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Visualizing the Bipolar Electrochemistry of Electrochemically Doped Luminescent Conjugated Polymers Shiyu Hu, Xu Chi, Shulun Chen, Faleh AlTal, and Jun Gao J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b00073 • Publication Date (Web): 28 Mar 2017 Downloaded from http://pubs.acs.org on March 29, 2017
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Visualizing the Bipolar Electrochemistry of Electrochemically Doped Luminescent Conjugated Polymers Shiyu Hu, Xu Chi, Shulun Chen, Faleh AlTal and Jun Gao* Department of Physics, Engineering Physics and Astronomy, Queen's University, Kingston, Ontario, K7L 3N6, Canada Abstract A micro-manipulated 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 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 offer innovative ways to modify the doping profiles in molecular devices. The allpolymer BPE also expands the realm of bipolar electrochemistry to beyond that of a conventional liquid cell containing metal or carbon electrodes. 1 ACS Paragon Plus Environment
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Introduction The simplest two-terminal electrolytic cell consists of a pair of driving electrodes separated by an electrolyte solution. Adding a wireless conductor to the electrolyte creates a bipolar electrode (BPE) that is capable of driving both reduction and oxidation reactions even though it is not directly connected to a power source. The redox reactions occur preferentially at the extremities of the BPE where the potential difference between the BPE and the electrolyte solution is the largest.1-3 Electrochemistry involving a bipolar electrode, or bipolar electrochemistry, has become an increasingly versatile and powerful method with applications in electrodeposition and patterning,4 growth of nanotubes and nanoparticles,5-7 fuel cells,8 and catalyst screening9-10 among others.11-15 Although many of the applications mentioned above are more commonly accomplished with a wired working electrode, a BPE offers distinctive advantages due to its wireless nature. For example, millions of dispersed micro-BPEs can be addressed remotely to generate homogenous electrochemiluminescence in bulk.16 Suspended, wireless BPEs can have net motion when propelled by bubbles generated asymmetrically at the poles.17-18 In the direction of the applied field, the solution potential along the BPE varies spatially. This property of the BPE has been exploited to produce materials with a compositional or doping gradient,7, 19-20 and Janus particles in bulk.21-22 These bipolar electrochemical systems contain a liquid or gel electrolyte in contact with bipolar electrodes that are either metallic or carbon conductors. Recently, our group demonstrated that bipolar electrochemistry could also be combined with solid-state electrochemistry in a polymer light-emitting electrochemical cell (PLEC).23 A PLEC was initially demonstrated as an alternative to the common polymer light-emitting diodes (PLED).24 Both PLEC and PLED employ a luminescent conjugated polymer as a light emitter. The PLEC, however, also contains a solid polymer electrolyte (SPE) mixed together with the luminescent polymer. A sufficiently large external voltage bias causes in situ electrochemical p- and n-doping of the polymer next to the driving electrodes. The doping fronts propagate inward until 2 ACS Paragon Plus Environment
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they meet to form an electroluminescent p-n or p-i-n junction. The PLEC is a solid-state electrochemical cell based on a mixed ionic/electronic conductor.25-26 An illustration of this process is seen in Figure 2 (ii). The electrochemical doping of a conjugated polymer involves the reduction or oxidation of the polymer chain, followed by the insertion of counterions either from a liquid electrolyte, or a polymer electrolyte in the case of PLEC. The electrochemically-doped polymers are electrically neutral, but have elevated conductivities. In addition, doping causes fluorescence quenching if the polymer being doped is luminescent. The following schemes depicts the electrochemical doping process of a PPV (poly(phenylene vinylene)) molecule in the presence of K+ and CF3SO3- counterions.
p − doping :
(PV )n + (nm)CF3SO3− − (nm)e− ↔ (PV m+ )(CF3SO3− )m
n − doping :
(PV )n + (nm)K + (nm)e ↔ (PV )(K )m n
n
+
−
m−
+
In the above schemes, n is the number of repeat units in a PPV molecule, m is the fraction of charge transfer per repeat unit, and e- is the elementary charge. Adding a floating metallic conductor to the PLEC film can cause additional doping if the external bias and/or the size of the conductor are sufficiently large. The electrochemical p- and n-doping originate from the extremities of the floating conductor and propagate in opposite directions. As many as four light-emitting junctions are formed when doping from the floating conductor interacts with doping that originated from the driving electrode. This is an example of bipolar electrochemistry at work in a solid-state device.23 In this study, we use a micro-manipulated vacuum probe station and fluorescence imaging to induce and visualize bipolar electrochemical redox reactions in an extremely large planar PLEC: a device configuration pioneered by our group and particularly suited for visualizing electrochemical and bipolar electrochemical phenomena via optical imaging.27-29 A pair of biased metallic probes are first placed into direct contact with the exposed polymer surface of the planar PLEC. This causes localised electrochemical doping and the formation of a strongly emitting p-n junction, as seen in Figure 1 (i) and (iii). With some delay, the biased probes are then moved to contact the aluminum electrodes of the PLEC to 3 ACS Paragon Plus Environment
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activate the device. We elucidate the complex doping patterns generated. We demonstrate, for the first time, bipolar electrodes in the form of electrochemically doped polymer, from which electrochemical por n-doping are induced wirelessly. Experimental Methods In this study we fabricated extremely large planar LECs with an interelectrode gap of over 10 mm, The planar LECs had an active layer consisting of three compounds. The luminescent polymer, poly[5-(2-ethylhexyloxy)-2-methoxy-1, 4-phenylene vinylene], MEH-PPV, was obtained from OLEDKing Optoelectronic Materials Ltd, China with a molecular weight of Mw=3.3×105 and a polydispersity index of 1.4. The electrolyte polymer, polyethylene oxide (PEO, Mw=2M), and the potassium triflate (KTf, 98%) salt, were purchased from Sigma Aldrich and used as received. Cyclohexnone solutions of MEH-PPV (10 mg/ml) and PEO:KTf (20mg:3.85mg/ml) were mixed to create a casting solution of MEH-PPV:PEO:KTf at a weight ratio of 1:1.3:0.25. 100 µl of the solution was dispensed onto square glass substrate (16×16 mm2) and spun at 2000 rpm. The resulting polymer film had a thickness of 300 nm, as determined with a DektakXT stylus surface profiler. The polymer film was dried at 50 for at least 5 hours to remove any residual solvent. Aluminum electrodes of 100 nm in thickness were thermally evaporated onto of the polymer film to create a planar LEC with an interelectrode spacing of either 10.7 mm or 2.1 mm. The above device processing steps were carried out in a nitrogen-filled glove box/evaporator system with low levels (
