Synthesis & characterization of panchromatic assemblies using CuAAC: developing light harvesting systems towards photovoltaic applications

Abstract

The pure versatility and ease of CuAAC based layer-by-layer fabrication of the two different panchromatic films implicates the potential for this technique to be used to create light harvesting arrays for molecular based solar cells. The versatility of this technique allows other dye systems to be studied. The simple process can easily be scaled to larger surface area depositions for global scale production.

Humankind's insatiable appetite for energy has led to many significant inventions from fire and watermills to combustion engines and nuclear power plants. With our rapidly expanding population looms an impending energy crisis, which is arguably the most significant scientific and technical challenge to face humankind. Society must find a solution to increasing energy demands for a peaceful and sustainable future. Solar energy may be the only renewable energy source that will provide enough energy globally, while simultaneously cutting down on carbon-emissions for a cleaner planet. Improving photovoltaic devices is essential for solar energy to be a viable and successful as an energy source on the global scale. Thus research and development of photovoltaic technologies is necessary to reduce costs and increase efficiency in order for solar to be cost effective. Dye-sensitized solar cells (DSSCs) are a next generation photovoltaic technology with the potential to realize these goals as a low cost, efficient alternative to the current silicon-based solar cells.

Herein, I describe efforts towards developing light-harvesting arrays of chromophores assembled onto oxide surfaces using a molecular multilayer layer-by-layer technique assisted by copper(I)-catalyzed azide-alkyne cycloaddition (CuAAC). This process has the potential to develop efficient panchromatic assemblies while maintaining low material and fabrication costs. Utilizing the simple, rapid, and versatile CuAAC reaction in sequential, self-limiting steps, multilayers of pigments are generated with desired composition and thickness. I have investigated two types of panchromatic assemblies containing films of perylene diimides (PDIs) and films of spectrally complementary porphyrins and boron-dipyrromethenes (BODIPY). Several characterization techniques were used to probe the properties of these films like UV-Visible spectroscopy, fluorescence spectroscopy, infrared spectroscopy (IR), surface electrochemical techniques, and X-ray reflectivity (XRR). These characterizations showed that the desired CuAAC reactions occurred with excellent linearity, exhibiting the control of this technique.

The successful incorporation of three different colored PDIs onto the same substrate demonstrates the versatility of the CuAAC reaction. Fairly synthetically simple, these PDIs were easily incorporated into the same film and the resulting film is the summation of the three individual components and absorbs strongly across the visible spectrum.

Energy transfer (EnT) was also investigated for the porphyrin-BODIPY systems. CuAAC linkage provides a non-invasive and straightforward method to create porphyrin-BODIPY arrays. Solution studies in tetrahydrofuran (THF) show that EnT occur in excess of 95% efficiency. Furthermore, mixed multilayer films were created with varying amount of BODIPY to achieve tunable doping of BODIPY within the porphyrin films. EnT is also demonstrated within these multilayer films for three levels of doping.