Quantum molecular dynamics study of polaron recombination in conjugated polymers (original) (raw)
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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).
Synthetic Metals, 2003
We report on an experimental study of spin-dependent exciton formation in films of -conjugated polymers. We employed a spectroscopic/magnetic resonance technique, namely the photoinduced absorption (PA)-detected magnetic resonance technique, and showed that non-geminate recombination pairs of polarons form singlet excitons more efficiently than triplets. The experimental proof for this is achieved by detection of a resonant decrease of triplet exciton absorption and resonantly enhanced singlet photoluminescence (PL). In addition, resonantly enhanced absorption is observed which we assign to a singlet exciton precursor. Our study has important implications regarding the efficiency that can be achieved in polymer light-emitting diodes (PLEDs).
The Journal of Physical Chemistry B, 2014
Using a modified version of the Su-Schrieffer-Heeger (SSH) model combined with the Extended Hubbard Model (EHB) the recombination between a singlet exciton pair is investigated under influence of an external electric field, electron-electron interactions, and temperature effects in the scope of a nonadiabatic evolution method. The excitons are positioned very close to each other in a way to mimic a high-density region in monomolecular conjugated polymer systems. Results show that there are mainly three possible channels resulting from singlet-singlet exciton recombination: (1) forming an excited negative polaron and an excited positive bipolaron; (2) forming two free and excited oppositely charged polarons, and (3) forming a biexciton. These results suggest that the recombination processes critically depends on the condition imposed to the system. The description of this dependence, as carried out in the present work, may provide guidance to improve the generation of free charge carriers in organic optoelectronic devices.
Concentration effects on intrachain polaron recombination in conjugated polymers
Phys. Chem. Chem. Phys., 2014
The influence of different charge carrier concentrations on the recombination dynamics between oppositely charged polarons is numerically investigated using a modified version of the Su-Schrieffer-Heeger (SSH) model that includes an external electric field and electron-electron interactions. Our findings show that the external electric field can play the role of avoiding the formation of excited states (polaronexciton and neutral excitation) leading the system to a dimerized lattice. Interestingly, depending on a suitable balance between the polaron concentration and the electric field strength, the recombination mechanism can form stable polaron-excitons or neutral excitations. These results may provide guidance to improve the electroluminescence efficiency in Polymer Light Emitting Diodes.
Tracking the coherent generation of polaron pairs in conjugated polymers
Nature communications, 2016
The optical excitation of organic semiconductors not only generates charge-neutral electron-hole pairs (excitons), but also charge-separated polaron pairs with high yield. The microscopic mechanisms underlying this charge separation have been debated for many years. Here we use ultrafast two-dimensional electronic spectroscopy to study the dynamics of polaron pair formation in a prototypical polymer thin film on a sub-20-fs time scale. We observe multi-period peak oscillations persisting for up to about 1 ps as distinct signatures of vibronic quantum coherence at room temperature. The measured two-dimensional spectra show pronounced peak splittings revealing that the elementary optical excitations of this polymer are hybridized exciton-polaron-pairs, strongly coupled to a dominant underdamped vibrational mode. Coherent vibronic coupling induces ultrafast polaron pair formation, accelerates the charge separation dynamics and makes it insensitive to disorder. These findings open up ne...
The Journal of Chemical Physics, 2000
We examine the dynamics of exciton self-trapping in conjugated polymer systems using mixed quantum-classical molecular dynamics. The model treats the exciton as a two-dimensional quantum mechanical wave function representing a particle/hole quasiparticle interacting with a classical vibrational lattice ͓M. N. Kobrak and E. R. Bittner, J. Chem. Phys. 112, 5399 ͑2000͔͒. We show that the dynamics are influenced strongly by thermal disorder in the lattice, and that there is a dramatic change in the self-trapping mechanism as temperature increases. At low temperatures, the rate of localization is limited by the time required for the vibrational lattice to respond to the creation of the particle-hole pair, while at higher temperatures thermal disorder permits localization on time scales limited primarily by electronic response. We simulate the time-resolved fluorescence spectrum for the model system, and compare the temperature dependence of the spectrum to recent time-resolved fluorescence upconversion studies on polydiacetylene derivatives.
Spin-dependent dynamics of polaron pairs in organic semiconductors
2010
Semiconducting organic polymers are an interesting class of materials, 1 with a wide range of applications. Arguably, the most important of these applications is electricity-light conversion, when the material is incorporated in either an organic light-emitting diode (OLED) or an organic solar cell. 2–7 The ability of this material to perform such functions is directly related to the generation, dissociation, and recombination of polarons and polaron pairs (PPs).
Dynamics of Photogenerated Polaron-Excitons in Organic Semiconductors
15th Brazilian Workshop on Semiconductor Physics, 2012
In this work we performed numerical simulations of one π-conjugated polymer chain subjected to photogeneration. Within the SSH model modified to include the Brazoviskii-Kirova symmetry breaking term, we investigate the dynamics of photoexcitations to address the generation mechanism of polaron-excitons using the unrestricted Hartree-Fock approximation. It was obtained that after the photoexcitation the system relaxes spontaneously into a polaron-exciton in a transient state in a range of 200 f s. Our results also show that charged polarons are generated directly after this transient state.
Reactive Scattering between Excitons and Charge Carriers in Conjugated Polymers
The Journal of Physical Chemistry C, 2014
The temperature influence on the scattering process between an exciton and a positively charged carrier (polaron or bipolaron) is theoretically investigated using a version of the Su−Schrieffer−Heeger (SSH) model modified to include temperature, Coulomb interactions, and an external electric field. In general, it is observed that the products of the reactive scattering are spin independent when thermal effects are taken into account. For the interaction between a polaron and an exciton, the polaron can be annihilated, when subjected to temperatures higher than a critical value, or pass through the exciton maintaining both their consistencies, if a lower temperature regime is considered. Regarding the recombination between a bipolaron and an exciton, it is observed that the bipolaron can be annihilated whereas the exciton dissociates into two trions or into one polaron and one trion. In all cases, the recombination mechanisms depend on a suitable balance between temperature and electric field. These results may extend the knowledge about electroluminescence process in conjugated polymers, thus being of potential use to improve internal quantum efficiency in polymer light-emitting diodes.
Exciton dissociation and charge carrier recombination processes in organic semiconductors
The Journal of Chemical Physics, 2011
Exciton dissociation and charge carrier recombination processes in organic semiconductors JCP: BioChem. Phys. 5, 12B602 Note: Scale-free center-of-mass displacement correlations in polymer films without topological constraints and momentum conservation J. Chem. Phys. 135, 186101 (2011) Directivity and isotropic band-gap in 12-fold symmetry plasmonic quasi-crystals with small index contrast Appl. Phys. Lett. 99, 181104 (2011) Structure-dependent band dispersion in epitaxial anthracene films J. Chem. Phys. 135, 124709 Electronic structure of the poly(3-hexylthiophene):indene-C60 bisadduct bulk heterojunction Exciton dissociation and charge recombination processes in organic semiconductors, with thermal effects taken into account, are described in this paper. Here, we analyzed the mechanisms of polaronexcitons dissociation into free charge carriers and the consequent recombination of those carriers under thermal effects on two parallel π -conjugated polymers chains electronically coupled. Our results suggest that exciton dissociation in a single molecule give rise to localized, polaron-like charge carrier. Besides, we concluded that in the case of interchain processes, the bimolecular polaron recombination does not lead to an usual exciton state. Rather, this type of recombination leads to an oscillating dipole between the two chains. The recombination time obtained here for these processes are in agreement with the experimental results. Finally, our results show that temperature effects are essential to the relaxation process leading to polaron formation in a single chain, as in the absence of temperature, this process was not observed. In the case of two chains, we conclude that temperature effects also help the bimolecular recombination process, as observed experimentally.