Issue 19, 2011

Surface and interface porosity of polymer/fullerene-derivative thin films revealed by contrast variation of neutron and X-ray reflectivity

Abstract

Contrast variation of neutron and X-ray reflections has been adapted to reveal the film in-depth (vertical) composition profiles of the blend of poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) for bulk heterojunction thin-film solar cells, with a PCBM/P3HT weight ratio of c = 0.6, 0.8 and 1.0. The X-ray scattering-length-density (SLD) profiles, extracted from X-ray reflectivity for the blend films spun-cast on Si wafer, exhibit a stratified film morphology of ca. 85 nm film thickness; the corresponding neutron SLD profiles extracted for the same films further elucidate a PCBM-enriched interfacial layer adjacent to the Si substrate. In contrast to the often assumed two-phase model, a three-phase model with porosity included as the third phase has to be used in deducing the absolute volume fractions of PCBM and P3HT from the complementary neutron and X-ray SLD profiles. In general, the thus deduced in-depth composition profiles for the blend films comprise a substantial surface layer (10–15 nm) of ca. 40% porosity, a 50 nm main layer with relatively uniform PCBM–P3HT composition, and a PCBM-enriched interface layer (∼20 nm) with ∼15% porosity. Formation of the surface porosity is related to interfacial instability occurred in a transient surface layer upon film drying. Annealing at 150 °C influences modestly the vertical phase separation of the film, but drastically activates local phase separation for formation and growth of PCBM and P3HT nanodomains, as revealed by grazing incidence small/wide angle X-ray scattering. The surface/interface porosity features (overlooked in nearly all the previous studies) and the composition-dependent vertical phase separation bear hints in advancing device performance via interfacial morphology optimization.

Graphical abstract: Surface and interface porosity of polymer/fullerene-derivative thin films revealed by contrast variation of neutron and X-ray reflectivity

Supplementary files

Article information

Article type
Paper
Submitted
30 May 2011
Accepted
25 Jul 2011
First published
19 Aug 2011

Soft Matter, 2011,7, 9276-9282

Surface and interface porosity of polymer/fullerene-derivative thin films revealed by contrast variation of neutron and X-ray reflectivity

H. Liu, U. Jeng, N. L. Yamada, A. Su, W. Wu, C. Su, S. Lin, K. Wei and M. Chiu, Soft Matter, 2011, 7, 9276 DOI: 10.1039/C1SM06005H

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