Energy Transfer in Organic Light Emitting Diodes (OLED): The secret of the missing excitons
Image: Felix Hofmann
Organic light-emitting diodes convert electrical energy into light and we encounter them every day in our cell phone displays, laptops and televisions. The great advantage of OLEDs: Their layers can be printed very thinly on flexible foil, they produce brilliantly colored and sharp images, and they consume less energy than conventional displays. However, the diodes would be even more efficient if a small but significant part of the energy did not diffuse away into the material in seemingly unpreventable ways.
The central component of an OLED is its semiconductor material, which usually consists of long carbon chains. On it, optically inactive areas alternate with sections that can be made to glow by electricity as so-called chromophores. As soon as electricity excites a molecular chain, an electron-hole pair (exciton) is created, which travels along the polymer chain as a kind of mobile excited state. The exciton moves from one chromophore to the next until, at some point, 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 annihilated and its energy is converted into heat instead of light.
The excitons behaviour
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 e-conversion scientist Prof. Philip Tinnefeld (Physical Chemistry, LMU Munich) has set out on the trail of this mystery - together with experts from the Universities of Glasgow, Regensburg and Bonn. Their 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 simultaneously determining the number of chromophores on a chain as well as their interactions. To do this, they excited a single chain molecule with a laser and observed how the chromophores reacted by emitting photons. Therefor they used detectors that can measure the number of emitted photons at intervals of a few picoseconds. For comparison: a picosecond relates to a second as a second relates to more than 30,000 years.
"To determine the number of chromophores, we count the detected photons per excitation cycle. Until now, however, annihilation processes have falsified these measurements," explains Tim Schröder, one of the first authors of the paper. "With our new method, we for the first time detect the photons in picosecond intervals and analyze the emission process in a time-resolved manner. Immediately after an excitation, no annihilation has taken place yet. That is when we count the chromophores. Then we measure how the annihilation develops over time by detecting fewer and fewer photons.”
Artificial DNA serves as 3D model system
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. For the model, the scientists used the so-called DNA origami technique. It enables them to fold a specific 3D structure out of artificial DNA building blocks with binding sites for various molecules. "What is crucial about our model is that we can define the number of chromophores and their distance from each other," explains Prof. Philip Tinnefeld. "And via the distance, we can control the probability for annihilation processes and validate our new method: from completely deactivated annihilation to very strong interaction. Our plan is to apply the method to many more materials."
In the first field experiment, the scientists were able to show that effective exciton transport in OLEDs depends on how the carbon chains are spatially organized. With solar cells, too, transport must be as efficient as possible and further optimized. In addition, the knowledge is 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 was a cooperation of the following 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).
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
Prof. Dr. Philip Tinnefeld
Department of Chemistry
Physical chemistry/ Nanobiosciences
Butenandtstr. 5 - 13
D- 81377 Munich
PD Dr. Jan Vogelsang
Faculty of Physics
Institute of Experimental and Applied Physics
University of Regensburg