[Paper Review] Ion-exchange doped polymers at the degenerate limit: what limits conductivity at 100% doping efficiency?

Ian E. Jacobs, Gabriele D’Avino+25 more|arXiv (Cornell University)|Jan 5, 2021

Conducting polymers and applications4 citations

TL;DR

This study uses ion-exchange doping to achieve near-100% doping efficiency in high-mobility conjugated polymers, demonstrating that electrical conductivity at degenerate doping levels is primarily limited by paracrystalline disorder rather than counterion-induced charge traps. The work identifies maximizing crystalline order as critical for achieving high conductivities up to 1200 S/cm.

ABSTRACT

Doping of semiconducting polymers has seen a surge in research interest driven by emerging applications in sensing, bioelectronics and thermoelectrics. A recent breakthrough was a doping technique based on ion-exchange, which separates the redox and charge compensation steps of the doping process. The improved microstructural control this process allows enables us for the first time to systematically address a longstanding but still poorly understood question: what limits the electrical conductivity at high doping levels? Is it the formation of charge carrier traps in the Coulomb potentials of the counterions, or is it the structural disorder in the polymer lattice? Here, we apply ion-exchange doping to several classes of high mobility conjugated polymers and identify experimental conditions that achieve near 100% doping efficiency under degenerate conditions with nearly 1 charge per monomer. We demonstrate very high conductivities up to 1200 S/cm in semicrystalline polymer systems, and show that in this regime conductivity is poorly correlated with ionic size, but strongly correlated with paracrystalline disorder. This observation, backed by a detailed electronic structure model that incorporates ion-hole and hole-hole interactions and a carefully parameterized model of disorder, indicates that trapping by dopant ions is negligible, and that maximizing crystalline order is critical to improving conductivity.

Motivation & Objective

• To systematically investigate the fundamental limits on electrical conductivity in degenerately doped conjugated polymers.
• To resolve the long-standing debate on whether counterion-induced charge traps or structural disorder dominate conductivity suppression at high doping levels.
• To achieve near-100% doping efficiency using ion-exchange doping for precise control over doping and microstructure.
• To determine the relative roles of ionic size and lattice disorder in limiting conductivity in semicrystalline polymer systems.

Proposed method

• Employing ion-exchange doping to decouple redox and charge compensation steps, enabling precise control over dopant incorporation.
• Applying the technique to multiple classes of high-mobility conjugated polymers to ensure generalizability of findings.
• Measuring electrical conductivity across a range of doping levels approaching one charge per monomer (degenerate regime).
• Correlating conductivity with ionic size and structural disorder using X-ray scattering and electronic structure modeling.
• Using a parameterized model of disorder and incorporating ion-hole and hole-hole interactions to simulate electronic behavior.
• Analyzing the correlation between conductivity and paracrystalline disorder while isolating the influence of counterion size.

Experimental results

Research questions

• RQ1 What limits electrical conductivity in conjugated polymers at near-100% doping efficiency and degenerate conditions?
• RQ2 To what extent do counterion Coulomb potentials contribute to charge carrier trapping in highly doped polymers?
• RQ3 How does structural disorder in the polymer lattice affect conductivity at high doping levels?
• RQ4 Is there a strong correlation between ionic size and conductivity in degenerately doped conjugated polymers?
• RQ5 Can a combined electronic structure and disorder model accurately predict conductivity trends in doped semicrystalline polymers?

Key findings

• Ion-exchange doping achieves near-100% doping efficiency in semicrystalline conjugated polymers, enabling systematic study at degenerate doping levels.
• Conductivity reaches up to 1200 S/cm in high-mobility polymer systems under optimized doping and microstructural conditions.
• Conductivity shows poor correlation with counterion size, indicating that ion-induced trapping is not the dominant limitation.
• Conductivity exhibits a strong inverse correlation with paracrystalline disorder, highlighting structural order as the key factor in conductivity enhancement.
• Theoretical modeling incorporating ion-hole and hole-hole interactions confirms that charge trapping by counterions is negligible under degenerate doping.
• Maximizing crystalline order is identified as the primary pathway to achieving ultra-high conductivity in doped conjugated polymers.