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Faculty of Mathematics and Natural SciencesInstitute of Physics II Dep. of Physics

Subgroup Wang

We are a group of physicists and materials scientists studying the photophysics in (templated) organic and hybrid semiconductors using ultrafast and steady-state optical spectroscopy.

We are always looking for talented and motivated master and bachelor students to join our (sub-)group for their theses. Interested students are welcome to take a look at the "Open Positions" section and to contact Dr. Tianyi Wang (twang@ph2.uni-koeln.de) for more details.

Direction A

Bernhardt et al, Journal of the American Chemical Society 144 (42), 19372-19381 (2022)

Structure-Property Relationships in Molecular Aggregates

In contrast to regular J- and H-aggregates, thin film squaraine aggregates usually have broad absorption spectra containing both J-and H-like features, which are favorable for organic photovoltaics. Despite being successfully applied in organic photovoltaics for years, a clear interpretation of these optical properties by relating them to specific excited states and an underlying aggregate structure has not been made. In this work, by static and transient absorption spectroscopy on aggregated n-butyl anilino squaraines, we provide evidence that both the red- and blue-shifted peaks can be explained by assuming an ensemble of aggregates with intermolecular dipole–dipole resonance interactions and structural disorder deriving from the four different nearest neighbor alignments─in sharp contrast to previous association of the peaks with intermolecular charge-transfer interactions. In our model, the next-nearest neighbor dipole–dipole interactions may be negative or positive, which leads to the occurrence of J- and H-like features in the absorption spectrum. Upon femtosecond pulse excitation of the aggregated sample, a transient absorption spectrum deviating from the absorbance spectrum emerges. The deviation finds its origin in the excitation of two-exciton states by the probe pulse. The lifetime of the exciton is confirmed by the band integral dynamics, featuring a single-exponential decay with a lifetime of 205 ps. Our results disclose the aggregated structure and the origin of red- and blue-shifted peaks and explain the absence of photoluminescence in squaraine thin films. Our findings underline the important role of structural disorder of molecular aggregates for photovoltaic applications.

Direction B

Rieland et al, Nano Letters 24 (32), 9824–9831 (2024)

Photophysics in Excitonic Materials

Using heterodyne transient grating spectroscopy, we observe a significant enhancement of exciton diffusion in a monolayer WSe2 stacked on graphene. The diffusion dynamics can be optically tuned within a few picoseconds by altering the photoexcited carrier density in graphene. The effective diffusion constant in initial picoseconds in the WSe2/graphene heterostructure is (40.3 ± 4.5) cm2 s–1, representing a substantial improvement over (2.1 ± 0.8) cm2 s–1, typical for an isolated WSe2 monolayer. This enhancement can be understood in terms of a transient screening of impurities, charge traps, and defect states in WSe2 by photoexcited charge carriers in graphene. Furthermore, diffusion within WSe2 is affected by interlayer interactions, such as charge transfer, varying with the incident excitation fluence. These findings underscore the dynamical nature of screening and diffusion processes in heterostructures of 2D semiconductors and graphene and provide insights for future applications of these systems in ultrafast optoelectronic devices.

Dr. Tianyi Wang

Avatar Tianyi Wang received his Bachelor degree in Materials Science and Engineering at Nanjing University of Aeronautics and Astronautics in 2012 with a thesis focusing on the chemical bath deposition of tin sulfide thin films for photovoltaic applications under the supervision of Prof. Honglie Shen. From 2012 to 2014, he studied Materials Science at Ludwig Maximilian University of Munich, Technical University of Munich and University of Montpellier under the Erasmus Mundus Master Program MAMASELF and graduated with a thesis on the low temperature fabrication of superstructured titania thin films via wet imprinting and characterization of these films using GISAXS under the supervision of Prof. Peter Müller-Buschbaum. He worked as a PhD student on the photophysics of organic and hybrid semicondutors under hydrostatic pressure in the Hybrid Solar Cells group at Insititute AMOLF in Amsterdam under the supervison of Prof. Bruno Ehrler from 2014 to 2019 and received his PhD in Physics at University of Amsterdam. After his PhD, he joined the Institute for Solar Fuels at the Helmholtz Center Berlin as a postdoctoral researcher, working on the exciton/charge carrier dynamics in organic semiconductors for solar water splitting under the supervision of Dr. Dennis Friedrich and Prof. Roel van de Krol. He joined the Optical Condensed Matter Science group led by Prof. Paul van Loosdrecht at Uniersity of Cologne as a sub-group leader in 2023. His current research interests include ultrafast optical spectroscopy and spin dynamics of excitons in organic and hybrid semiconductors.

E-Mail: twang@ph2.uni-koeln.de

Google Scholar profile: https://scholar.google.com/citations?user=aQntjWEAAAAJ&hl=en

M.Sc. Lukas Rieland

Lukas Rieland completed his Bachelor's degree in 2018 and his Master's degree in 2021 under the supervision of Prof. Stefan Linden at the University of Bonn. Lukas' Master's thesis focused on constructing a transient absorption setup to study TMD monolayers, leading him to develop an interest in time-resolved spectroscopy. Driven by this interest, Lukas decided to further his expertise by joining the research training group TIDE at the University of Cologne, under the mentorship of Prof. Paul van Loosdrecht. In his role within TIDE, Lukas utilizes ultrafast optical techniques to monitor the temporal evolution and diffusion of excitations in templated and non-templated organic systems.

E-Mail: rieland@ph2.uni-koeln.de

M.Sc. Aiman Rauf

Aiman Rauf completed her Bachelor's degree in 2022 and her Master's degree in 2024 from Jamia Millia Islamia in India, specializing in theoretical condensed matter physics under the supervision of Dr. S.K. Firoz Islam. Her Master's thesis focused on light-matter interaction, particularly how irradiation can give rise to non-topological massive bound states across smooth interfaces of topological semimetals. This work sparked her interest in the experimental aspects of optical condensed matter physics. Driven by this curiosity, she joined the group of Prof. Paul van Loosdrecht with support from the research training group TIDE, where she uses ultrafast transient absorption spectroscopy to study exciton dynamics in (templated) organic semiconductors.

E-Mail: arauf@ph2.uni-koeln.de

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This could be YOU! Please check Open Positions for further information.

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This could be YOU! Please check Open Positions for further information.

Dr. Robin Bernhardt

Avatar Robin Bernhardt earned his Bachelor's degree with a thesis centered on the fabrication of heterostructures using various two-dimensional materials at the University of Cologne under the guidance of Prof. Alexander Grüneis in 2017. Subsequently, he pursued a Master's degree in physics with a specialization in condensed matter physics at the same university from 2017 to 2020. His master's thesis, supervised by Prof. Paul van Loosdrecht, focused on the exciton population and valley dynamics in monolayer WSe2 using time-resolved photoluminescence spectroscopy. Robin completed his PhD under the support of the TIDE research training group, investigating the excitonic nature and dynamics of Squaraine molecules in aggregated thin films designed for photovoltaic and light-sensing applications. His research involves the use of transient absorption spectroscopy and exciton theory modeling.

E-Mail: bernhardt@ph2.uni-koeln.de

Google Scholar profile: https://scholar.google.com/citations?user=4uU10w8AAAAJ&hl=en

Current position: Data Scientist at parcIT

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Ultrafast Transient Absorption Spectroscopy

Ultrafast transient absorption spectroscopy is a pump-probe technique that allows us to track the dynamics of the excited states in molecules and semiconductors with sub-picosecond temporal resolution.

Laboratory equipment:

  • Laser source:
    • Regenerative amplified Yb:KGW laser (Pharos), maximum power at fundamental wavelength (1030 nm): ca. 15W, output wavelengths: 1450 nm, 800 nm and 515 nm, repetition rate: 150 kHz, pulse duration: ca. 150 fs;

  • Pump wavelengths:
    • 400 nm to 800 nm;

  • Probe wavelengths:
    • 500 nm to 1600 nm (with sapphire pumped at 1450 nm);

  • Maximum delay time:
    • 2000 ps (2 ns);

  • Sample form and environment:
    • Solid state: thin films and thin crystals (cryogenic environment possible) and liquid state: solution.

Avatar

Steady-state absorption and emission

Steady-state absorption and emission measurements provides fundamental information of electronic energy levels in the samples and the transitions between these levels.

Laboratory equipment:

  • UV/Vis/NIR spectrometer:
    • Perkin-Elmer Lambda 1050;

  • Absorption wavelengths range:
    • 200 nm to 3300 nm;

  • Fluorometer:
    • Perkin-Elmer LS55;

  • Excitation wavelengths range:
    • 200 nm to 800 nm;

  • Detection wavelengths range:
    • 200 nm to 900 nm;

  • Lukas Rieland, Julian Wagner, Robin Bernhardt, Tianyi Wang, Omar Abdul-Aziz, Philipp Stein, Eva A. A. Pogna, Stefano Dal Conte, Giulio Cerullo, Hamoon Hedayat*, Paul H. M. van Loosdrecht*, "Ultrafast Optical Control of Exciton Diffusion in WSe2/Graphene Heterostructures Revealed by Heterodyne Transient Grating Spectroscopy”, Nano Letters 24 (32), 9824–9831 (2024), selected as cover article.

  • Robin Bernhardt*, Marick Manrho, Jennifer Zablocki, Lukas Rieland, Arne Lützen, Manuela Schiek, Klaus Meerholz, Jingyi Zhu*, Thomas L.C. Jansen, Jasper Knoester, Paul H.M. Van Loosdrecht*, "Structural Disorder as the Origin of Optical Properties and Spectral Dynamics in Squaraine Nano-Aggregates”, Journal of the American Chemical Society 144 (42), 19372-19381 (2022).

  • Daniele T. Cuzzupè, Feray Ünlü, Khan Lê, Robin Bernhardt, Michael Wilhelm, Matthias Grosch, Rene Weißing, Thomas Fischer, Paul H.M. van Loosdrecht, Sanjay Mathur*, "Thermally-induced drift of A-site cations at solid–solid interface in physically paired lead halide perovskites”, Scientific reports 12 (1), 10241 (2022).

  • Jingyi Zhu*, Robin Bernhardt, Weili Cui, Raphael German, Julian Wagner, Boris V. Senkovskiy, Alexander Grüneis, Thomas Pichler, Yuxuan Li, Xueyang Li, Kaifeng Wu*, Rulin Liu, Xi Zhu, Paul H.M. Van Loosdrecht*, Lei Shi*, "Unraveling the Excitonic Transition and Associated Dynamics in Confined Long Linear Carbon Chains with Time‐Resolved Resonance Raman Scattering”, Laser & Photonics Reviews 15 (12), 2100259 (2021).

  • Julian Wagner, Henning Kuhn, Robin Bernhardt, Jingyi Zhu*, Paul H.M. van Loosdrecht*, "Trap induced long exciton intervalley scattering and population lifetime in monolayer WSe2”, 2D Materials 8 (3), 035018 (2021).

  • Henning Kuhn, Julian Wagner, Shuangping Han, Robin Bernhardt, Yan Gao, Liantuan Xiao, Jingyi Zhu*, Paul H.M. van Loosdrecht*, "Excitonic Transport and Intervalley Scattering Dynamics in Large‐Size Exfoliated MoSe2 Monolayer Investigated by Heterodyned Transient Grating Spectroscopy”, Laser & Photonics Reviews 14 (12), 2000029 (2020).

    • *: corresponding author, +: equal contributions

  • Jin Yang, Samrat Ghosh, Jérôme Roeser, Amitava Acharjya, Christopher Penschke, Yusuke Tsutsui, Jabor Rabeah, Tianyi Wang, Simon Yves Djoko Tameu, Meng-Yang Ye, Julia Grüneberg, Shuang Li, Changxia Li, Reinhard Schomäcker, Roel Van De Krol, Shu Seki, Peter Saalfrank, Arne Thomas*, "Constitutional isomerism of the linkages in donor–acceptor covalent organic frameworks and its impact on photocatalysis”, Nature Communications 13 (1), 6317 (2022)

  • Jin Yang, Amitava Acharjya, Meng‐Yang Ye, Jabor Rabeah, Shuang Li, Zdravko Kochovski, Sol Youk, Jérôme Roeser, Julia Grüneberg, Christopher Penschke, Michael Schwarze, Tianyi Wang, Yan Lu, Roel van de Krol, Martin Oschatz, Reinhard Schomäcker, Peter Saalfrank, Arne Thomas*, "Protonated Imine‐Linked Covalent Organic Frameworks for Photocatalytic Hydrogen Evolution”, Angewandte Chemie International Edition 60 (36), 19797-19803 (2021)

  • Moritz H. Futscher, Ju Min Lee, Lucie McGovern, Loreta A. Muscarella, Tianyi Wang, Muhammad Irfan Haider, Azhar Fakharuddin, Lukas Schmidt-Mende, Bruno Ehrler*, "Quantification of ion migration in CH3NH3PbI3 perovskite solar cells by transient capacitance measurements”, Materials Horizons 6 (7), 1497-1503 (2019)

  • Lin Song, Tianyi Wang, Volker Körstgens, Weijia Wang, Nitin Saxena, Christoph J. Schaffer, Thomas Fröschl, Nicola Hüsing, Sigrid Bernstorff, Peter Müller‐Buschbaum*, "Wet Imprinting of Channel‐Type Superstructures in Nanostructured Titania Thin Films at Low Temperatures for Hybrid Solar Cells”, ChemSusChem 11 (7), 1179-1186 (2018)

  • Stefan Wil Tabernig, Benjamin Daiber, Tianyi Wang, Bruno Ehrler*, "Enhancing silicon solar cells with singlet fission: the case for Förster resonant energy transfer using a quantum dot intermediate”, Journal of Photonics for Energy 8 (2), 022008-022008 (2018)

  • Tianyi Wang+, Benjamin Daiber+, Jarvist M. Frost, Sander A. Mann, Erik C. Garnett, Aron Walsh, Bruno Ehrler*, "Indirect to direct bandgap transition in methylammonium lead halide perovskite”, Energy & Environmental Science 10 (2), 509-515 (2017)

  • Lin Song, Amr Abdelsamie, Christoph J. Schaffer, Volker Körstgens, Weijia Wang, Tianyi Wang, Efi Dwi Indari, Thomas Fröschl, Nicola Hüsing, Tobias Haeberle, Paolo Lugli, Sigrid Bernstorff, Peter Müller‐Buschbaum*, "A Low Temperature Route toward Hierarchically Structured Titania Films for Thin Hybrid Solar Cells”, Advanced Functional Materials 26 (39), 7084-7093 (2016)

  • Weijia Wang, Christoph J. Schaffer, Lin Song, Volker Körstgens, Stephan Pröller, Efi Dwi Indari, Tianyi Wang, Amr Abdelsamie, Sigrid Bernstorff, Peter Müller-Buschbaum*, "In operando morphology investigation of inverted bulk heterojunction organic solar cells by GISAXS”, Journal of Materials Chemistry A 3 (16), 8324-8331 (2015)

    • *: corresponding author, +: equal contributions

PhD positions

We are currently looking for a talented and motivated PhD student (payscale: E13, TV-L, 75%) to join our (sub-)group immediately or on a later date upon mutual agreement. The selected candidate will be working within the TIDE research training group. The ideal candidate should have completed her/his/their master study by the desired starting date and should have a strong background in condensed matter physics and/or non-linear optics with hands-on experience in ultrafast optical spectroscopy/microscopy. Experience with organic/hybrid semiconductors, physical measurements at low temperature or in high magnetic field, or theoretical modeling of photophysics in organic materials will be considered as a plus. The description of the position can be found here.

Interested applicants should submit their applications (including CV, letter of motivation, and academic transcripts and certificates for bachelor and master studies) via the online jobportal of University of Cologne HERE. Informal inquiries regarding the position can be sent to Dr. Tianyi Wang (twang@ph2.uni-koeln.de).

Bachelor, Master and semester projects:

We are always looking for motivated and talented students to join our (sub-)group for their Bachelor, Master and semester projects. The ideal candidate should have an educational background in physics, chemistry, materials science, engineering or other relevant fields and should have strong interests (preferably also hands-on experience) in at least one of the following fields: 1. Time-resolved and steady-state optical spectroscopy; 2. Materials science of organic/hybrid semiconductors; 3. Computational modeling of photophysics in organic/hybrid semiconductors.

Interested students can contact Dr. Tianyi Wang (twang@ph2.uni-koeln.de) and Prof. Paul van Loosdrecht (pvl@ph2.uni-koeln.de) with their CVs, letters of motivation and their academic transcripts (with courses and grades) for potential projects.