The synthesis, properties and applications of a number of conjugated polymers and multi-component materials which provide a range of optical states that can be accessed for electrochromic and electroluminescent devices will be addressed. Results will be presented examining absorption and emission characteristics in the visible and near - infrared regions of the spectrum. Variable gap conjugated electrochromic polymers show high oxidative switching stabilities ( related to high HOMO levels for facile oxidation to the doped state) and narrow gap polymers exhibit three distinct colors as they can be both p-and n-type doped. The complementary nature of the electrochromic behavior of these polymers is demonstrated using reflection and transmission devices constructed using sandwich and lateral geometries ,along with new methods of patterning electrodes. Mutli-component polymer and organic LEDs that exploit energy transfer among emitting chromophores distributed within the active matrix allow precise control of emission. We will discuss near-IR photoluminescence (PL) and electroluminescence (EL) from blends of polymers and non-conjugated thermoplastics with a variety of lanthanide complexes.
We report on the photophysics of the pristine oligo(ethylene oxide) side-chain grafted polymer PPP-OR11 and the polymer blended with the lithium salt lithiumtriflate. The side-chains render the polymer soluble in common organic solvents and in addition provide ionic conductivity, which is important for the application of the polymer as mixed ionic-electronic conductor for instance in light-emitting electrochemical cells (LECs). The optoelectronic properties of the polymer were studied for two types of light-emitting devices, first in light emitting diodes and secondly in LECs. From these investigations it is evident that in polymer light-emitting diodes (PLEDs) several degradation processes caused by defects on the PPP backbone deteriorate the color stability. These defects are induced either by the oxidation of the polymer or the aluminum deposition process upon device fabrication. Contrarily, LECs fabricated from the same polymer provide color stable blue emission. The color stability of the LEC can be explained by the fact that the recombination zone is shifted from the cathode/polymer interface in PLEDs to the non-doped intrinsic zone between the p- and n-type regions of the LEC, avoiding emission from aluminum evaporation induced defects.
With the increasing use of night vision goggles and night missions, new methods to display information in the infrared region is of interest. We have developed both inorganic and organic electroluminescent thin films which emit at wavelengths between 700 nm and 1.8 μm. These thin films have been incorporated into simple devices and the feasibility of a NIR flat panel display has been demonstrated. Both inorganic zinc sulfide and organic polymers doped with rare earth lanthanide ions have been demonstrated. The wavelength of emission can be varied by choosing the appropriate lanthanide ion, such as dysprosium, erbium, thulium or neodymium. Power densities of ~30 μW/cm2 have been achieved with these devices.
By utilizing strain gage technology it is possible to directly and continuously measure the electrochemically induced strain response of EAP actuators. Strain sensitive actuators were constructed by directly vapor depositing gold (EvAu) on polyimide strain gages which are capable of measuring strain with an accuracy of +/- 1(mu) (epsilon) . Strain sensitive actuators were used to evaluate the strain response of polypyrrole (PPy), poly(3,4-ethylenedioxypyrrole) (PEDOP) and poly(3,6-bis(2-(3,4-ethylenedioxy)thienyl)-N-carbazole) (PBEDOT-Cz). PPy was shown to produce significantly higher strain when compared to PEDOP and PBEDOT-Cz. The resulting overall strain for the materials was: 236, 33, and 35 (mu) (epsilon) respectively. From the initial investigation, adhesion of the EAP to the EvAu layer was identified as a major factor in the resulting lifetime and strain response of these actuators. Therefore an electrochemically deposited Au layer (EcAu) was deposited on top of the EvAu layer to improve the adhesion of the EAP to the working electrode. By changing the surface roughness from requals3.43 (EvAu) to requals8.26 and 18.00 (EcAu) the normalized strain response after 2000 cycles increases from 45% to 60% and 68% respectively. Also by changing the surface roughness from 5 to 23, the resulting strain response increases from ~100 (mu) (epsilon) to 600-800 (mu) (epsilon) for Ppy.
We present a successful demonstration of controllable patterning of dual-color polymer light-emitting pixels using a hybrid inkjet printing technique. In this demonstration, the polymer buffer layer is a wide bandgap, blue emitting semiconducting polymer (PPP-NRt3+), prepared by the spin-casting technique. The inkjet printed layer is a red-orange semiconductor polymer, (MPS-PPV) which was printed onto the buffer layer.When a proper solvent was selected, MPS-PPV diffused into the buffer layer and efficient energy transfer took place from the PPP-NEt3+ to the MPS-PPV generating a red-orange photoluminescence and electroluminescence from the inkjet printed sites. Based on this principle, blue and orange-red dual-color polymer light-emitting pixels were fabricated on the same substrate. The use of this concept represents an entirely new technology for fabricating polymer multicolor displays with high-resolution, lateral patterning capability.
The redox switching of some conducting polymers was investigated. Poly(3,4- ethylenedioxythiophene) and two of its alkyl derivatives were investigated in a liquid electrolyte to probe its usefulness as a visible electrochromic device. The other type of device is based upon actively changing the surface conductivity of one active layer of the device using conducting polymer: in one state the surface conductivity is high, in the other state the surface conductivity is low. Switching speed, contrast ratio, and overall lifetime for both devices were determined and discussed.
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