ALBERT

Notes: We report the results of a spectroscopic investigation of the quadratic electro-optic nonlinearity in poly(2,5-thienylene vinylene). Electroabsorption spectra closely resemble the second energy derivative of the unperturbed π-π* absorption and show oscillations associated with the strong vibronic features present therein. The dispersion of both the real and imaginary parts of χ(3) (−ω;0,0,ω) has been determined and we find that Re{χ(3)} has a peak value of 7.5×10−9 esu at 1.83 eV and Im{χ(3)} has a peak value of 6.2×10−9 esu at 1.78 eV. This nonlinearity is responsible for a photorefractive effect under resonant photoexcitation.

Notes: Polarized confocal Raman mapping is used to characterize the degree of molecular alignment in spin coated conjugated polymer thin films. To this end, the polarization selection rules of a specific Raman active mode are employed to infer indirectly the local molecular alignment. The film can be monitored and characterized with a diffraction limited spatial resolution fixed by the microscope optics. In addition, we demonstrate examples of characterization of molecular alignment in highly oriented samples by monitoring the angular dependence of the polarized Raman signal. Finally, we show the use of temperature dependent Raman scattering to monitor thermal phase transitions in bulk conjugated polymer samples. These procedures can be used as in situ characterization methods in a wide variety of experimental situations and offer a sensitive probe of processing protocols used in the fabrication of polymer optoelectronic devices. © 2002 American Institute of Physics.

Notes: Impedance, current–voltage–luminosity and spectral measurements have been carried out on indium-tin-oxide/N,N′-diphenyl-N,N′-bis(3-methylphenyl)1-1′-biphenyl-4,4′-diamine (TPD)/Al light emitting diodes. The devices have a blue/violet emission with a spectrum peaked at 404 nm. Capacitance–voltage measurements show that at zero bias the devices are fully depleted. The impedance measurements show that the devices can be modeled on a single, frequency-independent parallel resistor-capacitor RPCP circuit with a small series resistance RS. RP changes with applied bias and temperature, while CP remains constant. The values of CP give cursive-epsilonr=3.0±0.3. Analysis of the current–voltage (J–V) characteristics show that the dominant conduction mechanism cannot be either ohmic, trap free space charge limited, or tunneling injection. The temperature and thickness dependence indicate that it must be either thermionic emission or thermally assisted tunneling, the carrier density varying from about 1010/1011 to 3×1013 cm−3 over the measured bias range. The EL efficiency increases 20 fold upon cooling but shows little variation with bias at all temperatures, indicating the same mechanism is responsible for the injection of both holes and electrons. Modeling the results with thermionic emission suggests that image force lowering is responsible for the variation of the current with applied bias, but the calculated injection barrier height and Richardson constant are much smaller than expected. This cannot be explained by models based on a backflowing surface recombination current due to the high carrier mobility found in TPD. © 1999 American Institute of Physics.

Notes: We report electroabsorption studies of poly(2-methoxy-5-ethyl(2′-hexyloxy) para-phenylene vinylene) light-emitting diodes. An electric field develops during operation which opposes the field of the applied bias. The counter field builds up within 5 s of turning on the device, increases in magnitude with the operating voltage, and decays exponentially with a time constant between 15 and 32 s. We attribute the counter field to bulk carrier traps and discuss its relevance to the increase of the turn-on voltage as organic light-emitting diodes degrade. © 1999 American Institute of Physics.

Notes: We report the use of a poly(phenyl quinoxaline) (PPQ) as an electron transporting conjugated polymer for electroluminescence (EL) applications. Single-layer PPQ devices with ITO anode and aluminium cathode show unipolar electron transport with current densities up to 60 mA/cm2 but no emission. Two-layer structures combining PPQ as electron transport material with the hole transporting poly(2,5-dialkoxy-p-phenylene vinylene) (PDAOPV) show strong emission from the PDAOPV with brightnesses up to 250 cd/m2. These two-layer structures have a maximum EL quantum efficiency of 0.35% which is ten-fold enhanced compared with the corresponding single-layer PDAOPV devices. © 1996 American Institute of Physics.

Notes: Current–voltage, impedance, and transient conductance measurements have been carried out on indium-tin-oxide/poly(phenylene vinylene)/Al light emitting diodes. In these devices injection and transport is expected to be dominated by positive carriers. Fowler–Nordheim tunneling theory cannot account for the temperature dependence, the thickness dependence, or the current magnitude of the current–voltage characteristics. Space-charge limited current theory with an exponential distribution of traps is however in extremely good agreement with all of the recorded current–voltage results in the higher applied bias regime (approximately 0.7≤V/d≤1.6×106 V cm−1). This gives a trap density Ht of 5(±2)×1017 cm−3 and the product of μNHOMO of between 1014 and 5×1012 cm−1 V−1 s−1. Assuming NHOMO is 1020 cm−3 gives an effective positive carrier mobility between 10−6 and 5×10−8 cm2 V−1 s−1. The characteristic energy Et of the exponential trap distribution is 0.15 eV at higher temperatures (190≤T≤290 K), but this decreases as the devices are cooled, indicating that the distribution is in fact a much steeper function of energy closer to the highest occupied molecular orbital (HOMO) levels. The current–voltage characteristics in the lower applied bias regime (approximately V/d≤0.7×106 V cm−1) can be fitted to pure space-charge limited current flow with a temperature and field dependent mobility of Arrhnenius form with a mobility at 290 K close to the above values. If NHOMO lies between 1021 and 1019 cm−3, then the trap filled limit bias gives a mobility independent value of Ht of 3(±1)×1017 cm−3. Capacitance–voltage measurements show that at zero bias the devices are fully depleted, and that the acceptor dopant density NA must be less than about 1016 cm−3. The impedance results show that the devices can be modeled on a single, frequency independent, parallel resistor-capacitor circuit with a small series resistor. The variation of the resistor and capacitor in the parallel circuit with applied bias and temperature are consistent with the space-charge limited current theory with the same exponential trap distribution used to model the current–voltage characteristics. Initial results for transient conductance measurements are reported. The transients have decay times greater than 300 s and exhibit a power-law dependence with time. This is shown to be exactly the behavior expected for the decay of an exponential trap distribution. Measurements at higher temperatures (290≥T≥150 K) give an Et of 0.15 eV, in excellent agreement with that found from the current–voltage measurements. This value of Et is exactly that found by similar analysis of the current–voltage characteristics in negative carrier dominated dialkoxy poly(phenylene vinylene) and Mq3 devices. It is proposed that this bulk transport dominated behavior is purely a consequence of hopping conduction through an approximately Gaussian density of states in which the deep sites act as traps. © 1997 American Institute of Physics.

Notes: The use of a new highly luminescent conjugated polymer as an emissive layer in single and multilayer electroluminescence devices is reported. Poly(m-phenylenevinylene-co-2,5 -dioctyloxy-p-phenylenevinylene) [PmPV-co-DOctOPV] was prepared via a Wittig synthesis reaction. The resulting polymer has a high photoluminescence quantum efficiency in the solid state with an emission spectrum peaked at 506 nm (2.45 eV) in the green. Electroluminescence devices were fabricated with an ITO anode and a MgAg cathode. Three different structures were studied: (i) single layer devices containing only PmPV-co-DOctOPV; (ii) double layer devices with PmPV-co-DOctOPV and an evaporated film of 1,3-bis(4-tert-butylphenyl-1,3,4-oxadiazoyl) phenylene [OXD-7] as an electron transport layer; (iii) triple layer devices containing PmPV-co-DOctOPV, OXD-7 and in addition a polyvinylcarbazole hole transport layer. Electroluminescence external quantum efficiencies for these devices were found to be up to 0.08%, 0.55%, and 1%, respectively, corresponding to luminous efficiencies of (approximate)0.5, (approximate)3, and (approximate)6 lm/W and power efficiencies of 8.5×10−5, 5.9×10−4, and 6.0×10−4 W/W. © 1997 American Institute of Physics.

Notes: The current–voltage (J–V) characteristics of ITO/polymer film/Al or Au structures of poly(phenylene vinylene) (PPV) and a dialkoxy PPV copolymer have been recorded for a range of different film thickness d and temperatures T. At high applied bias all the characteristics can be fitted over a given range to a power law J=KVm, where m increases with decreasing T, log(K) is proportional to m, and K is proportional to d−α m, where α∼2 (ITO/polymer film/Al devices) and ∼1 (ITO/polymer film/Au devices). Different single carrier space charge limited conduction theories have been used to try and explain this behavior. The analytical theory in which the carrier density is decreased by an exponential trap distribution lying below effectively isoelectronic transport states is in good agreement, but cannot explain the thickness dependence of the ITO/polymer film/Au devices and can be criticized as being physically unreasonable. A numerical analysis in which the mobility has the field and temperature dependence found for hopping transport in disordered systems is also in good agreement, but can only fit a small range of J and cannot explain the magnitude of K, the temperature dependence of m or the abrupt change in slope in the J–V characteristics with increasing bias. Mixed models are equally good but cannot explain the deviations from experiment. We consider that further experimental studies of carrier mobilities and the nature of the traps present in such materials is required to distinguish between these models and resolve the nature of bulk limited conduction in conjugated polymers. © 1998 American Institute of Physics.

Notes: We have fabricated light-emitting diodes with poly(p-phenylenevinylene) as the emissive layer, and with an electron-transporting layer formed from a solid state dispersion of 2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole in poly(methyl methacrylate), placed between this and the negative electrode. These structures show typically a tenfold improvement in efficiency in the low-voltage regime and an eightfold improvement in the high-voltage regime over devices without the electron-transporting layer. Typical efficiencies are about 0.8% photons/electron. We consider that the role of the electron-transport layer is to confine holes to the emissive layer.

Notes: We report the observation of electroluminescence and photoluminescence from microcavity structures containing poly(2,5-dialkoxy-p-phenylene vinylene) (PDAOPV) conjugated polymer emitting regions. Strong spectral narrowing from 112 nm full width half-maximum to 34 nm in electroluminescence and from 128 to 16 nm in photoluminescence and clear angular dependence of the peak emission wavelength are observed. These are characteristic signatures of the modification of spontaneous emission properties in microcavity structures. © 1995 American Institute of Physics.