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We investigate the optical properties of the dipole-like modes of an H1 nano-cavity consisting of a single missing airhole imbedded into a triangular two-dimensional silicon nitride (Si3N4) based photonic crystal coated with a red-fluorescent molecular dye. We modify the size and position of the first six neighboring airholes around the nano-cavity and demonstrate that this allows control over the energy and separation of two dipole-like optical modes (Mx and My). This allows us to produce either linearly polarized optical modes or an unpolarized optical mode composed of degenerate modes having orthogonal polarization. We confirm our findings using three-dimensional finite difference time domain calculations.

In this article we present a detailed comparison of ultrasonic spray coating and spin coating for the fabrication of polymer organic light-emitting diodes (OLEDs). Single-carrier devices of hole-transporting polymer poly[(9,9-dioctylfluorenyl-2,7-diyl)-co-(4,4’(N-(4-sec-butylphenyl))) diphenylamine] (TFB) were fabricated by ultrasonic spray coating. Uniform reference devices using spin coating were also made. We have shown, across a range of device thicknesses from 37 nm to 138 nm, typical of those used in OLED hole-transport layers, that there is no statistical difference in the hole-injection efficiency between ultrasonic spray coating and spin coating. We have also demonstrated the importance of controlling the roughness of the films and we determine a threshold of 10 nm average roughness below which injection efficiency is not controlled by roughness. However, above 10 nm roughness we find a reduction in injection efficiency up to an 86 % loss in performance for roughnesses of the order of 40 % the thickness of the film. By optimising the deposition parameters, in order to allow the wet films to start to equilibrate, we find a wide processing window for smooth uniform films with excellent injection efficiency. This work reinforces the importance of ultrasonic spray coating as a potential route to high volume manufacturing of OLED based technology.

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.

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...

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%, respectivel...

We have fabricated strongly coupled organic microcavities by laminating an organic thin film between two dielectric mirrors. The organic film consisted of J aggregates of a cyanine dye suspended in a transparent matrix. By applying a nonuniform force across the substrate during lamination, the optical path length of the cavity changed linearly across the sample, thereby forming a wedge. This enabled cavity mode tuning by changing the position of the incident light beam with respect to the surface of the cavity. Transmission measurements are employed to investigate strong exciton-photon coupling in such microcavities. The results show a well-resolved anti-crossing behavior, which is confirmed by numerical analysis.

We report the resonant pump–probe measurements used to study the dynamics of molecular vibrations in the conjugated polymer poly(9,9-dioctlyfluorene) (PFO). Free-electron-laser excited, pump–probe measurements on a drop-cast polymer film yield the lifetime of a series of different infrared active, high frequency vibrational modes at 5K. A general trend of decreasing lifetime with increasing frequency of the vibrational mode seems consistent with an enhanced density of accepting states for high frequency modes. Our measurements provide an insight into the dissipation of energy in conjugated polymers, and have implications for exciton generation and dissociation mechanisms in organic optoelectronic devices.

We report the fabrication of high-finesse strongly coupled microcavities composed of a polystyrene film doped with the dye tetraphenyl-porphyrin zinc positioned between two high reflectivity dielectric mirrors. The bottom mirror was deposited by plasma enhanced chemical vapor deposition, and was composed of 11 lambda/4 thick (silicon oxide/silicon nitride) pairs. The organic layer was deposited on to this by spin coating. Finally, the top mirror was deposited by thermal evaporation and consisted of 12 lambda/4 thick (tellurium oxide/lithium fluoride) pairs. Such cavities are characterized by Q factors of between 440 and 620. Strong coupling was evidenced via white light reflectivity measurements. Due to the high cavity Q factor, a Rabi splitting of 135 meV at resonance was very clearly resolved. (C) 2003 American Institute of Physics.

We report the experimental observation of strong exciton–photon coupling in a planar microcavity composed of an organic semiconductor positioned between two metallic (silver) mirrors. Via transmission and reflectivity measurements, we observe a very large, room temperature Rabi splitting in excess of 300 meV. We show that the Rabi-splitting is enhanced in all-metal microcavities by a factor of more than 2 compared to an organic film positioned between a silver mirror and a dielectric mirror. This enhancement results from the significantly larger optical fields that are confined within all-metal microcavities.

Unbalanced carrier transport is known to strongly affect the efficiency of polymer light-emitting diodes. Here, we report the results of time-of-flight (TOF), current density–voltage, and electroluminescence (EL) quantum efficiency measurements on single-layer poly(9,9-dioctylfluorene) (PFO) devices doped with the red-emitter tetraphenylporphyrin (TPP). TOF shows that PFO is a unipolar conductor, with hole transport much better than electron transport. At a field of 5×105 V/cm, a nondispersive hole mobility of 4×10−5–5×10−4 cm2/V s, dependent on sample morphology, is obtained. Upon the addition of 5% by weight TPP, hole transport becomes as highly dispersive as electron transport, having no measurable average mobility. This results in a decrease in the current for a given applied bias but an increase in the external EL quantum efficiency. TPP acts as a strong hole trap, reducing the dominant hole current and producing more balanced carrier transport. At TPP concentrations above 6%, the device characterist...

We have investigated the performance of conjugated polymer light emitting diodes (LEDs) as their active areas are reduced to dimensions typical for pixellated displays. We find that by reducing the active diameter of the device from our standard size (1.5×3 mm) to 50 μm, the current density that can be sustained before LED failure increases by a factor of 9. This increase in current density is mirrored by similar increases in maximum LED brightness. We propose that such effects occur due to improved thermal management in small area devices. Using pulsed driving schemes, devices with a diameter of 50 μm sustained current densities of 5.1 MA m−2, and have emitted electroluminescence with a brightness of 6.5 Mcd m−2.

We report on the fabrication and device properties of light-emitting resonant cavity conjugated-polymer diodes. The microcavity structures were constructed using a dielectric mirror, an indium–tin–oxide anode, a green light-emitting polyfluorene blend, and a metal (cathode) mirror. The best performance was obtained for a composite calcium–aluminum cathode, that combines high reflectivity with a low workfunction. This structure emitted electroluminescence with a linewidth of 12 nm and a maximum brightness of 300 cd m−2 at a bias of 35 V with an efficiency of 0.95 cd A−1.

We have studied the device degradation of single layer poly(2,5-dialkoxy-p-phenylenevinylene) light emitting diodes by electroabsorption spectroscopy. The applied direct current (dc) bias generates an opposing internal field. This internal field rises as the applied dc bias is increased. The development of the internal field is less pronounced in vacuum than in an ambient atmosphere and is no longer apparent for devices that were prepared and tested under an inert atmosphere in a glovebox. For the devices that were tested in air and under dynamic vacuum conditions we have also observed a change in the flat band voltage of the devices due to an aging effect on the electrodes. The combination of these two processes leads to an increase in the device turn-on voltage with increasing operating time.

The application of oxide buffer layers for improved carrier extraction is ubiquitous in organic electronics. However, the performance is highly susceptible to processing conditions. Notably, the interface stability and electronic structure is extremely sensitive to the uptake of ambient water. In this study we use density functional theory calculations to asses the effects of adsorbed water on the electronic structure of MoOx, in the context of polymer-fullerene solar cells based on PCDTBT. We obtain excellent agreement with experimental values of the ionization potential for pristine MoO3 (010). We find that IP and EA values can vary by as much as 2.5 eV depending on the oxidation state of the surface and that adsorbed water can either increase or decrease the IP and EA depending on the concentration of surface water.

We have made a comparative study of the relative operational stability of bulk-heterojunction organic photovoltaic (OPV) devices utilising different hole transport layers (HTLs). OPV devices were fabricated based on a blend of the polymer PCDTBT with the fullerene PC70BM, and incorporated the different HTL materials PEDOT:PSS, MoOx and V2O5. Following 620 h of irradiation by light from a solar simulator, we find that devices using the PEDOT:PSS HTL retained the highest efficiency, having a projected T80 lifetime of 14 500 h.

A scanning tunneling microscope was used to generate electroluminescence (EL) from thin films of the conjugated polymer poly(1,3-phenylenevinylene-co-2,5-dioctyloxy-1,4-phenylenevinylene). This allowed the spatial distribution of EL to be mapped across the film, and also measurements of local EL emission spectra to be recorded. It was observed that both the emission spectra and their intensity are highly nonuniform on length scales ≳2 nm. This is important in the context of light emitting diodes since it indicates that control of nanostructure can greatly improve device performance.

The distribution of a confined optical field within a microcavity was determined using a thin layer of the fluorescent polymer PPV as a probe of the local-field amplitude. It is shown that the photoluminescence emission intensity from the cavity is a function of the axial position of the polymer, and confirms that a microcavity can be used to control spontaneous emission.

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.

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.

We report a comparative study based on the fabrication of polymer:fullerene photovoltaic (PV) devices incorporating carbazole, fluorene, and a PTB based co-polymer. We have explored the efficiency and performance of such devices when the active polymer:fullerene layer is deposited by spin-casting either under nitrogen or ambient conditions. We show that PV devices based on carbazole and fluorene based materials have very similar power conversion efficiencies when processed under both air and nitrogen, with other photobleaching measurements suggesting that such materials have comparatively enhanced photostability. Devices based on the PTB co-polymer, however, have reduced efficiency when processed in air.

We construct a microcavity in which the extended optical path length of the cavity (5.9 μm) permits a series of closely spaced optical modes to be supported. By placing a J-aggregated cyanine dye into the cavity, we reach the strong-coupling regime and evidence a simultaneous optical hybridization between the organic-exciton and a number of the confined cavity modes, forming an effective ladder of polariton branches. We explore the emission from such cavities and evidence a polariton-population on adjacent polariton branches around k∥ = 0.

A key prerequisite for low-threshold polariton lasing in organic or inorganic microcavity systems is the efficient population of the lower polariton ground state. Here, we report the observation of a resonant phonon-mediated relaxation process which gives rise to nonthermal polariton population with sub 100 fs build-up times. This mechanism is manifested by discrete maxima of the angular-resolved photoluminescence intensity, with corresponding shortening of the photoluminescence rise time at respective phonon resonances. The realization of enhanced relaxation rates in disordered J-aggregate systems is important for developing room temperature organic laser sources with less fabrication complexity than their crystalline counterparts.

We report spectroscopic and electrical measurements to explore hole injection and conduction in devices comprising a molybdenum sub-oxide (MoOx) hole injection layers and poly[(9,9-dioctylfluorenyl-2, 7-diyl)-co-(4,4’(N-(4-sec-butylphenyl))) diphenylamine](TFB) hole transporting polymer. We report improvements in device conductivity over benchmark structures incorporating an ITO electrode and polyethylenedioxythiophene polystyrene sulfonate (PEDOT:PSS) hole injection layers and furthermore achieve injection from MoOx to TFB that is efficient even with an underlying low workfunction Al electrode. XPS spectroscopy has been used to investigate the electronic structure of the interfaces and we find discrete energy alignment regimes consistent with recent surface science studies by Tengstedt et al. [Appl. Phys. Lett. 88, 053502 (2006)], corresponding to Fermi level pinning for MoOx/TFB and vacuum level pinning in the case of Al/TFB. While the energetic alignment regime is measured to be independent of MoOx thi...

We model the optical properties of a SiN based one-dimensional waveguide photonic crystal nanocavity with three tapered air holes as a function of the photonic crystal mirror fill factor using finite difference time domain calculations. We show that the Q-factor of this type of cavity has a strong dependence on the mirror fill factor, with a Q-factor as high as 35 000 predicted. Experimental studies on a range of representative structures whose surface is coated with a layer of a fluorescent dye confirm our modeling and reveal that such structures are characterized by a single optical mode having a Q-factor of 2000.

To maximize the electroluminescence (EL) switching rate that can be generated in an organic-based light emitting diode, we fabricated a device having an active area of 0.018 mm2 which is based on a 1:1 blend of the high charge-carrier mobility conjugated polymers poly[dioctylfluoreneco-N-(4-butylphenyl) diphenylamine] and poly[9,9′-dioctylfluorene-co-benzothiadiazole]. Our devices have a 50% cutoff frequency of 26 MHz and a maximum EL modulation rate of 63 MHz at a drive voltage of 3.8 V. Such switching rates are suitable for short-range video transmission via plastic optical fibers, suggesting possible applications as optical communication light sources.

Single molecule fluorescence spectroscopy is used to study the optical properties of two polymers: a fluorene-based statistical copolymer that contains a low fraction (10%) of a red-emitting thiophene group, and the green-emitting polymer poly(9,9-dioctylfluorene-alt-benzothiadiazole). These polymers were studied when isolated at a low concentration in a polymer matrix (either polynorbornene or polystyrene). For the red-emitting polymer, we compare the relative emission intensity from the green-emitting benzothiadiazole groups with the red-emitting thiophene. We find that red emission from the thiophenes is significantly suppressed in the single molecule regime, suggesting that interchain energy transfer dominates intrachain processes in such polyfluorene copolymers. We then use fluorescence spectroscopy and polarization anisotropy measurements to show that the conformations of both polymers are dependent on their host matrix, adopting a more collapsed, globular conformation in polystyrene and a more extended chain conformation in polynorbornene.

An ultracompact near-field optical probe is described that is based on a single, integrated assembly consisting of a gallium nitride (GaN) light-emitting diode (LED), a microlens, and a cantilever assembly containing a hollow pyramidal probe with a subwavelength aperture at its apex. The LED emits ultraviolet light and may be used as a light source for near-field photolithographic exposure. Using this simple device compatible with many commercial atomic force microscope systems, it is possible to form nanostructures in photoresist with a resolution of 35 nm, corresponding to λ/10.

We have used time-resolved scanning near-field microscopy to image the fluorescence decay lifetime across a phase-separated blend of the photovoltaic-applicable polymers poly(9,9′-dioctylfluorene-alt-benzothiadiazole) (F8BT) and poly(9,9′-dioctylfluorene-alt-bis- N,N′-(4-butylphenyl)-bis-N,N′-phenyl-1,4-phenylenediamine) (PFB). We show that the efficiency of local fluorescence quenching is composition dependent, with excitons on F8BT molecules being more effectively quenched when F8BT is trapped at a low concentration in a PFB-rich phase. Despite such presumed differences in charge-carrier generation efficiency, our results demonstrate that charge extraction from F8BT:PFB devices is the most dominant mechanism limiting their operational efficiency.

We demonstrate a significant improvement in the external electroluminescence efficiency of red-emitting polymer light emitting diodes (LEDs) by modifying the optical structure of the device. By using a cathode composed of a thin (5nm) film of calcium backed with an optically thick film of silver, we measured improvements in the external efficiency of polymer LEDs by a factor of 1.6 times compared to a device using a cathode composed of calcium backed with lower reflectivity aluminum. By incorporating the LED into a microcavity structure (to form a resonant cavity LED) it is possible to obtain additional (but rather smaller) improvements in external efficiency of the order of 1.15 times, compared to a standard LED utilizing the same cathode. By combining high reflectivity cathode∕mirror materials with a low finesse cavity structure, we show that the external efficiency of a LED can be improved by as much as 1.8 times compared to a standard (noncavity) LED. Our results are in good agreement with those of th...