Tuning effective hyperfine fields in PEDOT:PSS thin films by doping

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Tuning effective hyperfine fields in PEDOT:PSS thin films by doping

M. Y. Teferi, J. Ogle, G. Joshi, H. Malissa, S. Jamali, D. L. Baird, J. M. Lupton, L. Whittaker Brooks, and C. Boehme

Phys. Rev. B 98, 241201(R) – Published 5 December 2018

Abstract

Using electrically detected magnetic resonance spectroscopy, we demonstrate that doping the conducting polymer poly(3,4-ethylenedioxythiophene):poly(styrene-sulfonate) (PEDOT:PSS) with ethylene glycol allows for the control of effective local charge carrier hyperfine fields through motional narrowing. These results suggest that doping of organic semiconductors could enable the tuning of macroscopic material properties dependent on hyperfine fields such as magnetoresistance, magneto-optical responses, and spin diffusion.

Received 17 April 2018

DOI:https://doi.org/10.1103/PhysRevB.98.241201

Physics Subject Headings (PhySH)

Spin relaxation

Organic semiconductors

Electron paramagnetic resonance Electron spin resonance

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

M. Y. Teferi¹, J. Ogle², G. Joshi¹, H. Malissa¹, S. Jamali¹, D. L. Baird¹, J. M. Lupton^1,3, L. Whittaker Brooks², and C. Boehme^1,*

¹Department of Physics and Astronomy, University of Utah, Salt Lake City, Utah 84112, USA
²Department of Chemistry, University of Utah, Salt Lake City, Utah 84112, USA
³Institut für Experimentelle und Angewandte Physik, Universität Regensburg, Universitätsstrasse 31, 93053 Regensburg, Germany

^*boehme@physics.utah.edu

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Issue

Vol. 98, Iss. 24 — 15 December 2018

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Images

Figure 1
Illustrations of spin randomization of the charge carrier's spin state while the carrier is undergoing hopping transitions through localized paramagnetic charge carrier states. The green arrows represent local random hyperfine field directions at the different hopping sites while the blue and black arrows represent the carrier's spin states for the times when the charge transitions into and out of the localized states, respectively. The dashed curved arrow indicates the hopping. (a) shows the case of slow hoping with the effective distribution of nuclear magnetic fields $p (B)$ sketched on the right. When fast hopping occurs (b), the charge carrier spin state precesses only marginally before the next hopping transition occurs, implying that the distributions of the effective hyperfine fields acting on the charge carrier spins are considerably narrower, i.e., motionally narrowed.
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Figure 2
Electrical characterization of the PEDOT:PSS thin-film devices for different EG doping concentrations. (a) Plot of the $I - V$ characteristics at 5 K for three different EG concentrations. The observed functions are non-Ohmic, yet overall the current still increases with doping concentration (cf. Ref. [26] for the full set of $I - V$ curves). (b) Plot of the low-temperature charge carrier mobilities that were obtained by fits of Mott-Gurney's law to the devices' low-temperature $I - V$ characteristics, as a function of the applied EG concentration.
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Figure 3
(a) Results of magnetic field modulated, lock-in detected, cw-EDMR measurements of PEDOT:PSS films with 0.125% EG doping at $T = 5$ K under application of various excitation frequencies between 100 MHz and 17 GHz. (b) Plot of two normalized spectra measured at $f = 0.4$ GHz for 0% (undoped) and 0.125% EG-doped PEDOT:PSS samples. A derivative double-Gaussian function is fitted to the data to establish the linewidth for each carrier species in the pair. (c) Line-shape analysis as a function of applied magnetic field, showing plots of the widths $σ_{1}$ and $σ_{2}$ of the two charge carrier states in PEDOT:PSS. These plots are based on fit results of the measurement series displayed in (a), acquired for all doping concentrations studied. The arrows indicate increasing EG concentration. The gray areas indicate uncertainty domains for the respective curves.
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Figure 4
(a) Plot of hyperfine field distribution width vs. EG concentration for the broad and narrow Gaussian lines of the EDMR spectrum. (b) Plot of normalized charge carrier spin echoes vs. interpulse delay for three different EG concentrations. The curves are vertically offset for clarity. Solid lines show the fit. Inset: The coherence time $T_{2}$ vs. EG concentration.
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