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Abstract

Planar polymer light-emitting electrochemical cells (LECs) with concentric circle electrodes are demonstrated. With a fixed outer electrode radius of 6.8 mm, the inner electrode radius is varied from 1 to 3 mm. All cells can be activated with a 40 V bias at 360 K. The extremely large electrode gap size allows for time-lapsed imaging of the in situ electrochemical p- and n-doping processes. When the advancing p- and n-doping fronts meet, a light-emitting junction in the shape of a jagged ring is formed near the center of the electrode gap. Reversing bias polarity pushes the light-emitting junction outward to near the negative electrode. It is possible to achieve a perfectly centered emitting junction by fine tuning the inner electrode radius. The sensitivity of junction position to relative electrode radius indicates the p- and n-doping reactions are strongly coupled. The concentric circular electrode configuration offers a cross sectional view of an LEC-based light-emitting fiber. In any fiber shaped light-emitting device, the inner and outer electrodes always differ in size. This electrode asymmetry should be considered and exploited to achieve optimal cell performance.

Figure 1. Time-lapsed images of a PLEC with an inner electrode diameter of 2 mm. The cell was kept at 360 K and under 365 nm UV illumination. a) cellimage before the voltage bias was applied. Time elapsed since a 40 V bias was applied: b) 40, c) 60, d) 115, e) 200, and f) 480 sec, with UV illuminationturned off. g) schematic of the PLEC with concentric circular electrodes. Note the polymer layer covered the entire surface in finished cells. The outerelectrode has been cropped out of these images.