The Bilayer Polymer Light-Emitting Electrochemical Cells

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Birdee, Kiran
doping , light-emitting electrochemical cell , polymer interface , solid polymer electrolyte , ion transport , electroluminescence , p-n junction
Polymer light-emitting electrochemical cells (LECs) are solid-state polymer devices that form light-emitting p-n junctions in situ from electrochemical doping, which occurs under an applied voltage or current bias. LECs typically consist of a combination of a luminescent polymer, an electrolyte polymer, and a salt. This creates a mixed ionic/electronic conductor. When comparing the LECs to other polymer/organic light-emitting devices, the LEC is advantageous as it is not dependent on the electrode's work functions. However, a key LEC limitation is the phase separation among the luminescent polymer and the electrolyte-salt complex. One solution to the phase separation issue is to separate the LEC into two layers: the luminescent polymer layer and the electrolyte layer, creating a bilayer LEC. In my thesis, multiple studies were conducted to explore whether the thickness of the luminescent polymer layer changes the performance of the bilayer LEC, the development of a blue bilayer LEC, and examining a different bilayer LEC configuration of a luminescent polymer layer on top of a LEC layer. In the varying CP thickness study, the thinner CP layer allowed the ions to penetrate the total depth of the CP layer and commence uniform doping propagation. Meanwhile, in the thicker CP layer, the ions do not fully penetrate the CP layer which leads to a broadened light-emitting junction formation. The bilayer LECs have faster turn-on times than the single-layer LECs. In addition, the current and doping propagations in bilayer LECs demonstrated a linear dependence with varying operating voltages and an Arrhenius dependence with varying operating temperatures. These faster doping speeds imply that the long-range ion transport in the CP layer or crossing the CP/SPE interfacial energy barrier has a minimal effect to doping speed; rather, the limiting factor to the doping speed is the SPE bulk-resistance. These studies are important to understand the fundamental of bilayer LECs and potentially be the gateway to high-performances LECs, and other organic devices.
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