The surface and bulk morphologies of blend films were investigated by
atomic force microscopy (AFM) and transmission electron microscopy
(TEM). As shown in Figure 4a, the J52:NoCA-19 blend film exhibits a
uniform surface with a higher root-mean-square
(R q) surface roughness of 1.57 nm compared to the
J52:NoCA-17 (1.09 nm) and J52:NoCA-18 (1.43 nm) blend films.
Furthermore, Figure 4b shows that
the J52:NoCA-19 blend film exhibits smaller domain sizes and nano-scale
phase separation. To provide further insight into the molecular packing
in the blend films, 2D-GIWAXS was employed (Figures 4c and S5). The
NoCA-17-based blend film demonstrates overlapping lamellar packing
diffraction peaks with the polymer J52. Interestingly, the lamellar
packing diffraction peaks of the acceptors in the NoCA-18 and
NoCA-19-based blend films remain clear, indicating that the order
packings of NoCA-18 and NoCA-19 were not vulnerable to be disturbed when
blending with J52. This features in film morphology are beneficial for
charge transport and thus device performance.
In conclusion, the asymmetric end-group engineering was employed to
construct a new NFREA, namely NoCA-19 containing two distinct
end-groups, IC-2Cl and NC-2F. Together with two symmetrical NFREAs
(NoCA-17 and NoCA-18), experimental and theoretical studies were
systematically carried out, showing that asymmetric acceptor NoCA-19
possesses broad light absorption range, more coplanarπ -conjugated backbone, and appropriate crystallinity. The
NoCA-19-based device delivered a champion PCE of 12.26%, mainly due to
the best and most balanced carrier mobility, least charge recombination,
shortest charge extraction time, and most favorable morphology among the
three blend systems. This work sheds light on the potential of
asymmetric end-group engineering in designing low-cost and
high-performance NFREAs.
The supporting information for this article is available on the WWW
under https://doi.org/10.1002/cjoc.2023xxxxx.
B. Liu and C. Li contributed equally to this work. The authors
acknowledge the financial support from the NSFC (2197505, U2001222,
52103352, 52120105006, and 51925306), National Key R&D Program of China
(2018FYA 0305800), Key Research Program of Chinese Academy of Sciences
(XDPB08-2), the Youth Innovation Promotion Association of Chinese
Academy of Sciences (2022165), and the Fundamental Research Funds for
the Central Universities. DFT results described in this article were
obtained from the National Supercomputing Centre in Shenzhen (Shenzhen
Cloud Computing Centre).
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