Cyano-Functionalized Bithiophene Imide-Based n-Type Polymer Semiconductors: Synthesis, Structure–Property Correlations, and Thermoelectric Performance

Abstract

n-Type polymers with deep-positioned lowest unoccupied molecular orbital (LUMO) energy levels are essential for enabling n-type organic thin-film transistors (OTFTs) with high stability and n-type organic thermoelectrics (OTEs) with high doping efficiency and promising thermoelectric performance. Bithiophene imide (BTI) and its derivatives have been demonstrated as promising acceptor units for constructing high-performance n-type polymers. However, the electron-rich thiophene moiety in BTI leads to elevated LUMOs for the resultant polymers and hence limits their n-type performance and intrinsic stability. Herein, we addressed this issue by introducing strong electron-withdrawing cyano functionality on BTI and its derivatives. We have successfully overcome the synthetic challenges and developed a series of novel acceptor building blocks, CNI, CNTI, and CNDTI, which show substantially higher electron deficiencies than does BTI. On the basis of these novel building blocks, acceptor–acceptor type homopolymers and copolymers were successfully synthesized and featured greatly suppressed LUMOs (−3.64 to −4.11 eV) versus that (−3.48 eV) of the control polymer PBTI. Their deep-positioned LUMOs resulted in improved stability in OTFTs and more efficient n-doping in OTEs for the corresponding polymers with a highest electrical conductivity of 23.3 S cm–1 and a power factor of ∼10 μW m–1 K–2. The conductivity and power factor are among the highest values reported for solution-processed molecularly n-doped polymers. The new CNI, CNTI, and CNDTI offer a remarkable platform for constructing n-type polymers, and this study demonstrates that cyano-functionalization of BTI is a very effective strategy for developing polymers with deep-lying LUMOs for high-performance n-type organic electronic devices.

n-Type organic semiconducting polymers: stability limitations, design considerations and applications

“…89,128 Cyanation is an effective way to deepen the LUMO level of OSCs, and in this case a series of cyanated P(BTimR) derivatives were demonstrated to effectively increase the EA from 3.47 to 3.78 eV, though this was still not fully effective to sufficiently improve ambient stability and did not increase m e,max (Table 6). 125…”

Incorporation of Cyano‐Substituted Aromatic Blocks into Naphthalene Diimide‐Based Copolymers: Toward Unipolar n‐Channel Field‐Effect Transistors

“…[ 29 ] Guo et al also synthesized n‐type polymeric semiconductors by adopting cyano‐functionalized strategy. [ 30 ] These results demonstrate that n‐type semiconductors would be achieved via this method. To further explore unipolar n‐type polymeric semiconductors and provide guidance for the development of this field, we synthesized a series of cyano‐substituted monomers of 3,3′‐dicyano‐2,2′‐bithiophene (BTCN), ( E )‐1,2‐bis(3‐cyanothiophene‐2‐yl)ethene (TVTCN), and ( E )‐1,2‐bis(3‐cyanoselenophene‐2‐yl)ethene (SVSCN).…”

Recent Progress on Addressing the Key Challenges in Organic Thermoelectrics

“…The polymer also exhibited a high PF value of 57.3 μW m −1 K −2 , which is among the highest in solution‐processed n‐doped conjugated polymers. For n‐type OTE materials, lowering LUMO energy level is an effective strategy, which has been widely employed recently [24] …”