Imaging the degradation of polymer light-emitting devices

Dane, Justin; Gao, Jun

doi:10.1063/1.1810213

Cited by 31 publications

(20 citation statements)

References 33 publications

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“…[24,26,28,[33][34][35][36][37] For each active material (X = Li, K, Rb), we recorded 4-5 independent sets, each containing approximately 300 photographs, of the entire p-n junction formation process. [38] By analyzing each set of photographs with a custom program in Matlab (The MathWorks), we were able to calculate the position of the p-type doping front as a function of time during device turn-on.…”

Section: Resultsmentioning

confidence: 99%

Polymer Light‐Emitting Electrochemical Cells: Doping Concentration, Emission‐Zone Position, and Turn‐On Time

Shin

Robinson

Xiao³

et al. 2007

Adv Funct Materials

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Direct optical probing of the doping progression and simultaneous recording of the current–time behavior allows the establishment of the position of the light‐emitting p–n junction, the doping concentrations in the p‐ and n‐type regions, and the turn‐on time for a number of planar light‐emitting electrochemical cells (LECs) with a 1 mm interelectrode gap. The position of the p–n junction in such LECs with Au electrodes contacting an active material mixture of poly(2‐methoxy‐5‐(2′‐ethylhexyloxy)‐p‐phenylene vinylene) (MEH‐PPV), poly(ethylene oxide), and a XCF3SO3 salt (X = Li, K, Rb) is dependent on the salt selection: for X = Li the p–n junction is positioned very close to the negative electrode, while for X = K, Rb it is significantly more centered in the interelectrode gap. Its is demonstrated that this results from that the p‐type doping concentration is independent of salt selection at ca. 2 × 1020 cm–3 (ca. 0.1 dopants/MEH‐PPV repeat unit), while the n‐type doping concentration exhibits a strong dependence: for X = K it is ca. 5 × 1020 cm–3 (ca. 0.2 dopants/repeat unit), for X = Rb it is ca. 9 × 1020 cm–3 (ca. 0.4 dopants/repeat unit), and for X = Li it is ca. 3 × 1021 cm–3 (ca. 1 dopants/repeat unit). Finally, it is shown that X = K, Rb devices exhibit significantly faster turn‐on times than X = Li devices, which is a consequence of a higher ionic conductivity in the former devices.

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Section: Resultsmentioning

confidence: 99%

Polymer Light‐Emitting Electrochemical Cells: Doping Concentration, Emission‐Zone Position, and Turn‐On Time

Shin

Robinson

Xiao³

et al. 2007

Adv Funct Materials

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“…LECs operating within the frozen-junction regime have also demonstrated fast turn-on times and relatively long light-emission lifetimes, [23,[27][28][29][30] but some anomalous experimental results, notably a low rectification ratio in conjunction with unipolar light emission and/or a significantly lower stability of the current compared to the light emission, have also been reported. [27,30,31] In this article, we employ a planar surface cell geometry (for a schematic of the structure see the bottom part of Fig. 1) to show that, for certain combinations of active material and electrode, the part of the proposed operational mechanism that assumes the formation of a continuous light-emitting p-i-n junction needs to be revised.…”

Section: Introductionmentioning

confidence: 99%

Polymer Light‐Emitting Electrochemical Cells: The Formation and Effects of Doping‐Induced Micro Shorts

Shin

Xiao²,

Edman

2006

Adv Funct Materials

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A planar surface cell structure has been utilized to investigate a polymer light‐emitting electrochemical cell (LEC), consisting of an active‐material mixture of poly[2‐methoxy‐5‐(2′‐ethylhexyloxy)‐1,4‐phenylenevinylene] (MEH‐PPV) and an ionic liquid, tetra‐n‐butylammonium trifluoromethanesulfonate, contacted by Au electrodes. It is shown that diffuse and needle‐shaped doping fronts originate from the electrodes (p‐type from the positive electrode and n‐type from the negative electrode) during the charging process at a temperature (T) of 393 K. After a turn‐on time, a significant portion of the doping fronts makes contact close to the negative electrode to form a light‐emitting p–i–n junction, but at some points p‐type doping protrudes all the way to the negative electrode to form what are effectively micro shorts. It is shown that the consequences of such doping‐induced micro shorts during a subsequent stabilized “frozen junction” operation at T = 200 K are drastic: the current‐rectification ratio (RR) is low and the quantum efficiency (QE) is far from optimum. Both RR and QE increase significantly during long‐term operation under frozen‐junction conditions, demonstrating that the lifetime of the doping‐induced micro shorts is limited even under such stabilized conditions. Still, it is clear that it is critical for the optimization and further development of LECs to find new types of active material and electrode combinations that allow for formation of a continuous p–i–n junction in the center of the interelectrode gap.

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“…[24][25][26][27] Shin et al further developed this concept when they demonstrated that it is possible to turn on wide-gap devices at low voltages. 28 In this article, we employ this type of planar device to demonstrate that doping front propagation ͑or turn-on time͒ in LECs is limited by ion motion in the undoped region, and consequently that a significant electric field is located in this undoped region between the two doping fronts during the turn-on process.…”

Section: Introductionmentioning

confidence: 99%

Doping front propagation in light-emitting electrochemical cells

et al. 2006

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Observations of the expansion of the p-and n-doped regions in planar ͑lateral͒ polymer light-emitting electrochemical cells with large ͑mm-sized͒ interelectrode gaps driven with 5 to 50 V have inspired a model describing the doping front propagation. We find that the propagation is limited by the movement of ions in the electronically insulating region between the p-and n-doped regions. Two consequences of an ion-limited front propagation are that the doping fronts accelerate as they approach each other, and that fingerlike protrusions are formed, particularly at larger drive voltages.

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Imaging the degradation of polymer light-emitting devices

Cited by 31 publications

References 33 publications

Polymer Light‐Emitting Electrochemical Cells: Doping Concentration, Emission‐Zone Position, and Turn‐On Time

Polymer Light‐Emitting Electrochemical Cells: Doping Concentration, Emission‐Zone Position, and Turn‐On Time

Polymer Light‐Emitting Electrochemical Cells: The Formation and Effects of Doping‐Induced Micro Shorts

Doping front propagation in light-emitting electrochemical cells

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