Visualizing the Bipolar Electrochemistry of Electrochemically Doped Luminescent Conjugated Polymers
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A micromanipulated vacuum probe station and fluorescence imaging are used to induce and visualize bipolar electrochemical redox reactions in a solid-state polymer light-emitting electrochemical cell (PLEC). The PLEC is a planar cell consisting of a composite polymer film partially covered by two parallel, vacuum-deposited rectangular aluminum electrodes at a separation of 10.7 mm. A pair of biased metallic probes are placed into direct contact with the exposed polymer surface, causing in situ electrochemical p- and n-doping of the luminescent polymer in the interior of the PLEC. Subsequently, the biased probes are moved to contact the rectangular aluminum electrodes of the PLEC to activate the device. An interesting phenomenon has been observed: in situ electrochemical doping is seen to originate from the previously doped regions that are isolated from the driving electrodes. By analyzing the complex doping patterns generated, it is concluded that the doped polymers have functioned as bipolar electrodes (BPEs), from which electrochemical p- or n-doping are induced wirelessly. In a separate planar cell of a smaller gap size, bipolar electrochemistry has been used to create five coupled and strongly emitting polymer p–n junctions. These results offer vivid visualization of the intriguing bipolar electrochemical phenomena in a solid-state polymer blend. The ability to form a BPE in situ and in the form of a heavily doped polymer offers innovative ways to modify the doping profiles in molecular devices. The all-polymer BPE also expands the realm of bipolar electrochemistry to beyond that of a conventional liquid cell containing metal or carbon electrodes.