title: Understanding and controlling morphology evolution via DIO plasticization in PffBT4T-2OD/PC71BM devices.
abstract: We demonstrate that the inclusion of a small amount of the co-solvent 1,8-diiodooctane in the preparation of a bulk-heterojunction photovoltaic device increases its power conversion efficiency by 20%, through a mechanism of transient plasticisation. We follow the removal of 1,8-diiodooctane directly after spin-coating using ellipsometry and ion beam analysis, while using small angle neutron scattering to characterise the morphological nanostructure evolution of the film. In PffBT4T-2OD/PC71BM devices, the power conversion efficiency increases from 7.2% to above 8.7% as a result of the coarsening of the phase domains. This coarsening process is assisted by thermal annealing and the slow evaporation of 1,8-diiodooctane, which we suggest, acts as a plasticiser to promote molecular mobility. Our results show that 1,8-diiodooctane can be completely removed from the film by a thermal annealing process at temperatures ≤100 °C and that there is an interplay between the evaporation rate of 1,8-diiodooctane and the rate of domain coarsening in the plasticized film which helps elucidate the mechanism by which additives improve device efficiency.
authors.affiliation: Department of Physics and Astronomy, The University of Sheffield, S3 7RH, UK.
authors.forename: Yiwei
authors.initials: Y
authors.lastname: Zhang
authors.affiliation: Department of Physics and Astronomy, The University of Sheffield, S3 7RH, UK.
authors.forename: Andrew J
authors.initials: AJ
authors.lastname: Parnell
authors.affiliation: Department of Physics and Astronomy, The University of Sheffield, S3 7RH, UK.
authors.forename: Fabio
authors.initials: F
authors.lastname: Pontecchiani
authors.affiliation: ISIS Pulsed Neutron and Muon Source, STFC, Rutherford Appleton Laboratory, Harwell Campus, Oxon, OX11 0QX, UK.
authors.forename: Joshaniel F K
authors.initials: JF
authors.lastname: Cooper
authors.affiliation: Department of Chemistry, Durham University, South Road DH1 3LE, UK.
authors.forename: Richard L
authors.initials: RL
authors.lastname: Thompson
authors.affiliation: Department of Physics and Astronomy, The University of Sheffield, S3 7RH, UK.
authors.forename: Richard A L
authors.initials: RA
authors.lastname: Jones
authors.affiliation: ISIS Pulsed Neutron and Muon Source, STFC, Rutherford Appleton Laboratory, Harwell Campus, Oxon, OX11 0QX, UK.
authors.forename: Stephen M
authors.initials: SM
authors.lastname: King
authors.affiliation: Department of Physics and Astronomy, The University of 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, S3 7RH, UK.
authors.forename: Gabriel
authors.initials: G
authors.lastname: Bernardo