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Influences of Non-fullerene Acceptor Fluorination on Three-Dimensional Morphology and Photovoltaic Properties of Organic Solar Cells
journal contribution
posted on 2019-06-26, 00:00 authored by Mengxue Chen, Dan Liu, Wei Li, Robert S. Gurney, Donghui Li, Jinlong Cai, Emma L. K. Spooner, Rachel C. Kilbride, James D. McGettrick, Trystan M. Watson, Zhe Li, Richard A. L. Jones, David G. Lidzey, Tao WangFluorination of conjugated
molecules has been established as an
effective structural modification strategy to influence properties
and has attracted extensive attention in organic solar cells (OSCs).
Here, we have investigated optoelectronic and photovoltaic property
changes of OSCs made of polymer donors with the non-fullerene acceptors
(NFAs) ITIC and IEICO and their fluorinated counterparts IT-4F and
IEICO-4F. Device studies show that fluorinated NFAs lead to reduced Voc but increased Jsc and fill-factor (FF), and therefore, the ultimate influence to efficiency
depends on the compensation of Voc loss
and gains of Jsc and FF. Fluorination
lowers energy levels of NFAs, reduces their electronic band gaps,
and red-shifts the absorption spectra. The impact of fluorination
on the molecular order depends on the specific NFA, and the conversion
of ITIC to IT-4F reduces the structural order, which can be reversed
after blending with the donor PBDB-T. Contrastingly, IEICO-4F presents
stronger π–π stacking after fluorination from IEICO,
and this is further strengthened after blending with the donor PTB7-Th.
The photovoltaic blends universally present a donor-rich surface region
which can promote charge transport and collection toward the anode
in inverted OSCs. The fluorination of NFAs, however, reduces the fraction
of donors in this donor-rich region, consequently encouraging the
intermixing of donor/acceptor for efficient charge generation.