Abstract
Electroluminescence (EL) from organic molecules and conjugated polymers has attracted wide interest because of the large potential for application in display fabrication. In organic light-emitting diode (OLED),holes and electrons injected from the anode and cathode can recombine to yield singlet fraction (χS) of 25% according to quantum statistics and some experiments. However, for polymer light-emitting diodes (PLED), this fraction still remains under debate. Therefore, it is extremely important for us to clarify χS in the PLED and investigate its relation with molecular structure of conjugated polymer so that high χS conjugated polymers can be designed accordingly. In the first part of this thesis, a method is proposed to measure χS based on the measurement of phosphorescence decay of phosphorescent dopant in the polymer under electrical field excitation in the working device. We find that the χS in the blue emitting PCBPF based PLED exceeds the quantum statistics limit (25%) and increases with electric field excitation. It is probably resulted from triplet-triplet annihilation by a collision between two triplet excitons in the same chain to yield a singlet exciton. For purely fluorescent device, only singlet exciton is emissive and most energy is wasted by non-emissive triplet exciton. By doping with phosphor as guests in small molecules or polymers as hosts, both singlet and triplet exciton formed under electric field excitation can be harvested by the phosphor and consequently the internal quantum efficiency is possible to be promoted toward 100%. However, in order to confine triplet exciton on phosphor guest, a host material with triplet energy level (ET) higher than the phosphor guest is intuitively required as a significant quenching of triplet exciton by a low ET host for a high ET guests can occur。 In the second part of thesis, we demonstrate that an effective reduction of quenching of triplet exciton for a high ET phosphor guest with a low ET polymer host is possible upon introducing dense side chains to the polymer to block a direct contact from the guest such that possibility of Dexter energy transfer between them is reduced to a minimum. The system investigated is CzPPP (ET = 2.39 eV) as host and Ir-G (ET=2.41 eV) as guest, which gives high device efficiency (30 cd/A) and is more efficient than that (23 cd/A) in the system with high host triplet energy (PCBP, ET = 2.53 eV). This observation suggests a new route for molecular design of electroluminescent polymers as host for phosphorescent dopant：The ET of polymer host is not necessary to be higher than that of phosphor guest for efficient electrophosphorescence. Such finding provides a more freedom for molecular design of low ET electroluminescent polymers as host for high ET phosphor dopant.