title: Morphology Control in Films of Isoindigo Polymers by Side-Chain and Molecular Weight Effects.
abstract: The performance of devices relying on organic electronic materials, such as organic field-effect transistors (OFET) and organic photovoltaics (OPV), is strongly correlated to the morphology of the conjugated material in thin films. For instance, several factors such as polymer solubility, weak intermolecular forces between polymers and fullerene derivatives, and film drying time impact phase separation in the active layer of a bulk heterojunction OPV device. In an effort to probe the influence of polymer assembly on morphology of polymer thin films and phase separation with fullerene derivatives, five terthiophene-alt-isoindigo copolymers were synthesized with alkyl side-chains of varying lengths and branching on the terthiophene unit. These P[T3(R)-iI] polymers were designed to have similar optoelectronic properties but different solubilities in o-dichlorobenzene and were predicted to have different tendencies for crystallization. All polymers with linear alkyl chains exhibit similar thin film morphologies as investigated by grazing-incidence wide-angle X-ray scattering (GIWAXS) and atomic force microscopy (AFM). The main differences in electronic and morphological properties arise when P[T3(R)-iI] is substituted with branched 2-ethylhexyl (2EH) side-chains. The bulky 2EH substituents lead to a blue-shifted absorption, a lower ionization potential, and reduced ordering in polymer thin films. The five P[T3-iI] derivatives span hole mobilities from 1.5 × 10-3to 2.8 × 10-2cm2V-1s-1in OFET devices. In OPV devices, the 2EH-substituted polymers yield open-circuit voltages of 0.88 V in BHJ devices yet low short-circuit currents of 0.8 mA cm-2, which is explained by the large phase separation observed by AFM in blends of P[T3(2EH)-iI] with PC71BM. In these P[T3(R)-iI] systems, the propensity for the polymers to self-assemble prior to aggregation of PC71BM molecules was key to achieving fine phase separation and increased short-circuit currents, eventually resulting in power conversion efficiencies of 5% in devices processed using a single solvent.
authors.forename: Caroline
authors.initials: C
authors.lastname: Grand
authors.affiliation: Max Planck Institute for Polymer Research , Ackermannweg 10, 55128 Mainz, Germany.
authors.forename: Wojciech
authors.initials: W
authors.lastname: Zajaczkowski
authors.forename: Nabankur
authors.initials: N
authors.lastname: Deb
authors.forename: Chi Kin
authors.initials: CK
authors.lastname: Lo
authors.forename: Jeff L
authors.initials: JL
authors.lastname: Hernandez
authors.forename: David G
authors.identifier: http://orcid.org/0000-0003-4558-6933
authors.initials: DG
authors.lastname: Bucknall
authors.affiliation: Max Planck Institute for Polymer Research , Ackermannweg 10, 55128 Mainz, Germany.
authors.forename: Klaus
authors.identifier: http://orcid.org/0000-0001-6630-8786
authors.initials: K
authors.lastname: Müllen
authors.affiliation: Max Planck Institute for Polymer Research , Ackermannweg 10, 55128 Mainz, Germany.|Department of Molecular Physics, Faculty of Chemistry, Lodz University of Technology , Zeromskiego 116, 90-924 Lodz, Poland.
authors.forename: Wojciech
authors.initials: W
authors.lastname: Pisula
authors.forename: John R
authors.identifier: http://orcid.org/0000-0002-7417-4869
authors.initials: JR
authors.lastname: Reynolds
keywords: bulk heterojunction solar cells; donor−acceptor polymers; isoindigo; organic field-effect transistors; organic photovoltaics; polymer fullerene blends; thin film morphology