Polymer Light Emitting Electrochemical Cells for High efficiency, Fast response and Multifunctional Electroluminescence Devices

Polymer light emitting electrochemical cells (PLECs) are of scientific importance because of the in-situ creation of a p-i-n junction in semiconducting polymer thin films. This represents an efficient and convenient approach to reduce interfacial energy barrier and could be more widely applicable in a large number of organic electronic devices. The work in this dissertation is focused on the improvement of PLECs to reach the response speed, efficiency and lifetime requirements for electroluminescent devices. Based on the unique properties of PLECs, multifunctional devices are also demonstrated including transparent and highly flexible PLECs, shape changing and stretchable PLECs, and dual-color emission PLECs.;Fast turn-on within a few milliseconds has been achieved through a novel method to stabilize the p-i-n junction in PLECs. This approach uses curable ionic conductive small molecules (TMPTMA). The role of the TMPTMA in stabilizing the junction is studied. The stability of the junction was characterized by impedance and photovoltaic measurements; no relaxation was observable after 72 hours idle without bias at room temperature. The PLECs with the static junction exhibit fast response (<5ms), efficient electroluminescence (450 cd/m2 and 3.0 cd/A at 10V), and a modest lifetime. The static p-i-n junction was also found to enhance the photovoltaic efficiency, demonstrating a 200% increase in short circuit current (I sc) and an 87.5% increase in open circuit voltage (Voc ) under 100 mW/cm--2 AM1.5 solar illumination as compared to the freshly prepared device without the stabilized p-i-n junction.;Very high efficiency and long lifetime PLECs have also been demonstrated by using a new class of ionic conductive oligomers combining the high ionic conductivity of PEO and curing capability of TMPTMA. Up to 15 lm/W power efficiency is realized in PLECs using a green-yellow emissive fluorescent light emitting polymer (Covion super-yellow). The half lifetime is estimated to exceed 20,000 hours with 100 cd/m2 initial brightness. The mechanism was analyzed and an idealized p-i-n junction model were proposed, in which the p doped and n doped polymers were well separated by a finite region of intrinsic semiconducting polymer, from which highly efficient light emission can be possibly generated.;In use of the simple device architecture and the tolerance to a high work function cathode and a thick active polymer layer, PLECs have been successfully made with single-walled carbon nanotubes (SWNTs) as both cathode and anode through a simple roll lamination process. In a blue emissive device, quite high electroluminescent performance was demonstrated: low turn-on voltage (3.8 V), high efficiency (2.2 cd/A at 480 cd/m2) and high brightness (1400 cd/m2 at 10 V). All the PLECs are highly flexible and can be bent to a 2.5 mm radius without causing failure. Such devices are also highly transparent with transmittance above 70% in the wavelength range where the emissive polymers do not absorb. Tandem configured PLECs have also been demonstrated through vertically stacking two of the transparent devices and the emission color was adjustable from sky-blue to yellow by tuning the relative biasing voltage on each device.;As stimulated by the superior compliance of the SWNTs electrodes, large deformation shape changing PLECs have been demonstrated with SWNTs electrodes on shape memory polymer substrates. High efficiency (2.1 cd/A) has been obtained in such devices with blue-green emission color. Such devices are very unique in terms of the combination of high mechanical robustness and large deforming capability. The shape changing property is reached at a higher temperature than the glass transition of the substrate polymer and any newly developed shapes can be subsequently preserved after cooling down the devices back to room temperature. Up to 54% linear strain has been realized in such PLECs without damaging the emission characteristics. The device remains the highly mechanical rigidity before and after the deformation.;Finally, dual-color emission PLECs have been demonstrated, in which distinctive color of light can be emitted simultaneously from either side of the PLECs. The dual emissive devices have been demonstrated by using either bilayer or single layer device architectures. They were also found to be useful for better understanding on the doping and junction formation mechanism in PLECs.