Structural and dynamical characterization of P3HT/PCBM blends Giuseppe Paternó a , Franco Cacialli a , Victoria García-Sakai a,b,⇑ a London Centre for Nanotechnology, Department of Physics and Astronomy, University College London, Gower Street, London WC1E 6BT, UK b ISIS Pulsed Neutron and Muon Source, Science and Technology Facilities Council, Rutherford Appleton Laboratory, Harwell Science and Innovation Campus, Didcot OX11 0QX, UK article info Article history: Available online 24 October 2013 Keywords: Quasi-elastic neutron scattering Dynamics Polymer blends X-ray scattering Photovoltaics abstract We report data from X-ray and neutron scattering on the structure and dynamics of P3HT–PCBM solid blends cast from three different solvents, characterized by different boiling points, namely chloroform (CF, boiling point T b = 61.2 °C), chlorobenzene (CB, T b = 131 °C) and ortho-dichlorobenzene (ODCB, T b = 180.5 °C). Whereas the blend cast from CB shows already a significant degree of order in its ‘‘pristine’’ (as cast) state, blends prepared from CF and ODCB develop their order only upon annealing at 160 °C. In addition, blending seems to frustrate the polymer dynamics. We propose that such change in dynamics might be related to the polymer confinement within PCBM domains. However, no appreciable difference in the degree of dynamics (and/or of its frustration) is observed as a function of the solvent choice. Ó 2013 Elsevier B.V. All rights reserved. 1. Introduction In the last few years large efforts have been made to gain a de- tailed understanding of the morphology-performance relationship in organic photovoltaic (OPV) devices. These are a promising low- cost alternative to the traditional silicon technology. The photo- conversion process in organic-based solar cells occurs in four steps: the generation of bound excitons (electron/hole pairs) upon photon absorption, the dissociation of excitons into free charges at the electron donor/electron acceptor interface, the transport of electrons and holes to their respective electrodes and the charge extraction from the donor or acceptor domains into the electrodes. However, an important disadvantage of OPVs is related to the relatively large binding energy of excitons [1] brought about by the relatively low dielectric constant of organic materials. This, combined with a relatively limited exciton diffusion length (5–10 nm), has led to the development of the bulk type II hetero- junction (BHJ) architecture as a way to achieve efficient dissocia- tion of excitons and large interfacial areas [2], for example in its most popular incarnation based on regioregular poly(3-hexylthi- ophene) (P3HT) as electron donor and [6,6]-Phenyl C61 butyric acid methyl ester (PCBM) as electron acceptor [3]. In the BHJ archi- tecture the formation of an interpenetrated network between donor and acceptor materials upon blending has the potential to provide both the large interface area and, upon careful selection of the materials, the driving force necessary for exciton splitting and efficient charge generation [4]. To limit losses due to charge recombination and/or trapping, it is paramount to understand how the processing parameters of these devices such as thermal annealing [5–7], solvent choice [8,9] and the use of chemical addi- tives, [10–16] affect the electron-acceptor/electron-donor inter- penetrated network. Although BHJ devices are among the best performing at present, a clear structure-performance relationship has yet to emerge due to the complex multicomponent nature of the solid blend [17], in which both amorphous and crystalline do- mains have been identified [18]. From a more fundamental view- point, this solid blend is a polymer nanocomposite system which encompasses not only polymer confinement by the composite par- ticles, but also from the crystalline/amorphous domains. In recent years neutron scattering techniques have been effi- ciently employed to investigate the morphological features as well as the structural evolution upon changing processing parameters of the polymer/fullerene blends [18–23]. Neutron scattering pro- vides a unique contrast for this organic blend as the scattering cross-section of fullerene derivatives is considerably different (7  10 7 Å 2 and 3.6  10 6 Å 2 for P3HT and PCBM respectively) from the protonated conjugated polymers and, therefore, enables highlighting their structural features, the study of their phase sep- aration and mutual diffusion at their interfaces. Despite work indi- cating that the dynamics and the relaxation mechanism in the P3HT heavily affects its crystallization processes [24] and charge transport properties [25], relatively little has been devoted to the investigation of the constituents dynamics inside the active layer. Quasi-elastic neutron scattering (QENS) is a well-established tech- nique to probe various dynamical processes in soft matter, ranging from fast vibrations and rotations to slower processes such as segmental relaxations and diffusion [26]. Therefore QENS can mon- itor the dynamics of multi-component systems such as polymer blends and nanocomposite systems, at a molecular level. QENS is 0301-0104/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.chemphys.2013.10.006 ⇑ Corresponding author at: ISIS Pulsed Neutron and Muon Source, Science and Technology Facilities Council, Rutherford Appleton Laboratory, Harwell Science and Innovation Campus, Didcot OX11 0QX, UK. E-mail address: victoria.garcia-sakai@stfc.ac.uk (V. García-Sakai). Chemical Physics 427 (2013) 142–146 Contents lists available at ScienceDirect Chemical Physics journal homepage: www.elsevier.com/locate/chemphys