Interface Modification to Improve Hole‐Injection Properties in Organic Electronic Devices

The performance of organic electronic devices is often limited by injection. In this paper, improvement of hole injection in organic electronic devices by conditioning of the interface between the hole‐conducting layer (buffer layer) and the active organic semiconductor layer is demonstrated. The conditioning is performed by spin‐coating poly(9,9‐dioctyl‐fluorene‐co‐N‐ (4‐butylphenyl)‐diphenylamine) (TFB) on top of the poly(3,4‐ethylene dioxythiophene): poly(styrene sulfonate) (PEDOT:PSS) buffer layer, followed by an organic solvent wash, which results in a TFB residue on the surface of the PEDOT:PSS. Changes in the hole‐injection energy barriers, bulk charge‐transport properties, and current–voltage characteristics observed in a representative PFO‐based (PFO: poly(9,9‐dioctylfluorene)) diode suggest that conditioning of PEDOT:PSS surface with TFB creates a stepped electronic profile that dramatically improves the hole‐injection properties of organic electronic devices.     

On‐Chain Fluorenone Defect Emission from Single Polyfluorene Molecules in the Absence of Intermolecular Interactions

The emission of semiconducting polyfluorenes is often accompanied by an undesired feature in the green spectral region. Whereas a number of previous investigations have argued in favor of a monomolecular origin of the emission species based on ketonic defects, recent experimental results suggested the necessity of excimer formation between individual fluorenone units. We provide a range of new evidence supporting the monomolecular origin of green band emission in polyfluorenes. Most importantly, we succeed in performing single‐molecule spectroscopy on fluorenone‐containing polyfluorene model compounds. Whereas most fluorenone‐containing molecules exhibit both blue backbone and green fluorenone emission independent of fluorenone concentration, it is the relative intensities of the two species which correlate strongly with the fluorenone concentration on the single‐molecule level. Furthermore, we consider a novel model compound with a bifacial arrangement of two fluorenone units. This compound does not provide any signatures of enhanced intramolecular excimer formation but does strongly indicate that concentration quenching effects occur once fluorenone units can interact electronically. The ability to detect on‐chain defect emission in a single polymer molecule demonstrates that photochemical reactions in conjugated polymers can be monitored by fluorescence spectroscopy down to the level of a few atoms, constituting an unprecedented degree of materials characterization.     

The Influence of UV Irradiation on Ketonic Defect Emission in Fluorene‐Based Copolymers

The influence of UV irradiation in inert atmosphere on the emission spectrum of fluorenone containing poly[9,9‐bis(2‐ethyl)hexylfluorene] (PF2/6) has been investigated by means of optical absorption, photoluminescence (PL) and Fourier transform infrared (FTIR) spectroscopy. It is shown that a substantial reduction of green emission arising from ketonic defect sites can be achieved by irradiation of thin films with UV light. This is found to be accompanied by partial cross‐linking of the films. FTIR measurements show no reduction of the C=O stretching mode upon irradiation, and, moreover, the degree of cross‐linking does not scale with the relative fluorenone content (0.1, 0.5, and 5%). Therefore, the reduced emission intensity in the green spectral region is rather associated with the occurrence of interruptions in the polymer backbone, which reduce the effective conjugation length and subsequently inhibit the energy transfer onto the ketonic defect sites. The found results enabled us to build organic light emitting devices (OLEDs) that can be structured by selective illumination of the emitting layer with an intense UV light source. This method allows for the fabrication of rather efficient (2000 cd m⁻² at 7 V) two‐color OLEDs.     

Recent development of polyfluorene-based RGB materials for light emitting diodes

As a continuation of our presentation at IUMRS-2000 in Hong Kong, we report the latest development of polyfluorene-based Red, Green and Blue (RGB) materials for light emitting diodes at The Dow Chemical Company. A modified Suzuki coupling process is used to synthesize RGB fluorene-based homopolymers and copolymers. Optimization of reaction conditions has led to a highly efficient procedure to generate polymers with controlled molecular weight (Mw), ranging from 10,000 to above 500,000 depending on the requirements of the desired applications. The optical and electronic properties of the polymers are tailored through selective incorporation of different aromatic units into the fluorene copolymer systems. By using this methodology, a portfolio of fluorene-based polymers has been designed and synthesized, achieving emissive colors that cover the entire visible spectrum.The performance of fluorene-based polymers in light emitting devices has been optimized by modifying the polymer compositions so as to increase charge mobilities and to improve the carrier injection balance. As a result of these compositional changes, devices based on Dow's green emitters, using bi-layer structures on indium tin oxide substrates and evaporated metal cathodes, have demonstrated unprecedented high efficiencies at high brightness levels and long lifetime performance. A device comprised of a Dow green emitting polymer has a low turn-on voltage of 2.25 V and exhibits a peak efficiency of 10.5 Cd/A at 6600 Cd/m² at 4.85 V. These devices maintain an efficiency of greater than 10.0 Cd/A up to 50,000 Cd/m²and demonstrate very good stability as exemplified by a device half-life of greater than 1500-h starting from 1100 Cd/m². Similar outstanding progress with red and blue emitters has also been made and will be discussed.     

Efficient White‐Electrophosphorescent Devices Based on a Single Polyfluorene Copolymer

An efficient white‐light‐emitting polymer ( W3 ) is realized by covalently attaching a green fluorophore and a red phosphor into the backbone and the side chains, respectively, of polyfluorene at a concentration of 0.04 mol %. In addition, charge‐transporting pendant units are included to improve carrier injection and transport. White‐electrophosphorescent devices with the structure ITO/PEDOT:PSS/ W3 /CsF/Al (ITO: indium tin oxide; PEDOT:PSS: poly(styrenesulfonate)‐doped poly(3,4‐ethylenedioxythiophene)) exhibit a low turn‐on voltage of 2.8 V and a luminance of ca. 10³ cd m^–2 at below 6 V. The peak luminance and power‐conversion efficiencies are 8.2 cd A^–1 and 7.2 lm W^–1, respectively. Furthermore, the device shows relatively stable white emission: the Commission Internationale d'Éclairage (CIE) chromaticity coordinates of the devices change only slightly from (0.35,0.38) at 10 mA cm^–2 to (0.33,0.36) at 100 mA cm^–2, with an almost constant color render index (CRI) value of 82 at all measured current densities.