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  • 1.
    Blazinic, Vanja
    et al.
    Karlstad University, Faculty of Health, Science and Technology (starting 2013), Department of Engineering and Physics (from 2013).
    Ericsson, Leif
    Karlstad University, Faculty of Health, Science and Technology (starting 2013), Department of Engineering and Physics (from 2013).
    Levine, Igal
    Hansson, Rickard
    Karlstad University, Faculty of Health, Science and Technology (starting 2013), Department of Engineering and Physics (from 2013).
    Opitz, Andreas
    Moons, Ellen
    Karlstad University, Faculty of Health, Science and Technology (starting 2013), Department of Engineering and Physics (from 2013).
    Impact of intentional photo-oxidation of a donor polymer and PC70BM on solar cell performance2019In: Physical Chemistry, Chemical Physics - PCCP, ISSN 1463-9076, E-ISSN 1463-9084, Vol. 21, p. 22259-22271Article in journal (Refereed)
    Abstract [en]

    A short lifetime is the main factor hindering the wider implementation of low-cost organic photovoltaics in large-area and outdoor applications. Ingress of oxygen and water vapour through non-ideal encapsulation layers is a known cause of degradation for polymer/fullerene based solar cells. To better understand the origin of this performance degradation, we study the effect of intentional exposure of the photo-active layer to simulated sunlight (AM1.5) in air both on the solar cell performance and on the molecular semiconductor materials. Cathode-free thin films of a blend of the electron donor polymer poly[2,3-bis-(3-octyloxyphenyl)quinoxaline-5,8-diyl-alt-thiophene-2,5-diyl] (TQ1) and the electron acceptor fullerene derivative [6,6]-phenyl-C70-butyric acid methyl ester (PC70BM) were exposed to simulated sunlight in air. Fourier-transform infrared spectra demonstrate the formation of carbonyl photo-oxidation products in the blend films, as well as in the pristine polymer and fullerene films. Solar cells prepared with photo-oxidized active layers show increasingly degraded electrical performance (lower short circuit current, open circuit voltage and fill factor) with increasing exposure time. The increased diode ideality factor indicates that trap-assisted recombination hinders device operation after exposure. The external quantum efficiency decreases drastically with increasing exposure time over the whole photon energy range, while the UV-vis absorption spectra of the blend films only show a mild photo-induced bleaching. This demonstrates that not only the photo-induced degradation of the solar cell performance is not predominantly caused by the loss in light absorption, but charge transport and collection are also hampered. This is explained by the fact that photo-oxidation of PC70BM causes bonds in its conjugated cage to break, as evidenced by the decreased ∏* intensity in C1s-NEXAFS spectra of PC70BM films. This degradation of unoccupied states of PC70BM will hinder the transport of photo-generated electrons to the electrode. Surface photovoltage spectroscopy gives direct evidence for gap states at the surface of a PC70BM film, formed after 2 hours of exposure and resulting in upward band bending at the PC70BM/air surface. These observations indicate that the photo-oxidation of PC70BM is likely to be the main cause of the performance degradation observed when the photoactive layer of a TQ1:PC70BM solar cell is intentionally exposed to light in air.

  • 2.
    Brumboiu, Iulia Emilia
    et al.
    KTH Royal Institute of Technology.
    Ericsson, Leif
    Karlstad University, Faculty of Technology and Science, Department of Physics and Electrical Engineering. Karlstad University, Faculty of Technology and Science, Materials Science. Karlstad University, Faculty of Health, Science and Technology (starting 2013), Department of Engineering and Physics (from 2013).
    Blazinic, Vanja
    Karlstad University, Faculty of Health, Science and Technology (starting 2013), Department of Engineering and Physics (from 2013).
    Hansson, Rickard
    Karlstad University, Faculty of Health, Science and Technology (starting 2013), Department of Engineering and Physics (from 2013).
    Opitz, Andreas
    Humboldt-Universität zu Berlin.
    Brena, Barbara
    Uppsala Universitet, Physics and Astronomy.
    Moons, Ellen
    Karlstad University, Faculty of Technology and Science, Department of Physics and Electrical Engineering. Karlstad University, Faculty of Technology and Science, Materials Science. Karlstad University, Faculty of Health, Science and Technology (starting 2013), Department of Engineering and Physics (from 2013).
    The Photooxidation of PC60BM: New Insights from SpectroscopyManuscript (preprint) (Other academic)
    Abstract [en]

    In the quest towards more durable solution-processed solar cells, the stability of the active layer materials under operation conditions is important. While lifetimes of several years have been demonstrated for encapsulated organic solar cells, it is generally known that degradation events can be accounted for by air components (O2 and/or water vapour) leaking into the cell through a non-ideal sealing. Here we present a fundamental study of intentional photo-degradation of the electron-acceptor PC60BM ([6,6]-phenyl-C61-butyric acid methyl ester) in air, with the purpose of improving the understanding of the electronic effects of fullerene photo-oxidation. We have studied spincoated thin films of PC60BM by X-ray Photoelectron Spectroscopy, Near-edge X-ray Absorption Fine Structure spectroscopy, and Fourier Transform Infrared Spectroscopy, before and after exposing them to simulated sunlight in air. The changes observed in the spectra obtained by these complementary methods were compared with calculated spectra of a large set of possible oxidation products of PC60BM where oxygen atoms have been attached to the C60 cage. The best fit with experimental IR spectra of photodegraded PC60BM films was obtained for a linear combination of calculated spectra for two degradation products, a dicarbonyl and an anhydride, both with open cages with 58 carbon atoms, and the pristine PC60BM molecule. From this comparison, we conclude that the conjugation of the fullerene cage is disturbed by the formation of several carbonyl-based derivatives on the C60 cage, accompanied by a transition from sp2 to sp3-hybridized carbon. The π* resonance in the C1s NEXAFS spectrum was found to be a very sensitive probe for small changes to the fullerene cage, and FT-IR was needed in combination with O1s NEXAFS, to identify the oxidation products.

  • 3.
    Grob, Stefan
    et al.
    Univ Augsburg, Inst Phys, D-86135 Augsburg, Germany..
    Bartynski, Andrew N.
    Humboldt Univ, Inst Phys, D-12489 Berlin, Germany..
    Opitz, Andreas
    Univ So Calif, Dept Chem, Dept Chem Engn, Los Angeles, CA 90089 USA..
    Gruber, Mark
    Univ Augsburg, Inst Phys, D-86135 Augsburg, Germany..
    Grassl, Florian
    Univ Augsburg, Inst Phys, D-86135 Augsburg, Germany..
    Meister, Eduard
    Univ Augsburg, Inst Phys, D-86135 Augsburg, Germany..
    Linderl, Theresa
    Univ Augsburg, Inst Phys, D-86135 Augsburg, Germany..
    Hoermann, Ulrich
    Univ Augsburg, Inst Phys, D-86135 Augsburg, Germany..
    Lorch, Christopher
    Univ Tubingen, Inst Angew Phys, D-72076 Tubingen, Germany..
    Moons, Ellen
    Karlstad University, Faculty of Technology and Science, Department of Physics and Electrical Engineering.
    Schreiber, Frank
    Univ Tubingen, Inst Angew Phys, D-72076 Tubingen, Germany..
    Thompson, Mark E.
    Humboldt Univ, Inst Phys, D-12489 Berlin, Germany..
    Bruetting, Wolfgang
    Univ Augsburg, Inst Phys, D-86135 Augsburg, Germany..
    Solvent vapor annealing on perylene-based organic solar cells2015In: Journal of Materials Chemistry A, ISSN 2050-7488, Vol. 3, no 30, p. 15700-15709Article in journal (Refereed)
    Abstract [en]

    Diindenoperylene (DIP) and tetraphenyldibenzoperiflanthene (DBP) are two commonly used donor materials in organic solar cell devices. Despite their structural similarities, DIP films are crystalline, exhibiting good charge and exciton transport, whereas DBP films are amorphous and have lower carrier mobility and a short exciton diffusion length. However, DBP reveals a distinctly higher absorption due to the lying orientation of its transition dipole moments. In this paper, we investigate the influence of solvent vapor annealing (SVA) on the solar cell performance of both materials. In general, SVA induces a partial re-solubilization of the material leading to enhanced crystallinity of the treated layer. For DBP, extended annealing times result in a strong aggregation of the molecules, creating inhomogeneous layers unfavorable for solar cells. However, in DIP cells, SVA leads to an increase in fill factor (FF) and also a slight increase in short-circuit current density (JSC) due to interface roughening. The best results are obtained by combining solvent vapor annealed DIP layers with strongly absorbing DBP and C-70 on top. Through this device architecture, we obtain the same increase in FF in addition to a higher gain in J(SC), elevating the power conversion efficiency by a factor of 1.2 to more than 4%.

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  • 4.
    Talyzin, Alexandr V.
    et al.
    Umea Univ, Dept Phys, S-90187 Umea, Sweden..
    Mercier, Guillaume
    Umea Univ, Dept Phys, S-90187 Umea, Sweden..
    Klechikov, Alexey
    Umea Univ, Dept Phys, S-90187 Umea, Sweden..
    Hedenstrom, Mattias
    Umea Univ, Dept Chem, S-90187 Umea, Sweden..
    Johnels, Dan
    Umea Univ, Dept Chem, S-90187 Umea, Sweden..
    Wei, Di
    Nokia Technol, Broers Bldg,21 II Thomson Ave, Cambridge CB3 0FA, England..
    Cotton, Darryl
    Nokia Technol, Broers Bldg,21 II Thomson Ave, Cambridge CB3 0FA, England..
    Opitz, Andreas
    Humboldt Univ, Dept Phys, D-12489 Berlin, Germany..
    Moons, Ellen
    Karlstad University, Faculty of Health, Science and Technology (starting 2013), Department of Engineering and Physics (from 2013).
    Brodie vs Hummers graphite oxides for preparation of multi-layered materials2017In: Carbon, ISSN 0008-6223, E-ISSN 1873-3891, Vol. 115, p. 430-440Article in journal (Refereed)
    Abstract [en]

    Graphite oxides synthesized by one and two step Brodie oxidation (BGO) and Hummers (HGO) methods were analyzed by a variety of characterization methods in order to evaluate the reasons behind the difference in their properties. It is found that the Brodie method results in a higher relative amount of hydroxyl groups and a more homogeneous overall distribution of functional groups over the planar surface of the graphene oxide flakes. The higher number of carbonyl and carboxyl groups in HGO, detected by several methods, including XPS, NMR and FTIR, unavoidably results in defects of the graphene "skeleton", holes and overall disruption of the carbon-carbon bond network, stronger deviation from planar flake shape and poor ordering of the graphene oxide layers. It is also suggested that functional groups in HGO are less homogeneously distributed over the flake surface, forming some nanometer-sized graphene areas. The presence of differently oxidized areas on the GO surface results in inhomogeneous solvation and hydration of HGO and effects of inter- and intra-stratification. The proposed interpretation of the data explains the higher mechanical strength of multi-layered BGO membranes/papers, which are also less affected by humidity changes, thus providing an example of a membrane property superior to that of HGO. (C) 2016 Published by Elsevier Ltd.

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