A comprehensive review of the literature on electron transport materials (ETMs) used to enhance the performance of organic light-emitting diodes (OLEDs) is presented. The structure-property-performance relationships of many classes of ETMs, both smallmolecule-and polymer-based, that have been widely used to improve OLED performance through control of charge injection, transport, and recombination are highlighted. The molecular architecture, electronic structure (electron affinity and ionization potential), thin film processing, thermal stability, morphology, and electron mobility of diverse organic ETMs are discussed and related to their effectiveness in improving OLED performance (efficiency, brightness, and drive voltage). Some issues relating to the experimental procedures for the estimation of relevant material properties such as electron affinity and electron mobility are discussed. The design of multifunctional electroluminescent polymers whereby light emission and electron-and hole-transport properties are combined in one material to achieve efficient single-layer OLEDs is also discussed. The review concludes with a brief perspective on the challenges that future research should address.

We use photoinduced absorption spectroscopy to measure long-lived photogenerated charge carriers in optically thin donor/acceptor conjugated polymer blend films near plasmon-resonant silver nanoprisms. We measure up to 3 times more charge generation, as judged by the magnitude of the polaron absorption signal, in 35 nm thin blend films of poly(3-hexylthiophene)/phenyl-C(61)-butyric acid methyl ester on top of films of silver nanoprisms (approximately 40-100 nm edge length). We find that the polaron yields increase linearly with the total sample extinction. These excitation enhancements could in principle be used to increase photocurrents in thin organic solar cells.

The near-field effects of plasmonic optical antennas are being explored in applications ranging from biosensors to solar cells. We demonstrate that photoluminescence emission enhancement from CdSe quantum dots (QDs) can be obtained in the absence of any excitation enhancement near single silver nanoprisms. The spectral dependence of the radiative and nonradiative decay rate of the QDs closely follows the silver nanoparticle plasmon scattering spectrum. Using both experiment and theory we show that, in the absence of excitation enhancement, the ratio of radiative to nonradiative decay rate enhancement is proportional to the silver nanoparticle scattering efficiency. These results provide guidelines both for separating excitation and emission enhancement effects in sensing and device applications and for tailoring emission enhancement effects using plasmonic nanostructures.

A series of four new statistical copolymers of 9,9-dihexylfluorene and 9-fluorenone with well-defined structures and a new fluorene-fluorenone-fluorene trimer model compound were synthesized and used to investigate the photophysics, origin of the green emission, and electroluminescence of this class of light-emitting materials. We show that the new oligofluorene trimer with a central fluorenone moiety is an excellent model of the green-emitting chromophore in polyfluorenes. From systematic studies of the steady-state photoluminescence (PL) and PL decay dynamics of solutions of the fluorenone-containing copolymers and oligomer and thin films of the copolymers, we show that the controversial 535-nm green emission band originates from the fluorenone defects in single-chain polyfluorenes and not from intermolecular aggregates or excimers. The green emission, centered at 535 nm, was observed in dilute toluene solutions of all fluorenone-containing copolymers and oligomer; it was long-lived with a single-exponential PL lifetime of ∼5 ns, compared to a lifetime of 240-400 ps for the blue emission. The PL decay dynamics of the 535-nm emission from thin films of all copolymers was also well-described by a single-exponential lifetime of 5-6 ns. The observed increased intensity of the green emission with increased intermolecular interactions in solution or solid state can be explained by the increased excitation energy transfer from fluorene segments to the fluorenone moieties. Bright green electroluminescence (EL) centered at 535 nm was achieved from single-layer copolymer lightemitting diodes (LEDs), ITO/PEDOT/copolymer/Al, with luminances of 1600-3340 cd/m 2 that varied with fluorenone content. The EL data suggest that the fluorene-fluorenone copolymers are very promising materials for green LEDs.

Spectrally stable blue electroluminescence (EL) is obtained from single-layer polymer lightemitting diodes fabricated from binary blends of conjugated poly(9,9-dioctylfluorene) (PFO) with either thermally stable poly(vinyl diphenylquinoline) (PVQ) or polystyrene. The brightness and EL efficiency of the polymer blend LEDs were enhanced by a factor of 5-14 compared to the PFO homopolymer devices. The additional green emission observed in the EL spectra of pure PFO devices was substantially suppressed in the blend LEDs. The electrical characteristics of the diodes and electric-field-modulated PL spectroscopy results indicate increased spatial confinement induced exciton stability and electronhole recombination efficiency in the blend devices. The variation of the device performance with blend composition is related to the phase-separated morphology of the blends. The origin of the spectral stability lies in the improved thermal stability of the PFO:PVQ blends due to the high glass transition temperature (T g) of PVQ (185 °C). These results demonstrate that blending of PFO with high Tg charge transport or charge blocking polymers is a simple strategy to overcoming the problem of poor spectral stability of blue-emitting polyfluorenes.