The effect of annealing/quenching on the performance of polymer light-emitting electrochemical cells

We report the effect of thermal annealing and quenching on the film morphology and device performance of polymer light-emitting electrochemical cells (LECs). The polymer films of LECs consist of a luminescent polymer, an ion-conducting polymer, and a lithium salt. The LECs studied have an extremely large planar configuration, which enables time-resolved fluorescence imaging of both doping and emission profiles of the devices. Annealing at temperatures above 350 K leads to the disappearance of many visible "white dots" initially present in the LEC film, and a much smoother surface. Annealed and quenched devices exhibit dramatically improved initial and peak current, peak electroluminescence (EL) intensity, doping propagation speed and response time. In addition, the emission zone of annealed devices is more centered than un-annealed devices. These improvements are attributed to the melting of electrolyte domains in the LEC film, which leads to better film quality and enhanced ion conductivity. Our results demonstrate that the simple annealing/quenching technique can be used to achieve the desired phase morphology in LEC films, which are often severely phase-separated due to incompatibility between the luminescent polymer and the electrolyte polymer. (C) 2007 Elsevier B.V. All rights reserved.