Study On The Photoelectric Properties Of Poly (P-phenylene Benzobisoxazole) And Its Device

Poly(p-phenylene benzobisoxazole)(PBO) is a high-performance polymer and has found its applications in parts of a machine or construction due to its high mechanical strength and modulus, in aviation due to its heat and chemical stabilities. This work focused on its photophysical properties and its applications in optoelectronics.Fundamental photophysical and optoelectronic properties have been investigated. Absorption spectra both from of PBO solutions or solid films were measured and extinction coefficient are determined to be105L-mol-1·cm-1in solution and105cm-1in solid film at their peak positions. Energy structure of PBO, the energy level of the highest occupied molecular orbit (HOMO) and lowest unoccupied molecular orbit (LUMO), are estimated using cyclic voltammetry and found to be-5.8eV and-3.1eV, respectively.Photoluminescence from PBO formed a key part of the work. PBO in methanesulfonic acid (MSA) emits with a multiple peals located at440～442,466and503nm. Their time-resolved spectra reveal a single experiential decay with a lift time about0.3ns. No concentration quenching and molecular aggregation in PBO solution can be found. PBO solid film shows a single emission peak at515nm, but with a multiple experiential decays in its time-resolved spectra, which demonstrates strong interaction. This proposes to be from excimer formation.Charge carrier mobility of the PBO was measured via a space-limited-charge current approach. The PBO films are found have a hole mobility of～1.0×10-6cm2/Vs, and electron mobility of～2×10-8cm2/Vs. The hole mobility is two orders of magnitude higher than that of the electron mobility, This suggests that PBO is a hole transport material.Polymer light emitting diode (PLED) based on PBO as a light emitting layer is investigated. In order to improve device performance from a single layered device, a multi-layered having a complex cathode of "Alq3/LiF/Al" is fabricated. The device shows good electroluminescent performance, with a maximum brightness of8.7×103cd/m2and a current efficiency of4.8cd/A. It was found that the complex cathode acts as a hole buffer layer in addition to its electron injection role. It is discovered in this work that if doping a non-fluorescent Copper (Ⅱ) phthalocyanine (CuPc) into PBO slightly, performance of PBO:CuPc based PLEDs can be substantially improved. The best brightness reaches3.4×104cd/m2and the highest current efficiency of10.9cd/A. It is an impressive result which levels to current state of art in PLED technology.The physical phenomenon of doping a non-fluorescent dopant and cause a substantial improved emission from the host was reported for the first time. From the current-voltage characteristics, it can be found that the CuPc act as a hole-trapper in the light emitting layer. It balances carrier transport in the devices. It may offer a simplified approach to fabricate single layer PLEDs via dopant selection. The approach will have important implications in device engineeringOptoelectronic application using PBO is further explored such as a photocell fabrication. A good rectify ratio over is103obtained. The work also indicates that the photovoltaic (PV) response of single layer device depends on the metal/polymer interface. Both PV response and luminescence are observed from a device based on PBO/Fullerenes(C60) bilayer heterojunction structure. A competition between combination and separation of excitons should be responsible for such phenomenon.