What's next for semiconducting polymers

Abstract

A decade has passed since a special issue on polymer electronics appeared in the Journal of Polymer Science in 2012. The field of semiconducting polymers has witnessed many breakthroughs. For instance, the record power conversion efficiency (PCE) for polymer solar cells was 9.2% in 2011, reported by Mitsubishi Chemical Corp. Michael McGehee, an expert in polymer cells, had wished that “the door is still open for a technology that gives you 15%, is cheap and uses abundant materials.”1 By 2021, not only 15% efficiency has been achieved, but also the field has moved from polymer-fullerene solar cells to nonfullerene acceptors. The latest development based on PM6 semiconducting polymers and Y6 nonfullerene acceptors offers PCEs of 18% for single-layered solar cells.2 Similarly, with the advancement of polymer solar cells, polymer-based field-effect transistors, thermoelectrics, and electrochromics all have made a significant stride towards technology development and commercialization. In addition, the field has also experienced the renaissance of mixed ionic and electronic conductors over the past few years. Thus, it motivates us to organize the second special issue on semiconducting polymers and invite active researchers to share their perspectives and research progress in this field. In this special issue, it contains 20 contributions, ranging from polymer design and synthesis, modeling, characterization, processing, and applications. Polymer design and synthesis are the major driving force and the foundation for plastic electronics. Noh et al. provide an overview of recent progress in lactam-based polymer semiconductors for organic electronic devices. Lactams are a key moiety in many leading electron acceptors in donor-acceptor semiconducting polymers, such as diketopyrrolo[3,4-c]pyrrole (DPP), naphthalene diimide (NDI), isoindigo (IID), 2,2-bithiophene-3,3-dicarboximide (BTI), and thieno[3,4-c]pyrrole-4,6-dione (TPD). The authors surveyed the literature and concluded that there had been no remarkable progress on the development of new lactam-based polymers in the past 3 years, and DPP-based polymers still dominate the field. They also point out that batch-to-batch variation, scalability, and processability are critical issues the community has to address in order to advance polymer electronics. A review on direct arylation polymerization (DArP) by Thompson et al. partially addresses the polymer scalability issue. By circumventing monomer, functionalization with toxic transmetallation reagents such as organostannane and organoboron required for Stille-Migita and Suzuki-Miyaura polymerization methods, DArP proceeds through a metal-catalyzed C-H activation pathway for the preparation of high-performance semiconducting polymers. This review highlighted recent advances in developing more sustainable first-row transition metal catalysts for DArP. It emphasizes the need to pursue the next generation of catalytic design to enable a more effective and environmentally friendly synthesis of semiconducting polymers. Several examples of molecular design of semiconducting polymers that highlight side-chain engineering are presented in this special issue. Rondeau-Gagné et al. report the effect of incorporating polyamidoamine (PAMAM) dendritic side chains to DPP-based semicrystalline polymers on their optoelectronic, thermomechanical, and solid-state properties. Pei et al. utilize mixed aliphatic and oligo(ethylene glycol) side chains to modulate the aggregation behaviors of 3,7-bis((E)-7-fluoro-1-(2-octyldodecyl)-2-oxoindolin-3-ylidene)-3,7-dihydrobenzo[1,2-b:4,5-b’]difuran-2,6-dione (FBDPPV) and study the impact of side chain ratios on doping efficiency. Savoie et al. perform the first dedicated coarse-grained molecular dynamics study of mixed side chains. Their simulations recapitulate the nonlinear progression of the morphology from an interfacially gated electrolyte when large fractions of hydrophobic side-chains are incorporated to an e