Synthesis and characterisation of boron dipyrromethenes for solar energy applications

Abstract

Solar energy is a vital part in the continued effort to fight climate change, and dyesensitised solar cells (DSCs) offer a low-cost alternative to the conventional silicon based solar cells. Tandem DSCs have emerged as a possible method of producing high efficiency solar cells at a low cost. These devices require the inclusion of dyes with intense absorption properties over 600 nm in order to harvest as much light as possible. As such, this thesis focuses on the synthesis and characterisation of novel BODIPY dyes for solar cell applications. Chapter 1 is a literature overview of the current state of DSCs. This focuses on how dye design can affect the overall performance of p-type DSCs. Chapter 2 overviews some of the fundamental science discussed in the thesis. It also overviews the experimental techniques used, and synthetic protocols. Chapter 3 discusses the synthesis of six compounds with fluorine-based acceptor groups. A trifluorophenyl acceptor or the more unique pentafluorosulfanyl acceptor. The photophysics, electrochemistry, crystal structure and calculated orbitals for the dyes are discussed, as well as their relation to both the dye structure and integration in DSCs. One problem identified with the use of bathochromically shifted dyes with extended π systems is excessive stacking on the semiconductor surface. This reduces device efficiency. The dyes 1, 3, 5, and 6 all exhibited solid state emission. This property suggests that there would be reduced stacking of the dye on the thin film. This was achieved via the addition of a pentafluorosulfanyl group, or addition of a trimethylsilyl acetylene on the boron which also caused dimerisation. The dimer displayed emission only in the solid state but quenched fluorescence in solution due to exciton. Chapter 4 discussed the synthesis and characterisation of three dyes, all with absorbances over 640 nm, with a pentafluorosulfanyl group as an acceptor. The synthetic strategy to produce these dyes was Knoevenagel condensation, this provided both low temperature and palladium free synthesis. By investigating the solvatochromism and utilising TD-DFT none of the dyes displayed any push pull character with either the pentafluorosulfanyl group in the α or meso position. However, RJ8 was successfully tested in p-type and tandem DSCs, which achieved 0.107 % and 0.149 %, respectively. Chapter 5 discusses the synthesis and characterisation of a BODIPY-POM triad, RJ14 system and the photophysics and electrochemistry involved. However, the short excited-state lifetime of the BODIPY donor means that PET would be inefficient. Therefore, a more rigid structure with a longer excited state lifetime is required for the development of a BODIPY-POM to be used in a DSC. The second aspect was the synthesis of a BODIPY with an anchoring group to couple to a POM. Chapter 6 discusses the overall thesis and potential future developments in dye sensitised solar cells.