Extrinsic n‑doping of a double B←N bridged bipyridine-based polymer containing oligoethylene glycol side chains

This research is led by Jian Liu (Key Laboratory of Polymer Science and Technology, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, China) and Jun Liu (Key Laboratory of Polymer Science and Technology, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, China).

Conjugated polymers, renowned for their flexibility, low cost, and large area, can be used to fabricate organic electronic devices through solution processing, and are widely applied in organic light-emitting diodes (OLEDs), organic photovoltaics (OPVs), organic field-effect transistors (OFETs), organic thermoelectrics (OTEs), and organic electrochemical transistors (OECTs), etc. Most conjugated polymers are intrinsically non-conductive due to insufficient charge carriers. Doping is indispensable for conjugated polymers to improve charge carriers, including but not limited to molecular doping or electrochemical doping. Conjugated polymers containing polar oligoethylene glycol side chains are emerging organic mixed ionic-electronic conductors (OMIECs). Such OMIEC polymers have received significant attention as a channel layer in research fields such as OTEs and OECTs. P-type and n-type OMIEC materials with similar performances are essential for building efficient organic electronic devices, such as thermal power generators and organic complementary circuits. However, n-type OMIECs must still catch up with p-type regarding

material diversity and performance. For OTEs, the performances of the organic materials are reflected by the power factor (PF = S²σ, where S and σ are the Seebeck coefficient and electrical conductivity, respectively). For OECTs, the properties of the channel materials are captured by the figure of merit μC*, where μ and C* are charge mobility and volumetric capacitance, respectively. To improve n-type OMIEC materials, many research efforts have been made to adjust the energetics or improve backbone planarity by innovating electron-deficient building blocks. Examples include naphthalene diimide (NDI), bithiophene imide (BTI), diketopyrrolopyrrole (DPP), isoindigo (IID), benzodifurandione-based oligo (p-phenylenevinylene) (BDOPV), and double B←N bridged bipyridine (BNBP), etc. The packing structure of backbone chains determines the generation and transport pathway of charge carriers in extrinsically doped polymer films. Additionally, side chains

initially employed to enhance solubility play significant roles in doping. For instance, Liu et al. modified n-type conjugated polymers using oligoethylene glycol side chains, enhancing the miscibility between the host and dopant as well as the doping efficiency. Furthermore, customizing amphipathic side chains can help regulate counterion electrostatics and balance electronic and ionic charge transport. A double B←N bridged bipyridine building block is an excellent electron-deficient unit for constructing acceptor-acceptor (AA) type conjugated polymers. Previous reported n-type conjugated polymers incorporating BNBP units were hydrophobic and typically used in un-doped electronic devices. The efficient n-doping of these BNBP-based polymers requires long-time soaking in specific dopant vapor. Moreover, the mixed conducting properties of BNBP-based n-type polymers have not yet been studied.

In this contribution, the authors successfully, for the first time, endowed the BNBP-based conjugated polymer with mixed conduction by incorporating oligoethylene glycol side chains. They functionalized the thieno[3,4-c]pyrrole-4,6-dione dimer (BTPD) with glycol side chains and copolymerized this unit with BNBP to obtain PBN-OEG. The contrast material produced through the copolymerization of the BNBP unit and the alkyl chain functionalized TPD unit is referred to as PBN-alkyl. Due to the improved host/dopant miscibility of the polar side chains, doped PBN-OEG films exhibited higher doping levels than the doped PBN-alkyl films. Thus, PBN-OEG achieves a higher σ of 1.95 S cm⁻¹ and PF of 4.7 μW m⁻¹ K⁻². Moreover, the oligoethylene glycol side chains promote the swelling of the PBN-OEG film in the aqueous electrolyte and facilitate ionic mobility of hydrated cations. As a result, PBNOEG enables a large volumetric capacitance C* of 97.7 F cm⁻³

and a higher μC* of 2.6 F cm⁻¹ V⁻¹ s⁻¹ for OECT devices. These results demonstrate the great potential of n-type BNBP-based OMIEC materials for OTEs and OECTs.

See the article:

Extrinsic n‑Doping of a Double B←N Bridged Bipyridine-Based Polymer Containing Oligoethylene Glycol Side Chains

https://doi.org/10.1021/polymscitech.5c00042