title: Molecular order in high-efficiency polymer/fullerene bulk heterojunction solar cells.
abstract: We report quantitative measurements of ordering, molecular orientation, and nanoscale morphology in the active layer of bulk heterojunction (BHJ) organic photovoltaic cells based on a thieno[3,4-b]thiophene-alt-benzodithiophene copolymer (PTB7), which has been shown to yield very high power conversion efficiency when blended with [6,6]-phenyl-C71-butyric acid methyl ester (PC(71)BM). A surprisingly low degree of order was found in the polymer-far lower in the bulk heterojunction than in pure PTB7. X-ray diffraction data yielded a nearly full orientation distribution for the polymer π-stacking direction within well-ordered regions, revealing a moderate preference for π-stacking in the vertical direction ("face-on"). By combining molecular orientation information from polarizing absorption spectroscopies with the orientation distribution of ordered material from diffraction, we propose a model describing the PTB7 molecular orientation distribution (ordered and disordered), with the fraction of ordered polymer as a model parameter. This model shows that only a small fraction (≈20%) of the polymer in the PTB7/PC(71)BM blend is ordered. Energy-filtered transmission electron microscopy shows that the morphology of PTB7/PC(71)BM is composed of nanoscale fullerene-rich aggregates separated by polymer-rich regions. The addition of diiodooctane (DIO) to the casting solvent, as a processing additive, results in smaller domains and a more finely interpenetrating BHJ morphology, relative to blend films cast without DIO.
authors.affiliation: National Institute of Standards and Technology, Gaithersburg, Maryland 20899, United States.
authors.forename: Matthew R
authors.initials: MR
authors.lastname: Hammond
authors.forename: R Joseph
authors.initials: RJ
authors.lastname: Kline
authors.forename: Andrew A
authors.initials: AA
authors.lastname: Herzing
authors.forename: Lee J
authors.initials: LJ
authors.lastname: Richter
authors.forename: David S
authors.initials: DS
authors.lastname: Germack
authors.forename: Hyun-Wook
authors.initials: HW
authors.lastname: Ro
authors.forename: Christopher L
authors.initials: CL
authors.lastname: Soles
authors.forename: Daniel A
authors.initials: DA
authors.lastname: Fischer
authors.forename: Tao
authors.initials: T
authors.lastname: Xu
authors.forename: Luping
authors.initials: L
authors.lastname: Yu
authors.forename: Michael F
authors.initials: MF
authors.lastname: Toney
authors.forename: Dean M
authors.initials: DM
authors.lastname: Delongchamp