Direct imaging and probing of polymer electrolyte/luminescent conjugated polymer mixed ionic/electronic conductors

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A polymer electrolyte is a solid-state ionic conductor and a key component of certain fuel cells, lithium ion batteries and electrochromic devices. A conjugated luminescent polymer is an organic semiconductor and the active component of polymer light-emitting diodes, polymer thin film transistors, polymer photovoltaic solar cells and other emerging "plastic" electronic devices. Combining the two materials creates a luminescent polymer mixed ionic/electronic conductor (MIEC) rich in interesting physics due to the interaction between electronic and ionic charges. A luminescent polymer MIEC was first studied in the so-called polymer light-emitting electrochemical cell (LEC). The fundamental operating mechanism of an LEC involves in situ electrochemical p- and n-doping of the luminescent polymer and the formation of a dynamic, light-emitting p-n junction. In 2003 Dr. Gao's group first demonstrated planar LECs with millimeter-sized interelectrode spacing. The extremely large and fully exposed interelectrode gap allows for time-lapse fluorescence imaging with high temporal and spatial resolution. For the first time the dynamic p- and n-doping as well as the junction formation processes were captured on camera. Dr. Gao's subsequent studies showed that the complex doping and electroluminescence processes were strongly affected by operating voltage and temperature, salt type, electrode work function, the presence of metallic particles, film morphology and thermal history. In the proposed research Dr. Gao will use a micro-manipulated cryogenic probe station (purchased through a NSERC RTI grant) to directly measure the electrical conductivity and electrical potential distribution across an extremely large planar LEC. A fiber optics probe will be used to detect the built-in electrical field or potential by measuring the induced photocurrent or photovoltage. The proposed research will address fundamental questions related to the properties of luminescent conjugated polymers, polymer electrolytes and their mixtures. The results will improve our understanding of LEC and may lead to more efficient polymer-based light-emitting devices, photovoltaic solar cells and rechargeable batteries.