Computational optimization of dyes for dye-sensitized solar cells

At present, the development of renewable and sustainable energy sources along with efficient storing strategies are key to future prosperity. Solar energy conversion technologies certainly have a large potential to become the primary energy source. Particularly, the production of solar fuels and chemicals through employing photoelectrochemical (PEC) water-splitting is considered a highly promising option. The challenge is still to make the solar energy captured, converted, and stored in a cost-effective fashion. In Chapter 1 we discuss the problems of fossil fuels as a primary energy source and solar energy as an alternative source of energy. We discuss the turning points in the development of solar energy conversion technologies that lead to photoelectrochemical devices. We introduce the assembly and operational principle of dye-sensitized photoelectrochemical cells, and the idea of a modular approach with optimizing individual components of the system is a pathway to improve the overall performance. In Chapter 2 we briefly introduced the basic principles of the computational chemistry methods used throughout the thesis. That includes: density functional theory (DFT) and its approximate method density functional based tight-binding (DFTB); time-dependent density functional theory (TD-DFT) and two types of approximate methods time-dependent density functional based tight-binding (TD-DFTB) and simplified time-dependent density functional theory (sTD-DFT); GW approximation and implicit solvation models. In Chapter 3 we introduce the perylene molecular family, suitable for the role of photo-sensitizer in DS-PECs. We review the process of automated chemical functionalization using the newly developed compound attachment tool (CAT). Generating a large number of unique molecular derivatives allows us to screen the versatile optical and electrochemical properties. In Chapter 4 we take the first step into the characterization of the generated derivatives. We assessed the reliability of low-cost screening methods: TD-DFTB and sTD-DFT, for determining the absorption properties of the molecules. The sTD-DFT, parameterized on TD-DFT(CAM-B3LYP) showed a satisfying accuracy/cost ratio and it was used in screening. The subsequent analysis provided 1400 derivatives that possess intense, lowest excitation in the energy range from 1.35 eV to 3.2 eV. We propose a set of dyes that can be suitable for panchromatic sensitization of the photoelectrode in DS-PECs. In Chapter 5 we further narrow the search for optimal dyes including the thermodynamic requirements. We investigate the accuracy of different theoretical approaches when compared to experimentally measured ground state oxidation potentials (GSOPs). We test how different methods and procedures perform under vertical and adiabatic approaches. We found for a given set of dyes, that it is crucial to take into account solvent effects as well as the structural relaxation of the dye after oxidation. Other thermodynamic contributions are negligible. We were able to select a set of dyes that fit the system’s thermodynamic requirements, in particular those that have a suitable GSOP, and to estimate the excited state oxidation potentials (ESOPs). In conclusion, we have built a reliable, automated computational chemistry procedure that includes structure generation and examination of optical and electrochemical properties. This computational research offers greater insight for optimizing the dye molecule and assists in further DS-PEC development. In addition, this research might serve as a small database for perylene-based organic sensitizes and inspire novel experimental chemistry avenues while saving resources.