The creation of molecular models and the finding and understanding of structure-property relationships are the most crucial steps when developing new materials. While many great findings and inventions in the history of science and technology strongly relied on a certain degree of randomness, it becomes vital at a certain stage of development to really understand why a certain material has beneficial properties in order to create better and better materials. In in the development of organometallic light-emitting materials, scientists often use structural models based on crystallography, e.g. data obtained by the investigation of single crystal samples. Based on these models, further analyses, and comparison to known substances or so-called "chemical intuition" then leads to the proposition of modified, next-generation materials, which may or may not be realized by chemical synthesis.
While this approach has been executed with great success in the past, problems arise in cases where the initial model is too simple, inaccurate or even false. In this article, we propose an alternate approach to prevent such problems: the use of X-ray absorption spectroscopy (XAS), a long-known technique, in material science. In several case studies, we highlight problematic examples from the past and show where and how XAS was and could be used to prevent erroneous models.
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.
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.
A series of highly luminescent mono-, di-, and trinuclear copper(I) complexes has been synthesized using modular ligand systems of easily accessible N^N, P^P or P^N ligands in order to show the rich structural diversity of copper(I) compounds. Those systems allow for the design of various emitting materials with desired photophysical properties, such as emission colors and high efficiencies. The complexes were characterized with well-established methods such as X-ray crystallographic studies or elemental analysis and, in addition, due to their interesting photoluminescence characteristics, their emission properties were further investigated by means of spectroscopic methods as well as DFT-calculations. In detail, various cationic and neutral mononuclear complexes have been synthesized in order to investigate the photophysical properties of this these different types of emitting compounds. It has been found that neutral copper(I) complexes show superior emission properties (with PLQY up to 89%) compared to their cationic counterparts. Furthermore, a series of dinuclear and trinuclear copper(I) complexes has been synthesized featuring an easy tunable emission maximum from sky blue to deep red (481 nm to 713 nm) with extraordinary high photoluminescence quantum yields up to 99%. In addition, a new crosslinking-technique has been developed to open up the door for a new way to fully solution processed OLED using these promising emitting compounds: Alkyne-substituted emitting complexes crosslink automatically with azide-polymers in a copper-catalyzed alkyne-azide Click reaction.