• PDF / 94,853 Bytes
• 6 Pages / 612 x 792 pts (letter) Page_size
• 40 Downloads / 273 Views

DOWNLOAD

REPORT

---

Near-field photoconductivity imaging of a conjugated polymer blend R. Riehn1, R. Stevenson1, J.J.M. Halls1, D.R. Richards1,2, D.-J. Kang3, M. Blamire3, F. Cacialli1,4 1 Cavendish Laboratory, University of Cambridge, Cambridge CB3 0HE, U.K. 2 Dept. of Physics, King's College London, Strand, London WC2R 2LS, England, U.K. 3 Department of Materials Science, University of Cambridge, Cambridge, U.K. 4 Department of Physics and Astronomy, University College London, Gower Street, London WC1E 6BT, U.K. ABSTRACT We report combined scanning near-field optical microscopy (SNOM) and near-field photocurrent (NPC) imaging of a binary conjugated polymer blend. We find phase separation on a scale of about 5 µm, with a good correspondence between topographic, fluorescence, and photocurrent images. We excited at 488 nm, a wavelength at which only one of the two polymers absorbs light. Under this illumination regions that are high in the topography image show high luminescence and photocurrent. The photoluminescence (PL) efficiencies in the different regions of the sample were determined by calculating the absorbed energy using the Bethe-Bouwkamp model, and knowledge about the chemical composition of the different phases of the polymer blend. The calculation also allowed us to conclude that the photocurrent generation efficiency (current/absorbed photons) of the different polymer phases is comparable within the limit of confidence of this experiment (±10 %).

INTRODUCTION Polymer-based electronic devices such as polymer light-emitting diodes (PLED), photovoltaic cells, and thin-film transistors are attracting considerable interest, especially because of cheap manufacturing over large areas, which originates from solutionprocessability of conjugated, polymeric semiconductors. Frequently polymer-polymer interfaces are used to enhance processes of charge separation, or blocking of currents of different charge carrier species [1]. A very efficient way of providing a large interface area between two polymers is the spin-casting of a thin polymer blend film, which can result in a solid-state solution or a network of phase-separated or de-wetted domains. In this paper we study thin films of a blend of equal amounts in weight of F8BT [poly(9,9’dioctylfluorene-alt-benzothiadiazole)] and PFB [poly(9,9’dioctylfluorene-altbis-N,N’-(4-butylphenyl)-bis-N,N’-phenyl-1,4-phenylene-diamine]. The offset between the highest occupied molecular orbitals (HOMO) and lowest occupied molecular orbitals (LUMO) is 0.8 eV and 1.24 eV, respectively [2], making it energetically favorable for excitons to split at polymer-polymer interfaces. This characteristic, together with the blend’s relatively high electron and hole mobilities, makes it a potentially interesting system for photovoltaic applications. Previous studies on this polymer blend have studied BB3.17.1

the phase separation [3], have shown spatial distribution of chemical species with a high resolution (100 nm) [4], and have quantified concentration of chemical species within the phases [5]. While the ch