Photosensitive nanostrukturierte Materialien: Kombinierte theoretische und experimentelle Untersuchung des ultraschnellen Energie- und Ladungstransfers auf molekularer Ebene
Physikalische Chemie von Festkörpern und Oberflächen, Materialcharakterisierung
Zusammenfassung der Projektergebnisse
This project is concerned with photoinduced energy and charge transfer processes in nanostructured organic donor-acceptor (DA) materials that are developed in view of applications in organic electronics. In this context, we carried out state-of-the-art theoretical and computational studies, complementary to experimental investigations by our collaboration partners at Strasbourg University (Dr. S. Méry, Prof. S. Haacke). These studies highlight that the photophysical properties of the molecular DA building blocks cooperate in a non-trivial way with molecular packing effects, impacting exciton migration and charge separation. As a result, the efficiency of the relevant DA materials in organic photovoltaics hinges upon various factors that go beyond conventional bandgap engineering rules. Among the investigated target systems are self-assembled mesomorphic nanostructures composed of DA co-oligomers based on perylene diimide (PDI) acceptor moieties. These materials were synthesized and spectroscopically investigated by our partner groups at Strasbourg University; the mesomorphically ordered structures are characterized by segregated channels for hole and electron transport, which makes these systems interesting for both photovoltaic and ambipolar transport applications. Based upon an atomistic structure obtained by electron diffraction, the energetics of charge separation was characterized by a microelectrostatics (ME) analysis, in collaboration with Dr. Gabriele D’Avino (Institut Néel, Grenoble). The ME treatment leads to a microscopic analog of an effective Coulomb barrier to electron-hole (e-h) separation, and includes non-Coulomb energetics at short e-h distances as well as the effects of local defects. Two complementary approaches were employed for the dynamical treatment: first, a quantum dynamical approach describing the dominant electron transfer pathway along the PDI stacking direction, and second, a kinetic Monte Carlo (KMC) approach adapted to the dynamics on the full two-dimensional lattice on longer time scales. The site-to-site transfer rates employed in the KMC approach were calibrated against the first-principles quantum dynamical treatment. In further detail, the quantum dynamical calculations were based upon a 25-site lattice and nearly 900 vibrational modes at finite temperature, using the Multi-Layer Multiconfiguration Time-Dependent Hartree (ML-MCTDH) propagation scheme in conjunction with the thermofield dynamics method. In turn, the KMC treatment relied on a standard Bortz-Kalos-Lebowitz (BKL) implementation. The KMC approach reveals that e-h dissociation exhibits two-dimensional transport features, where the predominant electron transport in the PDI stacking direction is assisted by a secondary mechanism that involves neighboring PDI stacks. Preliminary time-resolved experiments (Prof. S. Haacke) confirm that the resulting e-h dissociation rates of the order of 1–10 ns−1 are plausible, but also indicate that competing processes occur, which are not accounted for in our model. Besides geminate recombination, which could act more rapidly than foreseen in our model, excimer formation has been discussed as an additional loss pathway in PDI based systems. Future studies should be devoted to clarifying these spectroscopic observations and their implication for similar self-assembled DA nanostructures. A series of additional investigations were concerned with models for regioregular blends of poly(3- hexylthiophene) (P3HT) and phenyl-C61-butyric acid methyl ester (PCBM). Here, the focus was on the role of charge transfer excitons, and their potential impact on charge separation at donor-acceptor heterojunctions. While this paradigm system is of interest by itself, the conclusions of these studies are also relevant for the DA co-oligomer systems addressed in collaboration with the Strasbourg groups, where composite donor species of fluorene-thiophene-benzothiadiazole type are employed. Besides, our studies were also concerned with the first-principles computation of intra-chain exciton diffusion rates in oligothiophene materials. All studies combine first-principles parametrized lattice Hamiltonians and state-of-the-art multi-configurational (ML-MCTDH) dynamical calculations. This simulation strategy for nanostructured DA systems is summarized in two recent review articles.
Projektbezogene Publikationen (Auswahl)
- “Impact of Charge Transfer Excitons in Regioregular Polythiophene on the Charge Separation at Polythiophene-Fullerene Heterojunctions”, J. Phys. B, 51, 014003 (2018) (Special Issue Problems of Light Energy Conversion, Light Harvesting)
M. Polkehn, H. Tamura, and I. Burghardt
(Siehe online unter https://doi.org/10.1088/1361-6455/aa93d0) - “Quantum Dynamical Studies of Ultrafast Charge Separation in Nanostructured Organic Polymer Materials: Effects of Vibronic Interactions and Molecular Packing”, Int. J. Quant. Chem., 118, e25502 (2018)
M. Polkehn, P. Eisenbrandt, H. Tamura, and I. Burghardt
(Siehe online unter https://doi.org/10.1002/qua.25502) - “Coherent Charge Transfer Exciton Formation in Regioregular P3HT: A Quantum Dynamical Study”, J. Phys. Chem. Lett., 10, 3326 (2019)
W. Popp, M. Polkehn, R. Binder and I. Burghardt
(Siehe online unter https://doi.org/10.1021/acs.jpclett.9b01105) - “Vibronic coupling models for donor-acceptor aggregates using an effective-mode scheme: Application to mixed Frenkel and charge-transfer excitons in oligothiophene aggregates”, J. Chem. Phys., 150, 244114 (2019)
W. Popp, M. Polkehn, K. H. Hughes, R. Martinazzo, and I. Burghardt
(Siehe online unter https://doi.org/10.1063/1.5100529) - “First-principles quantum and quantum-classical simulations of exciton diffusion in semiconducting polymer chains at finite temperature”, J. Chem. Theor. Comput., 16, 5441 (2020)
R. Hegger, R. Binder, and I. Burghardt
(Siehe online unter https://doi.org/10.1021/acs.jctc.0c00351) - “First-principles quantum simulations of exciton diffusion on a minimal oligothiophene chain at finite temperature”, Faraday Discuss., 221, 406 (2020)
R. Binder and I. Burghardt
(Siehe online unter https://doi.org/10.1039/C9FD00066F) - “Quantum dynamics of electron-hole separation in stacked perylene diimide-based self-assembled nanostructures”, J. Phys. Chem. C, 125, 25030 (2021) (Special Issue ”125 Years of JPC”)
D. Brey, W. Popp, P. Budakoti, G. D’Avino, and I. Burghardt
(Siehe online unter https://doi.org/10.1021/acs.jpcc.1c06374) - “Quantum dynamics of exciton transport and dissociation in multi-chromophoric systems”, Annu. Rev. Phys. Chem., 72, 591 (2021)
W. Popp, D. Brey, R. Binder, and I Burghardt
(Siehe online unter https://doi.org/10.1146/annurev-physchem-090419-040306)
