Photoelectrochemical and photophysical studies of carbon nanotube and molybdenum disulfide based light harvesting devices

There is a critical need in utilizing solar radiation as a renewable energy source. While photovoltaic solar cells are widely used, much attention has been devoted in the past decade to developing nanotechnology for potential cost reduction and improved device efficiency and reliability. Low-dimensional materials offer unique physical properties which may be exploited for solar energy harvesting and conversion. Understanding their fundamental properties and developing relevant manufacturing strategies will thus pave the road toward high-performance, cost-effective, light-harvesting devices. This thesis has investigated single-wall carbon nanotubes (SWCNTs) and molybdenum disulfide (MoS2) nanolayers for their light-harvesting ability in donor-acceptor systems. These materials were studied with three specific goals: (i) introducing innovative light-harvesting designs, (ii) understanding their fundamental photophysical and photoelectrochemical properties, and (iii) providing potential solutions to improve the system performance. First, novel light-harvesting complexes were designed using semiconducting SWCNTs and cationic porphyrins as acceptors and donors, respectively. These complexes were assembled by synthetic DNA oligonucleotides that recognize porphyrins, while noncovalently functionalizing SWCNTs. The SWCNT-DNA-porphyrin hybrids were used to manufacture large-area thin films through solution-phase processing and membrane filtration methods. From extensive studies of optical absorption, emission, and photocurrents, new detailed insights on photo-processes were gained for photoelectrochemical conversion. A regenerative donor-acceptor light-harvesting system was introduced and demonstrated to counteract photoinduced degradation of porphyrin molecules. The photo-damaged chromophores were dissociated from the complex by modulating the chemical environment, while DNA-SWCNTs were preserved. When fresh porphyrins were reintroduced and reassociated with DNA-SWCNTs, photocurrents were fully recovered. As proof-of-principle, A 50% increase in photocurrents was demonstrated through four successive regenerations within 90 minutes, compared to the complex without regeneration. Such dynamic strategy could improve the overall device efficiency and extend the operation lifetime. Lastly, a novel solution-phase manufacturing process was developed to fabricate large-area two-dimensional MoS2 nanolayers for light harvesting applications. The MoS2 nanolayers were functionalized with 8 porphyrin species from 3 families to mitigate charge recombination by defects and small crystallites. A strong correlation between porphyrin species and photocurrents was observed, where interfacial porphyrins suppress charge recombination within MoS2 nanolayers, thus enhancing the photoelectrochemical performance of the devices. A photocurrent enhancement mechanism was proposed based on the energy difference between the valence band of MoS2 and highest occupied molecular orbital level of porphyrins. Overall, the innovative designs and the scientific insights on photophysics and photoelectrochemical conversion in this thesis will form the basis for developing next-generation solar energy harvesting devices