Quantification of energy losses in organic solar cells from temperature-dependent device characteristics

Ulrich Hörmann, Julia Kraus, Mark Gruber, Christoph Schuhmair, Theresa Linderl, Stefan Grob, Stephan Kapfinger, Konrad Klein, Martin Stutzman, Hubert J. Krenner, and Wolfgang Brütting
Phys. Rev. B 88, 235307 – Published 11 December 2013

Abstract

Owing to the excitonic nature of photoexcitations in organic semiconductors, the working mechanism of organic solar cells relies on the donor-acceptor (D/A) concept enabling photoinduced charge transfer at the interface between two organic materials with suitable energy-level alignment. However, the introduction of such a heterojunction is accompanied by additional energy losses compared to an inorganic homojunction cell due to the presence of a charge-transfer (CT) state at the D/A interface. By careful examination of planar heterojunctions of the molecular semiconductors diindenoperylene (DIP) and C60 we demonstrate that three different analysis techniques of the temperature dependence of solar-cell characteristics yield reliable values for the effective photovoltaic energy gap at the D/A interface. The retrieved energies are shown to be consistent with direct spectroscopic measurements and the D/A energy-level offset determined by photoemission spectroscopy. Furthermore, we verify the widespread assumption that the activation energy of the dark saturation current ΔE and the CT energy ECT may be regarded as identical. The temperature-dependent analysis of open-circuit voltage VOC and dark saturation current is then applied to a variety of molecular planar heterojunctions. The congruency of ΔE and ECT is again found for all material systems with the exception of copper phthalocyanine/C60. The general rule of thumb for organic semiconductor heterojunctions, that VOC at room temperature is roughly half a volt below the CT energy, is traced back to comparable intermolecular electronic coupling in all investigated systems.

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  • Received 8 October 2013

DOI:https://doi.org/10.1103/PhysRevB.88.235307

©2013 American Physical Society

Authors & Affiliations

Ulrich Hörmann1, Julia Kraus1, Mark Gruber1, Christoph Schuhmair1, Theresa Linderl1, Stefan Grob1, Stephan Kapfinger1,2, Konrad Klein3, Martin Stutzman2,3, Hubert J. Krenner1,2, and Wolfgang Brütting1,*

  • 1Institut für Physik, Universität Augsburg, Universitätsstrasse 1, 86159 Augsburg, Germany
  • 2Nanosystems Initiative Munich, Schellingstr. 4, 80799 München, Germany
  • 3Walter Schottky Institut, Technische Universität München, Am Coulombwall 4, 85748 Garching, Germany

  • *wolfgang.bruetting@physik.uni-augsburg.de

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Vol. 88, Iss. 23 — 15 December 2013

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