title: Spectroscopic imaging of photopotentials and photoinduced potential fluctuations in a bulk heterojunction solar cell film.
abstract: We present spatially resolved photovoltage spectra of a bulk heterojunction solar cell film composed of phase-separated poly(9,9'-dioctylfluorene-co-benzothiadiazole) (F8BT) and poly(9,9'-dioctylfluorene-co-bis-N,N'-(4-butylphenyl)-bis-N,N'-phenyl-1,4-phenylenediamine) (PFB) polymers prepared on ITO/PEDOT:PSS and aluminum substrates. Over both PFB- and F8BT-rich domains, the photopotential spectra were found to be proportional to a linear combination of the polymers' absorption spectra. Charge trapping in the film was studied using photopotential fluctuation spectroscopy, in which low-frequency photoinduced electrostatic potential fluctuations were measured by observing noise in the oscillation frequency of a nearby charged atomic force microscope cantilever. Over both F8BT- and PFB-rich regions, the magnitude, distance dependence, frequency dependence, and illumination wavelength dependence of the observed cantilever frequency noise are consistent with photopotential fluctuations arising from stochastic light-driven trapping and detrapping of charges in F8BT. Taken together, our findings suggest a microscopic mechanism by which intermixing of phases leads to charge trapping and thereby to suppressed open-circuit voltage and decreased efficiency in this prototypical bulk heterojunction solar cell film.
authors.affiliation: Department of Chemistry and Chemical Biology, Cornell University, Ithaca, New York 14853, United States.
authors.forename: Justin L
authors.initials: JL
authors.lastname: Luria
authors.forename: Nikolas
authors.initials: N
authors.lastname: Hoepker
authors.forename: Robert
authors.initials: R
authors.lastname: Bruce
authors.forename: Andrew R
authors.initials: AR
authors.lastname: Jacobs
authors.forename: Chris
authors.initials: C
authors.lastname: Groves
authors.forename: John A
authors.initials: JA
authors.lastname: Marohn
mesh_terms: D004574:Electromagnetic Fields; D008027:Light; D008422:Materials Testing; D008567:Membranes, Artificial; D011137:Polystyrenes / Q000737:chemistry* / Q000528:radiation effects*; D012993:Solar Energy; D013057:Spectrum Analysis / Q000379:methods*; D013876:Thiophenes / Q000737:chemistry* / Q000528:radiation effects*
chemical_list: D008567:Membranes, Artificial; D011137:Polystyrenes; D013876:Thiophenes; C533756:poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate)