Earth-Abundant Sensitizers for Solar Energy Capture: Developing a Practical Computational Approach

This dissertation describes a computational approach to investigating sensitization capabilities of earth-abundant chromophores using a combination of density functional methods and quantum dynamics simulations of interfacial electron transfer (IET). Sensitization of large band gap semiconductors, such as TiO2, via molecular chromophores is fundamentally important for solar energy harvesting via dye-sensitized solar cells (DSSCs). The focus of the work is primarily on Fe(II) polypyridine complexes, as these are earth-abundant analogues of popularly utilized Ru(II) polypyridines which have near perfect quantum efficiency of IET into TiO2. Unlike the Ru(II) chromophores, the Fe(II) complexes are known to be significantly less efficient at IET due to a rapid intersystem crossing (ISC) cascade which quenches the initially-excited, photoactive metal-to-ligand charge-transfer (MLCT) states on a sub-picosecond timescale. As the biggest obstacle to successful use of Fe(II) polypyridines in DSSCs is the competition between the IET and ISC processes that occur at the same timescale, there are essentially two ways to improve the efficiency of Fe(II) sensitizers: either the rate of IET must be increased, or the rate of ISC must be decreased. Both of these approaches are explored in this dissertation.;In Chapter 1 the general overview of the electronic structure and photophysical properties of Fe(II) polypyridine chromophores are described and contrasted with their Ru(II) analogues. Chapter 2 describes our efforts to overcome the error in the computational determination of energy differences between states of unlike spin using hybrid functionals, which is shown to be systematic for a series of related complexes. The computational approach developed in Chapter 2 is utilized in Chapter 3 to compare relative ligand field strengths in a series of Fe(II) polypyridines with systematic donor atom and ligand scaffold modifications. Since the ligand field strength of these complexes can have pronounced effects on the timescale of the deactivation of the photoactive MLCT states via ISC, the ability to identify modifications that result in Fe(II) complexes with the strongest ligand field strength is important. The results obtained in this work suggest that the strongest ligand field is achieved when forming Fe--C bonds. Chapter 4 focuses on IET studies in Fe(II) dye-TiO2 nanoparticle systems and elucidates a unique band-selective sensitization phenomenon observed in [Fe(bipyridine-4,4'-dicarboxylic acid)(CN)2]-TiO2 assemblies. The impact of TiO2 anchoring groups on IET efficiency in [Fe(bipyridine-4,4'-anchor)(CN)2]-TiO 2 assemblies is investigated in Chapter 5. The hydroxamate anchoring group was found to lead to the most efficient IET and better photon-to-current conversion efficiencies in Fe(II) polypyridine sensitized solar cells. Finally, Chapter 6 describes criteria for efficient IET in dye-semiconductor assemblies and highlights screening principles to aid in rapid exploration of efficient Fe(II) sensitizers.