title: Symmetry-breaking charge transfer in a zinc chlorodipyrrin acceptor for high open circuit voltage organic photovoltaics.
abstract: Low open-circuit voltages significantly limit the power conversion efficiency of organic photovoltaic devices. Typical strategies to enhance the open-circuit voltage involve tuning the HOMO and LUMO positions of the donor (D) and acceptor (A), respectively, to increase the interfacial energy gap or to tailor the donor or acceptor structure at the D/A interface. Here, we present an alternative approach to improve the open-circuit voltage through the use of a zinc chlorodipyrrin, ZCl [bis(dodecachloro-5-mesityldipyrrinato)zinc], as an acceptor, which undergoes symmetry-breaking charge transfer (CT) at the donor/acceptor interface. DBP/ZCl cells exhibit open-circuit voltages of 1.33 V compared to 0.88 V for analogous tetraphenyldibenzoperyflanthrene (DBP)/C60-based devices. Charge transfer state energies measured by Fourier-transform photocurrent spectroscopy and electroluminescence show that C60 forms a CT state of 1.45 ± 0.05 eV in a DBP/C60-based organic photovoltaic device, while ZCl as acceptor gives a CT state energy of 1.70 ± 0.05 eV in the corresponding device structure. In the ZCl device this results in an energetic loss between E(CT) and qV(OC) of 0.37 eV, substantially less than the 0.6 eV typically observed for organic systems and equal to the recombination losses seen in high-efficiency Si and GaAs devices. The substantial increase in open-circuit voltage and reduction in recombination losses for devices utilizing ZCl demonstrate the great promise of symmetry-breaking charge transfer in organic photovoltaic devices.
authors.forename: Andrew N
authors.initials: AN
authors.lastname: Bartynski
authors.affiliation: §Institute for Physics, Augsburg University, 86135 Augsburg, Germany.
authors.forename: Mark
authors.initials: M
authors.lastname: Gruber
authors.forename: Saptaparna
authors.initials: S
authors.lastname: Das
authors.affiliation: ∥Department of Physics and Astronomy, Rutgers University, Piscataway, New Jersey 08854, United States.
authors.forename: Sylvie
authors.initials: S
authors.lastname: Rangan
authors.affiliation: ⊥Department of Materials Science and Engineering, Stanford University, Palo Alto, California 94305, United States.
authors.forename: Sonya
authors.initials: S
authors.lastname: Mollinger
authors.forename: Cong
authors.initials: C
authors.lastname: Trinh
authors.forename: Stephen E
authors.initials: SE
authors.lastname: Bradforth
authors.affiliation: ⊥Department of Materials Science and Engineering, Stanford University, Palo Alto, California 94305, United States.
authors.forename: Koen
authors.initials: K
authors.lastname: Vandewal
authors.affiliation: ⊥Department of Materials Science and Engineering, Stanford University, Palo Alto, California 94305, United States.
authors.forename: Alberto
authors.initials: A
authors.lastname: Salleo
authors.affiliation: ∥Department of Physics and Astronomy, Rutgers University, Piscataway, New Jersey 08854, United States.
authors.forename: Robert A
authors.initials: RA
authors.lastname: Bartynski
authors.affiliation: §Institute for Physics, Augsburg University, 86135 Augsburg, Germany.
authors.forename: Wolfgang
authors.initials: W
authors.lastname: Bruetting
authors.forename: Mark E
authors.initials: ME
authors.lastname: Thompson