Since ytterbium (Yb) possesses a low work function of 2.6 eV and Yb fluoride generally has a high negative free energy of formation, it is conceivable to use Yb, either directly or parasitically, with a metal fluoride, as a cathode in organic light-emitting diodes (OLEDs). In this work, the electronic structure and chemistry at the interface of Yb/poly(9,9-dioctylfluorene-co-benzothiadiazole) (F8BT) were investigated by ultraviolet and x-ray photoemission spectroscopy (UPS and XPS, respectively). We found that the deposition of Yb on top of F8BT foremost formed organometallic complexes with the sulfur but interacted partly with the nitrogen of F8BT, and eventually formed a Yb–C complex at higher Yb coverages. In the UPS spectra, Yb deposition increased the relative peak intensities corresponding to the σ-bonds originated from the aliphatic side chains, implying that some of the π-conjugated carbons in the polymer backbone may be destroyed. These results agree well with the disappearance of the π-to-π* transition as observed from the shake-up peaks of the carbon 1s core level in the XPS. The chemistry at the interfaces of Yb/CsF/Au and Yb/CsF/F8BT was also examined by XPS. In both cases, Cs was liberated from the CsF upon Yb deposition and the Yb reacted with the liberated F to form YbF3. In the Yb/CsF/F8BT system, the dissociated Cs did not exist in the metallic state, but reacted with the N atoms and carbon backbone in the F8BT. The low work function of Cs (2.2 eV) may further enhance the injection of electrons into the polymer layer. More importantly, the utilization of Yb/CsF as an electrode is polymer independent, and the CsF layer between the Yb and F8BT could retard Yb diffusion into the bulk polymer. The present results are supported by a calculation of the free energy needed to liberate the Cs in the metal/CsF system using a simple thermodynamic model. It is suggested that the use of Yb/CsF cathode in the polymer offers an advantage over most other metal fluorides because the current electrode has a higher affinity to liberate Cs due to the high negative heat of formation of YbF3, although Yb alone may not be a good electron injector in OLEDs.

A glycerol-modified poly(3,4-ethylene dioxythiophene) (PEDOT): poly(styrene sulfonate) (PSS) layer was used as an anode buffer layer in polymer light-emitting devices using poly(9,9-dioctylfluorene) (F8) as the emitter. Devices with a configuration of indium tin oxide/PEDOT:PSS (with or without glycerol)/F8/CsF/Al were fabricated. It was found that the glycerol-modified device showed a much larger current density than the unmodified device. At an operating voltage of 6 V, the glycerol-modified device showed a luminance of 1300 Cd/m2 and a current efficiency of 1.7 Cd/A compared to the corresponding values of 500 Cd/m2 and 1.3 Cd/A in the unmodified device. Analysis by ultraviolet spectroscopy suggests that the two devices have the same energy level structure and the performance improvement should not be due to change in the PEDOT/polymer interface. It was further found that incorporating a suitable amount of glycerol into the PEDOT:PSS layer can increase its conductivity by six times. This leads to a better balance in the hole and electron currents and thus improved device efficiency.

The electronic structure of poly(9,9-dioctylfluorene)(PFO) in different molecular weights coated with alkali metals (K and Cs) has been studied by x-ray photoelectron spectroscopy (XPS) and ultraviolet photoelectron spectroscopy (UPS). No significant differences on the electronic structures were observed for all combinations of alkali metals and PFO in different molecular weights. These metals led to low injection barrier at the K/PFO or Cs/PFO interfaces and induced bipolaron gap states in the PFO. With increasing coverage of the alkali metals, the bipolaron states and the highest-occupied molecular orbitals gradually broadened, and the two shake-up peaks of Cls XPS peaks associated with the lowest-unoccupied molecular orbitals of PFO also broadened and greatly diminished. Upon slight oxygen exposure, the two bipolaron states disappeared and the deformed features in the UPS and XPS spectra were partially recovered.

The electronic structure of poly(9,9-dioctylfluorene) (PFO) coated with potassium has been studied by X-ray photoelectron spectroscopy (XPS) and ultraviolet photoelectron spectroscopy (UPS). The low work function of K led to low or no injection barrier at the K/PFO interface and induced bipolaron gap states. With increasing K coverage, the bipolaron states and the highest-occupied molecular orbitals gradually broadened, and the two shake-up peaks of C 1s XPS peaks associated with the lowest-unoccupied molecular orbitals of PFO also broadened, diminished and eventually vanished. Upon slight oxygen exposure, the two bipolaron states disappeared and the deformed features in the UPS and XPS spectra were partially recovered.

We report on the fabrication and properties of single layer blue light-emitting diodes (LEDs) based on conjugated Polymer blends of emissive and hole transport materials. Despite the fact that the photoluminescence quantum efficiency of the blend is lower compared to that of both the host and the guest polymers, an enhancement in both the electroluminescence quantum and paver efficiency is seen for the blend. This observation indicates that the hole transporting material leads to a significantly improved hole injection and thus a greatly improved charge carrier balance factor. Optimised single layer blue LEDs showed a maximum brightness of 6000 Cd/m(2) at 14 V and a maximum external ELQE of 1% (2.1Cd/A) at 15 Cd/m(2) corresponding to a power efficiency of 1 lm/W.

The effect of oxygen on the interface formation between Ca electrode and a poly (9,9-dioctylfluorene) (PFO) film was investigated using x-ray photoelectron spectroscopy (XPS) and ultraviolet photoelectron spectroscopy (UPS). The XPS results indicated that strong interaction between Ca and C occurred and the lowest unoccupied molecular orbitals (LUMO's) of PFO were affected upon a submonolayer Ca deposition. After O-2 exposure, the interaction between Ca and C was reduced and the LUMO of PFO was partially recovered. However, too much O-2 exposure (>10(5) L in our experiments) would again deteriorate the original LUMO and result ina very wide band gap, which might be due to changes in the chemical structures of PFO in the Ca-doped region after extra O-2 exposure. The UPS results confirmed that the deformed UPS spectrum after Ca deposition can be partially recovered and the Ca-induced bipolaron states in the former forbidden energy gap could be removed upon O-2 exposure.

We report the use of green and blue fluorene conjugated polymers doped with hole transport materials consisting of triarylamine copolymers to fabricate bright and efficient blue and green single-layer light-emitting diodes (LEDs). These blends show enhanced quantum and power efficiency, much higher brightness and current densities and lower turn on and operating voltages compared with undoped devices. Optimised blue emission devices exhibited a maximum brightness of 1550 cd/m2, a maximum external electroluminescence quantum efficiency of 0.9 cd/A or 0.4% and a maximum power efficiency of 0.3 lm/W. Optimised green emission devices showed a maximum brightness of 7400 cd/m2, a maximum external electroluminescence quantum yield of 0.9% or 2.75 cd/A and a maximum power efficiency of 0.64 lm/W at high brightness.

The electronic structure of poly (9,9-dioctylfluorene) (PFO) film on a Au-coated Si substrate was investigated by ultraviolet photoelectron spectroscopy (UPS) and x-ray photoelectron spectroscopy (XPS). From the UPS measurement, we obtained the ionization potential (Ip) of the PFO film, Ip=5.60±0.05 eV. From the XPS shake-up peaks of the C1s core level, we estimated the electron energy band gap (Eg) of the film, Eg=3.10±0.10 eV. By comparing the Eg with the optical absorption gap, we found that the value of Eg is closer to the optical absorption maximum than to the optical absorption edge. Therefore, we suggest that the optical absorption maximum may be a better approximation than the optical absorption edge in estimating Eg.

The surface of poly (9,9-dioctylfluorene) (PFO) film was modified by 1.0 keV Ar+ irradiation with a dose of 6.5×1014 ions/cm2 prior to Ca deposition. Ultraviolet and x-ray photoelectron spectroscopic studies indicated that the modified surface could effectively block Ca diffusion into the PFO film and prevent the formation of doping-induced bipolaron states in the former forbidden energy gap. As a result, a sharper metal contact on the surface of the PFO film could be formed, compared to that on the surface without Ar+ irradiation. The results suggest that the judicial surface modification of polymer surfaces may be useful for the improvement of metal/polymer contacts and thus device performance.

The effects of ambient storage, thermal annealing, and ion irradiation on the electronic structure of poly (9,9‐dioctylfluorene) (PFO) films were investigated using ultraviolet photoelectron spectroscopy and x‐ray photoelectron spectroscopy. It was shown that PFO is a rather stable material used for light‐emitting devices.

Ultraviolet photoelectron spectroscopy and X-ray photoelectron spectroscopy (XPS) have been used to study the interface formation between poly(9,9-dioctylfluorene) (PFO) and Ca electrode. As the Ca coverage increases, the vacuum energy level of PFO decreases gradually in order to match the Fermi level (E-F) of Ca. The original highest occupied molecular orbital of PFO moves away from the E-F, and vanishes eventually. Bipolaron states with 1.8 eV in peak interval are formed in the former energy gap. The original lowest occupied molecular orbitals of PFO are broadened resulting in a featureless shake-up in the XPS Cls core level. (C) 2000 Published by Elsevier Science B.V.

The hole transporting fluorene-triarylamine copolymer poly{9,9-dioctylfluorene-co-[N,N′-bis(4-methoxyphenyl)-N,N′-diphenyl-1,4-phenylenediamine]} [PFMO] has been investigated by absorption and emission spectroscopy as both spin-coated films and solutions. The peak wavelength of the steady-state photoluminescence emission and its decay time depend strongly on the dielectric constant of the solvent. The absorption spectrum is, however, largely insensitive to the choice of solvent. These observations suggest the presence of a strongly polar excited state with a charge-transfer character.

The efficiency of electroluminescent devices is strongly affected by charge carrier mobilities, in that electrons and holes must be able to be quickly transported for high quantum efficiency. Hole mobility data are reported here for a new family of five conjugated copolymers consisting of triarylamines-which are known to possess high time-of-flight (TOF) hole mobilities-combined with fluorenes via a Suzuki coupling reaction. All five display high room-temperature TOF hole mobilities, while one, "TFB", has the highest yet reported for a pi-conjugated polymer.

Holographic studies of a new guest-host system based on a highly photoconducting fluorene-triarylamine copolymer are presented. The photorefractive grating dynamics are investigated in detail. We propose a new approach to characterize the temporal behavior of these processes. By performing an inverse Laplace transform analysis using the algorithm CONTIN, we were able to identify several processes and to evaluate the according time constants. We compare this method with a conventional procedure to prove its applicability. Both approaches yield almost identical results for the fast time constant which is down to 1 ms for this material at a writing beam intensity of I-wb = 1.44 W/cm(2). (C) 1999 Elsevier Science B.V, All rights reserved.

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.