Determination of Volume Fractions and Ligand Layer Thickness of Polymer/CdSe Quantum Dot Blend Films by Effective Medium Approximations Martin Scha ¨ del, 1,2 Krischan F. Jeltsch, 1 Phenwisa Niyamakom, 3 Frank Rauscher, 3 Yunfei Zhou, 4 Michael Kru ¨ ger, 4 Klaus Meerholz 1 1 Department of Chemistry, University of Cologne, Luxemburger Strasse 116, Cologne, North Rhine-Westphalia 50939, Germany 2 Technology, Q-Cells SE OT Thalheim, Sonnenallee 17-21, Bitterfeld-Wolfen, Saxony-Anhalt 06766, Germany 3 Process Technology, Bayer Technology Services GmbH, Leverkusen, North Rhine-Westphalia 51368, Germany 4 Integrated Materials & Microsystem Engineering, Freiburg Materials Research Centre (FMF), University of Freiburg, Stefan-Meier Strasse 21, Freiburg, Baden-Wu ¨ rttemberg 79104, Germany Correspondence to: K. Meerholz (klaus.meerholz@uni-koeln.de) Received 24 June 2011; revised 6 September 2011; accepted 23 September 2011; published online 17 October 2011 DOI: 10.1002/polb.22380 ABSTRACT: We investigate hybrid organic/inorganic films using different polymers and CdSe quantum dots (QD) and nanorods (NR) with hexanoic acid (HA)-treated hexadecylamine (HDA) or pyridine as the capping ligands. The volume ratios of the poly- mer:nanoparticle (NP) blends are studied by spectroscopic ellipsometry and transmission intensity data. Effective medium approximation based on the results of the pristine films is applied. With this routine, the polymer/NP volume ratio of the blend can be identified. In combination with the mass ratio of the components, the mass density of the NP including the inor- ganic crystalline core and the organic ligand layer is obtained. A geometrical model for QD and NR allows for the estimation of the ligand layer thickness. We find pyridine and HDA after HA treatment to be 0.9 and 0.6 nm on the QD surface, respec- tively. By contrast, the effective thickness of the organic ligand is 3.0 nm on the investigated NR. In both cases, the organic layer is thicker than a monolayer of the expected pyridine due to the presence of extant synthesis ligands as a result of incomplete ligand exchange. V C 2011 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 50: 75–82, 2012 KEYWORDS: conjugated polymers; modeling; morphology; nanoparticles; thin films INTRODUCTION Since the breakthrough of organic photovol- taics (OPV) by Tang 1 with power conversion efficiencies (PCE) above 1%, the progress in the last decades led to PCE above 8% for solution-processed organic photovoltaics with device areas of more than 1 cm 2 . 2 The photoactive material of these solar cells (SCs) is based on thin layers of blended or stacked heterojunctions of polymers and fullerenes or fullerene derivates. Fullerenes and their derivates comprise many advantages like high electron affinity and mobility. 3 Some fullerene derivates show the ability to intrinsically form nanoscale domains with large donor–acceptor interface areas for increased charge separation and continuous path- ways for the separated charges to reach the electrodes. 4 However, fullerenes show weak light absorption in the rele- vant spectrum of the sun light. The polymer has to compen- sate this disadvantage to obtain good light harvesting. Thus, to further increase the PCE, many efforts have been made to replace the fullerenes by other species with higher and tuna- ble extinction coefficients like inorganic nanoparticles (NP). 5–9 So far, PCE of 2.7% 10 for CdSe quantum dots (QD) and 3.1% for CdSe tetrapods were achieved. 11 The synthesis route of the particles introduces organic ligands covering the surface of the inorganic nanocrystals. This capping layer can be exchanged or removed in subse- quent steps. 6 The ligands have a strong impact on the device performance, as they passivate the surface states of the NP, 5 control the charge separation between polymer and NP, the recombination of free charge carriers at the interface, the transport of separated charges between the particles and the morphology or rather the formation of nanoscale domains and pathways to the electrodes. 6 The ligands are further required for the solubility of the NP in common sol- vents to prevent precipitation. Its thickness is an important characteristic and is influenced by the ligand species, their surface density, and their binding angle with the particle Additional Supporting Information may be found in the online version of this article. V C 2011 Wiley Periodicals, Inc. WWW.MATERIALSVIEWS.COM JOURNAL OF POLYMER SCIENCE PART B: POLYMER PHYSICS 2012, 50, 75–82 75 WWW.POLYMERPHYSICS.ORG FULL PAPER