Molecular Lego Blocks

Chemical Data Mining Boosts Search for New Organic Semiconductors

14.02.2019

Organic semiconductors are lightweight, flexible and easy to manufacture. But they often fail to meet expectations regarding efficiency and stability. Researchers at the Technical University of Munich (TUM) are now deploying data mining approaches to identify promising organic compounds for the electronics of the future.

First author Christian Kunkel, PD Dr. Harald Oberhofer and Prof. Karsten Reuter (fltr). (Image: A. Battenberg / TUM)

Producing traditional solar cells made of silicon is very energy intensive. On top of that, they are rigid and brittle. Organic semiconductor materials, on the other hand, are flexible and lightweight. They would be a promising alternative, if only their efficiency and stability were on par with traditional cells.

Together with his team, Karsten Reuter, professor of Theoretical Chemistry at the Technical University of Munich, is looking for novel substances for photovoltaics applications, as well as for displays and light-emitting diodes – OLEDs. The researchers have set their sights on organic compounds that build on frameworks of carbon atoms.

Contenders for the Electronics of Tomorrow

Depending on their structure and composition, these molecules, and the materials formed from them, display a wide variety of physical properties, providing a host of promising candidates for the electronics of the future.

“To date, a major problem has been tracking them down: It takes weeks to months to synthesize, test and optimize new materials in the laboratory,” says Reuter. “Using computational screening, we can accelerate this process immensely.”

Computers instead of Test Tubes

The researcher needs neither test tubes nor Bunsen burners to search for promising organic semiconductors. Using a powerful computer, he and his team analyze existing databases. This virtual search for relationships and patterns is known as data mining.

“Knowing what you are looking for is crucial in data mining,” says PD Dr. Harald Oberhofer, who heads the project. “In our case, it is electrical conductivity. High conductivity ensures, for example, that a lot of current flows in photovoltaic cells when sunlight excites the molecules.”

Algorithms Identify Key Parameters

Using his algorithms, he can search for very specific physical parameters: An important one is, for example, the “coupling parameter.” The larger it is, the faster electrons move from one molecule to the next.

A further parameter is the “reorganization energy”: It defines how costly it is for a molecule to adapt its structure to the new charge following a charge transfer – the less energy required, the better the conductivity.

The research team analyzed the structural data of 64,000 organic compounds using the algorithms and grouped them into clusters. The result: Both the carbon-based molecular frameworks and the “functional groups”, i.e. the compounds attached laterally to the central framework, decisively influence the conductivity.

Both the carbon-based molecular frameworks and the functional groups decisively influence the conductivity of organic semiconductors. (Image: C. Kunkel / TUM)

Identifying Molecules Using Artificial Intelligence

The clusters highlight structural frameworks and functional groups that facilitate favorable charge transport, making them particularly suitable for the development of electronic components.

“We can now use this to not only predict the properties of a molecule, but using artificial intelligence we can also design new compounds in which both the structural framework and the functional groups promise very good conductivity,” explains Reuter.

Publication:

C. Kunkel, C. Schober, J. T. Margraf, K. Reuter, H. Oberhofer, Finding the Right Bricks for Molecular Legos: A Data Mining Approach to Organic Semiconductor Design, Chem. Mater. 2019, 31, 969-978. (DOI: 10.1021/acs.chemmater.8b04436)

Further information:

The project was funded by the “Solar Technologies go Hybrid” research initiative of the Bavarian state government and is part of the new Cluster of Excellence e-conversion of the Munich universities funded by the German Research Foundation.

The structural data for the analysis were taken from the Cambridge Structural Database. The conductivity data was generated in sophisticated electronic structure calculations on Super-MUC, the supercomputer of the Leibniz Supercomputing Center in Garching. The new computer-designed molecules will be produced in a laboratory within the Cluster of Excellence e-conversion.

High Resolution Images:

https://mediatum.ub.tum.de/1474021

Contact:

Prof. Dr. Karsten Reuter
Chair of Theoretical Chemistry
Lichtenbergstr. 4, 85747 Garching, Germany
Tel.: +49 89 289 13616
karsten.reuter@ch.tum.de

Source: https://www.tum.de/en/about-tum/news/press-releases/detail/article/35249/

More Efficient Solar Cells Imitate Photosynthesis

Power Stations Driven by Light

January 9, 2019

The smallest building blocks within the power stations of organisms which get their energy directly from the sun are basically miniature reactors surrounded by collectors which capture photons and forward them to the centre. The close correlation between structure and interaction of the components boosts productivity, a strategy which an international team of researchers is using for increasing the efficiency of solar technology. At FAU, research is being carried out in this area by the Chair of Physical Chemistry I, and the latest results have been published in the prestigious journal Nature Chemistry (https://doi.org/10.1038/s41557-018-0172-y).

Image: Colourbox.de

Green plants, algae and some bacteria use sunlight to convert energy. The pigments in chlorophyll absorb electromagnetic radiation which induces chemical reactions in electrons. These reactions take place in the nucleus of complex protein structures, referred to by experts as photosystems I and II. The processes which take place in these photosystems are induced by catalysts in a certain order. In the first step, oxygen is released from water. The following reaction prepares the production of carbohydrates for which no further source of energy is needed.

The reaction centres of the photosystems are encircled by light-absorbing pigments grouped into consolidated complexes. These antennae increase the area available for light rays to hit and extend the spectrum of usable wavelengths, both prerequisites for a favourable energy balance. Each reactor core is surrounded by approximately 30 antennae. Experiments conducted by scientists are still far from replicating the complexity of nature. In general, a ratio of 1:1 is the best that can be achieved: one light-absorbing molecule in combination with one catalyst for oxidising water.

The group of researchers led by Prof. Dr. Dirk Guldi and his former employee Dr. Konstantin Dirian hope to revolutionise solar technology by synthesising modules based on the correlation between structure and function in photosystem II. In the newly developed systems, light-absorbing crystals such as those which are already used in LEDs, transistors and solar cells are layered into a network of hexagonal honeycombs around a water-oxidising catalyst with four ruthenium metal atoms in the centre. When shown in a rather simplified manner, the compact, stable units made up of two components with a common long axis are reminiscent of cylindrical batteries. In the self-assembling chemical process, such ‘miniature power stations’ create two dimensional slats. Like layers in a gateau, they form a common block which collects the energy won from the sun’s rays.

This is not an entirely accurate reproduction of the ideal arrangement found in the natural photosystem, but the principle is the same. Five macromolecules in the shape of a honeycomb with the ability to capture light create a sheath around each reactor core, and it has been shown that these small power stations are efficient and successful at harvesting sun energy. They have an efficiency of over 40 percent, losses are minimal. Wavelengths from the green portion of the colour spectrum, which plants reflect, can also be used. These research results nourish the hope that solar technology can one day make use of the sun’s energy as efficiently as nature.

Further information:

Prof. Dr. Dirk Guldi
Phone: +49 9131 85 27340
dirk.guldi@fau.de

Source: https://www.fau.eu/2019/01/09/news/research/more-efficient-solar-cells-imitate-photosynthesis/

All-in-One Light-Driven Water Splitting

09/07/2018

Solar-powered water splitting is a promising means of generating clean and storable energy. A novel catalyst based on semiconductor nanoparticles has now been shown to facilitate all the reactions needed for “artificial photosynthesis”.

A new photocatalytic system works like a multitool which separates the bonds in water molecules. (Image: C. Hohmann, Nanosystems Initiative Munich NIM)

In the light of global climate change, there is an urgent need to develop efficient ways of obtaining and storing power from renewable energy sources. The photocatalytic splitting of water into hydrogen fuel and oxygen provides a particularly attractive approach in this context. However, efficient implementation of this process, which mimics biological photosynthesis, is technically very challenging, since it involves a combination of processes that can interfere with each other.

Now, LMU physicists led by Dr. Jacek Stolarczyk and Professor Jochen Feldmann, in collaboration with chemists at Julius-Maximilians-Universität Würzburg (JMU) in Bavaria, led by Professor Frank Würthner, have succeeded in demonstrating the complete splitting of water with the help of an all-in-one catalytic system for the first time. Their new study appears in the journal Nature Energy.

Mimicking Natural Photosynthesis

Technical methods for the photocatalytic splitting of water molecules use synthetic components to mimic the complex processes that take place during natural photosynthesis. In such systems, semiconductor nanoparticles that absorb light quanta (photons) can, in principle, serve as the photocatalysts.

Absorption of a photon generates a negatively charged particle (an electron) and a positively charged species known as a ‘hole’, and the two must be spatially separated so that a water molecule can be reduced to hydrogen by the electron and oxidized by the hole to form oxygen. “If one only wants to generate hydrogen gas from water, the holes are usually removed rapidly by adding sacrificial chemical reagents,” says Stolarczyk. “But to achieve complete water splitting, the holes must be retained in the system to drive the slow process of water oxidation.”

The problem lies in enabling the two half-reactions to take place simultaneously on a single particle – while ensuring that the oppositely charged species do not recombine. In addition, many semiconductors can be oxidized themselves, and thereby destroyed, by the positively charged holes.

Nanorods with Spatially Separated Reaction Sites

“We solved the problem by using nanorods made of the semiconducting material cadmium sulfate, and spatially separated the areas on which the oxidation and reduction reactions occurred on these nanocrystals,” Stolarczyk explains. The researchers decorated the tips of the nanorods with tiny particles of platinum, which act as acceptors for the electrons excited by the light absorption.

As the LMU group had previously shown, this configuration provides an efficient photocatalyst for the reduction of water to hydrogen. The oxidation reaction, on the other hand, takes place on the sides of the nanorod. To this end, the LMU researchers attached to the lateral surfaces a ruthenium-based oxidation catalyst developed by Würthner‘s JMU team.

The compound was equipped with functional groups that anchored it to the nanorod. “These groups provide for extremely fast transport of holes to the catalyst, which facilitates the efficient generation of oxygen and minimizes damage to the nanorods,” says Dr. Peter Frischmann, one of the initiators of the project in Würzburg.

SolTech: Concepts for the Conversion of Solar Energy

The study was carried out as part of the interdisciplinary project “Solar Technologies Go Hybrid” (SolTech), which is funded by the state of Bavaria.

“SolTech’s mission is to explore innovative concepts for the conversion of solar energy into non-fossil fuels,” says Professor Jochen Feldmann, holder of the Chair of Photonics and Optoelectronics at LMU.

“The development of the new photocatalytic system is a good example of how SolTech brings together the expertise available in diverse disciplines and at different locations. The project could not have succeeded without the interdisciplinary cooperation between chemists and physicists at two institutions,” adds Professor Frank Würthner who heads the JMU Chair of Organic Chemistry II. Together with Feldmann, Würthner initiated SolTech in 2012

Publication

C. M. Wolff, P. D. Frischmann, M. Schulze, B. J. Bohn, R. Wein, P. Livadas, M. T. Carlson, F. Jäckel, J. Feldmann, F. Würthner, J. K. Stolarczyk, All-in-one visible-light-driven water splitting by combining nanoparticulate and molecular co-catalysts on CdS nanorods, Nature Energy 2018, 3, 862-869. (DOI: 10.1038/s41560-018-0229-6)

http://dx.doi.org/10.1038/s41560-018-0229-6

Contact

Dr. Jacek Stolarczyk, Chair for Photonics and Optoelectronics, Department of Physics, and CeNS, LMU, T +49-89-2180-1356, jacek.stolarczyk@physik.uni-muenchen.de

Source: https://www.uni-wuerzburg.de/en/news-and-events/news/detail/news/all-in-one-light-driven-water-splitting/

Optical “Overtones” for Solar Cells

München, 04/19/2018

NIM scientists from LMU Munich have found a new effect regarding the optical excitation of charge carriers in a solar semiconductor. It could facilitate the utilization of infrared light, which is normally lost in solar devices.

Analogous to a phenomenon known for music instruments when overtones of two different fundamental notes get into resonance, scientists from LMU Munich have found a new effect regarding the optical excitation of charge carriers in a solar semiconductor. It could facilitate the utilization of infrared light. (Image: A Manzi, LMU/NIM)

Semiconductors are nowadays the most prominent materials to convert solar light into usable electric energy. The International Energy Agency (IEA) reported that half a million solar panels were installed every day around the world last year. However, semiconductor-based solar cells still suffer from relatively low energy conversion efficiencies. The reason for that mainly lies in the fact that semiconductors efficiently convert the light from a quite small portion of the solar spectrum into electrical power. The spectral position of this window of light that can be efficiently converted is strongly related to a property of the semiconductor involved (that is, its band-gap). This means that, if the semiconductor is designed to absorb yellow light, longer-wavelength light (such as red and infrared light), will pass through the material without producing currents. Additionally, shorter-wavelength light (green, blue and UV light), that is more energetic than yellow light, will lose its additional amount of energy into heat. Obtaining higher energy conversion efficiencies from semiconductors is therefore still a big challenge.

Perovskite Nanocrystals for Energy Conversion

To study these limitations, Aurora Manzi, a PhD student from the Chair for Photonics led by Prof. Jochen Feldmann, has measured the charge carrier density created by the absorption of multiple photons in perovskite nanocrystals, a novel and promising material for photovoltaic applications.

“Multiple photon absorption of long-wavelength light with an energy lower than the semiconductor absorption window is usually very inefficient”, highlights Manzi, first author of the publication in Nature Communications and a student of the NIM graduate program. “I was therefore totally surprised to observe that for specific excitation wavelengths the efficiency of this process becomes drastically enhanced. At the beginning this did not make any sense to us!”

Light and Exciton “Overtones” in Resonance

After intense discussions, the team of LMU scientists realized that these resonances occur when multiples of two distinct fundamental frequencies become equal, namely that of the frequency of the primary light oscillation and that of the frequency of the band gap or more precisely of the exciton at the band-gap.

One could draw an analogy to resonance or overtone phenomena in acoustics, commonly used in music instruments. When intense red light impinges on nano-structured perovskite nanocrystals, a process similar to the generation of overtones in a guitar string takes place. The fundamental light wavelength generates higher order optical harmonics, that are overtones whose frequencies are integer multiples of the primary light oscillation. When such a “light overtone” becomes resonant with an overtone of the excitonic band-gap, the energy exchange is enhanced leading to an increased generation of charge carriers or more precisely of multiple excitons at the band gap.

Starting Point for Further Research

“The resonances observed are analogous to the physical phenomena taking place in two different strings of a guitar”, continues Manzi. “If we associate the first string to the light excitation and the second string to the semiconductor excitonic band-gap, we know from acoustics that they will get into resonance if a certain harmonic of the first string will match another harmonic of the second string.”

“The observation of this novel resonance phenomenon for optical excitations in excitonic semiconductors could pave the way for solar cells to more efficiently convert long-wavelength light into usable electric power”, adds Prof. Feldmann, the leader of the research team. “This is an exciting new finding with a possible impact for future solar devices. Together with our colleagues from the Research Network “Solar Technologies Go Hybrid” (SolTech), we will now try to develop innovative applications by playing with such overtones.”

Publication:

A. Manzi, Y. Tong, J. Feucht, E.-P. Yao, L. Polavarapu, A. S. Urban, J. Feldmann, Resonantly enhanced multiple exciton generation through below-band-gap multi-photon absorption in perovskite nanocrystals, Nature Communications 2018, 9, 1518.

For further information on Jochen Feldmann’s work, see:

New research network: Leading the way to next-generation LEDs

Source: https://www.en.uni-muenchen.de/news/press-services/press-releases/2018/feldmann_solarcells.html