M. Inbasekaran

Progress with Light-Emitting Polymers

Light-emitting polymers have been studied intensively as materials for light-emitting diodes (LEDs). Here research efforts toward developing these materials for commercial applications are reviewed. The Figure shows the preferred two-layer device structure for commercial polymer LEDs as well as polyfluorene, one of the polymers discussed.

Mobility enhancement in conjugated polymer field-effect transistors through chain alignment in a liquid-crystalline phase

A method is demonstrated by which liquid-crystalline self-organization in rigid-rod nematic conjugated polymers can be used to control the microstructure of the active semiconducting layer in solution-processed polymer thin-film transistors (TFTs). Enhanced charge carrier mobilities of 0.01–0.02 cm2/V s and good operating stability have been achieved in polyfluorene copolymer TFTs by preparing the polymer in a nematic glassy state and by aligning the polymer chains parallel to the transport direction with the help of an alignment layer. Mobility anisotropies of 5–8 for current flow parallel and perpendicular to the alignment direction have been observed that are of the same order of magnitude as optical dichroic ratios.

Electrochemical determination of the ionization potential and electron affinity of poly(9,9-dioctylfluorene)

We report cyclic voltammetry measurements for the blue electroluminescent conjugated polymer poly(9,9-dioctylfluorene). Both oxidation and reduction potentials are determined and thus estimates of both the ionization potential Ip and electron affinity Ea of the polymer are obtained for the same sample under the same experimental conditions. We estimate Ip=5.80 eV and Ea=2.12 eV. These results disagree with the common assumption that Ea is, to good approximation, given by the difference between Ip and the optical gap. Measurements on indium tin oxide/polyfluorene/calcium light emitting diode structures are consistent with the deductions from the electrochemical data.

High brightness and efficiency blue light-emitting polymer diodes

Efficient blue electroluminescence, peaked at 436 nm, is demonstrated from polymer light-emitting diodes operating at high brightness. A dioctyl-substituted polyfluorene was used as the emissive layer in combination with a polymeric triphenyldiamine hole transport layer. The luminance reaches 600 cd/m2 at a current density of 150 mA/cm2 for a bias voltage of 20 V, corresponding to an efficiency of 0.25 cd/A and a luminosity of 0.04 lm/W. These values are optimized at a critical emissive layer thickness.

Nondispersive hole transport in an electroluminescent polyfluorene

Time-of-flight measurements have been used to study carrier transport in films of poly(9,9-dioctylfluorene). We find nondispersive hole transport with a room-temperature mobility, μp=4×10−4 cm2/V s at a field of 5×105 V/cm. The field dependence of the mobility is weak: μp=3×10−4 cm2/V s at 4×104 V/cm and increases only modestly to μp=4.2×10−4 cm2/V s at 8×105 V/cm. Both the relatively high mobility and weak field dependence point to a high degree of chemical regularity and purity that makes polyfluorene attractive for use as an electroluminescent polymer and for other device applications. We have not been able to measure clean electron current transients, suggesting highly dispersive transport with possible deep trapping, as found in many other conjugated polymers.

Chain geometry, solution aggregation and enhanced dichroism in the liquidcrystalline conjugated polymer poly(9,9‐dioctylfluorene)

We report on the physical characterization of a dioctyl-substituted polyfluorene, both in solution and in the solid state. We focus on studies of chain geometry both by molecular modeling and by gel permeation chromatography coupled with light scattering. We determine experimentally a Kuhn segment length, lk = 17.1 ± 2.1 nm and a characteristic ratio C∞ = 21.5 %plusmn; 4.3 indicative of a stiff polymer chain. The effects on absorption and emission spectra of intermolecular interactions that lead to gelation or precipitation from solution are reported. We discuss these results in the context of the strong current interest in the nature of aggregation phenomena and their role in controlling the emissive properties of conjugated polymers. We further show that a markedly enhanced dichroism can be achieved through suitable control of the polymer microstructure.

MOBILITY ENHANCEMENT THROUGH HOMOGENEOUS NEMATIC ALIGNMENT OF A LIQUID-CRYSTALLINE POLYFLUORENE

Homogeneous alignment of poly(9,9′-dioctylfluorene) films on rubbed polyimide results in a more than one order of magnitude increase in time-of-flight hole mobility normal to the alignment direction. We find μ=8.5±1×10−3 cm2/V s at an electric field of E=104 V/cm. Hole transport is found to be nondispersive, indicating a low degree of energetic disorder.

Transient and steady-state space-charge-limited currents in polyfluorene copolymer diode structures with ohmic hole injecting contacts

We report detailed measurements on diode structures containing the electroluminescent polyfluorene copolymer poly(9,9-dioctylfluorene-co-bis-N,N′-(4-methoxyphenyl)-bis-N,N′-phenyl-1,4 phenylenediamine). Ohmic injection of holes is achieved with an oxygen plasma cleaned indium tin oxide (ITO) electrode, untreated ITO coated with a film of poly(ethylenedioxythiophene)/polystyrenesulphonic acid (PEDOT/PSS) and plasma cleaned ITO with PEDOT/PSS. Transient dark injection and time-of-flight mobility measurements and steady state current density versus voltage measurements are then entirely consistent with a positive carrier, trap-free, space-charge-limited current. Injection limited behavior is observed, however, for untreated ITO without PEDOT/PSS and for evaporated Au contacts.

Blue-green organic light-emitting diodes based on fluorene-oxadiazole compounds

We explore the electroluminescent properties of multilayer vapor-deposited organic light-emitting diodes based on a novel set of blue-green emitting-fluorene-oxadiazole compounds. The compounds were used both for the transport of electrons and for the emission of light; in all cases, arylamines were used for the transport of holes. Reducing the ionization potential of the fluorene-oxadiazole compound suppressed the exciplex formation between the hole-transporting layer (HTL) and the electron transport layer (ETL) and increased the corresponding photometric efficiency from 0.6 to 8.7 cd/A. High power efficiencies of 4.5 lm/W at a luminance of 200 cd/m2 were also achieved at low operating voltages. However, the device lifetime was very limited which was attributed mainly to the exciplex formation in the HTL/ETL interface and the instability of the oxadiazole moiety in the excited state.

Polarized emission from liquid crystal polymers

Abstract Liquid crystalline conjugated polymers have potential applications as inexpensive, easy to process polarized back lights for liquid crystal displays. Three ‘Hairy rod’ type conjugated polymers, namely Poly(9,9-dioctylfluorene) [PFO] and two alternating co-polymers with benzothiadiazole and dithiophene moieties in the mainchain, have been investigated. In this paper, we present polarized absorption and photoluminescence for films aligned on rubbed polyimide [PI] alignment layers. We also report electroluminescence measurements for oriented multilayer devices with rubbed hole transporting alignment layers [HTAL]. Sample preparation, device fabrication and characterisation are discussed and a preview of future work is given.