Fluorescent, Carrier-Trapping Dopants for Highly Efficient Single-Layer Polyfluorene LEDs

The discovery of efficient organic light-emitting diodes (OLEDs) based on small molecules and conjugated polymers, has triggered an intense investigation of their applications in high-efficiency flat-panel displays and solid-state lighting. Significant progress in the materials synthesis and device construction has led to the realization of full color as well as white OLEDs with improved efficiencies and lifetimes. Great efforts have been made to develop high-performance materials with desirable properties, and devices with optimized architecture to develop marketable OLEDs. Fluorophores with attractive emission characteristics, such as a good range of colors, high quantum yield, and good hole/electron mobility are desired as emitters of OLEDs. Moreover, good durability, such as high glass-transition temperatures (Tgs) of materials, and thermally and morphologically stable amorphous films, is another important factor that can drastically improve the physical performance of OLEDs. OLEDs fabricated with conjugated polymers, so called PLEDs, are at a stage ready for commercial application. In this field, polyfluorenes (PFs) have recently emerged as promising candidates for blue-light-emitting materials because of their low turn-on voltages, high brightness, high efficiency, and high thermal stabilities. Moreover, PFs could be used as host materials to generate other colors by the transfer of energy to doped lower-energy chromophores such as fluorescent and phosphorescent small molecules or polymers. Doping the host polymers with highly fluorescent dyes has proven to be an efficient strategy to achieve higher color purity for a large number of colors and to increase the lifetime and efficiency of devices. Several mechanisms have been postulated to explain the light generation above, one is an energy-transfer mechanism, such as the Förster energy transfer between two chromophore segments with different energy levels, and the other one is exciton formation on the doping molecules. Anthracene derivatives have been extensively studied and developed as light-emitting materials in OLEDs because of their interesting photoluminescence (PL) and electroluminescence (EL) properties. By introducing bulky substitutes at the 9and 10-positions of anthracene, the whole molecule becomes highly twisted. Thus the fluorescence-quenching interactions caused by aggregates can be suppressed to some extent and the non-radiative energy decay can be reduced. A recent report has demonstrated the fluorescence-enhancement effect of an anthracene core. On the other hand, in addition to the Förster energy transfer in copolymers, Miller et al. reported that the incorporation of a small amount of low-bandgap chromophores, such as anthracene, pyrene, and perylene, into the polyfluorene main chain could successfully suppress excimer formation and result in stable color PL and EL. The excitons migrate from the excited polymer backbone and are trapped by low-bandgap chromophores. The rapid and efficient energy transfer also prevents the formation of the main-chain