Strong Doping and Electroluminescence Realized by Fast Ion Transport through a Planar Polymer/Polymer Interface in Bilayer Light-Emitting Electrochemical Cells

Abstract

Bilayer light-emitting electrochemical cells are demonstrated with a top conjugated polymer (CP) emitting layer and a solid polymer electrolyte (SPE) underlayer. Fast, long-range ion transport through the planar CP/SPE interface leads to doping and junction electroluminescence in the CP layer. All bilayer cells have pairs of aluminum electrodes separated by 2 or 11 mm at their inner edges, creating the largest planar (lateral) cells that can be imaged with excellent temporal and spatial resolutions. To understand how in situ electrochemical doping occurs in the CP layer without any ionic species mixed in, the planar bilayer cells are investigated for different CPs, CP layer thickness, operating voltage, and operating temperature. The bilayer cells are much faster to turn on than control cells made from a single mixed CP/SPE layer. The cell current and the doping propagation speed exhibit a linear dependence on the operating voltage and an Arrhenius-type temperature dependence. Unexpectedly, long-range ion transport in the CP layer and across the CP/SPE interface does not impede the doping reactions. Instead, the doping reactions are limited by the bulk resistance of the extra-wide SPE underlayer. In bilayer cells with a thin red-emitting CP layer, ion transport and doping reactions can penetrate the entire CP layer in the vertical direction. In thicker MEH-PPV or the blue-emitting cells, the doping did not reach the top of the CP layer. This led to broadened emitting junctions and/or unexpected junction locations. The bilayer LECs offer unique opportunities to investigate the ion transport in pristine CPs, the CP/SPE interface, and the SPE using highly sensitive and reliable imaging techniques. Removing the inert electrolyte polymer from the semiconducting CP can potentially lead to highperformance electrochemical light-emitting/photovoltaic cells or transistors.

Understanding of the Intrinsic Difference between n- and p-Doping Propagation in Polymer Light-Emitting Electrochemical Cells: A Numerical Modeling Approach

“…Anions exhibit an increased migration speed with a higher applied bias. Previous research has examined the temperature effect on p-doping speed, with the relationship characterized by the Arrhenius equation . Considering the presence of charge traps resulting from the interaction between anions and PEO segmental-grasped cations, we propose that anions experience scattering in their directed motion.…”

“…32 Below the melting point, a mixture of crystalline and amorphous phases coexists in high-molecularweight PEO, as demonstrated in the numerous studies of the kinetics and mechanism of crystallization, which are influenced by thermal history, polymer molecular weight, and other factors. 33 For PEO with a molecular weight of 100000, the melting point is approximately 340 K. 34 Hence, linear fitting was applied to the converted experiment data and can be divided into two segments with an inflection point occurring at 340 K. It should be noted that the data at 300 K were excluded from the fitting process as n-doping did not occur, as shown in Figure 2a. Figure 5b shows the linear fitting result for the voltage dependence of the n-doping speed based on eq 5.…”

Recent advances of organic emitters in deep‐red light‐emitting electrochemical cells

“…Polymers are the first category of organic active materials applied for LECs, based on which both the electroluminescence mechanisms [28] and operating modes [29] of LECs have been investigated. Pure polymer LECs showed unexpected performance early, which induced researchers to apply different design strategies, such as adjusting the structural units of the backbone [30] or modifying the side chain [31].…”

In Situ Surface-Enhanced Raman Spectroscopy on Organic Mixed Ionic-Electronic Conductors: Tracking Dynamic Doping in Light-Emitting Electrochemical Cells

“…In LECs, the active materials are heterogeneous blends or complex systems of electrically conductive conjugated polymers and solid polymeric electrolytes, where mobile ion carriers become free species during device operation. The diffusion of these ions is essential for various aspects of LEC performance, including device turn-on time and polymer doping. ,, Ion redistribution within LEC devices is influenced by factors such as ionic conductivity, , active material thickness, applied bias, and operating temperature. , These variables contribute to turn-on times, which span from milliseconds to hours, representing the duration required for the p- and n-doped regions to establish a p–n junction. ,, Two models, the electrodynamic model (ED) and the electrochemical doping model (ECD), have been proposed to explain the conduction mechanism in LECs when an external bias is applied. ,− In the ED model, the applied potential primarily drops over the electric double layers (EDLs) near the electrode interfaces, resulting in a weak electric field within the bulk polymer and dividing the active layer into three regions. In contrast, the ECD model suggests that the electric field drops over the EDLs only as much as needed to create ohmic contacts, establishing an efficient electric field that facilitates increased charge carrier injection into the active layer and leads to the oxidation/reduction of the conjugated polymer.…”

“… 1 , 14 , 15 Ion redistribution within LEC devices is influenced by factors such as ionic conductivity, 16 , 17 active material thickness, 18 applied bias, and operating temperature. 19 , 20 These variables contribute to turn-on times, which span from milliseconds to hours, representing the duration required for the p- and n-doped regions to establish a p–n junction. 14 , 21 , 22 Two models, the electrodynamic model (ED) and the electrochemical doping model (ECD), have been proposed to explain the conduction mechanism in LECs when an external bias is applied.…”