Competition between the charge transfer state and the singlet states of donor or acceptor limiting the efficiency in polymer: Fullerene solar cells: Fullerene solar cells

Mark A. Faist, Thomas Kirchartz, Wei Gong, Raja Shahid Ashraf, Iain McCulloch, John C. De Mello, Nicholas J. Ekins-Daukes, Donal D.C. Bradley, Jenny Nelson

Research output: Contribution to journalArticlepeer-review

203 Scopus citations

Abstract

We study the appearance and energy of the charge transfer (CT) state using measurements of electroluminescence (EL) and photoluminescence (PL) in blend films of high-performance polymers with fullerene acceptors. EL spectroscopy provides a direct probe of the energy of the interfacial states without the need to rely on the LUMO and HOMO energies as estimated in pristine materials. For each polymer, we use different fullerenes with varying LUMO levels as electron acceptors, in order to vary the energy of the CT state relative to the blend with [6,6]-phenyl C61-butyric acid methyl ester (PCBM). As the energy of the CT state emission approaches the absorption onset of the blend component with the smaller optical bandgap, E opt,min min{E opt,donor; E opt,acceptor}, we observe a transition in the EL spectrum from CT emission to singlet emission from the component with the smaller bandgap. The appearance of component singlet emission coincides with reduced photocurrent and fill factor. We conclude that the open circuit voltage V OC is limited by the smaller bandgap of the two blend components. From the losses of the studied materials, we derive an empirical limit for the open circuit voltage: V OC ≲ E opt,min/e - (0.66 ± 0.08)eV. © 2011 American Chemical Society.
Original languageEnglish (US)
Pages (from-to)685-692
Number of pages8
JournalJournal of the American Chemical Society
Volume134
Issue number1
DOIs
StatePublished - Jan 11 2012
Externally publishedYes

ASJC Scopus subject areas

  • Catalysis
  • Chemistry(all)
  • Biochemistry
  • Colloid and Surface Chemistry

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