Wojciech Haske

A Universal Method to Produce Low–Work Function Electrodes for Organic Electronics

A Sturdy Electrode Coating To operate efficiently, organic devices—such as light-emitting diodes—require electrodes that emit or take up electrons at low applied voltages (that is, have low work functions). Often these electrodes are metals, such as calcium, that are not stable in air or water vapor and have to be protected from environmental damage. Zhou et al. (p. 327; see the Perspective by Helander) report that a coating polymer containing aliphatic amine groups can lower the work functions of various types of electrodes by up to 1.7 electron volts and can be used in a variety of devices. Air-stable, physisorbed polymers containing aliphatic amine groups can improve the efficiency of organic electronic devices. Organic and printed electronics technologies require conductors with a work function that is sufficiently low to facilitate the transport of electrons in and out of various optoelectronic devices. We show that surface modifiers based on polymers containing simple aliphatic amine groups substantially reduce the work function of conductors including metals, transparent conductive metal oxides, conducting polymers, and graphene. The reduction arises from physisorption of the neutral polymer, which turns the modified conductors into efficient electron-selective electrodes in organic optoelectronic devices. These polymer surface modifiers are processed in air from solution, providing an appealing alternative to chemically reactive low–work function metals. Their use can pave the way to simplified manufacturing of low-cost and large-area organic electronic technologies.

Crosslinking Using Rapid Thermal Processing for the Fabrication of Efficient Solution‐Processed Phosphorescent Organic Light‐Emitting Diodes

Copolymers with a triscarbazole hole-transport group and an oxetane or benzocyclobutene crosslinkable group can be readily thermally crosslinked on timescales of 30 min or less, with rapid thermal processing (RTP) being highly effective for this purpose. Devices with RTP-crosslinked hole-transport layers and spin-coated emissive layers exhibit high external quantum efficiencies of up to 15%.

Highly efficient inverted top-emitting green phosphorescent organic light-emitting diodes on glass and flexible substrates

Green phosphorescent inverted top-emitting organic light-emitting diodes with high current efficacy and luminance are demonstrated on glass and polyethersulfone (PES) substrates coated with polyethylene dioxythiophene-polystyrene sulfonate (PEDOT:PSS). The bottom cathode is an aluminum/lithium fluoride bilayer that injects electrons efficiently into an electron transport layer of 1,3,5-tri(m-pyrid-3-yl-phenyl)benzene (TpPyPB). The cathode is found to be highly sensitive to the exposure of trace amounts of O2 and H2O. A high current efficacy of 96.3 cd/A is achieved at a luminance of 1387 cd/m2 when an optical outcoupling layer of N,N′-Di-[(1-naphthyl)-N,N′-diphenyl]-(1,1′-biphenyl)-4,4′-diamine (α-NPD) is deposited on the anode.

Inverted top-emitting blue electrophosphorescent organic light-emitting diodes with high current efficacy

Two different types of inverted top-emitting blue electrophosphorescent organic light-emitting diodes (OLEDs) are demonstrated that differ only in the choice of high electron mobility transport layers. The electron transport layer consists of either 1,3,5-tri(p-pyrid-3-yl-phenyl)benzene (TpPyPB) or 1,3,5-tri(m-pyrid-3-yl-phenyl)benzene) (TmPyPB). Devices with TpPyPB exhibit a current efficacy of 5.1 cd/A at 1259 cd/m2. OLEDs with TmPyPB show higher performance with a current efficacy of 33.6 cd/A at 1126 cd/m2. The difference in performance of OLEDs with TmPyPB is due to a combination of TmPyPBs higher triplet energy that decreases exciton transfer to the ETL and altered charge balance.

Efficient blue-emitting electrophosphorescent organic light-emitting diodes using 2-(3,5-di(carbazol-9-yl)phenyl)-5-phenyl-1,3,4-oxadiazole as an ambipolar host

2-(3,5-Di(carbazol-9-yl)phenyl)-5-phenyl-1,3,4-oxadiazole, 1, was synthesised from the reaction of 3,5-di(carbazol-9-yl)benzohydrazide and trimethyl orthobenzoate at 185 °C. Compound 1 exhibits a glass-transition temperature of 120 °C, a reversible reduction at a half-wave potential of −2.34 V vs. ferrocenium/ferrocene, and an adiabatic triplet energy of 2.73 eV. Organic light-emitting diodes were fabricated using: solution-processed poly[6-(9H-carbazol-9-yl)-9-(4-vinylbenzyl)-9H-3,9′-bicarbazole] as the hole-transport layer; a vacuum-deposited emissive layer composed of 1 as a host and bis[(4,6-di-fluorophenyl)pyridinato-N,C2′](picolinato-N,O)iridium or fac-tris(2-phenylpyridinato-N,C2′)iridium as blue- or green-emitting phosphorescent guest molecules, respectively; and vacuum-deposited 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline or 3-([1,1′-biphenyl]-4-yl)-5-(4-(tert-butyl)phenyl)-4-phenyl-4H-1,2,4-triazole as the electron-transport layer. External quantum efficiencies of up to 21 and 25% were obtained for blue- and green-emitting devices, respectively.

Phosphorescent light-emitting diodes using triscarbazole/bis(oxadiazole) hosts: comparison of homopolymer blends and random and block copolymers

Examples of blends of carbazole- and bis(oxadiazole)benzene-based side-chain polymers have recently been reported to be efficient host materials for phosphorescent emitters in organic light-emitting diodes. Here, the properties and performance of a physical blend of polynorbornene homopolymers with triscarbazole and bis(oxadiazole)benzene side chains are compared to those of random and block copolymers of the corresponding triscarbazole- and bis(oxadiazole)benzene-functionalized monomers. Green-emitting devices in which the blend is used a host for Ir(ppy)3 are significantly more efficient than those based on copolymers. Differential scanning calorimetry and solid-state NMR data show that there is no macroscale separation between the two polymers in the blend. The NMR data suggest that there are significant differences in the dimensionality and characteristic length of nanoscale domain structures in the block copolymer and the blend. Use of Ir(pppy)3 in place of Ir(ppy)3 leads to even more efficient light-emitting diodes, with external quantum efficiencies of up to ca. 21% (at 100 cd m−2).

Stacked inverted top-emitting white organic light-emitting diodes composed of orange and blue light-emitting units

Stacked inverted top-emitting white electrophosphorescent organic light-emitting diodes (OLEDs) are demonstrated. The OLEDs consist of orange and blue light-emitting units interconnected with a connecting unit of 1,4,5,8,9,11-hexaazatriphenylene hexacarbonitrile/Al/LiF. These OLEDs combine the features of having inverted electrode positions, top-emission, and a stacked architecture. They exhibit an average current efficacy of 26.5 cd/A at a luminance of 100 cd/m2. Single-unit inverted top-emitting OLEDs based on the constituent orange and blue light-emitting units are also characterized for comparison. The current efficacies of the orange and blue OLEDs are 21.2 cd/A and 32.6 cd/A, respectively, at a luminance of 100 cd/m2.

Efficient Green and Blue Electrophosphorescent Light-Emitting Diodes using a Combination of Solution and Vacuum-Processed Materials

The performance of organic light-emitting diode devices with a spin-coated hole-transporting layer and a thermally deposited emissive layer consisting of a bis-sulfone small molecule, as a host for the blue phosphorescent emitter will be presented.

Advances in Two-Photon 3D Microfabrication

The development of two-photon materials for the fabrication of features with 80 nm resolution in 3D microfabrication and the fabrication of a range of photonic crystals with mid-IR stop bands will be discussed.