Growing semiconductor nanocrystals directly in a conducting polymer Andrew Watt * , Halina Rubinsztein-Dunlop, Paul Meredith Soft Condensed Matter Physics Group and Centre for Biophotonics and Laser Science, School of Physical Sciences, University of Queensland, Brisbane, Ausralia Received 24 March 2005; accepted 12 May 2005 Available online 4 June 2005 Abstract We describe a single step method to synthesise lead sulphide (PbS) nanocrystals directly in the conjugated polymer poly (2-methoxy-5-(2V - ethyl-hexyloxy)-p-phenylene vinylene) (MEH-PPV). This method allows size control of the nanocrystal via co-solvent ratios. We find good agreement between nanocrystal sizes determined by transmission electron microscopy and sizes theoretically determined from the absorption edge of the nanocrystals. Finally we show that this synthesis technique is not restricted to MEH-PPV and demonstrate that nanocrystals can be grown in Poly(3-hexylthiophene-2,5-diyl) (P3HT). D 2005 Elsevier B.V. All rights reserved. Keywords: Nanocomposites; Optical materials and properties; Polymers; Solar energy materials 1. Introduction Optoelectronic devices based upon conducting polymers and nanocrystals are generating intense research interest [1– 4]. Creating these materials with well defined physical and electronic properties is an important step towards building efficient devices. We have recently demonstrated that PbS nanocrystals can be grown directly in the conducting polymer MEH-PPV [5]. The major advantage of this new method is the elimination of the initial insulating surfactant used to template the growth. In existing methods this surfactant has to be removed before mixing with a conducting polymer if charge separation and transfer is required between the two components [6]. This can result in nanocrystal aggregation leading to poor electronic interfacing and therefore ineffi- cient devices. Recently, groups have presented methods for making nanocrystals in conducting polymers using electro active ligands [7,8]. Our method does not need these ligands and allows more natural contact between nanocrystal and polymer. We believe that the conducting polymer acts as a steric surfactant. So far we have reported respectable photovoltaic devices from the first generation material with ¨1% power conversion efficiency [9]. Lead sulphide (PbS) was chosen as the nanocrystal material for a number of reasons. Firstly, PbS in the quantum regime (i.e., particles of size smaller than the Bohr radius) have tunable broad band absorption from 560 to 1600 nm [10,11]. Secondly, electrons and holes are equally mobile, [12] unlike cadmium selenide where electron transport dominates. Thirdly, PbS nanocrystals have been shown to exhibit a relatively long excited state lifetime [13,14]. Finally, high efficiency carrier multi- plication has recently been demonstrated [15]. This combination of properties make PbS nanocrystals a much more versatile charge acceptor and optically active component when blended with conducting polymers for optoelectronic applications. Non-ionic polymers can be used as steric surfactants to template the growth of nanocrystals [16]. Apart from the fact these polymers are insulating, the other major shortcoming of these syntheses is that they do not allow nanocrystal size control. In this paper, we present a significant advancement of these techniques whereby the size of nanocrystals grown in a polymer can be controlled and we show that the general synthetic 0167-577X/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2005.05.013 * Corresponding author. Tel.: +61 7 3365 1245; fax: +61 7 3365 1242. E-mail address: watt@physics.uq.edu.au (A. Watt). Materials Letters 59 (2005) 3033 – 3036 www.elsevier.com/locate/matlet