One potential solution to the world’s expanding energy needs is the harnessing of solar energy—an inexhaustible energy source. In part because of the relatively low efficiency, high cost, and short durability of solar cells, only 2% of energy in the US presently comes from solar. Thin film polymer solar cells offer the potential of making solar energy more affordable.[2- 5] However, one of the challenges of polymer solar cells is the limited absorption range. Certain conditions lead to a red shift in absorption offering the possibility of increased light absorption, but the effect is not fully understood. In order to understand what causes a red shift we must study morphology. The morphology of polymer chains refers to their form and structure. Two aspects of morphology are chain conformation and aggregation. Chain conformation refers to the structural arrangement of the chains and aggregation refers to direct mutual attraction of the molecules. The morphology of polymer chains in solution depends on the solvent used and the polymer concentration [6,7] and has a great influence on the conjugation lengths of the chains which in turn has a great influence on absorption. [6,8] Longer conjugation lengths cause the absorption spectrum to red shift. [6,7,9,10] Because of these effects, understanding solvent effects on absorption could make polymer solar cells more efficient. A popular polymer used in solar cells is MEH-PPV [Poly[2-methoxy-5-(2-ethylhexyloxy)-1,4- phenylenevinylene and in particular, the morphology of MEH-PPV chains in solution has a great influence on absorption [6,7,11]. This study will investigate the absorption spectrum of MEH-PPV in solution both experimentally and theoretically.
Over the last decade, conjugated polymer-based semiconductors have been developed as a novel class of photovoltaic materials that have the potential to lower costs. Solvent based polymers MEH-PPV, MDMO-PPV, P3HT, and P3OT have been reported as electron donors in photovoltaic devices. In this research, we studied the use of a water soluble polythiophene - Sodium poly[2-(3-thienyl)-ethoxy-4-butylsulfonate]) [PTEBS] in photovoltaic devices. Solar cells in the configuration of bilayer heterojunctions with TiO<sub>2</sub> were prepared. The water-soluble polythiophene showed significant photovoltaic effect and potential for use in solar cells. The use of this polymer would allow safe, environmentally friendly processing. In addition, due to the covalent bonding of the counterion to the polymer backbone chain simultaneously with electron loss in the doping and oxidation, the water-soluble polymer PTEBS can be self-doped by acids. The appearance and absorption spectra of the self-doped solutions and films have also been investigated. New absorption bands in the ultraviolet and infrared have been observed after acidic doping offering the possibility of improved light harvesting. Experimental results have shown that the polymer can be used as the active layer in photovoltaic applications. These photovoltaic devices had an energy conversion efficiency of 0.23% and a fill factor of 0.41 under the illumination of an 80 mW/cm<sup>2</sup> solar simulator. A simple mechanism has also been proposed to fit the open circuit voltage found in the devices.