Bisisoindigo–Benzothiadiazole Copolymers: Materials for Ambipolar and n-Channel OTFTs with Low Threshold Voltages

Abstract

The development of design strategies in both ambipolar and electron-conducting organic thin-film transistor (OTFT) materials is important for producing high-performance materials and devices in organic electronics. Isoindigo donor− acceptor polymers have been well studied as hole-conducting materials in OTFTs, while more electron-deficient, acceptor-rich isoindigo polymers have been underexplored. In this report, two common design strategies in isoindigo-based polymers, acceptor− acceptor polymers and core-expanded isoindigo structures, are combined to create copolymers of bisisoindigo, a core-expanded derivative of isoindigo, and electron-deficient benzothiadiazoles. These polymers exhibit the low energy lowest unoccupied molecular orbitals (LUMOs) required for electron transport and show ambipolar OTFT performance with hole and electron mobilities up to 4.0 × 10 −3 and 1.4 × 10 −3 cm 2 V −1 s −1 , respectively. Additionally, changing the source and drain contacts from Au to LiF/Al significantly lowered the threshold voltage due to improved alignment of the polymer and electrode energy levels. Comparisons of the optoelectronic properties between these polymers and previously reported bisisoindigo donor−acceptor polymers indicate that the increased acceptor strength lowers the frontier molecular orbital energies, while the dithienyl-benzothiadiazole unit impacts the film morphology by altering the shape of the polymer backbone.

Functionalized Benzothiadiazole Non‐Fused A−D−A’−D−A Small Molecules for Effective Electron Mobilities and Metal‐free Photocatalysis

“…[20] Kelly and co-workers reported polymer of A 1 -DÀ A 2 -D configuration based on bisisoindigo and BT via thiophene spacer, that served as hole conducting materials in an organic thin-film transistor (OTFT) with hole and electron mobilities up to 4.0 × 10 À 3 cm 2 V À 1 s À 1 and 1.4 × 10 À 3 cm 2 V À 1 s À 1 respectively. [7] However, to our knowledge, AÀ DÀ A'À DÀ A structures based on acceptors such as BT and hemi-isoindigo, and donors such as thiophene and bithiophene with appreciable charge transport properties and photocatalytic activities are not yet reported.…”

Recent Structural Engineering of Polymer Semiconductors Incorporating Hydrogen Bonds

“…When F atoms are introduced into dithienyl‐benzothiadiazole units, the intramolecular S···F interaction improves the planarity of the P65 skeleton. In addition, the source and drain electrode materials were optimized from Au to LiF/Al, which significantly lowers the threshold voltage to 5 ± 2 V. [ 134 ]…”

Recent progress in lactam‐based polymer semiconductors for organic electronic devices

“…Kelly et al designed two A 1 -D-A 2 -D copolymers, P268 and P269. 121 P268 and P269 have lowlying LUMO levels and exhibit ambipolar transport characteristics. The electron mobility of P269 was slightly higher than that of P268.…”