© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 1 wileyonlinelibrary.com www.MaterialsViews.com www.advenergymat.de FULL PAPER Influence of Thermal Annealing on PCDTBT:PCBM Composition Profiles Olesia Synooka,* Kai-Rudi Eberhardt, Chetan Raj Singh, Felix Hermann, Gernot Ecke, Bernhard Ecker, Elizabeth von Hauff, Gerhard Gobsch, and Harald Hoppe* O. Synooka, K.-R. Eberhardt, C. R. Singh, F. Hermann, Prof. G. Gobsch, Dr. H. Hoppe Department of Experimental Physics 1 Institut für Physik and Institut für Mikro- und Nanotechnologien TU Ilmenau, PF 100565, 98693, Ilmenau, Germany E-mail: olesia.synooka@tu-ilmenau.de; harald.hoppe@tu-ilmenau.de G. Ecke Department of Nanotechnology Institut für Mikro- und Nanotechnologien TU Ilmenau, PF 100565, 98693, Ilmenau Dr. B. Ecker, Prof. E. von Hauff Organic Photovoltaics & Electronics Group Institute of Physics University of Freiburg 79104 Freiburg, Germany Fraunhofer ISE, Heidenhofstr. 2, 79110, Freiburg, Germany DOI: 10.1002/aenm.201300981 1. Introduction Power generation from solar energy is one of the most impor- tant future technologies. In this context, conjugated polymer- based organic solar cells are of special interest because of their potential for low cost and their great flexibility, better transpar- ency, low light suitability, disposability, and large-scale indus- trial production. [1–6] The use of alternating donor (D) and acceptor (A) units in one copolymer initiated a generation of efficiency improve- ments with bulk heterojunction solar cells. These copolymers have a reduced band gap and are designed to offer a more efficient light harvesting of the solar spec- trum and higher open circuit voltages. [7,8] A prominent and intensively studied group of these new materials are the fluorene- or carbazole-based D-A polymers, [9–11] which showed high efficiencies in solar cells and improved light charge carrier mobility. One of the most extensively studied donor–acceptor copolymers is poly[N-9 ′′ - hepta-decanyl-2,7-car-bazole-alt-5,5-(4 ′ ,7 ′ - di-2-thienyl-2 ′ ,1 ′ ,3 ′ -benzothiadiazole)] (PCDTBT), which was first synthesized by Leclerc and co-workers in 2007. [12] PCDTBT has been reported to have rela- tively high solubility, [12] nearly perfect internal quantum efficiency, [12] high- power conversion efficiency [13,14] in sam- ples annealed below 80 °C, and excellent thermal stability, [15] with operating life- times estimated to be ≈ 7 years. [16,17] Earlier work by Leclerc and co-workers on PCDTBT:[6,6]- phenyl C 71 butyric acid methyl ester (PCBM) photovoltaic devices has demonstrated reproducible efficiencies of more than 3% in simple as-cast device structures, with efficiencies exceeding 5% upon optimizing the light management of the device. [18] Several groups recently investigated the reason of the high current and efficiency in PCDTBT-based solar cells. They could demonstrate that the ultrafast charge carrier generation [19–21] and different recombination dynamics (comparing with poly(3-hexylthio- phene) (P3HT)) [22–24] are favorable for this material system. In contrast to P3HT/PCBM bulk heterojunctions (BHJs), which require thermal annealing to achieve their highest effi- ciency, [25] BHJs based on PCDTBT are performing best in an unannealed state. [26,27] This has been assigned to the struc- tural properties of PCDTBT. Lu et al. [28] reported the formation of PCDTBT-backbone pairing, where the alkyl chains are all anchored on the same segment and this allows the alkyl-free side of the PCDTBT backbones to pair. This arrangement is energetically favored for annealing temperatures below 140 °C in blends with PCBM, as detected by X-ray scattering studies. For higher temperatures this feature vanished and another ordering occurred, indicating a phase transition within the PCDTBT-phase occurring at about 140 °C. Additionally, the glass transition temperature of the PCDTBT:PCBM blend is reduced upon annealing, [29] an observation that is consistent with disruption of π – π staking between PCDTBT molecules. Reduced π – π staking is correlated with reduced hole mobility in thermally annealed films. The decrease in structural order is matched by the movement of hole-traps deeper into the band A variety of measurement techniques including photothermal deflection spec- troscopy (PDS), auger electron spectroscopy (AES), (sub–bandgap) external quantum efficiency (EQE), and impedance spectroscopy are applied to poly[N- 900-hepta-decanyl-2,7-carbazole-alt-5,5-(40,70-di-2-thienyl-20,10,30-benzothi- adiazole (PCDTBT)/[6,6]-phenyl C 71 butyric acid methyl ester (PC 71 BM) films and devices to probe the stability under thermal annealing. Upon annealing, solar cell performance is drastically decreased for temperatures higher than 140 °C. Detailed investigation indicate changes in polymer:fullerene interac- tions resulting in the formation of a polymer wetting layer upon annealing at temperatures higher than 140 °C. Upon device completion this wetting layer is located close to the metal electrode and therefore leads to an increase in recombination and a decrease in charge carrier extraction, providing an expla- nation for the reduced fill factor (FF) and power conversion efficiency (PCE). Adv. Energy Mater. 2013, DOI: 10.1002/aenm.201300981