25-Sep-2019 - Max-Planck-Institut für Polymerforschung

How to design efficient materials for OLED displays

For applications such as light-emitting diodes or solar cells, organic materials are nowadays in the focus of research. These organic molecules could be a promising alternative to currently used semiconductors such as silicon or germanium and are used in OLED displays. A major problem is that in many organic semiconductors the flow of electricity is hampered by microscopic defects. Scientists around Dr. Gert-Jan Wetzelaer and Dr. Denis Andrienko of the Max-Planck-Institute for Polymer Research have now investigated how organic semiconductors can be designed such that the electric conduction is not influenced by these defects.

The basic principle of the first light bulb, invented by Thomas Edison in the 19th century, was quite simple: Electrons – negatively charged particles – flow through a carbon filament and create light by converting their energy to light and heat. Nowadays, the physics of the generation of light in semiconducting devices is more complex: Electrons flow through a device and release their energy at a given point in the device. For this, they have to find a free place, i. e. a place that isn’t occupied by an electron - at a deeper lying energy level. This free place can be viewed as a sort of positive charge, a so called hole. If the electron jumps down into the hole, its energy is released in the form of light. Based on this principle, an organic light-emitting diode (OLED) converts electric current into light.

The efficiency of such a device strongly depends on how good holes and electrons can be conducted. If either electrons or holes are trapped by defects, meaning that they cannot contribute to the current anymore, then an excess of one type of charge exists. For example, in the case that holes are trapped, there are more electrons than holes, meaning only a part of the electrons can create light and the efficiency of the OLED is reduced.

“In our newest experiments, we examined a large range of organic semiconductors and found out the main parameters that are essential for equal and defect-free conduction of both holes and electrons”, says Gert-Jan Wetzelaer (Department of Prof. Paul Blom). In a semiconductor, electrons are moving at a higher energy level, whereas holes move at a level lower (deeper) in energy. The scientists found that the conduction of both charge types strongly depends on the position of these energy levels. “Depending on the energy of these levels the charge transport can be dominated either by electrons or holes or, with the right choice of energy levels, they contribute equally to the charge transport,” says Wetzelaer.

In computer simulations, scientists around Denis Andrienko (Department of Prof. Kurt Kremer) had a deeper look at the origin of these charge traps: “In our simulations we introduced clusters of water molecules in the semiconductor, which may accumulate in little pockets in the semiconductor”, explains Andrienko. “We found that these clusters of water molecules can function as a hole trap, leading to electron-dominated organic semiconductors. By contrast, for hole-dominated semiconductors, oxygen related defects capture electrons. As a result, we could show that highly unipolar charge transport for either holes or electrons is governed by a very small amount of defects, such as water and oxygen.” Unfortunately, removing such defects completely has proven challenging.

Therefore, the Mainz researchers are able to define how to design highly efficient organic semiconductors in the future: The different energy levels of the material should be in a certain range, which strongly reduces the influence of oxygen and water molecules that are the main cause for charge trapping. Based on this concept, the first highly efficient OLEDs with defect-free electrical conduction have recently been realized.

Facts, background information, dossiers

  • electrons
  • organic light-emitt…
  • displays

More about MPI für Polymerforschung

  • News

    Combination Therapy against Cancer

    In their quest to destroy cancer cells, researchers are turning to combinational therapies more and more. Scientists from Germany and China have now combined a chemotherapeutic and photodynamic approach. All agents are encapsulated in nanocapsules with a protein shell to be delivered to the ... more

    When ions rattle their cage

    Electrolytes play a key role in many areas: They are crucial for the storage of energy in our body as well as in batteries. In order to release energy, ions - charged atoms - must move in a liquid such as water. Until now the precise mechanism by which they move through the atoms and molecu ... more

    "Make two out of one" - Division of Artificial Cells

    The success of life on earth is based on the amazing ability of living cells to divide themselves into two daughter cells. During such a division process, the outer cell membrane has to undergo a series of morphological transformations that ultimately lead to membrane fission. Scientists at ... more

More about Max-Planck-Gesellschaft

  • News

    First programmable photocatalyst developed

    Researchers at the Max Planck Institute of Colloids and Interfaces have developed a sustainable and "smart photocatalyst". The special feature: as a so-called smart material, it can distinguish between the colors of light (blue, red and green) and, in response, enables a specific chemical r ... more

    Molecular mechanisms of corona drug candidate Molnupiravir unraveled

    The United States recently secured 1.7 million doses of a compound that could help to treat Covid-19 patients. In preliminary studies, Molnupiravir reduced the transmission of the SARS-CoV-2 coronavirus. Researchers at the Max Planck Institute (MPI) for Biophysical Chemistry in Göttingen an ... more

    Cellular filaments keeping the pace

    A new model describes the coordination of beating cilia allowing to predict their functional behavior. Researchers from the Max Planck Institute for Dynamics and Self-Organization (MPIDS) analyzed the formation of metachronal waves in arrays of cilia and how external cues might influence th ... more

q&more – the networking platform for quality excellence in lab and process

The q&more concept is to increase the visibility of recent research and innovative solutions, and support the exchange of knowledge. In the broad spectrum of subjects covered, the focus is on achieving maximum quality in highly innovative sectors. As a modern knowledge platform, q&more offers market participants one-of-a-kind networking opportunities. Cutting-edge research is presented by authors of international repute. Attractively presented in a high-quality context, and published in German and English, the original articles introduce new concepts and highlight unconventional solution strategies.

> more about q&more

q&more is supported by: