Voltage-controlled color change in polymer light-emitting diodes via thermochromism

Abstract

We report a reversible color change in polymer light-emitting diodes (PLEDs) fabricated with a single layer of poly [2-methoxy, 5-(2' ethyl-hexyloxy)-1, 4-phenylene vinylene] (MEH-PPV). When we varied the applied voltage from 10 to 19 V, the devices changed their emission color from an orange-red (similar to 590 nm) to a, greenish-yellow (similar to 560 nm). Moreover, this process was reversible. We attribute this to an operating voltage-induced thermochromism in the PLEDs. At higher voltages, the temperatures of the devices were elevated due to Joule heating, which altered the conformation of the polymers from rod-like to coil-like. This conformational change resulted in a lesser degree of planarity of the polymer backbone, thereby broadening the pi - pi* gap of the polymers.