Polymer light-emitting electrochemical cells: Frozen-junction operation of an “ionic liquid” device

Shin, Joon‐Ho; Xiao, Steven; Fransson, Åke; Edman, Ludvig

doi:10.1063/1.1999009

Cited by 57 publications

(44 citation statements)

References 23 publications

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“…A second limiting disadvantage of PLECs has been the short operating lifetimes compared with those of PLEDs. [8,9] We report here the results of an initial study of light emission from a luminescent polymer blended with a dilute concentration of an ionic liquid. Even with an aluminum cathode, the devices turn on at low voltage (approximately equal to the bandgap of the luminescent semiconducting polymer).…”

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Long‐Lifetime Polymer Light‐Emitting Electrochemical Cells

2007

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Polymer light-emitting devices have been divided into two general types: polymer light-emitting diodes (PLEDs) and polymer light-emitting electrochemical cells (PLECs). [1][2][3][4][5] The advantages of PLEDs include a fast response and relatively long operating lifetime (with proper packaging). However, low-work-function cathodes and/or thin interfacial layers (e.g., LiF) between the metal and the emitting polymer layer are required. In contrast, PLECs have relatively low turn-on voltages (approximately equal to the bandgap of the luminescent semiconducting polymer), and low work-function metals are not required.One of the serious disadvantages of PLECs, however, is the slow response time (the time required for the mobile ions to diffuse during junction formation). A solution to this problem is to "freeze" the junction after ion redistribution. [6,7] A frozen junction system that operates at room temperature is necessary for practical use. A second limiting disadvantage of PLECs has been the short operating lifetimes compared with those of PLEDs. [8,9] We report here the results of an initial study of light emission from a luminescent polymer blended with a dilute concentration of an ionic liquid. Even with an aluminum cathode, the devices turn on at low voltage (approximately equal to the bandgap of the luminescent semiconducting polymer). These ionic-liquid-containing LECs were operated continuously in the glove-box (without packaging) for several days without significant degradation in brightness. After sealing with epoxy and a glass cover slide, the ionic-liquid-containing LECs were operated continuously in air for several weeks.The major difference between PLEDs and PLECs is that the latter possess mobile ions inside the polymer; therefore, the selection of the mobile ions is one of the keys to fabricating high-performance PLECs. Previously, the mobile-ion systems that have been used fall into three categories. The first is polyethylene oxide (PEO) containing Li salts. [2,3] Crown ethers (and derivatives) [9,10] and other organic salts [11,12] have also been used in combination with metal salts. Finally, polymers with ionic side chains (polyelectrolyte conjugated polymers) and appropriate mobile counterions have been used. [13] For almost all PLECs, the additives comprise at least 5 wt %. More importantly, these systems involve two-component phase separation with the emitting polymer in one phase and the mobile ions (e.g., dissolved in PEO) in a second phase. To create the p-type-intrinsic-n-type (p-i-n) junction of the LEC, ions must move from one phase into the other; for example, from the PEO into the luminescent polymer. This phase separation appears to degrade the device performance, especially the lifetime.[10] The phase separation results from the relatively poor compatibility of the ionic materials (hydrophilic) with the host light-emitting polymers (hydrophobic). In order to reduce the phase separation, surfactants or bifunctional additives were introduced into the emitting layer and better perform...

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Long‐Lifetime Polymer Light‐Emitting Electrochemical Cells

2007

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“…[3][4][5][6][7][8][9][10][11][12][13] One device that exploits this opportunity in an attractive manner is the light-emitting electrochemical cell (LEC). [14][15][16][17][18][19][20][21][22] The nominal difference between an LEC and an OLED is that the former contains mobile ions in the active material. [23][24][25][26][27][28][29][30] These ions rearrange during operation, which in turn allows for a range of attractive device properties, including low-voltage operation with thick active layers and stable electrode materials.…”

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The dynamic organic p–n junction

et al. 2009

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“…For this reason, several methods have been devised to stabilize the junction in the absence of bias, among them, "frozen" junctions, 6,9,10 where the p-i-n structure is formed at elevated temperature and then fixed at suppressed temperature, and additional polymer electrolyte/counterion layers near the interfaces 11 to fix the regions where dopants can be found. Another method, presented below, is to fix the p-i-n structure by covalently attaching the dopant ions-in the form of a self-assembled monolayer ͑SAM͒-to one of the interfaces.…”

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Fixed p-i-n junction polymer light-emitting electrochemical cells based on charged self-assembled monolayers

Simon

Stanislowski

Carter

2007

Applied Physics Letters

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The authors report on enhanced efficiency of polymer light-emitting electrochemical cells ͑LECs͒ by means of forming a n-doping self-assembled monolayer ͑SAM͒ at the cathode-polymer interface. The addition of the SAM, a silane-based salt with structural similarity to the commonly used LEC n-dopant tetra-n-butylammonium, caused a twofold increase in quantum efficiency. Photovoltaic analysis indicates that the SAM increases both the open-circuit voltage and short-circuit current. Current versus voltage data are presented which indicate that the SAM does not simply introduce an interfacial dipole layer, but rather provides a fixed doping region, and thus a more stable p-i-n structure. © 2007 American Institute of Physics. ͓DOI: 10.1063/1.2711769͔Polymer light-emitting electrochemical cells ͑LECs͒ have been given much attention in recent years as an enhancement over the simpler polymer light-emitting diode ͑LED͒. 1-3 These devices differ from the standard polymer LED structure 4,5 in that the active polymer layer is blended with "doping" counterions and a solid polymer electrolyte. When a bias is applied across the device, charge is created within the polymer by oxidation/reduction of the conjugated backbone. The counterions migrate toward the electrodes and stabilize ͑"dope"͒ the polymer charge, thereby creating p-and n-doped regions at the anode and cathode, respectively, and leaving a relatively undoped intrinsic region in the middle. The resulting p-i-n structure has the advantage over the standard polymer LED of higher conductivity at the doped interfaces, enabling a larger amount of charge to be injected and radiatively recombine in the central insulating region.The trade-off for this enhancement is that it takes time to move the ions and establish the p-i-n junction. [6][7][8] Furthermore, when the bias is removed, the ions forming the junction migrate away from the interfaces and back toward a neutral position within the bulk of the device. For this reason, several methods have been devised to stabilize the junction in the absence of bias, among them, "frozen" junctions, 6,9,10 where the p-i-n structure is formed at elevated temperature and then fixed at suppressed temperature, and additional polymer electrolyte/counterion layers near the interfaces 11 to fix the regions where dopants can be found.Another method, presented below, is to fix the p-i-n structure by covalently attaching the dopant ions-in the form of a self-assembled monolayer ͑SAM͒-to one of the interfaces. A similar method has been used to increase the efficiency of small-molecule organic LEDs, 12,13 but in these cases the effect is attributed to dipolar effects of the SAM and increased wettability of the modified substrate. The effect discussed below differs in that an interfacial dipole layer is insufficient to explain the data, leading to the conclusion that the SAM is electrochemically doping the polymer host.In this letter, we employ this SAM-based p-i-n structure using a LEC active layer ͑described below͒ sandwiched between a Ag anode an...

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Polymer light-emitting electrochemical cells: Frozen-junction operation of an “ionic liquid” device

Cited by 57 publications

References 23 publications

Long‐Lifetime Polymer Light‐Emitting Electrochemical Cells

Long‐Lifetime Polymer Light‐Emitting Electrochemical Cells

The dynamic organic p–n junction

Fixed p-i-n junction polymer light-emitting electrochemical cells based on charged self-assembled monolayers

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