Research Paper: Pinpointing the Dynamic p-i-n in a polymer-based light-emitting electrochemical cell (LEC)
Summary
This study investigates the steady-state p-i-n junction in a polymer-based light-emitting electrochemical cell (LEC) to understand how it changes with applied bias voltage (Vbias). The main goal was to reconcile a significant discrepancy between experimental observations and predictions from established drift-diffusion models. Key findings reveal that, contrary to model predictions, the device's effective conductance increases with Vbias. The study attributes this to an electrochemical doping (ECD) level that scales positively with the applied voltage. This hypothesis was validated by in-operando electron spin resonance (ESR) spectroscopy, which also showed that only a small fraction (1–3%) of the available ions participate in the doping process. The study concludes that the assumption of a constant number of mobile ions in LEC models is flawed and proposes a new model where the number of active ions is voltage-dependent.
Why it matters
These findings provide fundamental insights into the operational mechanism of LECs and the broader field of organic mixed ionic and electronic conductors (OMIECs). The discovery of a voltage-dependent doping efficiency and the fact that only a small fraction of ions are necessary for efficient operation challenge long-held assumptions in the field. This has direct implications for device optimization, suggesting that future improvements should focus not just on salt concentration but on enhancing ion dissociation and mobility to gain better control over the dynamic p-i-n junction. The research also highlights the need for more sophisticated ion-transport models for OMIEC devices.
How Fluxim tools used
The study utilized the drift-diffusion package within the software Setfos to simulate the optoelectronic characteristics of the LEC.
• Steady-State & Transient Simulations: Setfos was used to solve the continuity, current drift-diffusion, and Poisson equations to model both the steady-state and transient electrical behavior of the device.
• Model Comparison: Researchers first used a standard model with a constant ion density (ρconst). When this failed to match experimental data, they implemented a modified model (ρ(Vbias)) where the ion density was phenomenologically scaled with voltage, which successfully reproduced the experimental trends.
• j-V Snapshots & Optical Modeling: Setfos was employed to simulate "frozen" snapshots of the p-i-n junction by locking the ion profiles in place during a simulated fast I-V scan. The electrical simulation output was then coupled with Setfos's transfer-matrix optical model to calculate luminance and current efficacy, accounting for light propagation through the device layers. The use of Setfos was crucial for demonstrating the limitations of the existing model and validating the new hypothesis regarding voltage-dependent doping.
Publication details
Ràfols-Ribé, J., Sato, A., Kirch, A., Zhang, X., Jenatsch, S., Larsen, C., Marumoto, K. and Edman, L. (2025), Pinpointing the Dynamic p-i-n Junction. Published 11 September 2025. DOI: 10.1103/2vyr-4yp3.