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Details

Autor(en) / Beteiligte
Titel
The Energy of Charge‐Transfer States in Electron Donor–Acceptor Blends: Insight into the Energy Losses in Organic Solar Cells
Ist Teil von
  • Advanced functional materials, 2009-06, Vol.19 (12), p.1939-1948
Ort / Verlag
Weinheim: WILEY‐VCH Verlag
Erscheinungsjahr
2009
Link zum Volltext
Quelle
Wiley Online Library - AutoHoldings Journals
Beschreibungen/Notizen
  • Here, a general experimental method to determine the energy ECT of intermolecular charge‐transfer (CT) states in electron donor–acceptor (D–A) blends from ground state absorption and electrochemical measurements is proposed. This CT energy is calibrated against the photon energy of maximum CT luminescence from selected D–A blends to correct for a constant Coulombic term. It is shown that ECT correlates linearly with the open‐circuit voltage (Voc) of photovoltaic devices in D–A blends via eVoc = ECT − 0.5 eV. Using the CT energy, it is found that photoinduced electron transfer (PET) from the lowest singlet excited state (S1 with energy Eg) in the blend to the CT state (S1 → CT) occurs when Eg − ECT > 0.1 eV. Additionally, it is shown that subsequent charge recombination from the CT state to the lowest triplet excited state (ET) of D or A (CT → T1) can occur when ECT − ET > 0.1 eV. From these relations, it is concluded that in D–A blends optimized for photovoltaic action: i) the maximum attainable Voc is ultimately set by the optical band gap (eVoc = Eg − 0.6 eV) and ii) the singlet–triplet energy gap should be ΔEST < 0.2 eV to prevent recombination to the triplet state. These favorable conditions have not yet been met in conjugated materials and set the stage for further developments in this area. CT states at the electron donor–acceptor interface are key intermediates in bulk‐heterojunction organic solar cells. A new general method allows determining their energy and provides insight into energetic losses in these cells. CT states may form with only 0.1 eV loss from the singlet excited state, but a major loss of 0.5 eV occurs when going to open‐circuit, limiting the ultimate power conversion efficiency.
Sprache
Englisch
Identifikatoren
ISSN: 1616-301X
eISSN: 1616-3028
DOI: 10.1002/adfm.200900090
Titel-ID: cdi_crossref_primary_10_1002_adfm_200900090

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