title: Effects of Crystal Morphology on Singlet Exciton Fission in Diketopyrrolopyrrole Thin Films.
abstract: Singlet exciton fission (SF) is a promising strategy for increasing photovoltaic efficiency, but in order for SF to be useful in solar cells, it should take place in a chromophore that is air-stable, highly absorptive, solution processable, and inexpensive. Unlike many SF chromophores, diketopyrrolopyrrole (DPP) conforms to these criteria, and here we investigate SF in DPP for the first time. SF yields in thin films of DPP derivatives, which are widely used in organic electronics and photovoltaics, are shown to depend critically on crystal morphology. Time-resolved spectroscopy of three DPP derivatives with phenyl (3,6-diphenylpyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione, PhDPP), thienyl (3,6-di(thiophen-2-yl)pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione, TDPP), and phenylthienyl (3,6-di(5-phenylthiophen-2-yl)pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione, PhTDPP) aromatic substituents in 100-200 nm thin films reveals that efficient SF occurs only in TDPP and PhTDPP (τSF = 220 ± 20 ps), despite the fact that SF is most exoergic in PhDPP. This result correlates well with the greater degree of π-overlap and closer π-stacking in TDPP (3.50 Å) and PhTDPP (3.59 Å) relative to PhDPP (3.90 Å) and demonstrates that SF in DPP is highly sensitive to the electronic coupling between adjacent chromophores. The triplet yield in PhTDPP films is determined to be 210 ± 35% by the singlet depletion method and 165 ± 30% by the energy transfer method, showing that SF is nearly quantitative in these films and that DPP derivatives are a promising class of SF chromophores for enhancing photovoltaic performance.
authors.affiliation: Department of Chemistry and the Argonne-Northwestern Solar Energy Research Center, and ‡Department of Materials Science and Engineering and the Materials Research Center, Northwestern University , 2145 Sheridan Road, Evanston, Illinois 60208, United States.
authors.forename: Patrick E
authors.initials: PE
authors.lastname: Hartnett
authors.affiliation: Department of Chemistry and the Argonne-Northwestern Solar Energy Research Center, and ‡Department of Materials Science and Engineering and the Materials Research Center, Northwestern University , 2145 Sheridan Road, Evanston, Illinois 60208, United States.
authors.forename: Eric A
authors.initials: EA
authors.lastname: Margulies
authors.affiliation: Department of Chemistry and the Argonne-Northwestern Solar Energy Research Center, and ‡Department of Materials Science and Engineering and the Materials Research Center, Northwestern University , 2145 Sheridan Road, Evanston, Illinois 60208, United States.
authors.forename: Catherine M
authors.initials: CM
authors.lastname: Mauck
authors.affiliation: Department of Chemistry and the Argonne-Northwestern Solar Energy Research Center, and ‡Department of Materials Science and Engineering and the Materials Research Center, Northwestern University , 2145 Sheridan Road, Evanston, Illinois 60208, United States.
authors.forename: Stephen A
authors.initials: SA
authors.lastname: Miller
authors.affiliation: Department of Chemistry and the Argonne-Northwestern Solar Energy Research Center, and ‡Department of Materials Science and Engineering and the Materials Research Center, Northwestern University , 2145 Sheridan Road, Evanston, Illinois 60208, United States.
authors.forename: Yilei
authors.initials: Y
authors.lastname: Wu
authors.affiliation: Department of Chemistry and the Argonne-Northwestern Solar Energy Research Center, and ‡Department of Materials Science and Engineering and the Materials Research Center, Northwestern University , 2145 Sheridan Road, Evanston, Illinois 60208, United States.
authors.forename: Yi-Lin
authors.initials: YL
authors.lastname: Wu
authors.affiliation: Department of Chemistry and the Argonne-Northwestern Solar Energy Research Center, and ‡Department of Materials Science and Engineering and the Materials Research Center, Northwestern University , 2145 Sheridan Road, Evanston, Illinois 60208, United States.
authors.forename: Tobin J
authors.initials: TJ
authors.lastname: Marks
authors.affiliation: Department of Chemistry and the Argonne-Northwestern Solar Energy Research Center, and ‡Department of Materials Science and Engineering and the Materials Research Center, Northwestern University , 2145 Sheridan Road, Evanston, Illinois 60208, United States.
authors.forename: Michael R
authors.initials: MR
authors.lastname: Wasielewski