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1.3 PHYSICAL SCIENCES
Modern society has created big challenges in the area of sustainable supply of energy to satisfy the needs of growing population and to account for depleting fossil fuel resources. The Irish Government has set targets for the energy sector by 2020, with 33% of electricity to be generated from renewable sources. Organic photovoltaic devices offer several advantages over expensive silicon solar cells, including deposition of ultra-thin films by spin-coating, printing and spray-coating. This in turn provides for the exciting possibility to make lightweight, flexible solar cells for a broad range of existing and emerging applications for security, military and medicine. This research project was inspired by the current drive into finding alternative technologies and materials for the design and manufacture of advanced solar cells. The primary objective was to tailor the properties of Poly3Hexylthiophene: PhenylC60 Buturic Acid Methyl Esther composite (P3HT:PCBM) thin films for flexible organic solar cells performance. The extensive experimental work was conducted to reveal the effect of the solar irradiation and thermal annealing on the dielectric, optical and electrical properties of P3HT:PCBM thin films. A common degradation pattern was demonstrated in the films after UV exposure whereby the optical absorbance and the resistivity were shown to be inversely proportionate. These two correlating techniques showed similar patterns after exposure. It was also shown that annealing the structure after deposition increased the absorbance in the thin film and the quantum efficiency of the final prototype device was related to film morphology. The dielectric properties of these films were studied using a novel microwave spectroscopy technique and it is believed to be the first report on the application of this novel technique to photovoltaic materials characterisation. To examine the dielectric properties of the P3HT:PCBM films using microwave spectroscopy, two types of Electro Magnetic (EM) wave sensors were fabricated, one on a Rogers substrate with Cu patterns and a second on a flexible substrate with Ag patterns. Both types of EM sensors exhibited shifts in resonant peak frequencies and amplitude during exposure to solar irradiation. All other experimental parameters and environmental conditions were kept constant. Therefore it is reasonable to conclude that the proposed method of microwave spectroscopy is a reliable tool to trace the changes in the properties of the materials caused by solar irradiation. The optical properties of the P3HT:PCBM films displayed a decrease in absorbance after 40mins solar simulator irradiation and then an increase in absorbance from 40 min to 20hrs. The electrical properties of P3HT:PCBM films showed a resistance decrease as the films were illuminated by a solar simulator from 0 to 40 min, and a subsequent increase in resistance up to 20hrs. In addition, a bespoke solar cell on flexible Polyethylene terephthalate (PET) was constructed and tested. It exhibited a fill factor and an efficiency of 0.3238 and 0.49% respectively. Although the performance is poor compared to reported state of the art for organic solar cells, the work demonstrates that operational devices can be manufactured under non-optimised laboratory conditions.
Arshak, R. (2013.) Manufacture and Investigation of Organic Composite Polymer Based Films for Advanced Flexible Solar Cells. Masters Thesis. Dublin Institute of Technology. doi:10.21427/D7GC8C