Polymer Electrochemical Light-Emitting Devices and Photovoltaic Cells
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Light-emitting electrochemical cells (LECs) are solid state polymer devices operating through the formation of a light-emitting p-n junction by in situ electrochemical doping. The LEC film contains a luminescent polymer and a polymer electrolyte. A sufficiently high voltage bias initiates the electrochemical p-doping reaction at the anode interface and n-doping reaction at the cathode interface. With time the doped regions expand in volume until they make contact to form a light-emitting p-n junction. In this thesis, I present my original research on both the light-emitting and photovoltaic properties of LECs. I discovered that continued doping after p- and n-doped regions have made first contact accounts for most of LEC turn-on time. I showed that because the electronic charges must be injected from an external circuit for the electrochemical doping to occur, the LEC turn-on response is limited to no faster than milliseconds. I also demonstrated that the lifetime of LECs can be affected by various factors such as stress temperature, stress current, substrate thermal conductivity, and luminescent polymer end group. With the right combination of substrates and materials, LECs exhibit a remarkable half lifetime on the order of hundreds of hours when stressed at a current density of 1A/cm2. I also observed that an as-formed p-n junction can even relax into a p-i-n junction upon the removal of applied voltage bias. A p-i-n junction LEC exhibits more efficient electroluminescence due to less photoluminescence quenching in the quasi-intrinsic emission zone. Frozen p-i-n junction LECs also exhibit a much improved photovoltaic response. By carefully controlling the relaxation (dedoping) temperature and duration, I have demonstrated p-i-n junction photovoltaic cells with record-high open-circuit voltage of 2.25V and short-circuit current density in excess of 10mA/cm2 under simulated sunlight of ~300mW/cm2. By optimizing film thickness and electrolyte content, I have achieved a thirty-fold increase in power conversion efficiency of p-i-n junction photovoltaic cells. My results demonstrate that a polymer homojunction such a p-n or a p-i-n junction is a promising device concept that has potential application in high performance polymer-based photonic devices.