Competition between the
Charge Transfer State and
the Singlet States of Donor or Acceptor Limiting the Efficiency in
Polymer:Fullerene Solar Cells
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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, <i>E</i><sub>opt,min</sub> ≡ min{<i>E</i><sub>opt,donor</sub>; <i>E</i><sub>opt,acceptor</sub>}, 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 <i>V</i><sub>OC</sub> 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: <i>V</i><sub>OC</sub> ≲ <i>E</i><sub>opt,min</sub>/<i>e</i> – (0.66 ± 0.08)eV