Electrical and Optical Probing of Extremely Large Planar Polymer Light-Emitting Electrochemical Cells
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Polymer light-emitting electrochemical cells (LECs) are unique solid-state light-emitting devices operating on the principle of in situ electrochemical doping and the formation of a p-n junction. Compared to conventional polymer light-emitting diodes, LECs possess some favorable device characteristics, making them attractive candidates for flat panel display applications. However, the underlying operating mechanism of LECs has been the subject of intense scrutiny and debate. The extremely large planar devices demonstrated by Dr. Gao’s group offer an unparalleled spatial resolution for investigating the electrochemical doping process. With time-lapse fluorescence imaging, our group has demonstrated the existence of the electrochemical doping and the p-n junction in an operating LEC. However, the LEC electronic structure has never been adequately studied and understood. In this thesis, the original research on the electrical and optical scanning measurements on extremely large planar LECs is described. A fresh LEC film has, for the first time, been turned on to form a light-emitting p-n junction by biasing a pair of probes that form direct contacts with the LEC film. The quasi-one-dimensional probe tips allow insight into the electrochemical doping mechanism, without being disturbed by imperfections in the pre-deposited electrodes. Time-lapse fluorescence imaging has exhibited p- and n-doping of various shapes and shades, p-n junction electroluminescence, and the effects of bias reversal. The observed doping reaction kinetics suggests that n-doping is reaction-limited whereas p-doping is diffusion-limited. Furthermore, the electric potential and conductivity distributions of extremely large planar LECs have been mapped. The results show that the p-doping is more conductive than n-doping. More importantly, The results provide the first evidence that the p-n junction is indeed a graded one with decreasing conductivity and doping concentration from the electrode contact to the metallurgical junction. In addition, the optical beam induced current (OBIC) measurement has been explored to map the built-in electric field of extremely large planar LECs. This result clearly shows that a large electric field exists only around the junction region rather than in the electrode region. Moreover, the result confirms that the LEC electronic structure is a graded p-n junction.