Temperature Dependence of Exciton Diffusion in Conjugated Polymers (original) (raw)
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Exciton diffusion and dissociation in conjugated polymer/fullerene heterostructures
SPIE Proceedings, 2004
We investigate the exciton dynamics in composite systems of conjugated polymers and fullerene molecules by photoluminescence ͑PL͒ and femtosecond transient absorption experiments. In solid mixtures ͑blends͒ we find a strong concentration-dependent quenching of the polymer PL. This is attributed to an efficient electron transfer ͑ET͒ from the photoexcited conjugated polymer to the fullerene. The ET dynamics is directly monitored by measuring the transient stimulated emission of the conjugated polymer. The transfer rate depends linearly on the C 60 concentration and ranges between (66 ps) Ϫ1 and (5 ps) Ϫ1 for concentrations from 0.5% to 5%. This dependence is in accordance with an exciton diffusion process occurring prior to the ET. The exciton diffusion length in the conjugated polymer is directly determined by measuring the PL quenching in welldefined heterostructures comprising a self-assembled fullerene monolayer and a thin spin-coated polymer layer of variable thickness. From these measurements we infer a value of 14 nm for the exciton diffusion length in ladder-type poly ͑p-phenylene͒. Our results are of direct relevance for further optimization of polymer photovoltaic devices. ͓S0163-1829͑99͒07923-0͔
Dynamics of exciton diffusion in poly(p-phenylene vinylene)/fullerene heterostructures
Physical Review B, 2005
The exciton diffusion process in a poly͑p-phenylene vinylene͒-͑PPV-͒based derivative is investigated using time-resolved photoluminescence in conjugated polymer/fullerene heterostructures. The decay of the luminescence in the polymer/fullerene heterostructures is governed by exciton diffusion and subsequent dissociation at the polymer/fullerene interface. The decay curves of polymer layers with varying thickness are consistently modeled using an exciton diffusion constant of 3 ϫ 10 −4 cm 2 / s. The resulting exciton migration radius amounts to 6 nm, which is a measure for the active part of the PPV/fullerene heterojunction for photovoltaic applications.
The Journal of Physical Chemistry A, 2005
Exciton diffusion and photoluminescence quenching in conjugated polymer/fullerene heterostructures are studied by time-resolved photoluminescence. It is observed that heterostructures consisting of a spin-coated poly(pphenylene vinylene) (PPV)-based derivative and evaporated C 60 are ill-defined because of diffusion of C 60 into the polymer, leading to an overestimation of the exciton diffusion length. This artifact is resolved by the use of a novel, thermally side-chain polymerizing and cross-linking fullerene derivative (F2D) containing two diacetylene moieties, forming a completely immobilized electron acceptor layer. With this heterostructure test system, an exciton diffusion length of 5 (1 nm is derived for this PPV derivative from time-integrated luminescence quenching data.
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.
Chemical Physics Letters, 2013
We investigate temperature effects on exciton dissociation dynamics in conjugated polymer systems. Using a modified version of the tight-binding Su-Schrieffer-Heeger model, the dissociation is studied under the influence of impurity effects with a nonadiabatic evolution method. Our results show that temperature effects reduce the critical electric field for the exciton dissociation. In the small temperature regime, the exciton is not trapped by the impurity and it is observed to perform a random walk, a fact not observed in the absence of temperature. This letter might enlighten the description of electroluminescence yields and charge transport efficiency in organic based electronic devices.
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.
Influence of the photoexcitation process on the dynamics of triplet excitons in organic polymers
Computational and Theoretical Chemistry, 2013
In this work we adopt a model Hamiltonian approach to investigate exciton generation and its subsequent dynamics in conjugated polymers. By using a modified version of the Su-Schrieffer-Heeger model Hamiltonian to include temperature effects, interaction between polymer chains, and different photoexcitation processes, we study the dynamics of triplet excitons in the system. The temperature is incorporated in the method by means of a classical Langevin equation, and Ehrenfest molecular dynamics is used to describe the time evolution of the system. We were able to distinguish the response of the oscillating electric dipole for different excitations at nonzero temperatures based on the amplitude and frequency.
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.