Light Emission and Charge Transport in Reverse Biased Polymer Junctions
Polymer p-n, p-i-n junctions , Electrochemical doping , Electroluminescence, , Light-emitting electrochemical cells , Electrical treeing
A semiconductor homojunction such as a p-n junction is at the heart of many modern solid-state devices. Among organic electronic devices, light-emitting electrochemical cells (LECs) are unique in that they possess a true p-n junction formed by in situ electrochemical doping reactions. Similar to an inorganic p-n junction LED in many aspects, LECs have some very desirable device characteristics such as low operating voltage, high efficiency and high tolerance to film thickness variation. However, the LEC junction itself is still poorly understood, especially in its stable, frozen form. My research focuses on the elucidation of electroluminescence (EL) and electrical transport properties of frozen-junction polymer LECs (PLECs) made from a conjugated polymer emitter and a solid polymer electrolyte. Chapter 1 covers the basics and background of LECs, including the materials, operating mechanism and relevant theories on charge transport. Chapter 2 provides a complete account of experimental techniques and procedures used in this dissertation research. Chapter 3 uncovers the underlying mechanism of a puzzling EL phenomenon observed under reverse bias. Careful imaging and transport measurements determined that the reverse bias EL was caused by the tunnel injection and subsequent recombination of charge carriers into the dedoping "i" (intrinsic) region. In Chapter 4, the frozen PLECs were investigated for their temperature dependence. A biased dedoping scheme was used to achieve the strongest ever reverse bias EL. Temperature hypersensitivity of the reverse bias EL was discovered and explained with a recombination model taking account of carrier transit time versus recombination time in the newly formed intrinsic region. In Chapter 5, the as-formed frozen PLEC junction was subjected to a large reverse bias current at a fixed temperature. During this process, the light emission zone was observed to shift into the previously n-doped region. Optical and SEM imaging revealed that significant material loss had occurred in the process. The material loss led to the formation of electrical trees in the n-doped region. We conclude that the treeing was caused by hot electron bombardment, which also caused light emission to become cathodoluminescence. Finally, Chapter 6 provides a conclusion and some suggested directions for future work.