Accurate determination of the index of refraction of polymer blend films by spectroscopic ellipsometry

Annie Ng, Chi Ho Li, Man Kin Fung, Aleksandra B. Djuriŝić, J. A. Zapien, Wai Kin Chan, Kai Yin Cheung, Wai Yeung WONG

Research output: Contribution to journalJournal articlepeer-review

33 Citations (Scopus)


To model the performance of a bulk-heterojunction solar cell, it is necessary to obtain information about the index of refraction of the blend layer, which is typically determined by spectroscopic ellipsometry measurements. The optical functions of poly(3-hexylthiophene)-[6,6]-phenyl C 61-butyric acid methyl ester (P3HT-PCBM) blend films have been extensively studied. However, there is a large variation of the reported optical functions in the literature. Because of this fact, as well as the widespread use of P3HT-PCBM films in organic photovoltaics, we have selected this material system as an example and performed a detailed analysis of spectroscopic ellipsometry data. We illustrate the occurrence of multiple solutions and the importance of a dedicated methodology to reach a satisfactory unique solution. The proposed methodology involves the following steps: (1) multisample analysis; (2) independent thickness and surface characterization; (3) use of the adequate optical description of substrate; (4) thickness estimation from transparent range using Cauchy model; (5) fitting n and k in the entire range with fixed thickness; verify result is physically meaningful; (6) optimization of the parameters to be fitted; (7) repeating steps 5 and 6 with and without EMA layer to account for the surface roughness; (8) finally, and only if no satisfactory fit could be obtained from previous steps, attempts to introduce anisotropy, graded layers, or other nonideal models should follow.

Original languageEnglish
Pages (from-to)15094-15101
Number of pages8
JournalJournal of Physical Chemistry C
Issue number35
Publication statusPublished - 9 Sept 2010

Scopus Subject Areas

  • Electronic, Optical and Magnetic Materials
  • Energy(all)
  • Physical and Theoretical Chemistry
  • Surfaces, Coatings and Films


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