Quantum and Nanoscale Modelling of Exciton Dynamics in Polymeric Systems (original) (raw)
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Understanding excitons in optically active polymers
Polymer International, 2008
We review the solid-state physics approach to electronic and optical properties of conducting polymers, and bring together the languages of solid-state theory for polymers and the quantum chemistry of oligomers. We consider polymers as generic one-dimensional semiconductors with specific features of strongly correlated electronic systems. Our model combines the large distance electron-hole motion within an exciton, governed by long-range Coulomb attraction with strong intramonomer electronic correlations, which results in effective intramonomer electron-hole repulsion. We exploit the dielectric screening to go beyond the single chain picture and to compare excitons for polymers in solutions and in films. Our approach allows the connecting, explaining, exploiting and organizing of such different experimental and numerical findings as shallow singlet and deep triplet excitons in phenylenes, anomaly in singlet-triplet exciton formation ratio, A g-B u crossing in polyenes and common 1/N energy dependencies in oligomers.
Unravelling the effect of strand orientation on exciton migration in conjugated polymers
Computational Materials Science, 2013
The study of the average distance that singlet excitons travel during their lifetime in conjugated polymers has attracted considerable attention during the past decade, because of its importance in the functioning of many polymer-based optoelectronic devices, like solar cells and photodetectors. Intriguingly, different values of exciton diffusion length have been extracted from experiments on seemingly identical conjugated polymers. Here we use computer simulations to show that the observed discrepancies in the reported values of the exciton diffusion length may arise from differences in the orientation of conjugated polymer strands relative to the substrate surface, a factor which has been mostly overlooked. Our results show that, on pristine polymer nanodomains with conjugated strands perpendicular to the substrate surface, exciton migration length is approximately 30% and 40% lower than on those with parallel and random strand orientation relative to that surface, respectively, resulting from the different contents of physical traps present in nanodomains with different strand orientation. This work underlines the importance of molecular arrangement on exciton migration, and provides a 2 novel theoretical framework for estimating the dependence of the exciton diffusion length with the orientation of conjugated polymers strands within the nanodomains, as well as helping the design of more efficient polymer-based optical and optoelectronic devices, such as optical sensors, photodiodes, photovoltaic cells and white light-emitting diodes.
Getting excited: challenges in quantum-classical studies of excitons in polymeric systems
Phys. Chem. Chem. Phys., 2016
A combination of classical molecular dynamics (MM/MD) and quantum chemical calculations based on the density functional theory (DFT) and many-body Green's functions theory (GW-BSE) was performed to describe the conformational and optical properties of diphenylethyne (DPE), methylated-DPE and poly para phenylene ethynylene (PPE).
Physical theory of excitons in conducting polymers
2010
In this tutorial review, we cover the solid state physics approach to electronic and optical properties of conducting polymers. We attempt to bring together languages and advantages of the solid state theory for polymers and of the quantum chemistry for monomers. We consider polymers as generic one-dimensional semiconductors with features of strongly correlated electronic systems. Our model combines the long range electron-hole Coulomb attraction with a specific effect of strong intra-monomer electronic correlations, which results in effective intramonomer electron-hole repulsion. Our approach allows to go beyond the single-chain picture and to compare excitons for polymers in solutions and in films. The approach helps connecting such different questions as shallow singlet and deep triplet excitons, stronger binding of interchain excitons in films, crossings of excitons' branches, 1/N energies shifts in oligomers. We describe a strong suppression of the luminescence from free charge carriers by long-range Coulomb interactions. Main attention is devoted to the most requested in applications phenyl based polymers. The specifics of the benzene ring monomer give rise to existence of three possible types of excitons: Wannier-Mott, Frenkel and intermediate ones. We discuss experimental manifestations of various excitons and of their transformations. We touch effects of the time-resolved self-trapping by libron modes leading to formation of torsion polarons.
Exciton diffusion in poly(3-hexylthiophene) by first-principles molecular dynamics
Physical Chemistry Chemical Physics, 2023
Poly(3-hexylthiophene) (P3HT) is a polymer used in organic solar cells as a light absorber and an electron donor. Photogenerated excitons diffuse and dissociate into free charge carriers provided they reach the absorber boundaries. The device efficiency is therefore dependent on the exciton diffusion. Although measurements can be performed for example by time-resolved photoluminescence, a quantitative modeling is highly desirable to get an insight into the relationship between the atomic structure at finite temperature and the diffusion coefficient of the exciton. This is the objective of the present work, achieved by resorting to first-principles molecular dynamics in combination with the restricted open-shell approach to model the singlet excited state. The maximally localized Wannier functions and their centers are used to monitor and localize the electron and the hole along the dynamics. The resulting diffusion coefficient is in close agreement with available measurements.
Exciton and Charge-Transfer Dynamics in Polymer Semiconductors
Springer Series in Chemical Physics, 2007
Organic semiconducting polymers are currently of broad interest as potential low-cost materials for photovoltaic and light-emitting display applications. I will give an overview of our work in developing a consistent quantum dynamical picture of the excited state dynamics underlying the photo-physics. We will also focus upon the quantum relaxation and reogranization dynamics that occur upon photoexcitation of a couple of type II donor-acceptor polymer heterojunction systems. Our results stress the significance of vibrational relaxation in the state-to-state relaxation and the impact of curve crossing between charge-transfer and excitonic states. Furthermore, while a tightly bound charge-transfer state (exciplex) remain the lowest excited state, we show that the regeneration of the optically active lowest excitonic state in TFB:F8BT is possible via the existence of a steadystate involving the bulk charge-transfer state. Finally, we will discuss ramifications of these results to recent experimental studied and the fabrication of efficient polymer LED and photovoltaics.
Computational study of the presence of defects in semiconducting polymers on exciton formation
International Conference on Applications of Optics and Photonics, 2011
Although semiconducting polymers are very attractive to be used in optoelectronic devices due to their molecular structure, they are not pristine semiconductors. After deposition it is possible to find out several structural and chemical defects, with different origins, that strongly influence exciton dynamics since they create deep energetic sites, where excitons can migrate leading to their quenching or reducing exciton diffusion length. By using a self-consistent quantum molecular dynamics method we performed a computational study to understand the influence of well-known polymer defects on excitons dynamics. Our results show that these defects influences mainly intramolecular exciton localization and exciton energy.
Temperature Dependence of Exciton Diffusion in Conjugated Polymers
The Journal of Physical Chemistry B, 2008
The temperature dependence of the exciton dynamics in a conjugated polymer is studied using time-resolved spectroscopy. Photoluminescence decays were measured in heterostructured samples containing a sharp polymer-fullerene interface, which acts as an exciton quenching wall. Using a 1D diffusion model, the exciton diffusion length and diffusion coefficient were extracted in the temperature range of 4-293 K. The exciton dynamics reveal two temperature regimes: in the range of 4-150 K, the exciton diffusion length (coefficient) of ∼3 nm (∼1.5 × 10 -4 cm 2 /s) is nearly temperature independent. Increasing the temperature up to 293 K leads to a gradual growth up to 4.5 nm (∼3.2 × 10 -4 cm 2 /s). This demonstrates that exciton diffusion in conjugated polymers is governed by two processes: an initial downhill migration toward lower energy states in the inhomogenously broadened density of states, followed by temperature activated hopping. The latter process is switched off below 150 K.
Journal of Physics: Condensed Matter, 2014
Exciton diffusion in small molecules 3,6-bis(5-(benzofuran-2-yl)thiophen-2-yl)-2,5-bis (2-ethylhexyl)pyrrolo[3,4-c]pyrrole-1,4-dione [DPP(TBFu) 2 ] is studied using first-principles simulations. We have examined dependence of exciton diffusion on structure disorder, temperature and exciton energy. We find that exciton diffusion length and diffusivity increase with structural order, temperature and the initial exciton energy. Compared to conjugated polymer poly(3-hexylthiophene) (P3HT), DPP(TBFu) 2 small molecules exhibit a much higher exciton diffusivity, but a shorter lifetime. The exciton diffusion length in DPP(TBFu) 2 is 50% longer than that in P3HT, yielding a higher exciton harvesting efficiency; the physical origin behind these differences is discussed. The time evolutions of exciton energy, electronhole distance, and exciton localization are explored, and the widely speculated exciton diffusion mechanism is confirmed theoretically. The connection between exciton diffusion and carrier mobilities is also studied. Finally we point out the possibility to estimate exciton diffusivity by measuring carrier mobilities under AC electric fields.
First-principles simulations of exciton diffusion in organic semiconductors
Physical Review B, 2011
Exciton diffusion is crucial for the performance of organic semiconductors in photovoltaic and solid state lighting applications. We propose a first-principles approach that can predict exciton dynamics in organic semiconductors. The method is based on time-dependent density functional theory to describe energy and many-body wave-functions of excitons. Non-adiabatic ab initio molecular dynamics is used to calculate phonon-assisted transition rates between localized exciton states. Using Monte Carlo simulations, we determine exciton diffusion length, lifetime, diffusivity and harvesting efficiency in poly(3-hexylthiophene) (P3HT) polymers at different temperatures, which agree very well with experiments. We find that exciton diffusion is primarily determined by the density of states of low-energy excitons. A widely speculated diffusion mechanism, namely an initial downhill migration followed by thermally activated migration, is confirmed and elucidated by the simulations. Some general guidelines for designing more efficient organic solar cells are obtained from the simulations.