Unraveling the morphology of high efficiency polymer solar cells based on the donor polymer PBDTTT-EFT

Wenchao Huang, Eliot Gann, Lars Thomsen, Cunku Dong, Yi-Bing Cheng, Christopher R. McNeill

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The microstructure of the polymer PBDTTT-EFT and blends with the fullerene
derivative PC 71 BM that achieve solar conversion effi ciencies of over 9% is
comprehensively investigated. A combination of synchrotron techniques are
employed including surface-sensitive near-edge X-ray absorption fi ne structure
(NEXAFS) spectroscopy and bulk-sensitive grazing-incidence wide angle
X-ray scattering (GIWAXS). A preferential “face-on” orientation of PBDTTTEFT
is observed in the bulk of both pristine and blend thin fi lms, with π–π
stacking largely normal to the substrate, which is thought to be benefi cial for
charge transport. At the surface of the blend, a slight “edge-on” structure of
the polymer is observed with side-chains aligned normal to the substrate. The
effect of the solvent additive 1,8-diiodooctane (DIO) on solar cell effi ciency
and fi lm microstructure is also investigated, where the addition of 3 vol%
DIO results in an effi ciency increase from ≈6.4% to ≈9.5%. GIWAXS studies
indicate that the addition of DIO improves the crystallization of the polymer.
Furthermore, atomic force microscopy and transmission electron microscopy
are employed to image surface and bulk morphology revealing that DIO suppresses
the formation of large PC 71 BM aggregates.
Original languageEnglish
Pages (from-to)1 - 11
Number of pages11
JournalAdvanced Energy Materials
Issue number7
Publication statusPublished - 2015

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