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Dong et al. Energy Mater. 2026, 6, 600026 Page 9 of 16

in the untreated sample are relatively small, whereas after introducing 0.05 mol L NdCl , the grain size
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increases significantly, indicating that NdCl treatment promotes grain growth. A more complete
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comparison of grain morphologies across the concentration series is provided in Supplementary Figure 8.
The cross-sectional SEM images in Figure 3H and I further reveal that the Sb Se film treated with NdCl 3
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exhibits a more compact columnar crystal structure, and the CdS/Sb Se interface becomes smoother and
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more tightly packed; the visible gaps and voids within the film are markedly reduced, suggesting an overall
improvement in film continuity and structural quality. To further verify this trend, we conducted AFM
measurements on the samples with and without NdCl treatment. Supplementary Figure 9A and B shows
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that NdCl treatment reduces the surface roughness (Ra) of the Sb Se film from 26.3 to 23.7 nm, further
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confirming enhanced surface flatness and uniformity. Taken together, these results indicate that NdCl 3
treatment improves the compactness of the CdS/Sb Se interface, enhances the crystallinity of Sb Se , and
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consequently reduces both interfacial and bulk defect densities, thereby laying the foundation for improved
device performance.
To clarify how Nd doping modulates the interfacial band characteristics of the CdS/Sb Se heterojunction,
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KPFM measurements were conducted. As shown in Figure 4A and B, the untreated Sb Se film exhibits a low
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and spatially nonuniform contact potential difference (CPD), whereas the Nd-treated sample presents a
significantly higher and more uniform CPD distribution, indicating reduced surface defect density and
enhanced electronic homogeneity . The line-scan profiles in Figure 4C and D further confirm that local
[43]
CPD fluctuations are substantially suppressed after Nd incorporation, reflecting a more uniform surface
work function. The CPD histograms in Figure 4E and F show that the untreated sample exhibits a large
surface photovoltage (SPV≈122 mV), whereas the Nd-treated film displays a substantially reduced SPV
(≈36 mV), indicating weakened interfacial band bending and suppressed carrier recombination. The
definition and calculation of the KPFM-derived SPV are provided in Supplementary Note 2.

In conjunction with the KPFM analysis, the optical properties and band structures of CdS and Sb Se were
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systematically examined to elucidate the impact of NdCl treatment. Ultraviolet-visible (UV-vis)
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measurements [Supplementary Figure 10A] show that NdCl -treated Sb Se exhibits enhanced absorption
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across 400-1,000 nm, suggesting improved photon utilization and more efficient photocarrier generation .
[44]
Tauc analysis reveals that the bandgaps of untreated and Nd-treated CdS films are 2.31 and 2.38 eV,
respectively, whereas those of Sb Se are 1.34 and 1.22 eV [Supplementary Figure 10B]. UPS measurements
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further provide the Fermi level and valence band maximum positions for each sample
[Supplementary Figure 11], enabling the construction of the band alignment diagrams in Figure 4G and H.
For the untreated interface, the conduction band minima (CBM) of CdS and Sb Se are -3.39 and -4.09 eV,
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respectively, yielding a conduction band offset (CBO) of 0.70 eV. Upon Nd incorporation, the CBM
positions shift to -3.84 and -4.21 eV, reducing the CBO to 0.37 eV. A large CBO (> 0.4 eV) tends to promote
electron accumulation at the interface, thereby enhancing interfacial recombination and hindering carrier
extraction. By contrast, a moderate spike-like alignment (0 < CBO < 0.4 eV) is generally favorable for
suppressing interfacial recombination and improving charge separation and transport . Overall, the
[45]
combined KPFM, UV-vis, and UPS analyses confirm that NdCl treatment reduces interface defect density,
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mitigates band bending, enhances optical absorption, and tunes the CdS/Sb Se band alignment into the
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optimal spike-like regime. These improvements collectively establish a more favorable interfacial electronic
environment, ultimately contributing to the enhanced performance of Sb Se solar cells. The calculated
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density of states (DOS) and projected DOS of Sb Se are provided in Supplementary Figure 12.
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Figure 5A displays the J-V results of devices with and without NdCl treatment. Table 1 lists the specific
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photovoltaic device parameters. After NdCl treatment, the short-circuit current density (J ), open-circuit
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SC
voltage (V ), and fill factor (FF) were 30.54 mA/cm , 0.465 V, and 64.58%, respectively. Additionally, a
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OC