Published: October 17, 2011 r2011 American Chemical Society 8470 dx.doi.org/10.1021/ma201911a | Macromolecules 2011, 44, 8470–8478 ARTICLE pubs.acs.org/Macromolecules Thermal Stability of Poly[2-methoxy-5-(2 0 -phenylethoxy)-1,4- phenylenevinylene] (MPE-PPV):Fullerene Bulk Heterojunction Solar Cells J. Vandenbergh, † B. Conings, † S. Bertho, † J. Kesters, † D. Spoltore, † S. Esiner, ‡ J. Zhao, § G. Van Assche, § M. M. Wienk, ‡ W. Maes, †,|| L. Lutsen, ^ B. Van Mele, § R. A. J. Janssen, ‡ J. Manca, †,^ and D. J. M. Vanderzande* ,†,^ † Institute for Materials Research (IMO), Hasselt University, Universitaire Campus, Building D, B-3590 Diepenbeek, Belgium ‡ Molecular Materials and Nanosystems, Eindhoven University of Technology, P.O. Box 513, 5600MB Eindhoven, The Netherlands § Physical Chemistry and Polymer Science, Department of Materials and Chemistry, Vrije Universiteit Brussel, Pleinlaan 2, B-1050 Brussels, Belgium ) Molecular Design and Synthesis, Katholieke Universiteit Leuven, Celestijnenlaan 200F, B-3001 Heverlee, Belgium ^ Division IMOMEC, IMEC, Universitaire Campus, Wetenschapspark 1, B-3590 Diepenbeek, Belgium b S Supporting Information ’ INTRODUCTION In polymer:fullerene bulk heterojunction solar cells, the nanoscale morphology of the active layer influences the electronic properties of the devices to a great extent. 14 Via proper sample preparation, such as annealing conditions and the choice of proces- sing solvents, the (nano)morphology of the donoracceptor blend can be optimized, leading to improved exciton dissociation and charge transport. 36 It was already demonstrated, however, that this initial optimized morphology deteriorates rather fast (during storage at elevated temperatures). 717 Long-term annealing treatments induce severe phase separation between the donor polymer and the fullerene acceptor material, leading to a decreased interfacial area and less efficient exciton dissociation. 1820 To stabilize the initial (nano)morphology of a bulk heterojunction layer, several pathways can be followed. A first approach is to use donoracceptor diblock copolymers to covalently fixate the morphology via the scale of the block lengths. 13,14 A second way to obtain a stable network is to build in a certain amount of functional groups in the donor polymer which can cross-link and freeze in the ultimate morphology after a thermal or UV light treatment. 21 In this paper, a third approach is further investigated, which is based on the use of a donor polymer with a high glass transition temperature (T g ). 2226 In this context the question arises how to adjust the chemical structure of the polymer to induce a higher T g , without jeopardizing the opto- electronic characteristics of the polymer. Here we demonstrate that by changing the side-chain substituents of the polymer, the T g can be deliberately increased. Poly[(2-methoxy-5-(3 0 ,7 0 -dimethyl- octyloxy))-1,4-phenylenevinylene] (MDMO-PPV) has a quite low T g of around 45 °C due to the long flexible alkoxy side chains, which cause a plasticizing effect on the PPV backbone. 22 To reduce the side-chain flexibility, a novel poly(p-phenylenevinylene) derivative, poly[2-methoxy-5-(2 0 -phenylethoxy)-1,4-phenylenevinylene] Received: August 22, 2011 Revised: October 2, 2011 ABSTRACT: To improve the thermal stability of polymer:fullerene bulk hetero- junction solar cells, a new polymer, poly[2-methoxy-5-(2 0 -phenylethoxy)-1,4- phenylenevinylene] (MPE-PPV), has been designed and synthesized, which showed an increased glass transition temperature (T g ) of 111 °C. The thermal character- istics and phase behavior of MPE-PPV:[6,6]-phenyl C 61 -butyric acid methyl ester ([60]PCBM) blends were investigated by means of modulated temperature differ- ential scanning calorimetry and rapid heatingcooling calorimetry. The thermal stability of MPE-PPV:[60]PCBM solar cells was compared with devices based on the reference MDMO-PPV material with a T g of 45 °C. Monitoring of the photo- currentvoltage characteristics at elevated temperatures revealed that the use of high- T g MPE-PPV resulted in a substantial improvement of the thermal stability of the solar cells. Furthermore, a systematic transmission electron microscope study of the active polymer:fullerene layer at elevated temperatures likewise demonstrated a more stable morphology for the MPE-PPV:[60]PCBM blend. Both observations indicate that the use of high-T g MPE-PPV as donor material leads to a reduced free movement of the fullerene molecules within the active layer of the photovoltaic device. Finally, optimization of the PPV:fullerene solar cells revealed that for both types of devices the use of [6,6]-phenyl C 71 -butyric acid methyl ester ([70]PCBM) resulted in a substantial increase of current density and power conversion efficiency, up to 3.0% for MDMO-PPV:[70]PCBM and 2.3% for MPE- PPV:[70]PCBM.