ABSTRACT Optoelectronics of polymer light emitting diode (LED) depends significantly on polymer molecular structure and charge conjugation. This study focused on the optoelectronics of freestanding films and LEDs of a colinear, fully conjugated heterocyclic aromatic rigid-rod polymer (PBT) and its mixtures with a partially conjugated coil-like polymer (Pbi). A deuterated PBTd4 was also mixed with a fully conjugated coil-like polymer (ABPBI) for UV-Vis absorption spectrum, photoluminescence (PL), diodic current-voltage response, and electroluminescence (EL). Rigid-rod PBT was only soluble in strong protic acid. PBT films were processed using methanesulfonic acid. PBT free-standing films showed maximum absorptions at 468 nm and 640 nm; PBTd4 having all hydrogen atoms on the phenylene moiety substituted by deuterium retaining same electron orbitals thus showed same absorption and PL spectra. It was likewise for the PBTd4 and ABPBI mixtures at ABPBI concentrations of 1 % and 10 %. For mixtures of PBT and Pbi, the absorption spectra indicated super- position of individual optical absorption response and no energy transfer. However, PL spectra showed a blue shift with increasing Pbi content. This was attribed for PBT rod-like configuration, or PBT aggregation perturbed by mixing with Pbi. Monolayer LED of Al/PBT/ITO and Al/Pbi/ITO yielded a threshold voltage of 4 V. When PBT/Pbi mixtures of 75/25, 50/50, 25/75, were used as the light emitting layer, the threshold voltage altered to 10 V, 7 V and 17 V, respectively. This threshold voltage deviation from 4 V is due mainly to difference in layer thickness, or phase separation affecting the tunneling effect. To enhance LED stability, an Ag layer was evaporated onto the Al electron injection electrode. For Ag/Al/PBT/ITO devices and mixed PBT/Pbi (75/25,50/50,25/75) devices, the maximum EL wavelength exhibited no systematic change at 753 nm, 714 nm, 727 nm, and 697 nm, respectively, due to using different bias voltage.

Molecular light emitting diode, including organic light emitting diode (OLED) and polymer light emitting diode (PLED), is commonly consist of one or several molecular layer(s) sandwiched between an anode and a cathode. When electrons and holes are injected respectively from cathode and anode into the molecular layer by a bias voltage, these two types of carriers migrate towards each other and a fraction of them recombine to form light emission. The focus of this study is electrode modifications of molecular light emitting diode. The electrode modifications include using a low work function cathode material, a high work function anode material or inserting a very thin electrode modifier between molecular layer and electrode for enhancing the electron or the hole injection efficiency leading to higher electroluminescence emission and/or lower threshold voltage. Low work function metal, Mg, could effectively reduce the electron injection barrier between molecular layer and cathode leading to better emission brightness and threshold voltage. A monolayer rigid-rod poly-p-phenylenebenzobisthiazole (PBT) or poly-p-phenylenebenzobis- oxazole (PBO) PLED with Mg cathode demonstrated a low threshold voltage of 3 V. Besides, a very thin layer of LiF (or Al2O3) inserted between molecular layer and Al cathode was applied to enhance the electron injection efficiency leading to a stronger electroluminescence intensity and a low threshold voltage of 2.8 V. On anode modification, a thin PBO layer was inserted between molecular layer and the indium-tin-oxide (ITO) substrate for improving the electroluminescence emission brightness and the threshold voltage. The PBO modified anode could effectively enhance the electro- luminescence intensity and lower the threshold voltage to 1 V~ 3 V on several mono- or multi-layer molecular light emitting diodes. Besides, a novel ITO substrate cleaning method via acid treatment was applied for increasing the work function of ITO to effectively enhance the hole injection efficiency.