Figure 4. The AFM height images (2 μm ×2 μm) of a) PBDB-T:
BZ4F-O-1; b) PBDB-T: BZO-4Cl; c) PBDB-T: BZ4F-O-1: BZO-4Cl; The AFM
phase images (2 μm ×2 μm) of d) PBDB-T: BZ4F-O-1; e) PBDB-T: BZO-4Cl; f)
PBDB-T: BZ4F-O-1: BZO-4Cl; The TEM images of g) PBDB-T: BZ4F-O-1; h)
PBDB-T: BZO-4Cl; i) PBDB-T: BZ4F-O-1: BZO-4Cl
To further investigate the intermolecular stacking behavior of the neat
and blend films, 2D-grazing incidence wide-angle X-ray scattering
(GIWAXS) patterns and the corresponding 1D profiles are fully analyzed.
As shown in Figures 5, S10 and Table S7, all the neat films of the
acceptor materials show a strong (010) peak at ~1.8
Å-1 in the out-of-plane (OOP) direction and (100) peak
at ~0.4 Å-1 in the in-plane (IP)
direction, which indicates a predominant face-on orientation. The (010)
peak is further studied to understand the π-π stacking properties.
BZ4F-O-1 and BZO-4Cl show the π-π stacking distance (d-spacing) of 3.50
and 3.42 Å with the crystal coherence lengths (CCLs) of 22.5 and 27.0 Å,
respectively. BZO-4Cl presents a shorter d -spacing and longer CCL
than that of BZ4F-O-1, thus a closer π-π stacking, which could be
ascribed to the stronger intermolecular interaction. Compared with the
neat films, the binary blends retain the favorable face-on orientation.
PBDB-T: BZO-4Cl blend exhibits the closest d -spacing among the
binary devices with the longest CCLs, which is favorable for charge
transport and consistent with the higher J SC.
When BZ4F-O-1 is introduced into the PBDB-T: BZO-4Cl system, thed -spacing of the ternary blend is slightly increased and the CCL
is meanwhile reduced, indicating the slightly reduced crystallinity.
Combined with the suppressed recombination process and the improved
charge transport (Figure 3), we can conclude that a proper (not that
close) molecular π-π stacking is achieved in the ternary blends due to
the introduction of BZ4F-O-1.