Wenlan Liu
I joined the theory group at the Max Planck Institute for Polymer Research as a postdoctoral fellow in February 2019.
In April 2011, I received my PhD degree in the group of Prof. Bernd Engels at the Würzburg University, on the topic of "Exciton coupling in valence and core excited aggregates of pi-conjugated molecules." After two short post-doc periods in Heidelberg and Tübingen Universities, I joined Prof. Andreas Köhn's group at the Mainz University back in May 2012. At the first two years, I developed the "Explicitly-correlated multi-reference internal contracted coupled cluster theory" code in the “a string-based general contraction code (GECCO)” program package. Then I switched my research focus on the topic of electronically excited state analysis method, and proposed an novel diabatization method. The method is also implemented now in the Turbomole program package. In between, my working place also switched from Mainz to Heidelberg, InnovationLab GmbH. In the meantime, I was also working on the topic of “Quantum chemical studies of interactions and transfer processes at interfaces”.
I am one of the board members of the Chinese-German Chemical Association (www.cgca.de), which promotes the academic exchange among its members, as well as cooperation between Chinese and German academic and industrial institutions in chemistry and related fields.
Published in the group
2020
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Long-range exciton diffusion in molecular non-fullerene acceptors
Y. Firdaus, V. M. Le Corre, S. Karuthedath, W. Liu, A. Markina, W. Huang, S. Chattopadhyay, M. M. Nahid, M. I. Nugraha, Y. Lin, A. Seitkhan, A. Basu, W. Zhang, I. McCulloch, H. Ade, J. Labram, F. Laquai, D. Andrienko, L. J. A. Koster, T. D. Anthopoulos
Nature Communications,
11,
5220,
2020,
[doi]
[abstract]
The short exciton diffusion length associated with most classical organic semiconductors used in organic photovoltaics (5-20 nm) imposes severe limits on the maximum size of the donor and acceptor domains within the photoactive layer of the cell. Identifying materials that are able to transport excitons over longer distances can help advancing our understanding and lead to solar cells with higher efficiency. Here we measure the exciton diffusion length in a wide range of nonfullerene acceptor molecules using two different experimental techniques based on photocurrent and ultrafast spectroscopy measurements. The acceptors exhibit balanced ambipolar charge transport and surprisingly long exciton diffusion lengths in the range of 20 to 47 nm. With the aid of quantum-chemical calculations we are able to rationalize the exciton dynamics and draw basic chemical design rules particularly on the importance of the end-group substituent on the crystal packing of nonfullerene acceptors.
Molecular origin of balanced bipolar transport in neat layers of the emitter CzDBA
W. Liu, N. B. Kotadiya, P. W. M. Blom, G.-J. A. H. Wetzelaer, D. Andrienko
Adv. Mater. Technol.,
2000120,
2020,
[doi]
[abstract]
Recently an efficient single‐layer organic light‐emitting diode has been reported consisting of a neat layer of the diboron‐based thermally activated delayed fluorescence emitter 510‐bis(4‐(9H‐carbazol‐9‐yl)‐26‐dimethylphenyl)‐510‐dihydroboranthrene exhibiting remarkably balanced bipolar electron and hole transport. Here the donor–acceptor–donor architecture of the molecule is linked to the transport characteristics of its neat amorphous films. It is found that energetic disorder is larger for holes than for electrons explaining the experimentally observed difference in temperature activation of the mobility. Although a difference in energetic disorder would suggest unbalanced charge transport it is demonstrated that it is partly counteracted by larger coupling elements for holes.
Intrinsic efficiency limits in low-bandgap non-fullerene acceptor organic solar cells
S. Karuthedath, J. Gorenflot, Y. Firdaus N. Chaturvedi, C. S. P. De Castro, G. T. Harrison, J. I. Khan, A. Markina, A. H. Balawi, T. A. D. Pena, W. Liu, R.-Z. Liang, A. Sharma, S. H. K. Paleti, W. Zhang, Y. Lin, E. Alarousu, D. H. Anjum, P. M. Beaujuge, S. De Wolf, I. McCulloch, T. D. Anthopoulos, D. Baran, D. Andrienko, F. Laquai
Nature Materials,
2020,
[doi]
[abstract]
In bulk heterojunction (BHJ) organic solar cells (OSCs) both the electron affinity (EA) and ionization energy (IE) offsets at the donor–acceptor interface should equally control exciton dissociation. Here we demonstrate that in low-bandgap non-fullerene acceptor (NFA) BHJs ultrafast donor-to-acceptor energy transfer precedes hole transfer from the acceptor to the donor and thus renders the EA offset virtually unimportant. Moreover sizeable bulk IE offsets of about 0.5 eV are needed for efficient charge transfer and high internal quantum efficiencies since energy level bending at the donor–NFA interface caused by the acceptors’ quadrupole moments prevents efficient exciton-to-charge-transfer state conversion at low IE offsets. The same bending however is the origin of the barrier-less charge transfer state to free charge conversion. Our results provide a comprehensive picture of the photophysics of NFA-based blends and show that sizeable bulk IE offsets are essential to design efficient BHJ OSCs based on low-bandgap NFAs.