Temperature Dependence of Exciton Diffusion in Conjugated Polymers O. V. Mikhnenko,* ,†,‡ F. Cordella, † A. B. Sieval, § J. C. Hummelen, †,| P. W. M. Blom, † and M. A. Loi † Zernike Institute for AdVanced Materials, UniVersity of Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands, Dutch Polymer Institute, P.O. Box 902, 5600 AX, EindhoVen, The Netherlands, Solenne BV, Zernikepark 12, 9747 AN, Groningen, The Netherlands, and Stratingh Institute for Chemistry, UniVersity of Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands ReceiVed: May 13, 2008; ReVised Manuscript ReceiVed: June 27, 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. I. Introduction Optical excitations in conjugated polymers are strongly bound electron-hole pairs, which are called Frenkel excitons and usually are localized on a single conjugated segment. Once created, excitons tend to migrate toward the lower energy sites, i.e. longer conjugated segments, by means of energy transfer. Due to the disorder of the polymer chains in thin films such a migration is a random walk, which can be regarded as a diffusion process. Exciton diffusion is one of the physical phenomena that governs the performance of polymer optoelec- tronic devices. For instance, in solar cells, the average distance that excitons diffuse during their lifetime, the exciton diffusion length, determines the number of excitons that reach the dissociation interface where they can be cleaved into unbound electrons and holes and contribute to the photocurrent. 1 In light emitting diodes, the exciton diffusion length is a measure for the amount of parasitic quenching by the metallic electrodes. 2 Understanding the exciton diffusion process is of crucial importance to design new materials to improve device performance. Recently, great effort has been made to determine the exciton diffusion parameters in conjugated polymers at room tempera- ture. 1-9 For various derivatives of the conjugated polymer poly(p-phenylene vinylene) (PPV) a typical L D of 5-6 nm has been reported at room temperature. A direct way to measure the exciton diffusion length L D is to monitor photoluminescence (PL) quenching in polymer/fullerene bilayer heterostructures as a function of polymer thickness (Figure 1). The exciton diffusion coefficient D can be extracted from the PL time-resolved measurements (Figure 2) in these heterostructures, leading to a room temperature value 6 of D ) 3 × 10 -4 cm 2 /s. However, knowledge of the room temperature characteristics alone does not provide insight into the mechanisms that govern the exciton diffusion process in conjugated polymers. In order to elucidate these mechanisms, we have investigated the temperature dependence of the exciton diffusion in the range of 4-293 K. Measurements of L D and D as a function of temperature reveal that the diffusion of excitons is governed by two subsequent steps: upon exciton creation, a downhill migration toward lower energy sites takes place, which is followed by thermally activated hopping. Below 150 K, the latter process is switched off, leading to temperature independent exciton dynamics. II. Experimental Methods Our model material is MDMO-PPV (poly[2-methyl-5-(3′,7′- dimethyloctyloxy)-p-phenylenevinylene]), which is of particular * Corresponding author. E-mail: O.Mikhnenko@rug.nl. † Zernike Institute for Advanced Materials, University of Groningen. ‡ Dutch Polymer Institute. § Solenne BV. | Stratingh Institute for Chemistry, University of Groningen. Figure 1. PL decays of 240 nm thick reference MDMO-PPV film (solid line) and heterostructured samples of two polymer thicknesses, 32 nm (dashed line) and 13 nm (dotted line), measured at room temperature. The PL decays are normalized to their maximum value. The inset illustrates the composition of heterostructures. J. Phys. Chem. B 2008, 112, 11601–11604 11601 10.1021/jp8042363 CCC: $40.75  2008 American Chemical Society Published on Web 08/26/2008