acceptor composites, as compared to those in pristine donor films. In the optical tweezer based studies, work towards the development of high spatial resolution optical tweezer controlled nanoprobes is presented. In particular, the possibility of exploiting the optical resonance of a particle to increase the optical tweezer forces acting on it within the trap has been investigated. Such an increase in the force would improve the potential spatial resolution of an optical tweezer controlled probe. Experimental results and numerical simulations on micron sized resonant dielectric particles showed a small increase in the optical forces that confine such particles within the trap, when tweezer trapping is conducted at wavelengths on the red-side of the optical resonance. Preliminary work on optical tweezer controlled ion/pH sensitive probes and on surface charge measurements is also reported. / Graduation date: 2012

The behavior and application of organic semiconductor materials depend strongly on their molecular structure, and molecular interactions. Several studies of intermolecular interactions in functionalized polyacene materials are presented. The degree and onset of aggregation of a functionalized anthradithiophene derivative was studied as a function of concentration in two different host matrices. The molecular environment was found to influence the degree and onset, but not the nature of aggregate formed. The effect of aggregation on photoconductivity was also studied. In a blend of two different anthradithiophene derivatives, the intermolecular separation was found to affect the nature of the interaction, transitioning from energy transfer at large intermolecular distances to the formation of an emissive excited state complex at smaller intermolecular distances. This complex was shown to have effects on both photoluminescence and photoconductivity. Finally, a single molecule fluorescence microscopy system was built and characterized. Software was written to process data produced from the system and several classes of functionalized polyacenes were studied at the single molecule level. In particular, the photophysics and molecular orientation of various derivatives were quantified. A new solution-processable, photoconductive, polycrystalline host material was found to be suitable for single molecule imaging, and the molecular orientations of individual molecules were found to depend on both their molecular structures and their local nano-environment. / Graduation date: 2013

published_or_final_version / abstract / Physics / Master / Master of Philosophy

No description available.

The unique physical properties and low temperature solution processability of organic semiconductors have enabled many applications such as light emitting diodes, flexible logic and solar cells, they are unexploited in their potential for use in solid state devices for spintronics and spin-based information processing. Organic semiconductors composed of mainly light elements appeal to the field of spintronics due to their long spin lifetime originating from their weak spin-orbit coupling. The significant progress in improving carrier mobility of organic semiconductors in the past decade may lead to organic spin transport materials with both long spin diffusion length and spin lifetime which is important for spintronics applications. This dissertation explores the spin transport in organic semiconductors using a variety of experimental techniques from all electrical spin injection and detection to ferromagnetic resonance spin pumping and ISHE spin detection. Non-local spin valves and novel all electrical spin transport device architectures based on high mobility conjugated polymers were studied systematically. The intrinsic roadblocks for electrical spin injection-based measurements were identified as the current spreading effect (electrical cross-talk between the injector and detector electrodes) and the hopping conduction in organic semiconductors which makes all electrical nonlocal spin injection and detection measurements extremely challenging if not impossible for organic semiconductors. In addition, spin current transmission in the out of plane direction of organic semiconductors was studied by tri-layer spin pumping technique where the spin transport properties of organic semiconductors are correlated with their molecular structure and charge transport properties. Spin pumping, a charge-free spin injection method together with ISHE spin detection successfully overcome the impedance mismatch problem and the intrinsic roadblocks imposed by electrical spin injection-based techniques and enabled lateral spin current transport in organic semiconductors to be detected electrically. The lateral spin diffusion length of up to a micrometre was observed in doped conjugated polymers in agreement with theoretical calculations based on exchange mediated spin diffusion model and parameters obtained from first principle. Moreover, this non-local spin transport device structure provides a platform for studying spin transport in a wide range of organic semiconductors where the spin current propagates along the high mobility direction and could potentially be used as building blocks for high performance flexible spintronics devices.

Thesis (Ph. D.)--University of Texas at Austin, 2006. / Vita. Includes bibliographical references.

Thesis (M. Phil.)--Hong Kong University of Science and Technology, 2003. / Includes bibliographical references (leaves 79-82). Also available in electronic version. Access restricted to campus users.

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