Transport properties of doped conjugated polymers (CPs) have been widely analyzed with the
Gaussian Disorder Model (GDM) in conjunction with hopping transport between localized states.
These models reveal that even in highly doped CPs, a majority of carriers are still localized because
dielectric permittivity of CPs is well below that of inorganic materials, making Coulomb interactions
between carriers and dopant counter-ions much more pronounced. However, previous studies within
the GDM did not consider the role of screening the dielectric interactions by carriers. Here we
implement carrier screening in the Debye-H¨uckel formalism in our calculations of dopant-induced
energetic disorder, which modifies the Gaussian density of states (DOS). Then we solve the Pauli
Master Equation using Miller-Abrahams hopping rates with states from the resulting screened DOS
to obtain conductivity and Seebeck coefficient across a broad range of carrier concentrations and
compare them to measurements. Our results show that screening has significant impact on the
shape of the DOS and consequently on carrier transport, particularly at high doping. We prove
that the slope of Seebeck coefficient vs electric conductivity, which was previously thought to be
universal, is impacted by screening and decreases for systems with small dopant-carrier separation,
explaining our measurements. We also show that thermoelectric power factor is underestimated by
a factor of ∼ 10 at higher doping concentrations if screening is neglected. We conclude that carrier
screening plays a crucial role in curtailing dopant-induced energetic disorder, particularly at high
carrier concentrations.
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Raising Dielectric Permittivity Mitigates Dopant‐Induced Disorder in Conjugated Polymers
Conjugated polymers need to be doped to increase charge carrier density and reach the electrical conductivity necessary for electronic and energy applications. While doping increases carrier density, Coulomb interactions between the dopant molecules and the localized carriers are poorly screened, causing broadening and a heavy tail in the electronic density‐of‐states (DOS). The authors examine the effects of dopant‐induced disorder on two complimentary charge transport properties of semiconducting polymers, the Seebeck coefficient and electrical conductivity, and demonstrate a way to mitigate them. Their simulations, based on a modified Gaussian disorder model with Miller‐Abrahams hopping rates, show that dopant‐induced broadening of the DOS negatively impacts the Seebeck coefficient versus electrical conductivity trade‐off curve. Increasing the dielectric permittivity of the polymer mitigates dopant‐carrier Coulomb interactions and improves charge transport, evidenced by simultaneous increases in conductivity and the Seebeck coefficient. They verified this increase experimentally in iodine‐doped P3HT and P3HT blended with barium titanate (BaTiO3) nanoparticles. The addition of 2% w/w BaTiO3nanoparticles increased conductivity and Seebeck across a broad range of doping, resulting in a fourfold increase in power factor. Thus, these results show a promising path forward to reduce the dopant‐charge carrier Coulomb interactions and mitigate their adverse impact on charge transport.
more » « less- NSF-PAR ID:
- 10366891
- Publisher / Repository:
- Wiley Blackwell (John Wiley & Sons)
- Date Published:
- Journal Name:
- Advanced Science
- Volume:
- 8
- Issue:
- 19
- ISSN:
- 2198-3844
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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