Long‐Lifetime Polymer Light‐Emitting Electrochemical Cells

Abstract

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...

Electron injection into organic semiconductor devices from high work function cathodes

“…2b). Similar dependence of electron-injection barriers on time response can be seen in LECs (36,37).…”

On-demand photochemical stabilization of doping in light-emitting electrochemical cells

“…[50][51][52][53] In chemically stabilized LECs, with no added initiator compound, the cross-linking of the ions and/or ion-transport material is presumably initiated by the electrons/holes on the conjugated polymer. [39] This has the undesired consequence that the conjugation, i.e.…”

Hybrid organic-inorganic light-emitting electrochemical cells using fluorescent polymer and ionic liquid blend as an active layer

“…Therefore, the HyPLECs exhibit extremely low turn-on, and high luminance efficiency at low operating voltage without matching the work functions between each electrode and active layer. 23,24 The device performances of FTO/ZnO/SY:ILMs blend/Au device with different ILMs content and FTO/ SY:ILMs blend ͑25 wt %͒/Au device are presented in Fig. 2.…”