title: Molecular weight dependent vertical composition profiles of PCDTBT:PC₇₁BM blends for organic photovoltaics.
abstract: We have used Soxhlet solvent purification to fractionate a broad molecular weight distribution of the polycarbazole polymer PCDTBT into three lower polydispersity molecular weight fractions. Organic photovoltaic devices were made using a blend of the fullerene acceptor PC₇₁BM with the molecular weight fractions. An average power conversion efficiency of 5.89% (peak efficiency of 6.15%) was measured for PCDTBT blend devices with a number average molecular weight of Mn = 25.5 kDa. There was significant variation between the molecular weight fractions with low (Mn = 15.0 kDa) and high (Mn = 34.9 kDa) fractions producing devices with average efficiencies of 5.02% and 3.70% respectively. Neutron reflectivity measurements on these polymer:PC₇₁BM blend layers showed that larger molecular weights leads to an increase in the polymer enrichment layer thickness at the anode interface, this improves efficiency up to a limiting point where the polymer solubility causes a reduction of the PCDTBT concentration in the active layer.
authors.affiliation: Ossila Ltd, Kroto Innovation Centre, Broad Lane, Sheffield, S3 7HQ, UK.
authors.forename: James W
authors.initials: JW
authors.lastname: Kingsley
authors.affiliation: Ossila Ltd, Kroto Innovation Centre, Broad Lane, Sheffield, S3 7HQ, UK.
authors.forename: Pier Paolo
authors.initials: PP
authors.lastname: Marchisio
authors.affiliation: Department of Chemistry, The University of Sheffield, Sheffield, S3 7HF, UK.
authors.forename: Hunan
authors.initials: H
authors.lastname: Yi
authors.affiliation: Department of Chemistry, The University of Sheffield, Sheffield, S3 7HF, UK.
authors.forename: Ahmed
authors.initials: A
authors.lastname: Iraqi
authors.affiliation: ISIS Pulsed Neutron and Muon Source, Science and Technology Facilities Council, Rutherford Appleton Laboratory, Harwell Science and Innovation Campus, Didcot OX11 0QX, UK.
authors.forename: Christy J
authors.initials: CJ
authors.lastname: Kinane
authors.affiliation: ISIS Pulsed Neutron and Muon Source, Science and Technology Facilities Council, Rutherford Appleton Laboratory, Harwell Science and Innovation Campus, Didcot OX11 0QX, UK.
authors.forename: Sean
authors.initials: S
authors.lastname: Langridge
authors.affiliation: Department of Chemistry, Durham, DH1 3LE, UK.
authors.forename: Richard L
authors.initials: RL
authors.lastname: Thompson
authors.affiliation: Department of Physics and Astronomy, The University of Sheffield, Hicks Building, Hounsfield Road, Sheffield, S3 7RH, UK.
authors.forename: Ashley J
authors.initials: AJ
authors.lastname: Cadby
authors.affiliation: 1] Department of Physics and Astronomy, The University of Sheffield, Hicks Building, Hounsfield Road, Sheffield, S3 7RH, UK [2].
authors.forename: Andrew J
authors.initials: AJ
authors.lastname: Pearson
authors.affiliation: Department of Physics and Astronomy, The University of Sheffield, Hicks Building, Hounsfield Road, Sheffield, S3 7RH, UK.
authors.forename: David G
authors.initials: DG
authors.lastname: Lidzey
authors.affiliation: Department of Physics and Astronomy, The University of Sheffield, Hicks Building, Hounsfield Road, Sheffield, S3 7RH, UK.
authors.forename: Richard A L
authors.initials: RA
authors.lastname: Jones
authors.affiliation: Department of Physics and Astronomy, The University of Sheffield, Hicks Building, Hounsfield Road, Sheffield, S3 7RH, UK.
authors.forename: Andrew J
authors.initials: AJ
authors.lastname: Parnell