Synthesis and characterization of n-type organic semiconductor materials for optoelectronic devices
File(s)
Author(s)
Hu, Xiantao
Type
Thesis
Abstract
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.
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.
Version
Open Access
Date Issued
2023-01
Date Awarded
2023-03
Copyright Statement
Creative Commons Attribution NonCommercial Licence
License URL
Advisor
Heeney, Martin
Publisher Department
Chemistry
Publisher Institution
Imperial College London
Qualification Level
Doctoral
Qualification Name
Doctor of Philosophy (PhD)