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.
2024
On the critical competition between singlet exciton decay and free charge generation in non-fullerene-based organic solar cells with low energetic offsets
M. Pranav, A. Shukla, D. Moser, J. Rumeney, W. Liu, R. Wang, B. Sun, S. Smeets, N. Tokmoldin, Y. Cao, G. He, Th. Beitz, F. Jaiser, Th. Hultzsch, S. Shoaee, W. Maes, L. Lüer, C. Brabec, K. Vandewal, D. Andrienko, S. Ludwigs, D. Neher
Energy and Environmental Science,
0,
null,
2024,
[doi]
[abstract]
Reducing voltage losses while maintaining high photocurrents is the holy grail of current research on non-fullerene acceptor (NFA) based organic solar cell. Recent focus lies in understanding the various fundamental mechanisms in organic blends with minimal energy offsets - particularly the relationship between ionization energy offset (ΔIE) and free charge generation. Here we quantitatively probe this relationship in multiple NFA-based blends by mixing Y-series NFAs with PM6 of different molecular weights covering a broad power conversion efficiency (PCE) range: from 15% down to 1%. Spectroelectrochemistry reveals that a ΔIE of more than 0.3 eV is necessary for efficient photocurrent generation. Bias-dependent time-delayed collection experiments reveal a very pronounced field-dependence of free charge generation for small ΔIE blends which is mirrored by a strong and simultaneous field-dependence of the quantified photoluminescence from the NFA local singlet exciton (LE). We find that the decay of singlet excitons is the primary competition to free charge generation in low-offset NFA-based organic solar cells with neither noticeable losses from charge-transfer (CT) decay nor evidence for LE-CT hybridization. In agreement with this conclusion transient absorption spectroscopy consistently reveals that a smaller ΔIE slows the NFA exciton dissociation into free charges albeit restorable by an electric field. Our experimental data align with Marcus theory calculations supported by density functional theory simulations for zero-field free charge generation and exciton decay efficiencies. We conclude that efficient photocurrent generation generally requires that the CT state is located below the LE but that this restriction is lifted in systems with a small reorganization energy for charge transfer.
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Semitransparent Organic Photovoltaics Utilizing Intrinsic Charge Generation in Non-Fullerene Acceptors
A. Sharma, N. Gasparini, A. Markina, S. Karuthedath, J. Gorenflot, J. Han, A. Balawi, W. Liu, D. Bryant, J. Bertrandie, J. Troughton, S. H. K. Paleti, H. Bristow, F. Laquai, D. Andrienko, D. Baran
Advanced Materials,
36,
2024,
[doi]
[abstract]
In organic semiconductors a donor/acceptor heterojunction is typically required for efficient dissociation of excitons. Using transient absorption spectroscopy to study the dynamics of excited states in non-fullerene acceptors (NFAs) it is shown that NFAs can generate charges without a donor/acceptor interface. This is due to the fact that dielectric solvation provides a driving force sufficient to dissociate the excited state and form the charge-transfer (CT) state. The CT state is further dissociated into free charges at interfaces between polycrystalline regions in neat NFAs. For IEICO-4F incorporating just 9 wt% donor polymer PTB7-Th in neat films greatly boosts charge generation enhancing efficient exciton separation into free charges. This property is utilized to fabricate donor-dilute organic photovoltaics (OPV) delivering a power conversion efficiency of 8.3% in the case of opaque devices with a metal top-electrode and an active layer average visible transmittance (AVT) of 75%. It is shown that the intrinsic charge generation in low-bandgap NFAs contributes to the overall photocurrent generation. IEICO-4F-based OPVs with limited PTB7-Th content have high thermal resilience demonstrating little drop in performance over 700 h. PTB7-Th:IEICO-4F semitransparent OPVs are leveraged to fabricate an 8-series connected semi-transparent module demonstrating light-utilization efficiency of 2.2% alongside an AVT of 63%.
2023
An ab initio method on large sized molecular aggregate system: Predicting absorption spectra of crystalline organic semiconducting films
W. Liu, D. Andrienko
J. Chem. Phys.,
158,
094108,
2023,
[doi]
[abstract]
Theoretical description of electronically excited states of molecular aggregates at an ab initio level is computationally demanding. To reduce the computational cost we propose a model Hamiltonian approach that approximates the electronically excited state wavefunction of the molecular aggregate. We benchmark our approach on a thiophene hexamer as well as calculate the absorption spectra of several crystalline non-fullerene acceptors including Y6 and ITIC which are known for their high power conversion efficiency in organic solar cells. The method qualitatively predicts the experimentally measured spectral shape which can be further linked to the molecular arrangement in the unit cell.
Reduced bimolecular charge recombination in efficient organic solar cells comprising non-fullerene acceptors
Y. Wu, Y. Li, B. van der Zee, W. Liu, A. Markina, H. Fan, H. Yang, C. Cui, Y. Li, P. W. M. Blom, D. Andrienko, G.-J. A. H. Wetzelaer
Scientific Reports,
13,
4717,
2023,
[doi]
2021
Reduced Intrinsic Non-Radiative Losses Allow Room-Temperature Triplet Emission from Purely Organic Emitters
Y. Li, L. Jiang, W. Liu, S. Xu, T.-Y. Li, F. Fries, O. Zeika, Y. Zou, C. Ramanan, S. Lenk, R. Scholz, D. Andrienko, X. Feng, K. Leo, S. Reineke
Advanced Materials,
2101844,
2021,
[doi]
[abstract]
Persistent luminescence from triplet excitons in organic molecules is rare as fast non-radiative deactivation typically dominates over radiative transitions. This work demonstrates that the substitution of a hydrogen atom in a derivative of phenanthroimidazole with an N-phenyl ring can substantially stabilize the excited state. This stabilization converts an organic material without phosphorescence emission into a molecular system exhibiting efficient and ultralong afterglow phosphorescence at room temperature. Results from systematic photophysical investigations kinetic modeling excited-state dynamic modeling and single-crystal structure analysis identify that the long-lived triplets originate from a reduction of intrinsic non-radiative molecular relaxations. Further modification of the N-phenyl ring with halogen atoms affects the afterglow lifetime and quantum yield. As a proof-of-concept an anticounterfeiting device is demonstrated with a time-dependent Morse code feature for data encryption based on these emitters. A fundamental design principle is outlined to achieve long-lived and emissive triplet states by suppressing intrinsic non-radiative relaxations in the form of molecular vibrations or rotations.
Chemical design rules for non-fullerene acceptors in organic solar cells
A. Markina, K.-H. Lin, W. Liu, C. Poelking, Y. Firdaus, D. R. Villalva, J. I. Khan, S. H. K. Paleti, G. T. Harrison, J. Gorenflot, W. Zhang, S. De Wolf, I. McCulloch, T. D. Anthopoulos, D. Baran, F. Laquai, D. Andrienko
Adv. Energy Mater.,
2102363,
2-11,
2021,
[doi]
[abstract]
Efficiencies of organic solar cells have practically doubled since the development of non-fullerene acceptors (NFAs). However generic chemical design rules for donor-NFA combinations are still needed. Such rules are proposed by analyzing inhomogeneous electrostatic fields at the donor–acceptor interface. It is shown that an acceptor–donor–acceptor molecular architecture and molecular alignment parallel to the interface results in energy level bending that destabilizes the charge transfer state thus promoting its dissociation into free charges. By analyzing a series of PCE10:NFA solar cells with NFAs including Y6 IEICO and ITIC as well as their halogenated derivatives it is suggested that the molecular quadrupole moment of 75 Debye A balances the losses in the open circuit voltage and gains in charge generation efficiency.
Impact of Acceptor Quadrupole Moment on Charge Generation and Recombination in Blends of IDT-Based Non-Fullerene Acceptors with PCE10 as Donor Polymer
J. I. Khan, M. A. Alamoudi, N. Chaturvedi, R. S. Ashraf, M. N. Nabi, A. Markina, W. Liu, T. A. Dela Pena, W. Zhang, W. Alsufyani, D. Andrienko, I. McCulloch, F. Laquai
Adv. Energy Mater.,
2100839,
2021,
[doi]
[abstract]
Advancing non-fullerene acceptor (NFA) organic photovoltaics requires the mitigation of the efficiency-limiting processes. Acceptor end-group and side-chain engineering are two handles to tune properties and a better understanding of their specific impact on the photophysics could facilitate a more guided acceptor design. Here the device performance energetic landscape and photophysics of rhodanine and dicyanovinyl end-capped IDT-based NFAs namely O-IDTBR and O-IDTBCN in PCE10-based solar cells are compared by transient optical and electro-optical spectroscopy techniques and density functional theory calculations. It is revealed how the acceptors’ quadrupole moments affect the interfacial energetic landscape in turn causing differences in exciton quenching charge dissociation efficiencies and geminate versus non-geminate recombination losses. More precisely it is found that the open circuit voltage (VOC) is controlled by the acceptors’ electron affinity (EA) while geminate and non-geminate recombination and the field dependence of charge generation rely on the acceptors’ quadrupole moments. The kinetic parameters and yields of all processes are determined and it is demonstrated that they can reproduce the performance differences of the devices’ current–voltage characteristics in carrier drift-diffusion simulations. The results provide insight into the impact of the energetic landscape specifically the role of the quadrupole moment of the acceptor beyond trivial considerations of the donor–acceptor energy offsets.
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,
20,
378-384,
2021,
[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.
2020
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.
