Influence of Annealing and Interfacial Roughness on the Performance of Bilayer Donor/Acceptor Polymer Photovoltaic Devices

Authors

  • Hongping Yan,

    1. Department of Physics, North Carolina State University, Raleigh, NC 27695, USA
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  • Sufal Swaraj,

    1. Department of Physics, North Carolina State University, Raleigh, NC 27695, USA
    Current affiliation:
    1. Present address: Synchrotron SOLEIL, Ormes des Merisiers, Saint Aubin, 91192, France
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  • Cheng Wang,

    1. Department of Physics, North Carolina State University, Raleigh, NC 27695, USA
    Current affiliation:
    1. Present address: Advanced Light Source, Lawrence Berkeley National Laboratory, Berkeley, CA94720, USA
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  • Inchan Hwang,

    1. Cavendish Laboratory, Department of Physics, University of Cambridge, J J Thomson Ave, Cambridge, CB3 0HE, UK
    Current affiliation:
    1. Present address: Department of Chemistry, University of Toronto, Toronto, ON M5S 3H6, Canada
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  • Neil C. Greenham,

    1. Cavendish Laboratory, Department of Physics, University of Cambridge, J J Thomson Ave, Cambridge, CB3 0HE, UK
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  • Chris Groves,

    Corresponding author
    1. School of Engineering and Computing Sciences, Durham University, Durham, DH1 3LE, UK
    • School of Engineering and Computing Sciences, Durham University, Durham, DH1 3LE, UK.
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  • Harald Ade,

    Corresponding author
    1. Department of Physics, North Carolina State University, Raleigh, NC 27695, USA
    • Department of Physics, North Carolina State University, Raleigh, NC 27695, USA
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  • Christopher R. McNeill

    Corresponding author
    1. Cavendish Laboratory, Department of Physics, University of Cambridge, J J Thomson Ave, Cambridge, CB3 0HE, UK
    • Cavendish Laboratory, Department of Physics, University of Cambridge, J J Thomson Ave, Cambridge, CB3 0HE, UK
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Abstract

Through controlled annealing of planar heterojunction (bilayer) devices based on the polyfluorene copolymers poly(9,9-dioctylfluorene-co-bis(N,N′-(4,butylphenyl))bis(N,N′-phenyl-1,4-phenylene)diamine) (PFB) and poly(9,9-dioctylfluorene-co-benzothiadiazole) (F8BT) we study the influence of interface roughness on the generation and separation of electron–hole pairs at the donor/acceptor interface. Interface structure is independently characterized by resonant soft X-ray reflectivity with the interfacial width of the PFB/F8BT heterojunction observed to systematically increase with annealing temperature from 1.6 nm for unannealed films to 16 nm with annealing at 200 °C for ten minutes. Photoluminescence quenching measurements confirm the increase in interface area by the three-fold increase in the number of excitons dissociated. Under short-circuit conditions, however, unannealed devices with the sharpest interface are found to give the best device performance, despite the increase in interfacial area (and hence the number of excitons dissociated) in annealed devices. The decrease in device efficiency with annealing is attributed to decreased interfacial charge separation efficiency, partly due to a decrease in the bulk mobility of the constituent materials upon annealing but also (and significantly) due to the increased interface roughness. We present results of Monte Carlo simulations that demonstrate that increased interface roughness leads to lower charge separation efficiency, and are able to reproduce the experimental current-voltage curves taking both increased interfacial roughness and decreased carrier mobility into account. Our results show that organic photovoltaic performance can be sensitive to interfacial order, and heterojunction sharpness should be considered a requirement for high performance devices.

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