Page 80 - Read Online

Xia et al. Energy Mater. 2026, 6, 600022 Page 9 of 13

charge transport resistance (R ) characterizes the carrier collection capability, while the recombination
tr
resistance (R ) is inversely proportional to the carrier recombination rate of the device. Fitted parameters
rec
[Supplementary Table 1] revealed that reduced charge transport resistance (R = 3.2 kΩ target vs. 5.2 kΩ
tr
control) confirms enhanced carrier collection, and increased recombination resistance (R = 17 kΩ target vs.
rec
10 kΩ control), which inversely correlates with recombination rates . In summary, dark J-V and EIS
[21]
analyses demonstrate that Zn doping concurrently suppresses carrier recombination and enhances
transport/extraction efficiency, directly corroborating the observed performance improvements.

The foregoing analysis demonstrates that Zn doping at the Sb S back surface enhances carrier concentration,
2 3
suppresses recombination, and improves carrier transport efficiency, particularly boosting J . To elucidate
sc
the underlying physical origins, we systematically investigated the chemical bonding, electronic properties,
and energy-level alignments of Zn-doping Sb S films. Raman spectroscopy [Figure 3F] revealed no ZnO
2 3
characteristic peaks (dashed lines in Figure 3F) in either control or Zn-doping films. All observed peaks
correspond exclusively to Sb S vibrational modes . The Raman spectrum of Sb S arises from four
[23]
2 3
2 3
vibrational modes (2A + 2E) of its tetrahedral SbS units (C symmetry): symmetric stretch ν (A ),
3
3v
s
1
1
asymmetric stretch ν (E), symmetric bend δs (A ), and asymmetric bend δ (E), with typical intensity
1
a
a
ordering ν > ν and δ > δ . As shown in Figure 3F, both films exhibit dominant peaks at 281 cm [ν (Sb-S)]
[24]
-1
s
a
s
a
a
and 304 cm [ν (Sb-S)], alongside weaker features at 127 cm (A ), 156 cm (B ), 189 cm [δ > (S-Sb-S)],
-1
-1
-1
-1
s
1g
g
a
and 239 cm [δs (S-Sb-S)]. Critically, the intensity ratio I /I of symmetric-to-asymmetric stretching
-1
281
304
vibrations inversely correlates with sulfur vacancy (V ) concentration. Spectral deconvolution
S
[Supplementary Figure 8A and B] yielded I /I = 1.05 (control) versus 1.30 (Zn-doping), confirming
304
281
reduced V defects in the Zn-doping film. This reduction is attributed to suppressed Sb S volatilization and
2 3
S
enhanced crystallinity during ZnO-capped annealing, consistent with diminished non-radiative
recombination observed in carrier dynamics.
To further probe the local electrical properties, conductive atomic force microscopy (C-AFM) was
performed under dark conditions on both control and Zn-doping Sb S films [Figure 3G-J]. Significantly
2 3
elevated current signals were detected within the large-grained structures of the Zn-doping film compared to
the reference. Concurrently, regions passivated by Sb O nanobelts at grain boundaries exhibited further
2
3
suppressed current leakage. These observations collectively demonstrate that enhanced bulk conductivity in
Zn-doping Sb S directly contributes to higher J . While effective defect passivation at grain boundaries via
sc
2 3
oxygen incorporation, mitigating carrier recombination. These nanoscale electrical characteristics align
consistently with the macroscopic device performance enhancements.
To elucidate carrier transport mechanisms, ultraviolet photoelectron spectroscopy (UPS) was performed on
control and Zn-doping Sb S films. The Fermi level (E ) and VBM positions were determined by subtracting
2 3
F
the secondary electron cutoff edge from the He I excitation source (21.22 eV) [Figures 4A and B]. The
control film exhibited E = -3.48 eV and VBM = -6.04 eV, while Zn doping shifted these to E = -3.70 eV and
F
F
VBM = -5.99 eV. Given the mixed-phase surface composition (> 80% Sb O ), the conduction band minimum
2
3
(CBM) was referenced to Sb O ’s bandgap. The proposed energy band diagram (Figure 4C, partial data
2
3
adapted from Ref. ) reveals that the control device possesses high CBM and deep VBM at the back surface
[25]
form an electron-blocking barrier, which suppresses recombination and transport holes through the oxide
tunnel. For Zn-doping device, a 0.22 eV downward shift in E and 0.05 eV upward shift in VBM/CBM
F
collectively enhance back-surface field strength. Coupled with the ultrathin oxide layer (< 10 nm), this
configuration promotes hole separation, directly explaining the enhanced V and J and PCE.
oc
sc
The successful incorporation of Zn and the concomitant suppression of Sb O underscore the dual
2
3
functionality of our ultrathin ZnO layer strategy. This feature distinguishes it from conventional