The Design of Organic Polymers and Small Molecules to Improve the Efficiency of Excitonic Solar Cells
The harvesting of solar energy using photovoltaics has the potential to provide a significant portion of the world's energy. For this to happen, the cost per watt of power produced from photovoltaics must decrease. Excitonic solar cells, including organic solar cells and dye-sensitized solar cells, have the potential to provide the necessary cost savings. However, power conversion efficiencies must be improved before these devices can become practical. This dissertation describes the implementation of several strategies to improve the efficiency of organic polymer and dye-sensitized solar cells.
Chapter 1 provides an introduction to current research on excitonic solar cells with a focus on organic polymer and dye-sensitized cells. The detailed mechanisms of photocurrent generation in each type of cell are discussed, and the factors that determine efficiencies are outlined. In addition, an overview of the progress over the last decade in research on polymer photovoltaics is given. Finally, the future prospects for achieving high efficiency devices are described.
Chapter 2 details a strategy for controlling the morphology of photovoltaic blends of conjugated polymers and small molecule perylene diimide dyes. Blends of these two materials are subject to excessive phase separation that decreases photovoltaic performance. The development of compatibilizers that alleviate this phase separation is described. A diblock copolymer of poly(3-hexylthiophene) (P3HT) and a perylene diimide (PDI) side chain polymer is an effective compatibilizer. Addition of this material to blends of P3HT and PDI suppresses the formation of micron-sized crystals and results in a 50% improvement in solar cell efficiency. The synthesis of a diblock copolymer of poly(3-(4-octylphenyl)thiophene) and the PDI side chain polymer is also described. This material functions as a compatibilizer, but does not allow for improved photovoltaic efficiency.
Chapter 3 describes the synthesis and characterization of a low bandgap conjugated polymer with thermally removable solubilizing groups. Following solution-based deposition of thin films of this polymer, heat can induce the cleavage and evaporation of the alkyl solubilizing chains, resulting in an insoluble film. The optical properties change considerably during this process: the bandgap decreases, and the absorption coefficient increases dramatically. These properties were exploited to fabricate bilayers of the low bandgap material and a highly fluorescent commercial polymer. Fluorescence resonance energy transfer from the fluorescent material to the low bandgap polymer is efficient over 30 nm. Such a scheme could be utilized to overcome the exciton diffusion bottleneck in layered polymer solar cells.
In chapter 4, the synthesis of novel n-type polymers based on PDI monomers is described. With appropriate substitution of alkyl chains, highly rigid perylene benzimidazole ladder polymers can be made soluble in common organic solvents. These materials have exceptionally low-lying LUMOs and possess unusual fluorescence properties. Fully planar perylene ethynylene polymers are also synthesized and characterized. Despite their planarity, data from absorbance and fluorescence spectra suggest that the PDI units in these polymers are poorly coupled electronically. X-Ray diffraction shows that the π-stacking distance varies in these materials depending on the nature of the solubilizing groups and can be decreased through solvent annealing.
Chapter 5 describes how light harvesting can be improved in dye-sensitized solar cells by adding a second dye that transfers energy to the primary sensitizing dye. In liquid cells, incorporation of a PDI dye in the liquid electrolyte solution results in a 28% improvement in power conversion efficiency. Energy transfer is at least 50% efficient despite significant quenching of the perylene dye's fluorescence by the iodide redox couple. Efforts to employ energy transfer in solid-state dye-sensitized solar cells are also described. This is more challenging because the solid-state hole transporter is an excellent quencher of fluorescent dyes. However, it is shown that this quenching can be prevented by self-assembling dyes such as fluorescein 548 into a poly(propyleneimine) dendrimer. Unfortunately, processing conditions that allow the dendrimer/dye conjugates to penetrate the pores of the cell's titania films have not yet been found.