Pioneering the fabrication and characterization of organic solar cell devices at Mount Holyoke College
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A solar cell is a device that converts light energy into electrical energy. Solar cells hold plenty of potential in becoming a more widely available source of electricity that makes economic sense. However, improvements in the materials and fabrication are necessary to increase accessibility and power conversion efficiency. Organic, or carbon-based materials, pose an interesting alternative to traditional silicon-based solar cells. An organic solar cell (OSC) composed of thin films deposited onto a glass or flexible polymer substrate could be the future of economically viable solar energy. In this thesis, we describe the fabrication of a type of organic solar cell, the polymer:fullerene bulk-heterojunction solar cells. This is the first attempt to fabricate a device in the Solar Cell Fabrication and Characterization Laboratory. We have replicated the OSC fabrication process outlined by the Yang Group (Shrotriya et al., 2006) with a goal of demonstrating a properly functioning solar cell competitive with state of the art devices. Fabrication of the OSC begins with deposition of an organic, hole-conducting film poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS). Next, the active layer composed of poly(3-hexylthiophene-2,5-diyl) (polythiophene) and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM, a fullerene derivative), is mixed in a 1:1 ratio and deposited from solution as one film. These two organic thin-films were deposited by spin-casting solutions of different concentration at different spin speeds to achieve varying thicknesses. Lastly, thermal evaporation was used to deposit the calcium and silver metal cathode. The thickness of various layers in the OSC sandwich structure was measured with an atomic force microscope. Film uniformity was determined with an optical microscope. The I-V characteristics were measured with a parameter analyzer. Because the thickness is a critical parameter in the optimization of device efficiency, we have determined the relationship between thickness and spin speed of the organic thin films. Our results indicate that we have successfully achieved, for the first time, on par with recent results in the literature, a fully functioning solar cell with I-V characteristics reaching those of the Yang group.