Polymer photonic devices based on mixed ionic/electronic conductors: device and materials physics

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Semiconductor photonic devices such as light-emitting diodes (LEDs), solar cells, photodiodes, and diode lasers are indispensible in our technology- and information-driven world. While these devices are still mainly based on silicon or group III-V compounds, various organic semiconductors have emerged as potential candidates for next-generation applications that require even higher functionality and less energy consumption. Semiconducting polymers are especially attractive because they have the mechanical and processing advantages of polymeric materials as well as being optically and electrically interesting. To make “plastic electronics” a reality, polymer semiconductors need to be better understood and fully exploited not only in their pure form, but also when “doped.” The field of “conductive polymers” (2000 Nobel Prize in Chemistry) came into existence when polyacetylene was discovered to exhibit vastly improved conductivity when chemically doped. Most polymer devices to date are based on pristine polymers. An exception is the polymer light-emitting electrochemical cell (LEC), whose active layer is a mixed ionic/electronic conductor (MIEC). The MIEC of an LEC is electrochemically doped in situ during operation, and a light-emitting p-n or p-i-n junction is formed between the differently doped regions. The LECs possess many desirable device characteristics as a result of doping. The chief challenges facing MIEC devices such as LECs is their inferior operational lifetime and the lack of understanding of the underlying physics. This is in large part due to the complex nature of MIEC devices, which possess at least four different charge carriers that can interact. In our proposed research, various polymer MIEC devices will be explored and investigated in depth in order to deal with the challenges. In particular, we will perform unprecedented, concerted scanning photocurrent, photoluminescence and absorption measurements of extremely large, frozen planar cells in order to better understand the electronic structure of MIEC junctions.  In addition, we strive to achieve long lasting MIEC devices ready for practical applications. The proposed research will build on my group’s extensive experience working on MIEC light-emitting cells and photovoltaic cells that has resulted in nearly 30 publications in top international journals in recent years. My group also developed a suite of powerful experimental techniques ideally suited for the study of MIEC devices. These include time-lapse fluorescence imaging, frozen-junction and controlled junction relaxation, scanning electrical probing of conductivity and electric potential, and concerted scanning photocurrent and photoluminescence probing techniques. The proposed research offers excellent opportunities for the training of HQPs that will prepare them for a career in academic research or industrial R&D.