title: Determining the optimum morphology in high-performance polymer-fullerene organic photovoltaic cells.
abstract: The morphology of bulk heterojunction organic photovoltaic cells controls many of the performance characteristics of devices. However, measuring this morphology is challenging because of the small length-scales and low contrast between organic materials. Here we use nanoscale photocurrent mapping, ultrafast fluorescence and exciton diffusion to observe the detailed morphology of a high-performance blend of PTB7:PC71BM. We show that optimized blends consist of elongated fullerene-rich and polymer-rich fibre-like domains, which are 10-50 nm wide and 200-400 nm long. These elongated domains provide a concentration gradient for directional charge diffusion that helps in the extraction of charge pairs with 80% efficiency. In contrast, blends with agglomerated fullerene domains show a much lower efficiency of charge extraction of ~45%, which is attributed to poor electron and hole transport. Our results show that the formation of narrow and elongated domains is desirable for efficient bulk heterojunction solar cells.
authors.affiliation: Organic Semiconductor Centre, SUPA, School of Physics and Astronomy, University of St Andrews, North Haugh, St Andrews, Fife KY16 9SS, UK.
authors.forename: Gordon J
authors.initials: GJ
authors.lastname: Hedley
authors.affiliation: Organic Semiconductor Centre, SUPA, School of Physics and Astronomy, University of St Andrews, North Haugh, St Andrews, Fife KY16 9SS, UK.
authors.forename: Alexander J
authors.initials: AJ
authors.lastname: Ward
authors.affiliation: Materials and Condensed Matter Physics, SUPA, School of Physics and Astronomy, University of Glasgow, Glasgow G12 8QQ, UK.
authors.forename: Alexander
authors.initials: A
authors.lastname: Alekseev
authors.affiliation: Organic Semiconductor Centre, SUPA, School of Physics and Astronomy, University of St Andrews, North Haugh, St Andrews, Fife KY16 9SS, UK.
authors.forename: Calvyn T
authors.initials: CT
authors.lastname: Howells
authors.affiliation: Organic Semiconductor Centre, SUPA, School of Physics and Astronomy, University of St Andrews, North Haugh, St Andrews, Fife KY16 9SS, UK.
authors.forename: Emiliano R
authors.initials: ER
authors.lastname: Martins
authors.affiliation: Glasgow Centre for Physical Organic Chemistry, WestCHEM, School of Chemistry, University of Glasgow, Glasgow G12 8QQ, UK.
authors.forename: Luis A
authors.initials: LA
authors.lastname: Serrano
authors.affiliation: Glasgow Centre for Physical Organic Chemistry, WestCHEM, School of Chemistry, University of Glasgow, Glasgow G12 8QQ, UK.
authors.forename: Graeme
authors.initials: G
authors.lastname: Cooke
authors.affiliation: Organic Semiconductor Centre, SUPA, School of Physics and Astronomy, University of St Andrews, North Haugh, St Andrews, Fife KY16 9SS, UK.
authors.forename: Arvydas
authors.initials: A
authors.lastname: Ruseckas
authors.affiliation: Organic Semiconductor Centre, SUPA, School of Physics and Astronomy, University of St Andrews, North Haugh, St Andrews, Fife KY16 9SS, UK.
authors.forename: Ifor D W
authors.initials: ID
authors.lastname: Samuel