title: π-Conjugated Organometallic Isoindigo Oligomer and Polymer Chromophores: Singlet and Triplet Excited State Dynamics and Application in Polymer Solar Cells.
abstract: An isoindigo based π-conjugated oligomer and polymer that contain cyclometalated platinum(II) "auxochrome" units were subjected to photophysical characterization, and application of the polymer in bulk heterojunction polymer solar cells with PCBM acceptor was examined. The objective of the study was to explore the effect of the heavy metal centers on the excited state properties, in particular, intersystem crossing to a triplet (exciton) state, and further how this would influence the performance of the organometallic polymer in solar cells. The materials were characterized by electrochemistry, ground state absorption, emission, and picosecond-nanosecond transient absorption spectroscopy. Electrochemical measurements indicate that the cyclometalated units have a significant impact on the HOMO energy level of the chromophores, but little effect on the LUMO, which is consistent with localization of the LUMO on the isoindigo acceptor unit. Picosecond-nanosecond transient absorption spectroscopy reveals a transient with ∼100 ns lifetime that is assigned to a triplet excited state that is produced by intersystem crossing from a singlet state on a time scale of ∼130 ps. This is the first time that a triplet state has been observed for isoindigo π-conjugated chromophores. The performance of the polymer in bulk heterojunction solar cells was explored with PC61BM as an acceptor. The performance of the cells was optimum at a relatively high PCBM loading (1:6, polymer:PCBM), but the overall efficiency was relatively low with power conversion efficiency (PCE) of 0.22%. Atomic force microscopy of blend films reveals that the length scale of the phase separation decreases with increasing PCBM content, suggesting a reason for the increase in PCE with acceptor loading. Energetic considerations show that the triplet state in the polymer is too low in energy to undergo charge separation with PCBM. Further, due to the relatively low LUMO energy of the polymer, charge transfer from the singlet to PCBM is only weakly exothermic, which is believed to be the reason that the photocurrent efficiency is relatively low.
authors.affiliation: Department of Chemistry and Center for Macromolecular Science and Engineering, University, of Florida , P. O. Box 117200, Gainesville, Florida 32611-7200, United States.
authors.forename: Subhadip
authors.initials: S
authors.lastname: Goswami
authors.affiliation: Department of Chemistry, University of North Carolina at Chapel Hill , Chapel Hill, North Carolina 27599, United States.
authors.forename: Melissa K
authors.initials: MK
authors.lastname: Gish
authors.affiliation: Department of Chemistry and Center for Macromolecular Science and Engineering, University, of Florida , P. O. Box 117200, Gainesville, Florida 32611-7200, United States.
authors.forename: Jiliang
authors.initials: J
authors.lastname: Wang
authors.affiliation: Department of Chemistry and Center for Macromolecular Science and Engineering, University, of Florida , P. O. Box 117200, Gainesville, Florida 32611-7200, United States.
authors.forename: Russell W
authors.initials: RW
authors.lastname: Winkel
authors.affiliation: Department of Chemistry, University of North Carolina at Chapel Hill , Chapel Hill, North Carolina 27599, United States.
authors.forename: John M
authors.initials: JM
authors.lastname: Papanikolas
authors.affiliation: Department of Chemistry and Center for Macromolecular Science and Engineering, University, of Florida , P. O. Box 117200, Gainesville, Florida 32611-7200, United States.
authors.forename: Kirk S
authors.initials: KS
authors.lastname: Schanze
keywords: isoindigo; orthometalated platinum; polymer solar cell; transient absorption; triplet state