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Surface Layering and Supersaturation for Top‐Down Nanostructural Development during Spin Coating of Polymer/Fullerene Thin Films
Author(s) -
Wu WeiRu,
Su ChunJen,
Chuang WeiTsung,
Huang YenChih,
Yang PoWei,
Lin PoChang,
Chen ChunYu,
Yang TsungYu,
Su AnChung,
Wei KungHwa,
Liu ChihMing,
Jeng USer
Publication year - 2017
Publication title -
advanced energy materials
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 10.08
H-Index - 220
eISSN - 1614-6840
pISSN - 1614-6832
DOI - 10.1002/aenm.201601842
Subject(s) - materials science , supersaturation , crystallization , chemical engineering , spin coating , polymer , evaporation , crystallization of polymers , fullerene , thin film , coating , chemical physics , nanotechnology , composite material , organic chemistry , chemistry , physics , engineering , thermodynamics
This study provides new evidence on a long postulated mechanism of phase separation in a polymer/fullerene mixture during spin coating for controlled nanodomains of oriented crystallization and heterojunctions that favor applications in polymer solar cells (PSCs). The simultaneous nanoscale phase separation and crystallization during spin coating of the mixture are traced using in situ grazing‐incidence small‐ and wide‐angle X‐ray scattering. Combined with the complimentary results from time‐resolved optical reflectance spectroscopy, transient stratification of the liquid film during the transition from the flow‐ to evaporation‐dominated stage of spin coating is disclosed; the vertical liquid–liquid phase separation incubates a supersaturated skin layer where fullerene aggregation and polymer crystallization occur and develop concomitantly. Shortly after the transition, the near‐surface structural development is largely pinned, leaving the solvent‐rich bottom layer to diminish via solvent diffusion and evaporation through the thickened skin layer that finally condenses into the spin‐coated film upon solvent depletion. The shear‐enhanced surface layering and supersaturation for the surface‐down nanostructural development are unexpected in all the existing structural models for PSCs. The mechanistic understanding of coupled vertical phase separation and local nanosegregation provides new insights and alternative strategy to the morphology control of spin‐cast PSC active layers in particular and various solution‐processed polymeric films in general.