Chemistry

Organic light emitting diodes

Organic light emitting diodes



We are searching data for your request:

Forums and discussions:
Manuals and reference books:
Data from registers:
Wait the end of the search in all databases.
Upon completion, a link will appear to access the found materials.

The energy barrier in two-layer OLEDs

At the interface of the two layers there is an internal energy barrier χ 'for the holes, which normally function as majority charge carriersL. as well as a barrier χ 'for the electrons that usually appear as minority charge carriersE. . The heights of these energy barriers correspond to the difference between the ionization potentials and the electron affinities of the two materials. In this way, in particular, the majority charge carriers are collected at the inner interface, the hole density is thus increased and thus the probability of recombination with the electrons is increased.

An additional advantage is that, in contrast to single-layer OLEDs, the recombination zone is located further away from the metal cathode. This greater distance prevents the radiation from being extinguished on the metal and thus also contributes to increasing the efficiency. Furthermore, the accumulation of positive charges in excess inside the diode causes a change in the electrical field conditions along the diode cross-section: The field gradient in front of the anode is weakened and the migration speed of the charge carriers in the hole transport layer is reduced which facilitates the more problematic electron injection and thus to lowerturn-on-Tension leads.


OLED.Education - organic light-emitting diodes in curricular innovation

The current newsreel already reported in May on white LEDs and organic photovoltaics. The combination of both areas leads us to today's topic, so to speak.
Organic light emitting diodes (OLEDs) are innovative and highly efficient light sources that are increasingly being built into the displays of modern electronic devices such as smartphones, curved TVs and tablet computers. The new technology is also increasingly entering the market in the lighting sector. The main difference between OLEDs and conventional LEDs lies in the semiconductor materials used. Instead of traditional semiconductors such as silicon, germanium or cadmium telluride, conjugated polymers (or oligomers) and small metal complexes with organic ligands are used for electroluminescence (Fig. 1). A great advantage of organic semiconductors is the comparatively easy accessibility using organic synthesis routes and the great variety of their processing options, which range from wet chemical to vaporization to printing processes. OLEDs can even be produced off the roll [1]. In the following article, the organic light-emitting diodes are introduced from a technical perspective and dealt with within the framework of a didactic research project for the curricular connection.


Why organic light-emitting diodes don't turn on lights anymore

What will the screen of the future look like? Extremely flat, flexible, self-luminous, color-fast and rich in contrast - once organic light-emitting diodes, or OLEDs for short, have established themselves. The first displays are already on the market, and they mostly contain small molecules in their color layer (SMOLEDs). Other devices use polymers (PLEDs). So far it has been assumed that in the plastic layer responsible for lighting in PLEDs, the conversion of an excited state can occur, which enables higher efficiencies. A team led by Dr. John Lupton from the Department of Physics at the Ludwig Maximilians University (LMU) in Munich has now been able to refute this

"Our statement does not necessarily sound positive at first glance," says Lupton. "However, the result has considerable technological relevance, especially for companies that want to set up production lines."

The technology of organic light-emitting diodes is based on the principle of electroluminescence. The components are made up of several extremely thin layers. One of them, the cathode, injects electrons. Another, the anode, removes electrons, creating holes. Electrons and holes can move freely and meet between cathode and anode. There is a thin layer of organic dye there. Electrons and holes combine when they meet to form what is known as an exciton. This releases energy in the form of a photon in the dye layer. During this process it is important to put the exciton in a suitable state of excitation. This state is given by the quantum mechanical size of the electron spin. "When an electron and a hole meet in the dye layer, there are four possible spin combinations," reports Lupton. "One of them forms a so-called singlet, the other three triplets." However, only the singlet, i.e. one of four excitons, can emit visible light. Triplets, on the other hand, give off the energy in the form of heat. A considerable part of the electrical energy is thus lost in the dark triplet channels. The electrical efficiency of the LED is initially limited to a maximum of 25 percent.

Lupton and his team have now been able to demonstrate for the first time that, in principle, no conversion of triplets to singlets takes place in the polymers. They developed a special method to make the triplets in the polymer directly visible. The smallest metallic impurities in the polymer enable direct emission of the triplet - the dark state becomes light. "For years there has been a discussion as to whether long-chain polymers are better suited for organic light-emitting diodes, because the proportion of electrically generated triplet excitations could be lower than with small ones Molecules, "says Lupton. "We have now clearly denied the hope that triplets will become singlets. The ratio of singlet to triplet cannot be more than 1: 3."

The mistakenly assumed spin conversion of the triplets was previously considered to be a great advantage of PLEDs over SMOLEDs, which are considerably more complex to manufacture. SMOLEDs have to be manufactured in a complex process under vacuum. PLEDs, on the other hand, can be produced relatively easily, for example with a type of inkjet printer. On the technical side, the polymers lag behind the small molecules by a few years in some respects, especially in terms of efficiency - and also in solving the triplet problem.

In SMOLEDs, molecular complexes have already been successfully incorporated into the dye layer. Their special chemical properties enable the triplets to disintegrate directly with the emission of light. "With these so-called phosphorescent emitters, quantum yields of almost 100 percent can be achieved," says Lupton. In principle, the process is also possible with polymers, but not so far advanced because the question of the triplet conversion seemed to have been clarified. The new results are not so much breaking up a research field as they are initiating a new one: the incorporation of phosphorescent emitters into the polymers. But the results are also of interest for other areas.

Original publication: M. Reufer, M. J. Walter, P. G. Lagoudakis, A. B. Hummel, J. S. Kolb, H. G. Roskos, U. Scherf, J. M. Lupton "Spin-conserving carrier recombination in conjugated polymers" Nature Materials 2005, 4 (4), 340.


Organic light-emitting diodes (OLEDs) as compass needles & # 8211 researchers are developing new types of magnetic sensors

Because OLEDs create a brilliant image, are relatively easy to manufacture, very thin, save energy and can even be used on flexible carrier films. Researchers at the University of Regensburg have now also been able to show that the generation of electrical light is very much dependent on magnetic fields. This means that OLEDs can be used as sensitive magnetic sensors - for example, to measure the earth's magnetic field in navigation devices.

Prof. Dr. John Lupton from the Institute for Experimental and Applied Physics at the University of Regensburg, in cooperation with scientists from the University of Utah and the University of Sydney, developed a device that combines the properties of OLEDs with the precision of conventional magnetic sensors. The unit does not require calibration and works even in extreme temperatures.

To convert electricity into light, OLEDs bring together positive and negative charges, so-called electrons and holes. In addition to their electrical properties, these elementary charges have another characteristic: From a microscopic point of view, an electron behaves like a small bar magnet. If many of these bar magnets are aligned in the same direction, one speaks of magnetism. While the static properties of magnetic fields dominate in everyday life, the dynamic magnetic processes - such as spin resonance - are of particular interest to physicists. This can be easily illustrated. If you run a compass under a power line, the compass needle deflects because the current generates a magnetic field that superimposes the earth's magnetic field. If the direction of the current changes regularly, it is possible to deflect the compass needle evenly or even to make it rotate.

The bar magnets of the electrons can also experience such a rotation in OLEDs. As with a series of bar magnets, the interaction between the magnets depends on the respective direction: two north poles repel each other, north and south poles attract each other. With a wire through which current flows, the electrons in the OLED can now be excited to vibrate. The smallest changes in magnetic fields can be measured exactly as a change in vibration. Thus, the - OLED-based - display of a navigation device becomes the navigation instrument itself.

In organic semiconductors from which OLEDs are made, electrons can show off their properties as bar magnets particularly well. OLED-based magnetic field sensors are therefore extremely sensitive. Such sensors could also be used in medical diagnostic processes. With an OLED display, for example, a magnetic field could be mapped so precisely that even biological processes could be investigated.

The results of the Regensburg physicists are published in the renowned journal "Nature Communications" (DOI: 10.1038 / ncomms1895).

Original publication title:
"Robust absolute magnetometry with organic thin-film devices"


Metal-Free OLED Triplet Emitters by Side-Stepping Kasha's Rule †

This work was supported by the Volkswagen Foundation through a collaborative research project. Financial support by the DFG (SFB 813) is gratefully acknowledged. J.M.L. and T.V.V. are indebted to the David & Lucile Packard Foundation.

Abstract

Light and luminous: Most organic fluorophores do not show useful emission from the triplet excited state. A new material class for organic light-emitting diodes, which also phosphoresce at room temperature in response to electrical excitation, does not require the heavy atom effect, because the internal conversion is blocked so effectively that emissions from higher-lying triplet states (above the singlet) can be observed at low temperatures.

As a service to our authors and readers, this journal provides supporting information supplied by the authors. Such materials are peer reviewed and may be re-organized for online delivery, but are not copy-edited or typeset. Technical support issues arising from supporting information (other than missing files) should be addressed to the authors.

Filename Description
ange_201307601_sm_miscellaneous_information.pdf900.6 KB miscellaneous_information

Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.

Note for articles published since 1962:

A search error may have occurred.

This is the German version of applied Chemistry.

Do not cite this version alone.

Take me to the International Edition version with citable page numbers, DOI, and citation export.


Organic solar cells and light emitting diodes in one

Organic solar cell consisting of vapor-deposited organic molecules and vapor-deposited metal contacts.

Johannes Benduhn, Kai Schmidt, Institute for Applied Physics, TU Dresden

Current-voltage characteristic of an organic, optoelectronic diode that absorbs ultraviolet and blue photons.

In organic semiconductor research over the past 25 years, organic solar cells and organic light-emitting diodes (OLEDs) were considered incompatible in one component. A team of physicists led by Prof. Koen Vandewal from the Technical University of Dresden has now succeeded in producing organic solar cells that also function as efficient OLEDs.

A fundamental loss mechanism in semiconductors is the emission of light to maintain the thermodynamic equilibrium between the material and the environment. It is precisely this balance between light absorption and emission in the semiconductors that is responsible for ensuring that “an ideal solar cell is also an ideal light-emitting diode,” says Johannes Benduhn, explaining the basic assumption of the Organic Solar Cells (OSOL) group at the Institute for Applied Physics.

In organic solar cells, however, further loss mechanisms occur that have hitherto opposed this assumption. These mechanisms cause the recombination of charge carriers in the form of heat without emitting light (“non-radiating”) and thus reduce the voltage that can be tapped and consequently the efficiency of the solar cell. These non-radiative voltage losses are one of the main reasons for the lower efficiency of organic solar cells compared to established technologies that are currently used on house roofs. With the newly developed organic solar cells, the OSOL group was able to keep these voltage losses comparatively low and thus pave the way for efficient and completely new areas of application.

The international research team has succeeded in developing combinations of organic semiconductors that are based on electron acceptor and donor transitions and that function as both a solar cell and an LED. The results of this research work expand the current understanding of organic semiconductors. For the first time they combine the physical description of organic solar cells and OLEDs.


Organic light-emitting diodes become brighter and more durable: layers made as ultrastable glasses improve device performance

Organic light-emitting diodes (OLEDs) truly have matured enough to allow for first commercial products in the form of small and large displays. In order to compete in further markets and even open new possibilities (automotive lighting, head-mounted displays, micro displays, etc.), OLEDs need to see further improvements in device lifetime while operating at their best possible efficiency. Currently, intrinsic performance progress is solely driven by material development.

Now, researchers from the Universitat Autònoma de Barcelona and Technische Universität Dresden have demonstrated the possibility of using ultrastable film formation to improve the performance of state-of-the-art OLEDs. In their joint paper published in Science Advances with the title 'High-performance organic light-emitting diodes comprising ultrastable glass layers', the researchers show in a detailed study that significant increases of efficiency and operational stability (& gt 15% for both parameters and all cases, significantly higher for individual samples) are achieved for four different phosphorescent emitters. To achieve these results, the emission layers of the respective OLEDs were grown as ultrastable glasses - a growth condition that allows for thermodynamically most stable amorphous solids.

Illustration summarizing the nanoscale difference of ultrastable glasses compared to conventional ones and the impact on the layer and device properties of organic light-emitting diodes (OLEDs).

This finding is significant, because it is an optimization which involves neither a change of materials used nor changes to the device architecture. Both are the typical starting points for improvements in the field of OLEDs. This concept can be universally explored in every given specific OLED stack, which will be equally appreciated by leading industry. This in particular includes thermally activated delayed fluorescence (TADF) OLEDs, which are seeing tremendous research and development interest at the moment. Furthermore, the improvements that, as shown by the researchers, can be tracked back to differences in the exciton dynamics on the nanoscale suggest that other fundamental properties of organic semiconductors (e.g. transport, charge separation, energy transfer) can also be equally affected.

The research leading to these results was partly carried out in the project ‘Modeling stability of organic phosphorescent light-emitting diodes (MOSTOPHOS)’ funded by the European Union’s Horizon 2020 research and innovation program (grant agreement no. 646259). Currently, this concept is being explored together with cynora GmbH, a MOSTOPHOS partner and a world-leading company in development of TADF emitters.

Publication: J. Ràfols-Ribé, P.-A. Will, C. Hänisch, M. González-Silveira, S. Lenk, J. Rodríguez-Viejo, S. Reineke, High-performance organic light-emitting diodes comprising ultrastable glass layers. Sci. Adv. 4, eaar8332 (2018).

About the Group of Nanomaterials and Microsystems (GNaM) : GNaM is part of the Physics Department at the Universitat Autònoma de Barcelona and led by Prof. Javier Rodríguez-Viejo. The group has been deeply involved in the growth and characterization of stable organic glasses, focusing on their thermal, thermodynamic and transport properties. Due to their unique properties including among others higher densities, better kinetic and thermodynamic stabilities and higher modulus, stable glasses are presently at the core of the research in the glass community providing a unique framework for a better comprehension of the glassy state and new opportunities for applications. GNaM is also developing new tools to characterize thermoelectric properties of low-dimensional and disordered solids and has recently founded a start-up company, FutureSisens, for the commercialization of Si-based thermoelectric sensors.

About the Light-Emitting and eXcitonic Organic Semiconductor (LEXOS) group : The LEXOS group is part of the Dresden Integrated Center for Applied Physics and Photonics Materials (IAPP) and the Institute of Applied Physics of the Technische Universität Dresden and led by Prof. Sebastian Reineke. The LEXOS group has long-standing expertise in the research and development of organic light-emitting diodes (OLEDs). The current OLED research comprises stack and concept development, devices optics, charge transport and recombination studies, long-term stability investigations, material development (dopant and emitter materials), and device integration. A second research focus of the LEXOS group is the investigation of excitonic and luminescent systems covering organic and other related emerging materials. The group has strong expertise in the optical spectroscopy of such systems. One current example is the investigation of organic biluminescence, where luminophores show both fluorescence and phosphorescence at room temperature.

Media inquiries
Prof. Sebastian Reineke
Dresden Integrated Center for Applied Physics and Photonic Materials (IAPP)
Tel: +49 (0) 351 463-38686
& # 115eb & # x61 & # x73 & # 116 & # x69 & # 97 & # 110 & # x2e & # 114 & # 101i & # x6ee & # x6be @ tu & # 45d & # x72 & # 101 & # x73d & # x65 & # x6e & # 46de


Energy transfer in organic light-emitting diodes (OLED) - the secret of the missing excitons

Organic semiconductors such as those in OLEDs would be more efficient if some of the energy did not disappear into the material undetected. A team from the e-conversion cluster has successfully started the search - with an elegant method that shows how many excited electrons were in the material and how they are lost.

Organic light-emitting diodes convert electrical energy into light and we encounter them every day in our cell phone displays, laptops and televisions. The greatest advantages of OLEDs: Their layers can be printed very thinly on flexible foils, brilliantly colored and sharp images are created and they consume less energy than conventional displays. But the diodes would be even more effective if a small but significant part of the energy did not diffuse away in the material in an unknown way.

The central component of an OLED is its semiconductor material, which usually consists of long carbon chains. Then optically inactive areas alternate with sections that can be made to glow as so-called chromophores by electricity. As soon as current excites a molecular chain, an electron-hole pair (exciton) is created, which moves along the polymer chain as a kind of mobile excited state. The exciton moves from one chromophore to the next until its energy is converted into a light particle (photon) at one of the chromophores. However, if an exciton wants to jump onto a chromophore on which another exciton is already located, it is extinguished (annihilation) and its energy is converted into heat instead of light.

Influence on the whole optoelectronics

How quickly do the excitons jump from chromophore to chromophore and how is this related to the annihilation processes? Can the annihilation process possibly be controlled? The group of the e-conversion scientist Prof. Philip Tinnefeld (physical chemistry, LMU Munich) has set out on the trail of this riddle - together with experts from the universities of Glasgow, Regensburg and Bonn. Your results not only play a role in the development of OLEDs. The behavior of excitons in organic semiconductors affects many materials in the field of optoelectronics. This also includes the core elements of solar cells.

For the first time, the scientists have succeeded in determining the number of chromophores on a chain and their interactions at the same time. To do this, they aimed a laser beam at a single chain molecule for excitation and observed how the chromophores responded by emitting photons. Detectors were used that can measure the number of emitted photons at intervals of a few picoseconds. For comparison: a picosecond is to a second like a second is to more than 30,000 years.

“To determine the number of chromophores, we count the detected photons per excitation cycle. So far, however, the annihilation processes have falsified these measurements, ”explains Tim Schröder, one of the first authors of the publication. “With our new method, we can for the first time detect the photons at picosecond intervals and thus observe the emission process in a time-resolved manner. No annihilation has taken place immediately after a suggestion. We count the chromophores. Then we measure how the annihilation develops over time. by detecting fewer and fewer photons. "

Artificial DNA as a 3D test model

The new method of picosecond detection and time-resolved data analysis first had to prove itself on a precisely defined model from Philip Tinnefeld's group. To do this, they used the DNA origami technique, with which artificial DNA building blocks can be folded in a targeted manner in 3D and molecules can be attached. “The decisive factor in our model is that we can define the number of chromophores and their distance from one another,” explains Prof. Philip Tinnefeld. “And over the distance we can control the probability of annihilation processes and validate our new method: from completely switched off annihilation to very strong interaction . Our plan is now to apply the method to many more materials. "

In the first field test, the scientists were able to show that effective exciton transport in OLEDs depends on how the carbon chains are spatially organized. Even with solar cells, transport must be as efficient as possible and be further optimized. And the knowledge is also helpful for understanding and adopting similar processes from nature: One of the reasons why plants are so successful in photosynthesis is that nature has perfected exciton transport in its light-harvesting complexes over millions of years.

The publication is the result of his cooperation between the following research groups: Prof. Philip Tinnefeld (LMU Munich), PD Dr. Jan Vogelsang and Prof. John M. Lupton (University of Regensburg), Prof. Sigurd Höger (University of Bonn) and Dr Gordon J. Hedley (University of Glasgow).

publication
Picosecond time-resolved photon antibunching measures nanoscale exciton motion and the true number of chromophores. Gordon J. Hedley, Tim Schröder, Florian Steiner, Theresa Eder, Felix Hofmann, Sebastian Bange, Dirk Laux, Sigurd Höger, Philip Tinnefeld, John M. Lupton and Jan Vogelsang. Nature Communications 12, 1327 (2021). https://doi.org/10.1038/s41467-021-21474-z

Contact
Prof. Dr. Philip Tinnefeld
Chemistry Department
Physical chemistry / nanobiochemistry
LMU Munich
Butenandtstrasse 5 - 13
81377 Munich

PD Dr. Jan Vogelsang
Faculty of Physics
Institute for Experimental and Applied Physics
University of Regensburg
Universitätsstrasse 31
93053 Regensburg


OLED.Education - organic light-emitting diodes in curricular innovation

The current newsreel already reported in May on white LEDs and organic photovoltaics. The combination of both areas leads us to today's topic, so to speak.
Organic light emitting diodes (OLEDs) are innovative and highly efficient light sources that are increasingly being built into the displays of modern electronic devices such as smartphones, curved TVs and tablet computers. The new technology is also increasingly entering the market in the lighting sector. The main difference between OLEDs and conventional LEDs lies in the semiconductor materials used. Instead of traditional semiconductors such as silicon, germanium or cadmium telluride, conjugated polymers (or oligomers) and small metal complexes with organic ligands are used for electroluminescence (Fig. 1). A great advantage of organic semiconductors is the comparatively easy accessibility using organic synthesis routes and the great variety of their processing options, which range from wet chemical to vaporization to printing processes. OLEDs can even be produced off the roll [1]. In the following article, the organic light-emitting diodes are introduced from a technical perspective and dealt with within the framework of a didactic research project for the curricular connection.