Polydispersity of the photoluminescence quantum yield in single conjugated polymer chains O. Mirzov, I.G. Scheblykin * Chemical Physics, Lund University, P.O. Box 124, 22100 Lund, Sweden Received 8 April 2005; accepted 10 June 2005 Available online 14 July 2005 Abstract The conjugated polymer poly(2-methoxy-5-(2 0 -ethylhexyloxy)-1,4-phenylene vinylene) (MEH-PPV) was studied by a single-mol- ecule imaging technique. A comparison of statistical distributions of fluorescence intensity with molecular weight distributions revealed that the distribution of the photoluminescence quantum yield of the single polymer chains under study is significantly asym- metric, with a polydispersity J 2. The result implies that there are molecules whose quantum yield is a few times higher than the ensemble quantum yield. This conclusion suggests a possibility of a great improvement of the photoluminescence quantum yield of MEH-PPV, which is known to be several times less than 1. Ó 2005 Elsevier B.V. All rights reserved. Keywords: MEH-PPV; Single-molecule spectroscopy; Photoluminescence quantum yield; Polydispersity 1. Introduction Conjugated polymers have been attracting attention of researchers due to their great potential for various applications such as plastic solar cells, molecular elec- tronics, and organic light-emitting diodes. One of the main characteristics of conjugated polymers, which is very important for applications, is the fluorescence quantum yield. Any information about correlation of quantum yield with other properties of conjugated poly- mers may be very important because it can help creating better recipes for polymer synthesis and device preparation. During the last years, single-molecule spectroscopy and imaging proved to be powerful tools for studying a variety of systems, including conjugated polymers [1]. For conjugated polymers they have been applied to study such phenomena as fluorescence intensity fluc- tuations (see e.g. [2–5] and references therein) and spec- tral fluctuations (see e.g. [3,6,7]). To the best of our knowledge, no experimental results on the fluorescence quantum yield of single polymer chains have been re- ported yet [8]. Nevertheless, application of single-mole- cule optics to the issue of quantum yield may be especially fruitful, because it can provide information about the statistical distribution of quantum yields for single molecules (SMs), which in turn can give some in- sight into the nature of imperfections decreasing the quantum yield. By definition, the photoluminescence quantum yield is the ratio of the photon emission rate to the rate of pho- ton absorption. To determine this ratio, one can use sin- gle-molecule imaging. The photon emission rate can be calculated from the image, provided that the experimen- tal setup has been calibrated with a reference sample. To determine the absorption rate, one needs to know the absorption cross-section and excitation intensity. The latter can be measured independently (see Section 2.1). As to the absorption cross-section, for conjugated poly- mer chains it can be assumed to be proportional to the 0301-0104/$ - see front matter Ó 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.chemphys.2005.06.017 * Corresponding author. Fax: +46 46 2224119. E-mail address: ivan.scheblykin@chemphys.lu.se (I.G. Scheblykin). URL: http://www.chemphys.lu.se (I.G. Scheblykin). www.elsevier.com/locate/chemphys Chemical Physics 318 (2005) 217–222