The focus of this study is mono-layer polymer light emitting diode (PLED). The emitting layer is poly-p-phenylenebenzobisoxazole (PBO). PBO is a fully conjugated heterocyclic aromatic rigid-rod polymer. Anode is indium-tin-oxide (ITO). Cathode is aluminum (Al). We used UV epoxy resin to package PLED devices, then measured current-voltage response, electroluminescence (EL) emission, and device lifetime. We demonstrate that the packaged mono-layer PBO LED reduced its demise from water and oxygen. Device lifetime increased from 1 hour to several hundred hours. At a larger bias voltage or current, emission intensity and device efficiency became higher. But decay rate increased leading to shortened device lifetime. Device temperature appeared linearly with current density. A red shift of the EL emission was observed. The £fmax. of emission spectra moved from 534 nm (initial) to 582 nm (after 100 hrs). After thermal annealing at 120¢J for ten hours, threshold voltage increased from 5 V to 12 V, current density decreased to several 10 mA/cm2, luminous intensity improved several ten times to 10-2 cd/m2, emission color changed from yellow-green to orange, luminous efficiency improved from 10-7 to 10-4 cd/A, but device lifetime declined to less than 20 hrs.

Poly-p-phenylenebenzobisoxazole (PBO) and carbon nanotube (CNT) contain fully conjugated rod like backbone entailing excellent mechanical properties, thermo -oxidative stability and solvent resistance. Rigid-rod PBO is commonly processed by dissolving in methanesulfonic acid or Lewis acid. A CNT of multi-wall carbon nanotube (MWNT) was dissolved in a Lewis acid solution of PBO for dispersion, and then spun for thin film. MWNT concentration in the films was from zero up to 5 wt. %. Compared to that of pure PBO film, all PBO/MWNT composite films retained same but enhanced UV-Vis absorption peaks, according to MWNT concentration, showing that PBO and MWNT did not have overlapping electron orbitals affecting their energy gaps. The composite films were excited at 325 nm using a He-Cd laser for photoluminescence (PL) emission. All PL spectra had maximum intensity at 540 nm indicative of yellow-green light emission. The composite films were fabricated as light emitting diodes using indium-tin-oxide/glass as substrate and anode, as well as vacuum evaporated Al as cathode for respectively hole and electron injectors. In these light emitting devices, MWNT doped PBO would decrease threshold voltage for about 2 V. Up to 0.1 wt. % of MWNT, the device emission current was increased two orders of magnitude than those of the devices without MWNT. Further increase of MWNT caused a successive decrease in electroluminescence emission intensity attributed to a quench effect from aggregations of MWNTs. UV epoxy resin was applied to package the mono-layer and bilayer PBO light emitting devices. The UV epoxy resin had some gas release during encapsulation. The devices were packaged with vacuum and without vacuum encapsulation. It was demonstrated that the device encapsulation reduced its demise from water and oxygen. The vacuum encapsulation could remove gaseous volatile of the device to inhibit oxygen and moisture to prolong device lifetime. The main degradation of light emitting device was the oxidization of cathode. The interactions between nitrogen of PBO and H2O caused the formation of hydrogen bonding at room temperature. Oxygen and moisture diffused into PBO polymer and were suspected to form mid-gap state for the polymer. The mid energy band disappeared upon heat treatment before encapsulation. A device under a higher bias voltage was found to have a shorter lifetime, but a larger EL emission intensity. The EL emission intensity was not a constant under a constant current bias. The vacuum encapsulated device had two or twenty times lifetime than, respectively, the device encapsulation without vacuum evacuation or in ambient conditions. The sandwich structure of ITO/PBO/Al had no observable photovoltaic effect due to insufficient exciton separation into electrons and holes. Poly(2,3-dihydro thieno-1,4-dioxin):polystyrenesulfonate (PEDOT:PSS), a hole transferring medium, was spun into a thin-film between PBO and indium-tin-oxide to facilitate photovoltaic (PV) effect by forming a donor-acceptor interlayer to separate and to transport photoinduced charges. Optimum PBO thickness for the PV heterojunctions was about 71 nm at which the hole transferring PEDOT:PSS generated the maximum short circuit current (Isc) at a thickness of 115 nm. By using a layer of lithium fluoride (LiF) as an electron transferring layer adhering to Al cathode, the most open circuit voltage (Voc) and the maximum short circuit current (Isc) were achieved with a LiF thickness of 1-2 nm due to possible electric dipole effect leading to an increase of Voc from 0.7 V to 0.92 V and of Isc from about 0.1

In this research, we investigated a novel rigid-rod polymer sPBI for mono-layer polymer light emitting diode (PLED) fabrication and luminescence emission. sPBI could be a luminescent polymer with a low threshold voltage of 4.5 V and green light electroluminescence emission (530 nm). Its SO3H pendant attached to the p-phenyl ring improved electronic delocalization along the backbone resulted in a red shift of the absorption spectrum. By attaching propanesulfonated pendants to the heterocyclic moiety of intractable fully conjugated sPBI, water-soluble rigid-rod polyelectrolyte sPBI-PS(Li+) was synthesized to promote its processibility in water or common organic solvent. This water-soluble rigid-rod polyelectrolyte sPBI-PS(Li+) was fabricated for polymer light-emitting electrochemical cells (PLECs) with LiCF3SO3 (LiTf) or LiN(CF3SO2)2 (LiTfSI) dopants for investigating the influence of propanesulfonated pendants as well as dopants on the opto-electronic emission and the room-temperature DC conductivity. The effect of lithium salts (LiTf or LiTfSI) on photoluminescence color of doped sPBI-PS(Li+) films was negligible. sPBI-PS(Li+) PLECs doped with 0.41 and 1.01 wt. % of LiTfSI showed higher green light electroluminescence emission (514 nm) with a lower threshold voltage of 3.0 V and -4.6 V, respectively. Emission brightness of the sPBI-PS(Li+) PLEC did not raise upon increasing the ionic conductivity of the luminescent layer.

Poly-p-phenylenebenzazoles (PBXs) are heterocyclic aromatic rigid-rod liquid-crystalline polymers with fully conjugated backbone having excellent thermo-oxidative, as well as dimensional stabilities. PBXs are considered to be multifunctional polymers of superior mechanical tenacity, non-linear optical response, and electrical properties. The fully conjugated PBX polymers are deemed to have excellent opto-electronic properties. In the last decade, molecular light emitting diodes (LEDs) have been investigated intensively for having distinct advantages as an advanced opto-electronic technology. This dissertation leads to rigid-rod polymer thin-films and mono-layer devices fabricated from acidic solutions. Photoluminescence (PL) spectra for poly-p-phenylenebenzobisthiazole (PBT) freestanding film were measured over a temperature range of 67 K to 300 K showing distinct electron-phonon interaction. Using an Mg cathode, the mono-layer PBT LEDs displayed a diodic electric response with a threshold voltage as low as 1 V. A blue shift in the maximum emission wavelength of the electroluminescence (EL) spectra was also observed with increasing electrical injection energy. For the multi-layer LEDs based on PBT using the same electrodes, the p-type/n-type bi-layer structure showing the most enhanced EL emission, and the tri-layer heterojunction had the least threshold voltage using the same electrodes. Our results indicated that the heterojunction architecture could be applied to balance charge carriers for increasing EL intensity. Meanwhile, the investigation also revealed the advantage in using the extra PBT layer for increasing both EL emission intensity and injection efficiency by lowering its threshold voltage. Two schemes for making uniaxial freestanding films and LED devices for polarized optical absorption and emission were processed from uniaxial poly-p-phenylenebenzobisoxazole (PBO) fiber. The PL of the uniaxial PBO films demonstrated an emission intensity ratio I¡ü/I¡æas high as 5. Anisotropically processed mono-layered PBO LED showed a markedly decreased threshold voltage from 7 V of the isotropic PBO device to 5 V. The polarization effects in optical absorption, PL and EL emissions were acquired and correlated with the uniaxial orientation of the rigid-rod PBO polymer. The molecular modification investigated the opto-electronic properties of poly-2,2'-m-phenylene-5,5'-bibenzimidazole (Pbi) with PBT physical blends, and monolithic 6F-PBO-OH-co-6F-PBO-di(OC10H21) copolymers. Partially conjugated polymer Pbi and fully conjugated polymer PBT were mixed for luminescence study. Their absorption spectra showed superposition of individual absorption response indicating no inter-molecular energy transfer. However, the PL and the EL emission demonstrated a blue shift with increasing Pbi content. This was attributed to the rigid-rod configuration or the aggregation of PBT perturbed by mixing with coil-like Pbi. It was recognized that the backbone of the fully conjugated rigid-rod PBT was collinear having more charge delocalization than that of not fully conjugated coil-like Pbi. The diode threshold voltage of the physical blends varied from 4 V to 14 V with decreasing PBT content. Another molecular modification was changing the composition of 6F-PBO copolymers. Their PL emission exhibited excellent chromatic tuning range from green to blue emission. The Commission Internationale de l¡¦Eclairage (C. I. E.) coordinates of the copolymer EL emission were from (0.25, 0.53) to (0.24, 0.31) covering a wide visible range and demonstrating a white light emission. Atomic substitution of the rigid-rod polymers was utilized to examine individual atomic contribution for luminescence emission. The hydrogen bond effect for PBO-OH and PBO was evidenced in a major Stoke¡¦s shift to a longer wavelength because of protonic transfer on the excited state. Elemental electronegativities affected the delocalization of the £k electron leading to a blue shift in absorption spectra as shown in case of PBO and PBT. The PBO molecule was more collinear and co-planar, providing more charge delocalization than PBT. However the absorption edge of the PBT was about 30 nm higher than that of PBO. This suggested that the electronegativities affected the molecular delocalization. Using the solid-state physics with pseudofunction (PSF) calculation, there was good match between absorption spectra and calculated excitation energies for the rigid-rod polymer systems.