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Pinpointing the Dynamic p-i-n Junction

Joan Ràfols-Ribé1,*, Atsushi Sato2, Anton Kirch1, Xiaoying Zhang1, Sandra Jenatsch3, Christian Larsen1, Kazuhiro Marumoto2, and Ludvig Edman1,4,†

  • 1Organic Photonics and Electronics Group, Department of Physics, Umeå University, SE-90187 Umeå, Sweden
  • 2Department of Materials Science, University of Tsukuba, Tsukuba, Ibaraki, 305-8573, Japan
  • 3Fluxim AG, Katharina-Sulzer-Platz 2, 8400 Winterthur, Switzerland
  • 4Wallenberg Initiative Materials Science for Sustainability, Department of Physics, Umeå University, SE-90187 Umeå, Sweden

  • *Contact author: joan.rafols-ribe@umu.se
  • †Contact author: ludvig.edman@umu.se
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PRX Energy 4, 033015 – Published 11 September, 2025

DOI: https://doi.org/10.1103/2vyr-4yp3

Abstract

The p-i-n junction structure that develops dynamically under an applied bias via electrochemical doping (ECD) is of key importance for the performance of light-emitting electrochemical cells (LECs). While the complex electronic and ionic processes that govern its transient formation have been extensively studied by both experiments and drift-diffusion modeling, less attention has been given to the steady-state junction as a function of voltage. Here we study the formed p-i-n structure of a polymer LEC at the steady state by measuring and analyzing its most distinctive feature: the current density–voltage-luminance characteristics. Unexpectedly, we find that the effective conductance of the p-i-n structure exhibits a positive correlation with the applied bias, a behavior not predicted by existing LEC drift-diffusion models. We attribute this discrepancy to the assumption in these models of a constant density of mobile ions. Hence, we present a modified model in which the ECD level—represented by the number of ions with which the organic semiconductor is doped—scales with the applied voltage, implying a voltage-dependent doping efficiency. We validate this hypothesis using electron spin resonance spectroscopy and drift-diffusion modeling, while additionally establishing that only a small fraction of the available ions in our system, which increases from 1% to 3% with increasing bias, contributes to the ECD and is necessary for efficient LEC operation. These findings not only provide fundamental insights into the operational mechanism of LECs but also have direct implications for the broader organic mixed ionic and electronic conductor community.

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Popular Summary

Light-emitting electrochemical cells (LECs) are thin-film devices consisting of an organic mixed ionic electronic conductor (OMIEC) material sandwiched between two electrodes. Their OMIEC character grants them a distinct operating mechanism: a p−i−n junction that forms dynamically when voltage is applied in a process called electrochemical doping (ECD). While the transient formation of this junction is well studied, less attention has been paid to how the applied voltage shapes the p−i−n structure in the steady state. This paper addresses this gap and uncovers findings that challenge the existing physical model, simply by looking at the p−i−n junction's current-voltage-luminance (j−V−L) characteristics curve.

Using j−V−L measurements of a polymer LEC, the authors find that the dynamic p−i−n junction rearranges into a more conductive state proportional to the applied voltage, an effect not predicted by current models. The authors attribute this effect to a voltage-dependent density of dissociated and mobile ions. A combination of electron spin resonance spectroscopy and an adapted numerical drift-diffusion model are in line with this hypothesis. Additionally, the authors show that only a small fraction of the total available ions in the film contribute to the ECD process that drives device operation.

These results have significant implications for understanding ion transport in LECs. Further studies across different OMIEC systems may lead to improved models for ion transport, potentially improving their performance in future applications.

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PRX Energy 4, 033015– Published 11 September, 2025
  • Received 26 February 2025
  • Revised 18 June 2025
  • Accepted 19 August 2025
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