Reverse bias activation of salt-doped polymer light-emitting devices

Current Opinion in Electrochemistry

2018

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Polymer light-emitting electrochemical cells (PLECs) are electroluminescent devices whose operation involves both ionic and electronic charges. A key reaction in PLEC is the electrochemical doping of the light emitter, a luminescent polymer, which eventually leads to the formation of a light emitting p-n or p-i-n junction in the interior of the cell. This review opens with a general introduction to the operating mechanism of PLECs. This is followed by a summary of key advancements in the field in the last two years. The focus of the review is on the chemical mapping and optical probing of the PLEC doping process and junction structures. The results showcased in this review provide a coherent picture of the PLEC operating mechanism and point in new research directions in both device applications and fundamental sciences of mixed ionic/electronic polymer conductors.

Section: Plecs With Bipolar Electrodesmentioning

confidence: 99%

Polymer light-emitting electrochemical cells—Recent advances and future trends

Current Opinion in Electrochemistry

2018

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“…Surprisingly, the ITO/MEH‐PPV:LiTf/Al cells can be activated with a large voltage bias to emit under either a forward or a reverse bias. The activated cells displayed strong evidence of doping and a photovoltaic response that was consistent with the formation of a doping‐induced junction . More importantly, the activated state was stable at room temperature.…”

Section: Freezing or Chemically Fixing The Plec Junctionmentioning

confidence: 99%

Strategies toward Long‐Life Light‐Emitting Electrochemical Cells

2017

ChemPlusChem

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This Minireview examines the operational lifetime of light‐emitting electrochemical cells (LECs). Under continuous operation, both polymer‐based LECs (PLECs) and ionic transition‐metal complex (iTMC)‐based LECs (iTMC‐LECs) now exhibit a luminance half‐life exceeding 1000 h. This improved performance was accomplished with several effective strategies aimed at optimizing the operating scheme, the material composition, and the device architecture. These strategies are presented in detail with PLECs as an example. iTMC‐LECs are also highlighted owing to their excellent stress stability with regards to both luminance and operating voltage. The survey of literature data points to clear trends, as well as some unexpected results in LECs stressed for an extended period. Major challenges still exist, but long‐lasting LECs are possible when the proven strategies are combined with innovative materials and device design.

“…19−21 Another exception is polymer LECs containing a molecular salt such as the lithium triflate without any electrolyte polymer. 22,23 The salt-doped cells require a high voltage to activate. Recent reviews discuss efficacies of various LEC electrolytes and strategies to circumvent dedicated electrolytes in LECs.…”

Section: ■ Introductionmentioning

confidence: 99%

“…To improve the phase morphologies of the LEC composite, researchers have employed thermal annealing, a surfactant-like additive, crown-ether, or ionic-liquid-based electrolytes, , and ultimately, bifunctional polymers that possess both ionically and electronically conductive moieties. − These innovations led to some success, but fundamentally, most LECs still contained an immobile, nonemitting material that does not directly contribute to the light emission processes. A notable exception is the ionic transition-metal complex (iTMC)-based LECs, , although high-performance iTMC LECs often contain added ionic species to compensate for the poor ionic conductivities of iTMCs. − Another exception is polymer LECs containing a molecular salt such as the lithium triflate without any electrolyte polymer. , The salt-doped cells require a high voltage to activate. Recent reviews discuss efficacies of various LEC electrolytes and strategies to circumvent dedicated electrolytes in LECs. , …”

Section: Introductionmentioning

confidence: 99%

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

Birdee

ACS Appl. Mater. Interfaces

2020

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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.