| dc.description.abstract | Global emissions need to decline steeply towards 2020, according to the International Energy Agency (IEA). As stated by the IEA, solar photovoltaics accounted for 27% of the overall renewable energy growth in 2017. Organic solar cells are a more environmentally friendly and low-cost alternative to traditional alternatives such as inorganic silicone solar cells. The bulk-heterojunction (BHJ) solar technology offer advantages such as flexibility, semitransparancy, low manufacturing cost, easy fabrication and shorter energy payback time. The traditional fullerene acceptors employed in BHJ organic solar cells are hampered by invariable energy levels, limited absorption in the visible spectrum, difficulties with purity and morphology and high production cost. Non-fullerene acceptors have thus been extensively studied in order to offer a wider pool of acceptors capable of blending well with existing donors, easy synthesis and purification, high solubility and improved optical properties.
The research group's extensive work with phenothiazine derivates for dye-sensitized solar cells, coupled with the emerging BHJ non-fullerene acceptor technology, lead to the aim of this master's thesis, which was the synthesis and characterization of four target molecules, T1-T4. This study describes the synthesis of building blocks 9a, 9b, 12a and 12b and the subsequent Knoevenagel condensation attempts in order to form the target molecules.
The synthetic routes were either three, four or six steps. The dibrominated phenothiazine building blocks 3 and 6 were prepared by a two-step synthesis in an overall yield of 47% and 67% respectively. The compounds were borylated, followed by Suzuki cross-coupling to give 12a and 12b in 45% and 60% yield respectively, calculated over two steps. Target molecule T2 was formed by Knoevenagel condensation with INCN according to NMR and HRMS analyses, but was not isolated. Target molecule T4 was not synthesized. An alternative target molecule T6 was synthesized by Knoevenagel condensation of 12a and malononitrile in 26% yield. The overall yield of this six-step synthesis was 6%.
Compounds 3 and 4 were formylated in 62% and 36% yields respectively. Target molecule T1 was synthesized by Knoevenagel condensation of 9a and INCN according to NMR and HRMS analyses, but was not isolated. Target molecule T3 was synthesized by condensation of 9b and INCN in 26% yield. The overall yield of this three-step synthesis was 7%. An alternative target molecule T5 was synthesized by Knoevenagel condensation of 12a and malononitrile in 42% yield. The overall yield of this three-step synthesis was 18%.
New products and intermediates were fully characterized by NMR, HRMS, IR and melting point when applicable. The optical and electrochemical properties of acceptors T3, T5 and T6 were evaluated. Acceptor T3 was found to have an absorption maximum at 495 nm and emission maximum at 629 nm. The optical bandgap was calculated to be 2.08 eV, and the HOMO and LUMO energy levels were calculated to be 5.58 eV and 3.50 respectively. Acceptor T5 was found to have an absorption maximum at 497 nm and emission maximum at 698 nm. The optical bandgap was calculated to be 2.10 eV, while the HOMO and LUMO energy levels were calculated to be 5.74 eV and 3.64 respectively. Acceptor T6 was found to have an absorption maximum at 506 nm and emission maximum at 652 nm. The optical bandgap was calculated to be 2.10 eV, while the HOMO and LUMO energy levels were calculated to be 5.77 eV and 3.67 respectively. The absence of a reduction peak in the voltammograms of T3, T5 and T6 suggested poor electron deficient character. | en |
