title: The impact of donor-acceptor phase separation on the charge carrier dynamics in pBTTT:PCBM photovoltaic blends.
abstract: The effect of donor-acceptor phase separation, controlled by the donor-acceptor mixing ratio, on the charge generation and recombination dynamics in pBTTT-C14:PC70 BM bulk heterojunction photovoltaic blends is presented. Transient absorption (TA) spectroscopy spanning the dynamic range from pico- to microseconds in the visible and near-infrared spectral regions reveals that in a 1:1 blend exciton dissociation is ultrafast; however, charges cannot entirely escape their mutual Coulomb attraction and thus predominantly recombine geminately on a sub-ns timescale. In contrast, a polymer:fullerene mixing ratio of 1:4 facilitates the formation of spatially separated, that is free, charges and reduces substantially the fraction of geminate charge recombination, in turn leading to much more efficient photovoltaic devices. This illustrates that spatially extended donor or acceptor domains are required for the separation of charges on an ultrafast timescale (<100 fs), indicating that they are not only important for efficient charge transport and extraction, but also critically influence the initial stages of free charge carrier formation.
authors: Max Planck Research Group for Organic Optoelectronics, Max Planck Institute for Polymer Research, Ackermannweg 10, D-55128, Mainz, Germany.
authors: Dominik W
authors: DW
authors: Gehrig
authors: Institute of Microstructure Technology (IMT), Karlsruhe Institute of Technology (KIT), Hermann-von-Helmholtz-Platz 1, D-76344, Eggenstein-Leopoldshafen, Germany.
authors: Ian A
authors: IA
authors: Howard
authors: Materials Science and Engineering, Stanford University, 476 Lomita Mall, Stanford, CA, 94305-4045, USA.
authors: Sean
authors: S
authors: Sweetnam
authors: Materials Science and Engineering, Stanford University, 476 Lomita Mall, Stanford, CA, 94305-4045, USA.
authors: Timothy M
authors: TM
authors: Burke
authors: Materials Science and Engineering, Stanford University, 476 Lomita Mall, Stanford, CA, 94305-4045, USA.
authors: Michael D
authors: MD
authors: McGehee
authors: Max Planck Research Group for Organic Optoelectronics, Max Planck Institute for Polymer Research, Ackermannweg 10, D-55128, Mainz, Germany.
authors: Frédéric
authors: F
authors: Laquai
mesh_terms: D004579:Electron Transport; D037741:Fullerenes / Q000737:chemistry*; D011108:Polymers / Q000737:chemistry*; D012993:Solar Energy
chemical_list: D037741:Fullerenes; D011108:Polymers
keywords: carrier dynamics; pBTTT; phase separation; polymer solar cells; transient absorption spectroscopy