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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 currently have a fully-funded PhD position opening on ultrafast optical spectroscopy of (templated) organic semiconductors in our (sub-)group starting immediately or on a later date upon mutual agreement. Please check the "Open Positions" section for more information and apply here via the online jobportal of University of Cologne until May 31st, 2024. Informal inqueries of the positions can be sent to Dr. Tianyi Wang (twang@ph2.uni-koeln.de) and Prof. Paul van Loosdrecht (pvl@ph2.uni-koeln.de).

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. 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. Currently, Robin is engaged as a PhD student in 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

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



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


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


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


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.


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;

  • 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 apply here via the online jobportal of University of Cologne until May 31st, 2024. Informal inquiries regarding the position can be sent to Dr. Tianyi Wang (twang@ph2.uni-koeln.de) and Prof. Paul van Loosdrecht (pvl@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.