Members from projects B2 and A2 explored the dynamics of charge-transfer excitons at the internal interface in heterostructures of the organic donor-acceptor molecules pentacene and perfluoropentacene.

Reprinted with permission from A. Rinn et al, 2017, 9, 48, 42020–42028. Copyright 2017 American Chemical Society.

Organic photovoltaics is a vivid research field as it promises the fabrication of large scale and thin film devices on flexible supports. One of the key challenges is the separation of optically excited bound electron-hole pairs, so-called excitons. Of particular relevance are charge-transfer (CT) excitons at donor-acceptor interfaces with the electron in the acceptor and the hole in the donor molecule as they are prime candidates as intermediaries for charge separation. Studying these CT excitons in state-of-the-art devices is challenging as these consist of blends of molecular donors and acceptors in order to provide maximum internal interface area. The resulting, complex interface geometry structure hampers microscopic characterization of such CT-excitons and, in particular, denies a well-defined correlation of the electronic properties with the molecular packing at the interface. Hence, detailed studies on the underlying physical mechanism of charge-separation in organic photovoltaics rely on highly-ordered model heterojunctions.

In the present study, the researchers combine their expertise in the fabrication of such highly ordered crystalline molecular heterostructures with precise, time-resolved optical microspectroscopy to study the energetics and dynamics of CT-excitons at donor-acceptor interfaces for selected pentacene / perfluoropentacene heterostructures. Based on previous work, various molecular heterostructures with different molecular orientation are realized and further compared with the dynamics of blends, i.e., completely intermixed heterostructures, as well as the respective pure materials.

The spatial separation of electrons and holes in the CT-excitons leads to extended lifetimes compared to the pristine species. Intriguingly, the energetics reveal that the common description of such excitons based on a straight-forward orbital picture as a mere linear combination of the involved individual constituents is insufficient and a more intricate description of CT-excitons is needed. The present study provides the first experimental data systematically investigating such excitations in highly-ordered crystalline molecular heterostructures, which will enable further theoretical calculations on the involved electronic effects.

Publication

Andre Rinn, Tobias Breuer, Julia Wiegand, Michael Beck, Jens Hübner, Robin C. Döring, Michael Oestreich, Wolfram Heimbrodt, Gregor Witte, Sangam Chatterjee, Interfacial Molecular Packing Determines Exciton Dynamics in Molecular Heterostructures: the Case of Pentacene – Perfluoropentacene,
ACS Applied Materials & Interfaces, (2017) DOI: 10.1021/acsami.7b11118

Contact
Prof. Dr. Sangam Chatterjee
Institute of Experimental Physics I, Justus-Liebig-University Giessen
SFB 1083 Project B2 (Chatterjee)
https://www.uni-giessen.de/ipi
Tel. +49 (0)641 99-33100
Email: Sangam.Chatterjee@physik.uni-giessen.de

Prof. Dr. Gregor Witte
AG Molekulare Festkörperphysik, Philipps-Universität Marburg
SFB 1083 Project A2 (Witte)
https://www.uni-marburg.de/sfb1083/projects/A2
Tel. +49 (0)6421 28-21384
Email: Gregor.Witte@physik.uni-marburg.de

Congratulations to Dr. Klaus Stallberg, who recently joined project B6 (Höfer/Wallauer) as a postdoc, for being awarded a dissertation-prize.

Congratulations to Dr. Klaus Stallberg, who recently joined project B6 as a postdoc, for being awarded a prize for his excellent dissertation at TU Clausthal on “Spectromicroscopic investigations of thin porphyrin layers and their influence on plasmonic excitations in silver structures by multi-photon-photoemission”.

In his thesis Klaus Stallberg investigated thin porphyrin films and plasmonic silver structures with multi-photon photoemission electronmicroscopy (nP-PEEM). Based on energy-resolved, time-resolved and laterally resolved nPPE experiments he porposed a model for the complex electronic excitation and relaxation processes in porphyrin films and was able to show how a thin porphyrin layer affects the dispersion of plasmons propagating on an extended silver island.

For these investigations he received one of three prizes (Förderpreis 1000 EUR) of the Friends of TU Clausthal (Verein von Freunden der Technischen Universität Clausthal e.V.).

Since July 2017 Klaus Stallberg explores the dynamics of charge transfer processes at interfaces of organic heterostructures by means of time-resolved two-photon photoemission in the framework of project B6 (Höfer/Wallauer).

Link to dissertation on TU Clausthal server.

TU Clausthal press release.

We congratulate Johannes Reimann, PhD-student in the research group of Prof. Höfer, who received a poster-prize for his outstanding presentation at the 16th International Conference on the Formation of Semiconductor Interfaces (ICFSI-16) in Hannover.

In his contribution, Johannes Reimann presented new insights into the generation and control of spin polarized photocurrents in the Dirac Cone of the topological insulator Sb₂Te₃ by means of time and angle resolved two-photon photoemission (2PPE). The experimental results allow for a better understanding of the fundamental processes governing the electron dynamics inside the topological surface states of this material class.

The work was undertaken within the DFG priority program “Topological Insulators: Materials-Fundamental Properties-Devices” (SPP 1666) and in close collaboration with project B6 of Prof. Höfer.

ICFSI-16 is part of a biannual conference series primarily focused on phenomena at surfaces, interfaces, and nanostructures that are of strong current interest, ranging from characterization at the atomic scale to prospects of applications.

Poster “Optical control of ultrafast currents in the topological surface state of Sb₂Te₃” by J. Reimann¹, K. Kuroda^1,2, K. A. Kokh³, O. E. Tereshchenko³, A. Kimura⁴, J. Güdde¹, U. Höfer¹ (¹ Philipps-Universität-Marburg, ² University of Tokyo, ³ Russian Academy of Sciences and Novosibirsk State University, ⁴ Hiroshima University) – 16th International Conference on the Formation of Semiconductor Interfaces (ICFSI-16) July 02 – 07, 2017, Hannover, Germany.

The SFB congratulates Dr. Alrun Hauke, a young postdoc in project A2 (Witte), for being awarded the annual Dr.-Walter-Seipp-Prize of Commerzbank Dresden in recognition of her outstanding PhD-dissertation on “Vertical Organic Field-Effect Transistors – On the Understanding of a Novel Device Concept” submitted at Technical University Dresden in 2016.

Initiated in the Nineties, Commerzbank Dresden each year honors the three highest-ranked theses submitted at the Technical University Dresden during the year before. On June 30, the Commerzbank in a small ceremony in its Event-Lounge in Dresden honored the three top-rated PhD-theses of 2016. The prize for the best dissertation, the Dr. Walter-Seipp-Preis and 4000 EUR, was awarded to Dr. Hauke.

Dr. Alrun Hauke’s thesis research was in the field of organic electronics under the guidance of Prof. Karl Leo (Inst. of Applied Physics) which led to a better understanding of a novel organic transistor, the so-called Vertical Organic Field-Effect Transistor (OFET). By combining experiments and data gained from device simulations she was able to identify the key parameters governing device operation as well as demonstrate improved devices with MOSFET-like inversion mode operation (MOSFET metal-oxide-semiconductor field-effect transistor=. The latter was facilitated by the concept of molecular doping, one of the primary research interests at the Institute of Applied Physics at TU Dresden.

We are pleased to have attracted Dr. Hauke to Marburg, where she joined project A2 in fall 2016. Within this project she now investigates interfaces between organic semiconductors and 2D materials with respect to their structural and electronic properties.

See also a press release in German issued by TU Dresden for more detail.

Dr. Kuroda and Prof. Dr. Höfer in the lab at the University of Marburg (Photo: Univ. Marburg).

At the 36th academic lecture sponsored by the Japan Surface Science Association (SSSJ) held at the Nagoya International Conference Hall on November 29th in 2016, Kuroda gave an excellent general presentation on “Ultrafast dynamics of Dirac surface states in topological insulators” which is expected to make a significant contribution to the development of surface science.

Topological insulators are insulators in which only the outermost surface reveals a metallic state. As surface electrons are crucial in shaping the material functions, understanding details of the mechanics and processes at work has drawn worldwide attention. Kuroda and colleagues were the first to successfully apply short laser pulses and time-resolved photoelectron spectroscopy in the mid-infrared region in their investigations. As the researchers analyzed the photo excitation of Dirac surface-electrons generated in topological insulators at femtosecond resolution, they were able to show a direct optical transition of the Dirac surface condition in response to the mid-infrared excitation with a resulting spin-polarization of the surface current. The SSSJ sees this discovery as an important step to achieving optical spin control of electrons solely by the means of light.

The award is granted to promising young researchers under the age of 32 years and the presentation ceremony took place at the Surface Science Society of Japan’s regular meeting on May 20th, 2017.

The underlying research work was performed in close collaboration with colleagues in Japan and from SFB-project B6 (Höfer) during Dr. Kuroda’s research stay as a JSPS fellow and guest scientist of SFB 1083 at the Philipps-Universität Marburg in 2014 and 2015.