Journal
ACS NANO
Volume 8, Issue 1, Pages 323-331Publisher
AMER CHEMICAL SOC
DOI: 10.1021/nn404172m
Keywords
phase segregation; confinement; spectroscopic ellipsometry; polymer:fullerene bulk heterojunction; organic photovoltaics; thin films
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Funding
- U.S. Department of Energy, Office of Basic Energy Sciences [DE-AC02-98CH10886]
- U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences [DE-AC02-98CH10886]
- U.S. Department of Energy, Sustainable Energy Technologies Department [DE-AC02-98CH10886]
- Materials Sciences and Engineering Division [DE-AC02-98CH10886]
- Laboratory Research and Development Initiative at Brookhaven National Laboratory
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The effects of thin-film confinement on the material properties of ultrathin polymer (electron donor):fullerene (electron acceptor) bulk heterojunction films can be important for both fundamental understanding and device applications such as thin-film photovoltaics. We use variable angle spectroscopic ellipsometry and near edge X-ray absorption fine structure spectroscopy to measure the optical constants, donor acceptor volume fraction profile, and the degree of interchain order as a function of the thickness of a poly(3-hexythlophene-2,5-diyl) and phenyl-C61-butyric acid methyl ester bulk heterojunction film. We find that as the thickness of the bulk heterojunction film is decreased from 200 nm to the thickness confinement regime (less than 20 nm), the vertical phase segregation gradient of the donor and acceptor phases becomes less pronounced. In addition, observing the change in exciton bandwidth and the shift of absorption resonances (0-0 and 0-1) relative to neat donor and acceptor films, we find that the conjugation length and disorder in ultrathin films (20 nm) are less affected than thicker (200 nm) films by the addition of fullerene into the polymer. We believe that these findings could be important for discovering methods of precisely controlling the properties of bulk heterojunction films with crucial implications for designing more efficient organic-based photovoltaics.
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