The dynamic bipolar electrode in polymer light-emitting electrochemical cells

The redox doping reactions in polymer light-emitting electrochemical cells (PLECs) have been analyzed to gain insight into a new type of bipolar electrode (BPE) formed within the cells. Planar (lateral) PLECs are constructed on a glass substrate with an exposed active polymer layer contacted by two aluminum (Al) driving electrodes at 11 mm apart. A horizontal array of 13 silver (Ag) microdiscus is deposited in-between the driving electrodes. When a 200 V bias is applied between the driving electrodes, the electrically floating Ag discs functioned as BPEs to induce redox p- and n-doping reactions in polymer layer that initially occur at the extremities of the BPEs. The doping reactions are visualized as strong photoluminescence quenching of the luminescent polymer. Over time, the doped regions grow in size, and the silver disc BPEs transform into dynamic, composite BPEs consisting of a p- and n-doped polymer conductor connected by the initial BPE disc. Unlike a homogenous BPE, the dynamic BPE is not a quasi-equipotential body under operation. Moreover, the n/p area ratios of all 13 BPEs in the array converge to a single value which is strongly affected by the composition of the PLEC layer. The n/p ratios ranging from 0.24 to 2.19 have been observed in PLECs made with Li, Na, and K triflate salts as well as two different luminescent polymers. The dynamic BPE array thus presents a novel essay to evaluate and screen the mixed luminescent conductor for device applications.