Morphology Control in Films of Isoindigo Polymers by Side-Chain and Molecular Weight Effects

Caroline Grand, Wojciech Zajaczkowski, Nabankur Deb, Chi Kin Lo, Jeff L. Hernandez, David Bucknall, Klaus Mullen, Wojciech Pisula, John Reynolds

Research output: Contribution to journalArticle

11 Citations (Scopus)
24 Downloads (Pure)

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 × 10–3 to 2.8 × 10–2 cm2 V–1 s–1 in 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.
Original languageEnglish
Pages (from-to)13357-13368
Number of pages12
JournalACS Applied Materials and Interfaces
Volume9
Issue number15
Early online date5 Apr 2017
DOIs
Publication statusPublished - 19 Apr 2017

Keywords

  • bulk heterojunction solar cells
  • donor−acceptor polymers
  • isoindigo
  • organic field-effect transistors
  • organic photovoltaics
  • polymer fullerene blends
  • thin film morphology

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