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Page 6 of 14 Yu et al. Energy Mater. 2026, 6, 600044

To understand the origin of the improved film morphology of P-4PACz, we first investigated its intrinsic
molecular packing and solution-state behavior. Single-crystal structures of the π-scaffolds and full molecules
were analyzed. For the pure π-scaffolds, pristine carbazole exhibits a vertical stacking distance of 3.37 Å and a
longitudinal slip of 5.73 Å [Supplementary Figure 4A]. In contrast, the 3-phenyl-substituted carbazole shows
a substantially shortened vertical distance of 2.47 Å but an increased slip of 6.46 Å
[Supplementary Figure 4B], reflecting a shift in the optimal stacking geometry induced by the phenyl
substituent. Notably, when the full molecules (including alkyl chains and phosphonic acid groups) are
considered, P-4PACz still exhibits a much shorter vertical distance (2.73 Å) compared to 4PACz (3.29 Å)
[Supplementary Figure 4C and D], while the slip distances are approximately 6.02 and 5.96 Å, indicating that
the π-π stacking is reasonably moderated. This unique stacking mode also influences the solution-state
behavior and the film formation of the SAM. As shown in Supplementary Figure 5, at identical concentration
(0.5 mg/mL), the 4PACz solution in ethanol exhibits visible turbidity with undissolved particulates, whereas
the P-4PACz solution is completely transparent and clear. This difference is also evident in chloroform, a
nonpolar solvent, indicating that the superior solubility of P-4PACz is an intrinsic property of its asymmetric
molecular structure rather than being solvent-specific. The reduced aggregation tendency in solutions arising
from the weakened intermolecular packing in the solid state.

The surface morphologies of bare ITO, ITO/SAMs, and ITO/PEDOT:PSS were characterized by AFM. As
shown in Supplementary Figure 6A-D, the ITO/P4PACz exhibits a reduced arithmetic average roughness
(Ra = 4.53 nm) compared to bare ITO (Ra = 4.81 nm) and ITO/4PACz (Ra = 4.67 nm), indicating a
smoothing effect that is visually supported by the corresponding height profile in Supplementary Figure 6E.
In contrast, the SAM-modified surfaces display Ra values similar to that of pristine ITO, which can be
attributed to their ultrathin monolayer structure. In contrast, the PEDOT:PSS exhibits a significantly lower
Ra value (Ra = 1.89 nm), reflecting its thicker polymeric film that effectively planarizes the underlying ITO
surface . Notably, the smoother substrate induced by P4PACz translates into improved activelayer
[36]
morphology. As shown in Supplementary Figure 7, the PM6:BTPeC9 blend deposited on ITO/P4PACz
exhibits a lower surface roughness (Ra = 1.82 nm) compared to that on ITO/4PACz (Ra = 1.99 nm), and
approaches that of the PEDOT:PSS reference (Ra = 1.78 nm). This suggests that the P4PACz interlayer not
only smooths the ITO surface but also promotes the formation of a more uniform and homogeneous bulk
heterojunction film. Surface energy plays a critical role in determining both the efficiency and long-term
stability of organic solar cells. To quantify the interfacial wettability, contact angle measurements were
conducted using water (H O) and formamide (FA) as probe liquids. The water contact angles of
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ITO/P-4PACz and ITO/4PACz were measured to be 86° and 80°, respectively, significantly higher than that
of bare ITO (46°) [Supplementary Figure 8 and 9], indicating enhanced hydrophobicity upon SAM
modification. Using the Owens-Wendt-Rabel-Kaelble method, the calculated surface energies of
ITO/PEDOT:PSS, ITO/4PACz, and ITO/P-4PACz are 76.7, 40.8, and 38.0 mN m , respectively, showing a
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progressive decrease. Notably, the surface energy of ITO/P-4PACz is closer to that of the active layer
PM6:BTP-eC9 (28.9 mN m ) [Supplementary Table 1], suggesting improved interfacial compatibility. In
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addition to increased hydrophobicity, this optimized surface energy alignment facilitates better film
formation of the active layer, thereby contributing to enhanced device performance.

The formation of SAMs on ITO substrates was confirmed by XPS. The fullspectrum scans of 4PACz and
P4PACz [Supplementary Figure 10] showed marked changes in peak intensities relative to bare ITO,
indicating successful SAM coverage. Highresolution O 1s corelevel spectra further revealed systematic
variations [Figure 2B and Supplementary Figure 11]: the relative area of the component around 531.5 eV,
commonly assigned to surface hydroxyl groups or oxygendeficient sites, decreased progressively from 0.95
(bare ITO) to 0.85 (ITO/4PACz) and to 0.65 (ITO/P4PACz). Moreover, the main O 1s peak for ITO/P4PACz
exhibited a noticeable positive bindingenergy shift compared to both bare ITO and ITO/4PACz. The changes