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Yu et al. Energy Mater. 2026, 6, 600044 Page 7 of 14

suggest that phosphonic acid groups likely consume reactive sites on the ITO surface, thereby contributing to
the formation of a more stable interface. Direct evidence for this interfacial bonding is provided by the P 2p
spectra [Figure 2C]. Specifically, for P4PACz, the P 2p and P 2p doublet is observed at 133.85 and
1/2
3/2
134.75 eV, respectively. Compared with the corresponding peaks of 4PACz (134.05 and 135.25 eV), this
represents a systematic negative shift of 0.2-0.5 eV, indicating an increase in electron density around the
phosphorus atom in P4PACz upon coordination with the ITO surface. Furthermore, the spectral weight of
the lower-binding-energy component (133.85 eV) is more pronounced in P-4PACz, which may reflect an
altered distribution of phosphonate binding motifs induced by the asymmetric molecular structure.
Furthermore, the In 3d spectra [Figure 2D] reveal that the In 3d and In 3d peaks of bare ITO are located
2/3
5/2
at 451.725 and 444.225 eV. Upon SAM deposition, these peaks shift to 452.350 and 444.850 eV for 4PACz,
and further shift to 452.725 and 445.225 eV for P4PACz. The observed binding energy shifts primarily in the
O 1s, P 2p, and In 3d regions collectively indicate an enhanced chemical coupling at the SAM/ITO interface.
Notably, the negative shift in the P 2p spectrum suggests increased electron density around the phosphorus
atom, consistent with stronger bonding between the phosphonate groups and the ITO substrate.
Concurrently, analysis of the C 1s and N 1s spectra [Supplementary Figure 12] reveals that the area
[37]
proportion of the C-P component increased from 19% to 23%. This change reflects an altered electronic
environment around the phosphorus center, which is consistent with the P 2p shift and further supports the
formation of stronger interfacial interactions.

To further investigate the optoelectronic properties of asymmetric SAM, KPFM measurements were
conducted. As shown in Figure 3A and B, ITO/P4PACz exhibits the highest surface potential. The work
functions (WFs) of the functionalized ITO substrates were characterized by UPS [Figure 3C and
Supplementary Figure 13] . The measured WFs for ITO/P4PACz and ITO/4PACz are -5.13 and -5.01 eV,
[38]
respectively, which aligns with the trend observed in KPFM. The WFs of bare ITO and PEDOT:PSS were
determined to be -4.56 and -4.95 eV, respectively, indicating that P4PACz further deepens the WF compared
to both 4PACz and PEDOT:PSS [39,40] . This leads to improved energy-level alignment across the interface,
thereby enhancing hole extraction and promoting more efficient charge carrier transport.

Local conductivity was characterized by C-AFM under a low bias of 8.7 mV. The average current measured
on ITO/P4PACz (4.44 nA) was significantly higher than that on ITO/4PACz (3.77 nA) [Figure 3D and E],
suggesting enhanced local charge transport. Considering the smoother morphology observed earlier, this
improvement likely reflects that the more uniform and continuous monolayer formed by P-4PACz helps to
minimize defects that would otherwise impede current flow. Furthermore, electrical characterization based
on sandwich-structured devices showed that the conductivities of 4PACz, P4PACz, and PEDOT:PSS are
1.969 × 10 , 2.188 × 10 , and 1.364 × 10 S·cm , respectively [Figure 3F], consistent with the improved
-1
-5
-5
-5
current characteristics observed in device performance.
Binary OSCs with the structure ITO/HTLs/PM6:BTPeC9/PNDIT-F3N/Ag were fabricated, with the device
architecture and energy level alignment illustrated in Figure 4A and B, respectively .
[41]

The SAM modification process and optimization conditions for the ITO substrates are detailed in
Supplementary Table 2, and the J-V curves of the best-performing devices are shown in Figure 4C. The
champion device based on 4PACz achieved a PCE of 18.28%, with a J of 27.42 mA cm , an V of 0.865 V,
-2
SC
OC
and a fill factor (FF) of 76.54%. In comparison, the champion device employing the optimized P-4PACz as
the hole transport layer attained a higher PCE of 19.03%, along with improved J (27.87 mA cm ) and FF
-2
SC
(78.86%), based on these values, we find that the improvement in FF from 76.54% to 78.86% contributes
dominantly to the overall PCE enhancement. According to the relationship PCE = V × J × FF, the increase
SC
OC
in FF accounts for approximately 65% of the total efficiency gain, whereas the contributions from V and J SC
OC