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.