Triplet energy transfer in conjugated polymers. I. Experimental investigation of a weakly disordered compound

Lekshmi Sudha Devi, Mohammad K. Al-Suti, Carsten Dosche, Muhammad S. Khan, Richard H. Friend, and Anna Köhler

Phys. Rev. B 78, 045210 – Published 25 July 2008

Abstract

Efficient triplet exciton emission has allowed improved operation of organic light-emitting diodes (LEDs). To enhance the device performance, it is necessary to understand what governs the motion of triplet excitons through the organic semiconductor. Here, we have investigated triplet diffusion using a model compound that has weak energetic disorder. The Dexter-type triplet energy transfer is found to be thermally activated down to a transition temperature $T_{T}$ , below which the transfer rate is only weakly temperature dependent. We show that above the transition temperature, Dexter energy transfer can be described within the framework of Marcus theory. We suggest that below $T_{T}$ , the nature of the transfer changes from phonon-assisted hopping to quantum-mechanical tunneling. The lower electron-phonon coupling and higher electronic coupling in the polymer compared to the monomer results in an enhanced triplet diffusion rate.

Received 5 March 2008

DOI:https://doi.org/10.1103/PhysRevB.78.045210

Authors & Affiliations

Lekshmi Sudha Devi^1,2, Mohammad K. Al-Suti³, Carsten Dosche⁴, Muhammad S. Khan³, Richard H. Friend¹, and Anna Köhler^2,*

¹Cavendish Laboratory, University of Cambridge, Madingley Road, Cambridge CB3 OHE, United Kingdom
²Experimental Physics, Department of Physics, University of Bayreuth, Bayreuth 95440, Germany
³Department of Chemistry, College of Science, Sultan Qaboos University, Al-Khod 123, Sultanate of Oman
⁴Physical Chemistry, Department of Chemistry, University of Potsdam, Golm 14476, Germany

^*anna.koehler@uni-bayreuth.de

Triplet energy transfer in conjugated polymers. II. A polaron theory description addressing the influence of disorder

Ivan I. Fishchuk, Andrey Kadashchuk, Lekshmi Sudha Devi, Paul Heremans, Heinz Bässler, and Anna Köhler

Phys. Rev. B 78, 045211 (2008)

Triplet energy transfer in conjugated polymers. III. An experimental assessment regarding the influence of disorder on polaronic transport

Sebastian T. Hoffmann, Esther Scheler, Jan-Moritz Koenen, Michael Forster, Ullrich Scherf, Peter Strohriegl, Heinz Bässler, and Anna Köhler

Phys. Rev. B 81, 165208 (2010)

Article Text (Subscription Required)

Click to Expand

References (Subscription Required)

Click to Expand

Issue

Vol. 78, Iss. 4 — 15 July 2008

Reuse & Permissions

Access Options

Author publication services for translation and copyediting assistance advertisement

Images

Figure 1
(Color online) The normalized phosphorescence spectra at 10 K and absorption spectra at 300 K of the polymer (solid line) and the monomer (dotted line). The chemical structures of the polymer and the monomer are shown as inset. Excitation for luminescence was provided by a continuous-wave argon-ion laser at 3.49 eV.Reuse & Permissions
Figure 2
(Color online) (a) The dependence of normalized integrated phosphorescence intensity on temperature for the polymer and the monomer. (b) The triplet lifetimes $(τ)$ plotted against temperature for the polymer and the monomer as described in the text. The lifetimes at 10 K for the polymer $(45 μ s)$ and the monomer $(120 μ s)$ are normalized to unity for comparison. (c) Arrhenius plot of the phosphorescence decay rate $(1 / τ)$ against inverse temperature (1000/T). The black curves correspond to the equation $\frac{1}{τ} = a \times \sqrt{\frac{1}{T}} exp [- \frac{E_{a}}{k_{B} T}] + b$ with an activation energy $(E_{a})$ of 60 meV for the polymer and 100 meV for the monomer. The dotted lines indicate the transition temperatures 80 and 250 K. (d) Arrhenius plot of the phosphorescence decay rate $(1 / τ)$ in the polymer. Both the long lived and short-lived components of the triplet lifetimes are shown as filled circles and crosses, respectively. The lifetimes at 10 K are normalized to unity to allow for comparison. A fit corresponding to the equation $\frac{1}{τ} = a \times \sqrt{\frac{1}{T}} exp [- \frac{E_{a}}{k_{B} T}] + b$ is also shown, yielding an activation energy $(E_{a})$ of approximately 60 meV for both components.Reuse & Permissions
Figure 3
(Color online) The integrated phosphorescence intensity is plotted against the number of excitons created per repeat unit (bottom $X$ axis) for the polymer (closed circles) and the monomer (open circles). The corresponding excitation density per pulse is shown on the top $X$ axis. Also shown are lines corresponding to the slopes of $m = 0.9$ and $m = 0.4$ on the double-logarithmic scale. Excitation was provided by pulsed YAG laser at 3.49 eV.Reuse & Permissions
Figure 4
(Color online) The temperature dependence of the photoluminescence spectra for (a) the polymer and (b) the monomer. The spectra are displayed on a logarithmic scale for clarity. The arrow indicates the direction of increasing temperature.Reuse & Permissions
Figure 5
(Color online) Schematic illustration of Dexter transfer in a Marcus theory description. (a) Frontier orbital representation of the energy-transfer reaction $A^{*} + B \to A + B^{*}$ . (b) Marcus electron transfer in the configuration coordinate representation for a self-reaction $(Δ G^{0} = 0)$ . The activation energy required for the transfer $E_{a}$ and the reorganization energy $λ$ are shown. (c) Exciton migration from donor $A^{*}$ to acceptor B along a polymer chain. $E_{rel}^{A}$ and $E_{rel}^{B}$ represent the relaxation energies of the donor and the acceptor, respectively. (d) The processes of donor de-excitation and acceptor excitation in a configuration coordinate diagram. The associated relaxation energies $E_{rel}^{A}$ and $E_{rel}^{B}$ are indicated.Reuse & Permissions

Physical Review B

covering condensed matter and materials physics