Synthesis and characterization of n-type organic semiconductor materials for optoelectronic devices
Creators: name: Hu, Xiantao
Publisher: name: Imperial College London
Year: 2023

description: Organic electronics focuses on developing next-generation electronic devices, including organic photovoltaics, organic field-effect transistors and organic photodetectors, which rely heavily on the development of organic semiconductor materials, owing to their unique advantages like mechanical flexibility and stretchability, low processing temperature, low fabrication cost. This thesis focuses on the synthesis, characterization and application of novel organic semiconductor materials. Benzothiadiazole, as one of the most widely used electronwithdrawing units, has attracted great interest and extensive effort in the design of smallmolecular or polymerized semiconductor materials. Most of the approaches to tune the energetics of benzothiadiazole focuses on changing the heteroatom in the thiadiazole ring or functionalizing the 5 and/or 6 position of the benzo ring. Here in my work, we annulated an additional ring (2-(1,3-dithiol-2-ylidene)malonitrile, DTYM) in the 4,5-positions of the benzo ring for the first time, and coupled with indacenodithiophene core to get two small-molecular non-fullerene acceptors (BTSCN-IDT, FBTSCN-IDT). The photovoltaic performance of both acceptors was evaluated in organic photovoltaics when blended with donor polymer PM6, and the fluorinated acceptor FBTSCN-IDT delivered a comparatively better power conversion efficiency of 13.7% under dim light condition, which can be ascribed to the more red-shifted absorption and more coplanar molecular backbone. In addition, we annulated the same additional ring to the 5,6-positions of the benzothiadiazole to allow polymerization through 4,7-positions with four different donor units and received four polymer semiconductors (P1- P4). The effect of introducing DTYM group on benzothiadiazole was evaluated and found that the incorporation of DTYM group leads to a more twisted backbone but an enhanced dipole moment. As a result, all polymers presented n-type semiconductor behaviour in organic fieldeffect transistors, with the polymer P3 showing the highest saturation electron mobility up to 5.2 × 10-3 cm2 V-1 s-1 among them. However, most of the reported organic semiconductor materials have long conjugated molecular backbone which may induce chemical instability, low solvent solubility and high fabrication cost from the complicated synthesis. On this basis, two fully non-fused nonfullerene acceptors (BTIC-1, BTIC-2) were synthesized and applied in organic photovoltaics and photodetectors. Both acceptors share the same backbone but have the same alkoxy group on different positions to form varying number of intramolecular conformational locks to tune the molecular structure. Theoretical calculation and optical spectrum indicate that BTIC-2 presents a more planar conformation and thus a more red-shifted absorption up to 900 nm. As a result, BTIC-2 outperforms BTIC-1 in photodetector performance, with a low dark current of 2.4 × 10-7 A cm-2, leading to a high responsivity of 0.357 A W-1 and a specific detectivity of 1.7 × 1011 Jones at 828 nm under the reverse bias of -2 V. Lastly, we are also interested in developing novel fused-ring building blocks for constructing semiconductor materials. Two novel core molecules were designed and synthesized by bridging benzothiadiazole unit and adjacent thiophene units using bridging atom oxygen instead of the commonly used nitrogen atom, as oxygen is more electronegative than nitrogen which may lead to lower-lying energy levels and thus high open-circuit voltage.