Recombination in polymer:Fullerene solar cells with open-circuit voltages approaching and exceeding 1.0 V
AuthorsHoke, Eric T.
Bartelt, Jonathan A.
Mateker, William R.
Douglas, Jessica D.
McGehee, Michael D.
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AbstractPolymer:fullerene solar cells are demonstrated with power conversion efficiencies over 7% with blends of PBDTTPD and PC 61 BM. These devices achieve open-circuit voltages ( V oc ) of 0.945 V and internal quantum efficiencies of 88%, making them an ideal candidate for the large bandgap junction in tandem solar cells. V oc 's above 1.0 V are obtained when the polymer is blended with multiadduct fullerenes; however, the photocurrent and fill factor are greatly reduced. In PBDTTPD blends with multiadduct fullerene ICBA, fullerene emission is observed in the photoluminescence and electroluminescence spectra, indicating that excitons are recombining on ICBA. Voltage-dependent, steady state and time-resolved photoluminescence measurements indicate that energy transfer occurs from PBDTTPD to ICBA and that back hole transfer from ICBA to PBDTTPD is inefficient. By analyzing the absorption and emission spectra from fullerene and charge transfer excitons, we estimate a driving free energy of -0.14 ± 0.06 eV is required for efficient hole transfer. These results suggest that the driving force for hole transfer may be too small for efficient current generation in polymer:fullerene solar cells with V oc values above 1.0 V and that non-fullerene acceptor materials with large optical gaps ( > 1.7 eV) may be required to achieve both near unity internal quantum efficiencies and values of V oc exceeding 1.0 V. © 2013 WILEY-VCH Verlag GmbH and Co.
SponsorsThis publication was supported by the Center for Advanced Molecular Photovoltaics (Award No KUS-C1-015-21), made possible by King Abdullah University of Science and Technology (KAUST). Additional support was provided for E.T.H. by the Fannie and John Hertz Foundation and for J.A.B. by the NDSEG fellowship. The authors would like to thank Laxman Pandey and Chad Risko for calculating the PBDTTPD triplet absorption cross section and Ian Howard for helpful discussions.
JournalAdvanced Energy Materials