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

Abstract

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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