Experimental Physics VI

7th May 2013:

Single photon sources constitute an essential building block for the implementation of an almost 100% secure optical data communication. Motivated by their benefits of chemical variability, solution processing and strong excitonic binding forces, we started to examine organic semiconductors as active material in electrically driven single photon sources operating at room temperature. Due to its innovative character and the ability of cooperation with the local industry this ambitious project by Prof. Jens Pflaum has been awarded the Universitätsförderpreis der Mainfränkischen Wirtschaft 2013 by the IHK Würzburg-Schweinfurt. [Official press release by the IHK]

16th April 2013:

Generation of single photons has been demonstrated in several systems. However, none of them satisfies all the conditions, e.g. room temperature functionality, telecom wavelength operation, high efficiency, as required for practical applications. In collaboration with Ioffe Institute, we have fabricated light-emitting diodes (LEDs) based on intrinsic defects in silicon carbide (SiC). The room temperature electroluminescence of our LEDs reveals two strong emission bands in the visible and near infrared spectral ranges, associated with two different intrinsic defects. As these defects can potentially be generated at a low or even single defect level, our approach can be used to realize electrically driven single photon source for quantum telecommunication and information processing. These results have been recently published in Scientific Reports.

2nd January 2013:

"Better than diamond" – we have demonstrated that silicon vacancy spins in silicon carbide (SiC) can be spectroscopically addressed, manipulated and selectively read out by means of the so-called double radio-optical resonance. Remarkably, these silicon vacancy qubits show quantum behavior even at room temperature. The device technologies (e. g., LED, MOSFETS, MEMS, sensors) are highly developed for SiC, facilitating new opportunities to control spin qubits while preserving the unique properties of color centers in diamond. These results are published in Physical Review Letters as collaboration with the Prof. Baranov's group (Ioffe Institute). Additional information is also available here (german).

10th December 2012:

Two of our recent works have been published in the current issue of Advanced Energy Materials.In a collaboration with the group of Prof. Scherf, University of Wuppertal we investigated the excitation dynamics in low bandgap donor-acceptor copolymers. We found that the effective coupling between copolymer building blocks governs the performance of the photovoltaic blends.
In a second collaboration with the IMEC institute in Leuven, two prominent organic solar cell device architectures, planar and bulk heterojunction - both based on CuPc and C60 - were directly compared in terms of nongeminate recombination and charge carrier distribution. Transient photovoltage and charge extraction as well as a macroscopic device simulations were applied to reveal the origin of different Voc vs. light intensity dependence.

20th November 2012:

We participate in the EU Project POCAONTAS ("Polymer-Carbon Nanotubes Active Systems for Photovoltaics") to enhance the performance of solar cells based on organic semiconductors with carbon nanotubes. The EU commission has granted about 3.45 Mio. Euro to pursue this research and training network within the coming four years. See press release here (german).

19th August 2012:

Our publication on Printed Paper Photovoltaic Cells, published in 2011, belongs to the most read papers of Advanced Energy Materials from 8/2011-7/2012.

14th June 2012:

Recently, our work based on our newly developed transient charge extraction technique was accepted for publication in Advanced Materials with the title "A New Approach for Probing the Mobility and Lifetime of Photogenerated Charge Carriers in Organic Solar Cells Under Real Operating Conditions". For the first time it is now feasible to investigate the mobility and lifetime of photogenerated charge carriers in organic solar cells under real operating conditions. Thereby we prevent the major drawback of charge injection typically occurring in the conventional photo-CELIV technique by implementing an adaptive field control during the delay time keeping the solar cell at open circuit conditions during recombination. In the paper, the practical benefit of the new technique is demonstrated by determining an important device parameter - the mobility-lifetime product - of solar cells based on the material blend systems of PCDTBT:PC71BM and P3HT:PC61BM.

17th January 2012:

A paper on "Electrically driven photon antibunching from a single molecule at room temperature", featuring Prof. Jens Pflaum as coauthor, was just published in Nature Communications.

12th December 2011:

Our PhD student Nadine Wolf received one of the Wilhelm-Conrad-Röntgen Studienpreis 2011 Awards for excellent achievements concerning their diploma thesis. Congratulations!

14th September 2011:

Our paper on printed paper photovoltaics (3PV) was published online in Advanced Energy Materials as collaboration with the group of Prof. Hübler, Technical University Chemnitz. The polymer-fullerene solar cells were printed on paper using a combination of gravure and flexographic printing techniques. The printed paper photovoltaic cells are free from expensive electrodes made with indium tin oxide, silver, or gold. Oxidized zinc film is used as the electron-collecting layer.

13th July 2011:

Hannes Kraus and Andreas Sperlich recently won the Energy & Environmental Science Poster Prize with their work on angle dependent magnetic resonance spectroscopy.

5th July 2011:

Our 2010 review article on Polymer-fullerene bulk heterojunction solar cells is one of the most popular articles of Reports on Progress in Physics.

19th May 2011:

Dr. Carsten Deibel presented his concluding Habilitation talk titled: Fukushima - Origin and Consequences from a Physical Perspective.

16th May:

Recently, our investigation of trap states in three methanofullerenes was published by Advanced Energy Materials (Schafferhans et al. (2011)). Thermally stimulated current measurements revealed significant differences in their trap distributions, with impact on the performance of organic solar cells.

In another publication (Liedtke et al., J. Am. Chem. Soc. (2011)) we investigated organic solar cells containing the trimetallic nitride endohedral fullerene Lu3N@C80-PCBEH, an acceptor leading to a high open circuit voltage. We found that intramolecular charge transfer along with electron back transfer to the triplet state in Lu3N@C80-PCBEH are loss mechanisms responsible for a reduced photocurrent.

11th January and 16th March 2011:

Recently, again two of our contributions to organic solar cell research were accepted for publication in high ranking international journals.

In one work (Strobel et al., Phys. Rev. Lett. 105, 266602 (2010)) we used Monte Carlo simulations to understand the role of the diffusion of polaron pairs in donor acceptor bulk heterojunctions. We found that the extraction of these bound polaron pairs at the electrodes is an important loss factor limiting the efficiency of organic optoelectronic and photovoltaic devices.

In a second work, published online by Adv. Funct. Mater. (Baumann et al. (2011)), we investigated the recombination dynamics in organic solar cells based on pBTCT-C12:PCBM in 1:1 and 1:4 ratio. This work is novel in as far as a combination of a local probe technique (microwave conductivity) with a charge extraction experiment is unique, and as the experimental findings can be generalized to other polymer:fullerene blend systems allowing for phase segregation as well.

17th September 2010:

The International Cluster Conference on Organic Photovoltaics, organised by Bayern Innovativ in cooperation with ZAE Bayern, was held on 16th September 2010 in Würzburg: with 270 attendees from 11 countries, it was a success. Several world-reknown experts presented their view on a range of topics from materials, devices and simulation to production and stability aspects. The conference was concluded by a panel discussion on the prospects of organic photovoltaics. The conference will be established as annual event in Würzburg, taking place next time on 22nd September 2011.

6th September 2010:

In the second round of proposals of the DFG priority programme SPP1355, we won four of the about 16 granted projects Germany-wide. This success proves and further strengthens the position of our Würzburg-based research in the field of organic semiconductors for applications as photovoltaics devices.

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