Manuela Wallesch

Bright coppertunities: multinuclear Cu(I) complexes with N-P ligands and their applications.

Easy come, easy go: the great structural diversity of Cu(I) complexes is an ambivalent trait. Apart from the well-known catalytic properties of Cu(I), a great number of potent luminescent complexes have been found in the last ten years featuring a plethora of structural motifs. The downside of this variety is the undesired formation of other species upon processing. In here, strategies to avoid this behavior are presented: Only one favorable structural unit often exists for multinuclear Cu(I) complexes with bridging ligands. In addition, these complexes exhibit favorable photophysical properties due to cooperative effects of the metal halide core. Furthermore, we demonstrate the broad range of applications of emitting Cu(I) compounds.

From iridium and platinum to copper and carbon: new avenues for more sustainability in organic light-emitting diodes

Recently, the first commercially successful applications for organic light-emitting devices (OLEDs) have entered the lighting and display markets, especially in smaller devices such as tablets and smartphones. In this article, we analyse materials and techniques used in OLED manufacturing in terms of sustainability and highlight upcoming trends which are supposed to further enhance this technologies sustainability.

Bridging the efficiency gap: fully bridged dinuclear Cu(I)-complexes for singlet harvesting in high-efficiency OLEDs.

The substitution of rare metals such as iridium and platinum in light-emitting materials is a key step to enable low-cost mass-production of organic light-emitting diodes (OLEDs). Here, it is demonstrated that using a solution-processed, fully bridged dinuclear Cu(I)-complex can yield very high efficiencies. An optimized device gives a maximum external quantum efficiency of 23 ± 1% (73 ± 2 cd A(-1) ).

Labile or Stable: Can Homoleptic and Heteroleptic PyrPHOS–Copper Complexes Be Processed from Solution?

Luminescent Cu(I) complexes are interesting candidates as dopants in organic light-emitting diodes (OLEDs). However, open questions remain regarding the stability of such complexes in solution and therefore their suitability for solution processing. Since the emission behavior of Cu(I) emitters often drastically differs between bulk and thin film samples, it cannot be excluded that changes such as partial decomposition or formation of alternative emitting compounds upon processing are responsible. In this study, we present three particularly interesting candidates of the recently established copper-halide-(diphenylphosphino)pyridine derivatives (PyrPHOS) family that do not show such changes. We compare single crystals, amorphous bulk samples, and neat thin films in order to verify whether the material remains stable upon processing. Solid-state nuclear magnetic resonance (MAS (31)P NMR) was used to investigate the electronic environment of the phosphorus atoms, and X-ray absorption spectroscopy at the Cu K edge provides insight into the local electronic and geometrical environment of the copper(I) metal centers of the samples. Our results suggest that--unlike other copper(I) complexes--the copper-halide-PyrPHOS clusters are significantly more stable upon processing and retain their initial structure upon quick precipitation as well as thin film processing.

ortho-Bromo(propa-1,2-dien-1-yl)arenes: Substrates for Domino Reactions

o-Bromo(propa-1,2-dien-1-yl)arenes exhibit novel and orthogonal reactivity under Pd catalysis in the presence of secondary amines to form enamines (concerted Pd insertion, intramolecular carbopalladation, and terminative Buchwald-Hartwig coupling) and of amides to form indoles (addition, Buchwald-Hartwig cyclization, and loss of the acetyl group). The substrates for these reactions can be accessed in a reliable and highly selective two-step process from 2-bromoaryl bromides.

Singlet harvesting copper-based emitters: a modular approach towards next-generation OLED technology

Copper(I)-based emitters show great potential for addressing the challenges of current organic light-emitting diode (OLED) technology. They can match current state-of-the-art phosphorescent materials for efficiency and can be tuned in color from red to blue. This paper gives an overview, describing examples of mono- and dinuclear Cu(I) complexes in terms of structures and properties. In particular, the modular structure of dinuclear compounds allows the independent tuning of emission color and solubility, making these materials perfect candidates for large area OLEDs produced from solution.

Towards Printed Organic Light-Emitting Devices: A Solution-Stable, Highly Soluble CuI–NHetPHOS

The development of iridium-free, yet efficient emitters with thermally activated delayed fluorescence (TADF) was an important step towards mass production of organic light-emitting diodes (OLEDs). Progress is currently impeded by the low solubility and low chemical stability of the materials. Herein, we present a CuI -based TADF emitter that is sufficiently chemically stable under ambient conditions and can be processed by printing techniques. The solubility is drastically enhanced (to 100 g L-1 ) in relevant printing solvents. The integrity of the complex is preserved in solution, as was demonstrated by X-ray absorption spectroscopy and other techniques. In addition, it was found that the optoelectronic properties are not affected even when partly processing under ambient conditions. As a highlight, we present a TADF-based OLED device that reached an efficiency of 11±2 % external quantum efficiency (EQE).

Bright coppertunities: efficient OLED devices with copper(I)iodide-NHetPHOS-emitters

The mass market application of OLEDs is currently hindered because i) the materials are too expensive and contain rare metals such as iridium and ii) current processing techniques are elaborate and cannot easily be up-scaled. Solution processable Cu(I)-complexes promise to solve both problems with one blow: Copper is an abundant metal, which offers new opportunities to develop materials for OLEDs. Due to their structural diversity, Cu(I) emitters allow for the design of materials with tunable properties. Beside this, it is also possible to adjust solution properties and introduce functionalities for cross-linking. The new materials feature exciting photophysical properties such as PLQY values close to unity and a tunable emission. The emission decay times are in the range of common emitters or lower, which is expected to reduce efficiency roll-off at high driving voltages. Cu(I)-complexes often feature thermally-activated delayed fluorescence (TADF). As a consequence, they can make use of triplet and singlet excitons in a process called Singlet Harvesting, which paves the way for high efficiencies. Unlike Ir(III)-complexes such as Irppy3, triplet-triplet annihilation does not occur when using Cu(I), even in very high doping concentrations. The feasibility of NHetPHOS-type Cu(I)-complexes is demonstrated as well as strategies that enable a smart crosslinking process, where the Cu(I) emitters themselves play an important role. In addition, high-brightness devices, which were operated at medium voltages, yielding 50.000 cd m-2 are shown. In a showcase example, we recently presented a device with an external quantum efficiency greater than 20% with a solution processed Cu(I)-PyrPHOS-device without using outcoupling techniques.

Reduced concentration quenching in a TADF-type copper(I)-emitter

Phosphorescent OLEDs are now being used in first commercial products, mainly in displays. Typically, such devices operate at low-to-moderate brightnes s (<500 cd m-2), while it would be beneficial for actual lighting applications to also reach a very high luminance. However, a phenomenon called efficiency roll-off contradicts this aim. The reducing of the device efficiency with rising triplet exciton concentration due to triplet-triplet annihilation (TTA) is the most relevant factor causing roll-off for such compounds. Photophysically, this is reflected by strong concentration quenching in concentrated samples of phosphorescent materials. We present a potential solution for this issue. In this article we identify a copper(I) emitter showing thermally-activated delayed fluorescence (TADF) that seems to be much more immune to concentration quenching than conventional phosphorescent materials, even though triplet states are also populated in a similar manner.

Late bloomers: copper complexes in organic LEDs

In recent years, displays based on organic LEDs (OLEDs) were successfully introduced into consumer electronics as a new, efficient means of converting electricity into visible light. However, the real potential of OLED technology has yet to be unveiled. The dream of using OLEDs for lighting, smart packaging, and other mass-market applications is still in the future, and in fact is even contradicted by the partial reliance of modern commercial OLEDs on metal-organic, phosphorescent iridium compounds as emitting materials. Iridium is one of the rarest metals in the earth’s crust (abundance: 0.0003ppm).1 The reason people still choose it is efficiency. Using triplet harvesting (a technique for converting excitation energy into light), a theoretical efficiency of 100% can be achieved,2 whereas the maximum efficiency of classic fluorescent emitters (such as aluminum oxinate) is limited to 25% as predicted by quantum statistics (see Figure 1).3 Nevertheless, the high price, low abundance, and the special photophysical properties of iridium emitters hamper the manufacturing of affordable large-scale devices as well as the production of large quantities of OLEDs. Consequently, OLED displays have to date been restricted to high-price applications such as smartphones and tablet PCs. Recently, however, new avenues to more abundant emitter materials have begun to open. The so-called singlet-harvesting approach4 (see Figure 1) allows for substitution of phosphorescent heavy-metal emitters with ones that use no metal at all5 or more abundant metals such as copper6 (abundance: 68ppm).1 The idea that copper(I) complexes are now seen as a new, promising class of emitters is interesting, considering that the first OLEDs incorporating such materials were announced mere weeks7, 8 after the groups of Forrest and Thompson published the two papers2, 9 introducing the triplet-harvesting approach Figure 1. Triplet harvesting and singlet harvesting. In an organic LED (OLED), electrical energy is transformed into so-called excitons. Due to quantum statistics, 25% of the excitons have singlet character, while 75% have triplet character. The triplet-harvesting approach (left) transforms all incoming excitons into triplet excitons and uses those to generate light (transition from T1 to S0/. The recently established singlet-harvesting approach (right) transforms all excitons into singlet excitons for the same purpose (transition from S1 to S0/. E: Energy difference. kb: Boltzmann constant. T: Temperature.