Carrier Screening Controls Transport in Conjugated Polymers at High Doping Concentrations

Area of Science:

• Materials Science
• Condensed Matter Physics
• Polymer Science

Background:

• Doped conjugated polymers (CPs) exhibit localized charge carriers due to low dielectric permittivity, enhancing Coulomb interactions.
• Existing Gaussian disorder models (GDM) often neglect the crucial role of carrier screening in dielectric interactions.

Purpose of the Study:

• To incorporate carrier screening within the Debye-Hückel formalism into GDM calculations for doped CPs.
• To investigate the impact of screening on dopant-induced energetic disorder and the density of states (DOS).
• To analyze the effects of screening on charge transport properties like conductivity and Seebeck coefficient.

Main Methods:

• Implementing carrier screening using Debye-Hückel theory to modify the Gaussian DOS.
• Solving the Pauli master equation with Miller-Abrahams hopping rates on the screened DOS.
• Comparing calculated transport properties (conductivity, Seebeck coefficient) with experimental measurements.

Main Results:

• Carrier screening significantly alters the DOS shape, impacting charge transport, especially at high doping concentrations.
• The previously assumed universal slope of Seebeck coefficient vs. conductivity is shown to be affected by screening, decreasing with smaller dopant-carrier distances.
• Neglecting screening leads to an underestimation of the thermoelectric power factor by approximately a factor of 10 at higher doping levels.

Conclusions:

• Carrier screening is essential for accurately modeling transport properties in doped conjugated polymers.
• Screening effectively curtails dopant-induced energetic disorder, particularly at high carrier concentrations.
• The study provides a more accurate theoretical framework for understanding and optimizing thermoelectric materials based on conjugated polymers.