Revealing the interfacial electronic structure of Sb 2 S 3 on NH 4 Cl-modified CdS by Kelvin probe force microscopy

Revealing the interfacial electronic structure of Sb 2 S 3 on NH 4 Cl-modified CdS by Kelvin probe force microscopy | Authorea try { document.documentElement.classList.add('js'); } catch (e) { } var _gaq = _gaq || []; _gaq.push(['_setAccount', 'G-8VDV14Y67G']); _gaq.push(['_trackPageview']); (function() { var ga = document.createElement('script'); ga.type = 'text/javascript'; ga.async = true; ga.src = ('https:' == document.location.protocol ? 'https://ssl' : 'http://www') + '.google-analytics.com/ga.js'; var s = document.getElementsByTagName('script')[0]; s.parentNode.insertBefore(ga, s); })(); Skip to main content Preprints Collections Wiley Open Research IET Open Research Ecological Society of Japan All Collections About About Authorea FAQs Contact Us Quick Search anywhere Search for preprint articles, keywords, etc. Search Search ADVANCED SEARCH SCROLL This is a preprint and has not been peer reviewed. Data may be preliminary. 30 March 2026 V1 Latest version Share on Revealing the interfacial electronic structure of Sb 2 S 3 on NH 4 Cl-modified CdS by Kelvin probe force microscopy Authors : Alexandra Tsekou 0009-0001-9520-5900 , Geumha Lim , Evgeniia Gilshtein , Eugen Stamate , William Jo [email protected] , and Stela Canulescu Authors Info & Affiliations https://doi.org/10.22541/au.177490739.90357147/v1 187 views 94 downloads Contents Abstract 1 Acknowledgments 2 Introduction 3 Results and discussion 4 Conclusion 6 Experimental section References Information & Authors Metrics & Citations View Options References Figures Tables Media Share Abstract Understanding how interfacial chemistry influences the nanoscale electronic properties of absorber layers is critical for optimizing the performance of the emerging Sb 2 S 3 solar cells. In this work, we employ Kelvin probe force microscopy (KPFM) and surface photovoltage (SPV) to reveal the local surface potential, grain-boundary band bending and interfacial potential step in hydrothermally grown Sb 2 S 3 deposited on CdS buffer layers prepared with and without NH 4 Cl. Complementary SEM and AFM reveal morphology changes, while Raman spectroscopy, cross-sectional KPFM and ultraviolet photoelectron spectroscopy (UPS) are used to probe the electronic structure and the buried CdS/Sb 2 S 3 interface. Sb 2 S 3 grown on CdS:NH 4 Cl exhibits a stronger SPV response and a larger interfacial potential step than Sb 2 S 3 deposited on untreated CdS, indicating more efficient photoinduced charge separation. UPS analysis indicates a reduction in the cliff-like conduction band offset at the CdS/Sb 2 S 3 interface upon NH 4 Cl treatment. KPFM analysis also reveals substrate-dependent grain-boundary band bending under illumination, highlighting how local potential changes are modified by CdS surface chemistry. Consistent with these nanoscale observations, solar cells fabricated on CdS:NH 4 Cl show improved fill factor, thereby increasing the maximum power conversion efficiency from 4.75% to 4.92%. These results demonstrate that NH 4 Cl treatment of CdS alters the nanoscale electronic properties of the Sb 2 S 3 absorber and its buried interface, thereby improving device performance. Revealing the interfacial electronic structure ofSb 2 S 3 on NH 4 Cl-modified CdS by Kelvin probe force microscopy Alexandra Tsekou, † Geumha Lim, ‡ Evgeniia Gilshtein, † Eugen Stamate, ¶ William Jo, ∗,‡ and Stela Canulescu ∗,† † Department of Electrical and Photonics Engineering, Technical University of Denmark, 4000 Roskilde, Denmark ‡ Department of Physics, Ewha Womans University, Seoul 03760, South Korea ¶ National Centre for Nano Fabrication and Characterization, Technical University of Denmark, 2800 Kgs. Lyngby, Denmark E-mail: [email protected] ; [email protected] 1 Acknowledgments E. G. acknowledges financial support from Independent Fund Denmark, DFF Project 1 (Inge Lehmann) entitled New, tunable n-type material for emerging thin-film solar cells, grant number 1134-00005B. A. T. acknowledges travel support grant from Otto Mønsted Foundation. This work was also supported by the National Research Foundation of Korea (NRF) grants funded by the Korean government (Ministry of Science and ICT, MSIT) (RS-2024-00355905, RS-2025-02315803, RS-2025-02311266, RS-2025-16063688) and by the Ministry of Education (RS-2018-NR031064). Abstract Understanding how interfacial chemistry influences the nanoscale electronic properties of absorber layers is critical for optimizing the performance of the emerging Sb 2 S 3 solar cells. In this work, we employ Kelvin probe force microscopy (KPFM) and surface photovoltage (SPV) to reveal the local surface potential, grain-boundary band bending and interfacial potential step in hydrothermally grown Sb 2 S 3 deposited on CdS buffer layers prepared with and without NH 4 Cl. Complementary SEM and AFM reveal morphology changes, while Raman spectroscopy, cross-sectional KPFM and ultraviolet photoelectron spectroscopy (UPS) are used to probe the electronic structure and the buried CdS/Sb 2 S 3 interface. Sb 2 S 3 grown on CdS:NH 4 Cl exhibits a stronger SPV response and a larger interfacial potential step than Sb 2 S 3 deposited on untreated CdS, indicating more efficient photoinduced charge separation. UPS analysis indicates a reduction in the cliff-like conduction band offset at the CdS/Sb 2 S 3 interface upon NH 4 Cl treatment. KPFM analysis also reveals substrate-dependent grain-boundary band bending under illumination, highlighting how local potential changes are modified by CdS surface chemistry. Consistent with these nanoscale observations, solar cells fabricated on CdS:NH 4 Cl show improved fill factor, thereby increasing the maximum power conversion efficiency from 4.75% to 4.92%. These results demonstrate that NH 4 Cl treatment of CdS alters the nanoscale electronic properties of the Sb 2 S 3 absorber and its buried interface, thereby improving device performance. Keywords : Sb 2 S 3 , KPFM, SPV, interface, band alignment 2 Introduction Commercial thin-film photovoltaic (PV) technologies are currently dominated by CdTe and Cu(In,Ga)Se 2 (CIGS) solar cells, which have reached high power conversion efficiencies (PCEs) of 21% and 23.35%, respectively, in single-junction cells, while module efficiencies are slightly lower, reaching up to 19.9% for CdTe and 19.2% for CIGS. 1–3 Despite their high performance, approaching that of silicon (Si) solar cells (27.8% cell and 26% module PCE), 3 large-scale deployment of CdTe and CIGS remains constrained by material sustainability concerns, including the scarcity and cost of indium, gallium, and tellurium, as well as toxicity issues associated with cadmium. 4,5 These limitations motivate the development of alternative thin-film absorbers based on earth-abundant, environmentally benign elements that deliver competitive efficiencies while advancing sustainable PV technologies. The search for scalable and low-cost thin-film PV absorbers has increasingly focused on antimony chalcogenide materials, due to their favorable optical properties and the ease of processing with more earth-abundant elements, 6 compared to other PV technologies. This material family, denoted as Sb 2 X 3 , includes the pure sulfide compound (Sb 2 S 3 ), the pure selenide one (Sb 2 Se 3 ) and mixed chalcogenide (Sb 2 (S,Se) 3 ). 7 Their bandgap can be tuned from 1.0 eV to 1.8 eV, depending on the S/Se ratio, while they exhibit a high absorption coefficient in the visible range of the solar spectrum ( > 10 5 cm −1 ) and excellent physical and chemical stability. 8,9 Structurally, Sb 2 X 3 consists of (Sb 4 X 6 ) n ribbons, weakly bonded with Van der Waals forces, forming a quasi-1D crystal structure with orthorhombic crystallographic phase, resulting in anisotropic electrical and optical properties that depend on the crystal orientation. 10,11 Also, Sb 2 X 3 possesses relatively low melting points at 550 ◦ C for Sb 2 S 3 and 608 ◦ C for Sb 2 Se 3 , enabling low-temperature fabrication of high-quality absorbers. 10,12 The PCE of Sb 2 X 3 solar devices has seen a significant increase over the past years, rising from 0.66% for Sb 2 Se 3 solar cells in 2009 to above 10% today, with Sb 2 (S,Se) 3 holding the record PCE of 10.81%, followed by Sb 2 Se 3 with 10.58%. 13–15 In contrast, Sb 2 S 3 solar cells exhibit lower efficiencies, with a reported maximum of 8.26%, leaving room for further optimization. 16 According to the Shockley-Queisser limit, the theoretical PCE of Sb 2 S 3 solar cells, with a bandgap of approximately 1.7 eV, is 28.6% with an open-circuit voltage (V OC ) limit of 1.4 V. 17 Particularly, V OC deficit remains one of the main limiting factors in Sb 2 S 3 solar cells, originating from defect recombinations in the absorber, where intrinsic donor and acceptor defects, as well as hole traps, act as recombination centres. 18,19 Also, voltage losses are strongly influenced by suboptimal energy band alignment at both absorber interfaces: the electron transport layer (ETL/Sb 2 S 3 ) and hole transport layer (HTL/Sb 2 S 3 ) interfaces, making ETL selection particularly critical for device performance. 20,21 Various ETLs have been explored for Sb 2 X 3 solar cells in an effort to optimize band alignment at the ETL/absorber interface, including TiO 2 , 22 ZnO, 23 and SnO 2 . 24 Nevertheless, CdS remains the most commonly used ETL in Sb 2 X 3 solar cells, incor- porated in Sb 2 X 3 devices that currently hold the record efficiencies. 14–16 CdS is also a well-established ETL in other commercial and emerging solar cells such as CIGS, CdTe, and Cu 2 ZnSn(S,Se) 4 (CZTSSe). 25 Chemical bath deposition (CBD) is the most widely employed method for CdS deposition due to its low-temperature processing, conformal coverage and compatibility with large-area substrates. 26 However, the structural and electronic properties of CBD-grown CdS are highly sensitive to bath parameters, such as precursor materials concentrations, dopants, deposition time and temperature, which influence carrier density, crystallinity, bandgap, and morphology. 27–29 In Sb 2 S 3 devices, voltage losses are strongly associated with the Sb 2 S 3 /CdS junction, where the conduction band minimum (CBM) of CdS lies at a lower energy than that of Sb 2 S 3 , resulting in a cliff-like band alignment. 30,31 Variations in CdS chemistry can therefore modify its electronic properties, influencing interfacial band bending and charge extraction. 32,33 Also, the quasi-1D growth of Sb 2 S 3 is sensitive to substrate properties, meaning that changes in CdS surface chemistry may affect absorber nucleation and defect formation. 34,35 The incorporation of NH 4 Cl during CBD has been reported to enhance CdS crystallinity and uniformity by modifying nucleation kinetics and grain growth, as well as induc- ing slight changes in the bandgap, depending on the NH 4 Cl concentration used. 36,37 Despite these known effects, the influence of NH 4 Cl-modified CdS on the structural and electronic properties of Sb 2 S 3 and on the resulting heterojunction remains largely unexplored. In this work, we systematically compare Sb 2 S 3 absorbers grown by hydrothermal synthesis on two types of CBD CdS buffer layers: standard CdS (recipe with CdSO 4 , CH 4 N 2 S, NH 4 OH, and H 2 0) and chlorine-modified CdS (prepared from a bath additionally containing NH 4 Cl). The aim is to determine how chemical modification of the CdS affects the morphological, chemical, electronic and interfacial properties of the Sb 2 S 3 layer, ultimately affecting device performance. Surface morphology was investigated by SEM and AFM, while interfacial potential, grain-boundary band bending, photoinduced charge separation and band alignment were investigated using KPFM, SPV and UPS. By combining KPFM profiling with UPS, we reveal that NH 4 Cl incorporation into CdS modifies the difference between the conduction-band minima and the interfacial potential step at the Sb 2 S 3 /CdS junction. We also show that subtle changes in CdS chemistry can directly influence the performance of Sb 2 S 3 solar cells by altering charge separation, grain-boundary recombination and interfacial band alignment. 3 Results and discussion Figure 1a illustrates the stack configuration (glass/FTO/CdS/Sb 2 S 3 ) used in this study. The Sb 2 S 3 absorbers were hydrothermally grown on CdS buffer layers prepared CBD, either with or without NH 4 Cl additive. Details of the hydrothermal growth process are provided in the Experimental section. For simplicity, Sb 2 S 3 grown on untreated CdS is denoted as Sb 2 S 3 -CdS, whereas samples grown on NH 4 Cl-modified CdS are referred to as Sb 2 S 3 -CdS:NH 4 Cl. Representative top-view SEM images of the Sb 2 S 3 absorbers are shown in Figure 1b, c for films deposited on CdS without and with NH 4 Cl, respectively. Both absorbers exhibit a compact polycrystalline morphology with average grain size exceeding 6 µ m. Localized regions containing clustered micro-grains are observed in both samples, although these regions appear slightly more pronounced for Sb 2 S 3 grown on CdS:NH 4 Cl. Similar morphologies have previously been reported by Zhou et al. , who attributed them to elevated temperatures during hydrothermal growth. 38 In addition, spherical particles are visible in the SEM images of the Sb 2 S 3 -CdS absorber (Figure 1b). These particles are likely secondary Sb 2 S 3 precipitates formed by homogeneous nucleation in the liquor during hydrothermal synthesis, leading to the formation of stoichiometric Sb 2 S 3 crystallites, 39 as supported by EDS analysis (see Figure S1 and Table S1). Notably, the particles appear to be embedded within the absorber layer rather than located on the surface. This suggests that precursor species diffuse through grain boundaries, pores or other defects in the growing film under hydrothermal conditions. Such diffusion pathways allow the local supersaturation and promote internal nucleation for the growth of Sb 2 S 3 crystallites. The Sb 2 S 3 solar cells were fabricated in the standard superstrate configuration (FTO/CdS/Sb 2 S 3 /Spiro-OMeTAD/Au) and the representative J-V boxplots are shown in Figure 2a-d, based on 23 Sb 2 S 3 -CdS and 16 Sb 2 S3-CdS:NH4Cl solar cells. The statistical distributions show a notable improvement in the mean FF, from 40.1% to 45.6%, resulting in a slight increase in the mean power conversion efficiency (PCE) for Sb 2 S3-CdS:NH4Cl devices, from 3.78% to 3.89%. The V OC remains relatively stable for both solar cells at 0.7-0.71 V on average, while J SC exhibits a small reduction of 1.3 mA/cm 2 for Sb 2 S 3 -CdS:NH 4 Cl solar cells, reaching a mean value of 12 mA/cm 2 . The J-V curves of the champion devices, measured under AM1.5G illumination, are shown in Figure 2e, with the corresponding device parameters summarized in the inset. The data are consistent with the statistical analysis, as the device fabricated on CdS:NH 4 Cl exhibits a higher FF of 49.33% compared to 45.03% for the CdS reference, while J SC follows the same trend as in the statistics, showing a slight decrease of 1.25 mA cm −2 . For the champion devices, V OC shows an increase of 22 mV for Sb 2 S3-CdS:NH4Cl compared to the reference. Accordingly, the maximum PCE increases from 4.75% for Sb 2 S 3 -CdS to 4.92% for Sb 2 S 3 -CdS:NH 4 Cl. The most pronounced improvement is observed in the FF, which increases by approximately 13% after NH 4 Cl incorporation in CdS. In contrast, V OC remains nearly unchanged within the experimental variation, while a slight decrease in J SC is observed. As a result, the overall increase in device efficiency is primarily driven by the enhancement in FF. These results demonstrate that tuning the CdS chemistry can positively influence device performance. Similar behavior has been reported by Gopi et al. , who observed an increase in PCE of Sb 2 Se 3 solar cells upon the addition of NH 4 Cl during CBD of CdS, driven by improved V OC and FF, while excessive NH 4 Cl concentrations led to decreased performance. 40 While the achieved efficiencies remain below the current state of the art for Sb 2 S 3 solar cells, the emphasis of this study is on mechanistic understanding rather than on record efficiency, providing insights that are broadly relevant to interface engineering in emerging thin-film photovoltaics. Figure 3a–d shows AFM images of the Sb 2 S 3 absorbers acquired at two scan sizes to assess surface morphology. Large-area scans (30 × 30 µ m) capture the overall morphology, while smaller regions (10 × 10 µ m) avoid particle clustering. RMS roughness values, averaged over seven scans per absorber, are 24.7 nm (large-area) and 22 nm (zoomed) for Sb 2 S 3 grown on CdS:NH 4 Cl. In contrast, Sb 2 S 3 on CdS alone exhibits higher roughness in large-area scans (32.4 nm) due to the large Sb 2 S 3 crystallites, while smoother zoomed regions show an RMS roughness of 14 nm. Figure 3e,f shows the contact potential difference (V CPD ) maps of the Sb 2 S 3 absorbers obtained via KPFM. The work function (WF) was calculated from V CPD using the relation: 41 VCPD = (Φ tip − Φ sample ) /e (1) Here, the Φ tip was determined by measuring V CPD of a highly oriented pyrolytic graphite (HOPG) reference with a known WF of 4.6 eV. 42 V CPD values were extracted by fitting the CPD distribution with a Gaussian function and taking the peak (center) value of the fitted curve, as shown in Figure 3i,j (black curves). Our data show that Sb 2 S 3 grown on CdS has a WF of 4.24 eV, whereas Sb 2 S 3 grown on CdS:NH 4 Cl exhibits a slightly lower WF of 4.15 eV, corresponding to a decrease of approximately 90 meV and an upward shift of the Fermi level. This shift indicates a modification of the Sb 2 S 3 surface potential induced by chlorine incorporation in the CdS layer. Figure 3g,h shows the SPV-derived CPD maps of the Sb 2 S 3 absorbers under whitelight illumination. The SPV, which is defined as the difference between the V CPD,Illuminated and V CPD,dark was calculated according to: 43 SPV = VCPD,Illuminated − VCPD,dark (2) The SPV distributions, fitted with Gaussian functions (red curves in Figures 3i,j), reveal positive SPV values for both samples. Sb 2 S 3 grown on CdS exhibits an SPV of 0.112 V, whereas Sb 2 S 3 grown on CdS:NH 4 Cl shows a higher SPV of 0.177 V. This increase suggests that chlorine modification of the CdS substrate influences chargecarrier separation and defect-mediated recombination in the overlying Sb 2 S 3 layer. Statistical analysis of CPD-derived WF and SPV values is presented in Figure 3k,l, based on measurements collected from multiple independent regions (5-6 per sample). The averaged values confirm the trends discussed above. The average WF of Sb 2 S 3 grown on CdS is 4.29 eV, whereas Sb 2 S 3 grown on CdS:NH 4 Cl shows a slightly lower value of 4.22 eV, corresponding to a slight difference of approximately 70 meV. For SPV measurements, Sb 2 S 3 absorbers were illuminated with white light. Although the illumination contains photon energies both above and below the bandgap of Sb 2 S 3 , the dominant photoexcitation mechanism is assumed to be band-to-band transitions induced by photons with energies exceeding the bandgap, while trap states may act as recombination centers and reduce SPV response. 43,44 The average SPV value for Sb 2 S 3 grown on CdS is 0.122 V, whereas Sb 2 S 3 grown on CdS:NH 4 Cl exhibits a higher value of 0.191 V, corresponding to an increase of approximately 70 mV. This suggests more efficient photo-induced charge separation and reduced defectassisted recombination in Sb 2 S 3 grown on CdS:NH 4 Cl. Conversely, the lower SPV value for Sb 2 S 3 on CdS alone may be associated with enhanced recombination through deep-level defects. 44,45 This trend is consistent with the observed increase in FF for Sb 2 S 3 -CdS:NH 4 Cl, suggesting improved charge extraction and reduced recombination losses under operating conditions. Defect-assisted excitation may also occur under white-light illumination; however, a selective assessment of donor and acceptor defect densities would require monochromatic sub-bandgap illumination. 46 The contact potential difference between grain interiors (IGs) and grain boundaries (GBs) was calculated from 30 line profiles per sample using Delta KPFM = V CPD,IGs,dark - V CPD,GBs,dark . Representative GBs are shown in Figure 4a,c,g for Sb 2 S 3 on CdS and Figure 4b,d,h for Sb 2 S 3 on CdS:NH 4 Cl. Figure 4e,i and 4f, j show representative CPD line profiles on the GBs under dark and light, for Sb 2 S 3 on CdS and CdS:NH 4 Cl, respectively. The averaged CPD value on both sides of the GB was calculated and the mean value of the two was used for determining the CPD difference at the grain boundary, shown as dotted lines in the line profiles. Figure 4k presents the statistical distribution of ∆KPFM of the Sb 2 S 3 samples. Since the bias was applied to the tip during measurements, a higher V CPD corresponds to a lower work function. 47 Both samples exhibit downward potential bending at the grain boundaries, corresponding to upward band bending in the band diagram. The average potential difference ranges from 8.6 to 10.5 mV, with Sb 2 S 3 on CdS alone showing slightly stronger upward band bending, although the difference remains relatively small. These values are lower than those reported for selenide absorbers. 48 Upward band bending is typically associated with negatively charged acceptor defects at grain boundaries. 49,50 The statistical distribution of grain-boundary potential difference under illumination is shown in Figure 4l and was calculated using ∆SPV=V CPD,IGs,ill -V CPD,GBs,ill . A clear distinction between the two samples is observed. Sb 2 S 3 on CdS:NH 4 Cl exhibits a negative value (–8.5 mV), whereas Sb 2 S 3 on non-Cl-assisted CdS shows a positive value (6.4 mV). Under illumination, Sb 2 S 3 on non-Cl-assisted CdS still exhibits upward band bending at grain boundaries, although its value is reduced compared to dark conditions. Strong upward band bending can spatially separate carriers and reduce recombination at grain boundaries. 51 The reduced barrier height under illumination, therefore, suggests weaker carrier separation in Sb 2 S 3 grown on non-Cl-assisted CdS. In contrast, Sb 2 S 3 on CdS:NH 4 Cl exhibits downward band bending under illumination. Similar behavior has been reported for CIGS and CZTSSe absorbers, where such potential redistribution attracts electrons toward grain boundaries while repelling holes. 52,53 This mechanism can enhance carrier separation and suppress recombination at grain boundaries. 53 Schematic diagrams for upward and downward band bending are presented in Figures 4m, n, indicating the movement of charges at the GBs in both cases. The illumination-induced redistribution of band bending at grain boundaries has important effects on device performance. In particular, the transition to downward band bending observed for Sb 2 S 3 -CdS:NH 4 Cl is expected to facilitate electron transport along grain boundaries while suppressing hole accumulation, suggesting improved charge separation at grain boundaries relative to Sb 2 S 3 -CdS. This provides a possible explanation for the improved device performance, primarily reflected in increased FF and corresponding efficiency. However, additional characterization is required to fully assess the role of grain boundaries as recombination centers. 54 Figure 5a,b shows optical images of the mechanically dimpled Sb 2 S 3 samples, exposing the Sb 2 S 3 /CdS interface. Raman spectra were acquired at selected positions across the interface, as indicated in Figures 5a,b, and are presented in Figure 5c,d. For the first 4–5 measurement points corresponding to the Sb 2 S 3 region, deconvoluted peaks are observed at 300 cm −1 assigned to the B 1 g mode and 156 cm −1 , 197 cm −1 and 282 cm −1 , attributed to A g vibrational modes of orthorhombic Sb 2 S3. 55–57 For Sb 2 S 3 on non-NH 4 Cl assisted CdS, an extra peak at 128 cm −1 appears, also assigned to B 1 g mode, while the metallic phase of Sb is present in both absorbers, with a peak at 147 cm −1 . 56,58 The deconvolution of the Raman spectra is shown in Figure S2. These spectra confirm the formation of orthorhombic Sb 2 S 3 for both samples. The last 2–3 points, corresponding to the exposed substrate region, the peaks indicating Sb 2 S 3 phase disappear and a peak at 299 cm −1 emerges, associated with the 1LO phonon mode of CdS, 59 confirming successful exposure of the CdS layer through dimpling in both cases. AFM topography and corresponding KPFM maps of the Sb 2 S 3 /CdS interface are shown in Figures 5e,f. The red lines indicate the locations of the KPFM line profiles used to evaluate the interfacial potential bending, with the extracted potential profiles presented in Figures 5g,h. A clear difference in the interfacial potential step is observed between the two samples. Sb 2 S 3 -CdS exhibits a potential difference of 133 mV across the interface, whereas Sb 2 S 3 -CdS:NH 4 Cl shows an increased potential step of 157 mV. This increase indicates that NH 4 Cl incorporation in CdS modifies the interface at the Sb 2 S 3 /CdS junction. The CPD difference between Sb 2 S 3 and CdS reflects the built-in interfacial potential barrier, which influences charge carrier separation and back recombination at the heterojunction. 60 A larger interfacial potential step can enhance carrier separation and reduce interfacial recombination losses. 61 Therefore, the increased barrier observed for Sb 2 S 3 on CdS:NH 4 Cl suggests improved suppression of electron–hole recombination at the interface. This trend aligns with the higher FF of Sb 2 S 3 -CdS:NH 4 Cl, suggesting reduced resistive losses and more efficient charge extraction across the junction. High-resolution XPS spectra of Sb 3d and S 2p for Sb 2 S 3 deposited on CdS without and with NH 4 Cl are shown in Figure S3. The Sb 3d 5 / 2 spectrum is dominated by the Sb 2 S 3 component at 529.9 eV, confirming Sb 2 S 3 as the primary chemical state in both absorbers. A small Sb metallic contribution may originate from defective Sb 2 S 3 or slight reduction of Sb 2 S 3 during Ar + cluster etching used for surface cleaning prior to XPS measurements. 62 Similarly, the weak Sb 2 O 3 contribution suggests small surface oxidation of Sb 2 S 3 , which can occur during the annealing. 63 UPS measurements were performed to determine the valence band maximum (VBM) and work function (WF) of Sb 2 S 3 and CdS films. Figure 6a–d shows the secondary electron cut-off (SECO) at high binding energy and the valence band edge at low binding energy, from which the WF and VBM values were extracted, respectively. For Sb 2 S 3 , the VBM is located at -1.05 eV when deposited on CdS alone and shifts to -1.18 eV with respect to the Fermi level when grown on CdS:NH 4 Cl. The corresponding WF values are 4.04 eV and 4.21 eV, respectively. This yields VBM with respect to the vacuum level as -5.09 eV and -5.39 eV for Sb 2 S 3 -CdS and Sb 2 S 3 -CdS:NH 4 Cl, respectively. The extracted WF values are consistent with those reported in the literature, which range from 4.0 to 4.7 eV. 64,65 We note, however, that the WF of Sb 2 S 3 -CdS lies at the lower end of the reported range. Our data reveal a slight n-type character for the Sb 2 S 3 solar absorbers, a feature widely reported in the literature for absorbers produced by various methods, including hydrothermal synthesis and atomic layer deposition. 66,67 Compared to the control Sb 2 S 3 film, the Fermi level of NH 4 Cl-treated Sb 2 S 3 exhibits a shift of the Fermi level closer to the conduction band minimum (CBM), indicating a slight increase in electron carrier concentration in the n-type Sb 2 S 3 . It should also be noted that the accuracy of the precise determination of the VBM is limited by poor tailing in the UPS spectra of Sb 2 S 3 and deviations from an ideal linear slope of the valence band edge. Nevertheless, a slight reduction in the valence band tailing is observed for CdS:NH 4 Cl compared to CdS alone, suggesting a reduced defect density. The UPS spectra of the CdS buffer layer shown in Figure S4 indicate a VBM with respect to the vacuum level of -6.75 eV for Sb 2 S 3 -CdS and -6.81 eV for Sb 2 S 3 CdS:NH 4 Cl. This denotes an n-type character, in agreement with previous reports. 68 The VBM and WF values are comparable for both substrates, suggesting that NH 4 Cl incorporation during CBD does not significantly affect the electronic structure of CdS. This is consistent with previous studies reporting only minor changes in the bandgap and WF of CdS upon NH 4 Cl addition. 37,40 Lastly, energy band diagrams of the n–i–p device architecture (i.e., CdS/Sb 2 S 3 /spiroOMeTAD) derived from the UPS measurements are shown in Figure 6e,f. The energy level alignment at the heterojunction, quantified by the magnitude of the conduction band offset (CBO), plays a critical role in determining charge-carrier extraction, recombination rates and overall solar cell performance. 6 In particular, a negative CBO (i.e., a cliff-like alignment) can lead to increased non-radiative recombination at the interface via trap states and a reduced open-circuit voltage ( V OC ). Our findings reveal that the CBO at the CdS/Sb 2 S 3 interface is −0 . 91 eV for Sb 2 S 3 -CdS and decreases to −0 . 68 eV for the NH 4 Cl-treated sample. For these calculations, bandgaps of 2 . 46 eV for CdS and 1 . 71 eV for Sb 2 S 3 were used. 18,68 In both cases, the negative CBO indicates a cliff-like band alignment, consistent with the literature. 66,69 Such a large conduction band offset (CBO) may contribute to the significant V OC deficit commonly observed in Sb 2 S 3 solar cells, which is a characteristic limitation of emerging antimony chalcogenide photovoltaics. The V OC achieved in this work lies at the upper end of the reported range, with the highest value to date being 0.796 V, as reported by Liu et al. upon Ce 3+ incorporation. 63 For example, our Sb 2 S 3 /CdS devices with a V OC of 0 . 7 V exhibit a V OC deficit, expressed as V OC /V OC SQ , of approximately 49%. 17 Although the Sb 2 S 3 -CdS:NH4Cl solar cells still exhibit a cliff-type alignment compared to the reference, the reduced CBO is expected to lower non-radiative recombination at the interface and potentially increase V OC , which is not aligned with our observations. 31 Despite UPS data revealing a reduction in the CBO, and KPFM profiling showing an increased interfacial potential step for Sb 2 S 3 -CdS:NH4Cl, these modifications do not translate into a prominent increase in the average V OC . This observation suggests that V OC is not completely governed by the CdS/Sb 2 S 3 interface, but may also be limited to a great extent by trap-assisted defects in the bulk or recom- bination at the Sb 2 S 3 /HTL interface. 6,7 On the other hand, a less pronounced cliff-like alignment can facilitate carrier extraction and reduce recombination losses at the junction, which may contribute to the observed increase in FF of Sb 2 S 3 -CdS:NH4Cl solar cells. In general, the magnitude of the CBO experimentally determined in this work is larger than typically reported values for CdS/Sb 2 S 3 interfaces. Nevertheless, similarly large offsets have been reported for Sb 2 S 3 /TiO 2 , 65,70 whereas for CdS/Sb 2 S 3 solar cells, the CBO is typically below -0 . 4 eV. 31,66 For example, hydrothermally grown Sb 2 S 3 solar cells with a CBO close to 0 eV have shown V OC values ranging from 0 . 72 V to 0 . 75 V, comparable to those reported in this work. 69 Similarly, CdS/Sb 2 S 3 systems with a CBO as low as -0 . 13 eV exhibit comparable V OC values. 31 These observations suggest that the relationship between CBO and V OC is not straightforward and is influenced by additional factors, including defect-trap states in the bulk, interface quality, and recombination pathways. A similar limitation may also contribute to the slightly reduced J SC observed for Sb 2 S 3 -CdS:NH 4 Cl solar cells, as carrier extraction can be affected by recombination losses or transport limitations at the Sb 2 S 3 /HTL interface. 71 Supporting this interpretation, EQE spectra measured on representative Sb 2 S 3 solar cells from the same batch (not corresponding to the champion devices), shown in Figure S5, exhibit a reduced photoresponse predominantly in the long-wavelength region. Since this spectral range corresponds to carriers generated deeper in the absorber, closer to the Sb 2 S3/HTL interface, the observed reduction suggests that limitations at this interface contribute to the decrease in J SC . Overcoming the V OC bottleneck is critical for further progress in Sb 2 S 3 photovoltaics, which hold significant potential not only as standalone devices but also for indoor photovoltaics and tandem solar cells. 4 Conclusion This work demonstrates that NH 4 Cl incorporation during chemical bath deposition of CdS modifies the nanoscale electronic properties of hydrothermally grown Sb 2 S 3 absorbers. Sb 2 S 3 grown on CdS:NH 4 Cl shows a stronger positive SPV response and substrate-dependent grain-boundary band bending under illumination, all of which indicate enhanced photoinduced charge separation and reduced recombination. Crosssectional KPFM reveals an increased interfacial potential step, while UPS shows a reduction in the magnitude of the cliff-like conduction-band offset from −0 . 91 to −0 . 68 eV. These observations indicate reduced charge recombination at the interface in the NH 4 Cl-treated system. Consistent with these nanoscale measurements, devices fabricated on CdS:NH 4 Cl exhibit improved FF, increasing the maximum PCE from 4.75% to 4.92%, while V OC remains nearly unchanged. Overall, the results show that tuning CdS chemistry is an effective route to modify absorber electronic properties, suggesting reduced interfacial recombination and improving Sb 2 S 3 solar cell performance. 5 Conflicts of Interest The authors declare no conflicts of interest. 6 Experimental section 6.1 Device Fabrication Substrate cleaning Fluorine-doped tin oxide (FTO) substrates were cleaned prior to film deposition to remove surface contamination. The substrates were sequentially ultrasonicated in deionized water for 10 minutes, followed by acetone and isopropanol for an additional 10 minutes each. After drying under a nitrogen stream to remove residual solvents, the substrates were UV-cleaned for 10 minutes using a LOCTITE 97034 spot-cure system. Prior to deposition, part of each cleaned substrate was masked with Kapton tape to define a non-exposed FTO region for subsequent metal-contact deposition during device fabrication. Chemical bath deposition of CdS CdS buffer layers were deposited using a controlled chemical bath deposition (CBD) process. In all cases, glass/FTO substrates were immersed vertically to ensure uniform film growth. For the NH 4 Cl-containing recipe, the precursor solution consisted of 0.12 M cadmium sulfate (CdSO 4 ) as the cadmium source, 0.30 M thiourea (CH 4 N 2 S) as the sulfur source, 33% w/w aqueous ammonia (NH 4 OH) and 0.20 M ammonium chloride (NH 4 Cl). While immersed in an outer water bath at room temperature (RT), the CdSO 4 , water and NH 4 Cl were mixed in the inner bath and stirred together until a 50 ◦ C temperature was reached. Then, NH 4 OH was further added to the solution. When the bath reached 70 ◦ C, CH 4 N 2 S was immediately added. The deposition was carried out at 70 ◦ C for 4 min 30 s. Under these conditions, a CdS layer with a thickness of 50±5 nm was obtained. Lastly, it was annealed at 400 ◦ C for 10 min to improve crystallinity and remove excess residues. For the recipe without NH 4 Cl, the bath was prepared by mixing aqueous solutions of 0.0015 M CdSO 4 , 0.05 M (NH 2 )CS, and 1.5 M NH 4 OH. In this protocol, the outer bath temperature was first set to 90 ◦ C. When the outer bath temperature reached the above-mentioned temperature, the solutions were mixed in the inner bath and placed in the preheated outer water bath under constant stirring. This ensured a gradual natural heating up of the solution. Once the temperature of the inner bath with solution reached 45 ◦ C the timer was set and the CdS deposition occurred for 7 min 55 s. Under these conditions, the CdS layer has a thickness of approximately 50 nm. Lastly, it was annealed using the conditions mentioned above. Hydrothermal growth of SbS Sb 2 S 3 thin films were synthesized on glass/FTO/CdS substrates using a hydrothermal route. An aqueous precursor solution containing antimony and sulfur sources was prepared by dissolving KSbC 4 H 4 O 7 ·0.5H 2 O, 20 mM and Na 2 S 2 O 3 ·5H 2 O, 80 mM in 35 mL DI water, followed by stirring for 5 min to ensure complete homogenization. The solution was transferred to a 50 mL Teflon-lined stainless-steel autoclave and the cleaned glass/FTO/CdS substrates were immersed in the precursor solution and positioned at a tilt angle of 75 ◦ to ensure high reproducibility. The autoclave was sealed and hydrothermal treatment was carried out at 135 ◦ C for 180 min. After cooling naturally to room temperature, the films were rinsed with DI water and dried under nitrogen. A post-annealing crystallization step was performed at 350 ◦ C for 10 min in an N 2 -filled tube furnace at 175 mbar, selected as the optimized annealing condition. SbSdevice fabrication Solar cell devices were completed by sequentially depositing the hole-transport layer (HTL) and the metal contact. Spiro-OMeTAD HTL was deposited by spin coating onto the glass/FTO/CdS/Sb 2 S 3 substrates at 3000 rpm for 30 s, followed by heating on a hot plate at 100 ◦ C for 10 min in air. The spiro-OMeTAD solution was prepared by dissolving 36.6 mg of spiro-OMeTAD powder, 14.5 µ L of 4- tert -butylpyridine (tBP) and 9.5 µ L of 520 mgmL −1 lithium bis(trifluoromethanesulfonyl)imide (Li-TFSI) solution in acetonitrile, in 1 mL of chlorobenzene. Finally, an Au contact electrode of 95 nm was deposited by thermal evaporation under a vacuum pressure of 3 . 0 × 10 −5 Torr. The active area of the device was defined as 0.09 cm 2 . 6.2 Characterization The surface morphology of the Sb 2 S 3 absorbers was examined using field-emission SEM Zeiss Merlin. Plan-view images were obtained at an accelerating voltage of 5 kV and a working distance of 10 mm. Elemental distribution was analyzed by energydispersive X-ray spectroscopy (EDS) integrated into the same system, operated at 15 kV. AFM and KPFM measurements were performed in non-contact mode using an MFP3D Origin from Oxford Instruments, equipped with a Pt/Ir-coated tip. Topography images and contact potential difference (CPD) maps were collected over scan areas of 30x30 µm and 10x10 µm . From these measurements, the root-mean-square (RMS) roughness and the local work function were extracted. Surface photovoltage (SPV) measurements were performed under white light illumination and the photo-induced CPD shift (∆CPD) was extracted to evaluate carrier photogeneration and recombination behavior. KPFM and SPV line profiles were recorded across grain boundaries to quantify lateral potential variations and local band-bending gradients. Raman spectra were collected using a micro-Raman system with a 532 nm excitation source in back-scattering geometry. The laser power at the sample surface was limited to 1 mW to prevent thermal degradation of Sb 2 S 3 absorbers. Spectra were recorded with an acquisition time of 1 minute and a spectral resolution of 0.1 nm. Bare Sb 2 S 3 absorbers deposited on CdS substrates were mechanically dimpled using a dimple grinder (model 657, Gatan Inc.) to expose the Sb 2 S 3 /CdS interface. A 15 mm-diameter polishing wheel was used for 3 min, employing an alumina suspension, while maintaining a stage rotation speed of 3 rpm. Raman, AFM and KPFM measurements were performed on selected regions exhibiting smooth polishing without pronounced surface features or scratches. XPS analysis was performed using a Nexsa system (Thermo Fisher Scientific) equipped with a monochromatic Al K α source (h ν = 1486.6 eV). Survey spectra and high-resolution scans of Sb 3d, S 2p, and O 1s were acquired at pass energies of 200 eV and 50 eV, respectively. Binding energy calibration was performed using a sputter- cleaned Au reference sample by setting the Au 4f 7 / 2 peak to 84.0 eV. Charge correction was applied by referencing the C 1s peak to 284.8 eV. Spectra were fitted with Thermo Fisher Scientific Avantage software using Smart background subtraction to determine chemical states and elemental composition. UPS measurements were carried out using He I radiation (21.22 eV) to determine the work function and valence-band edge of Sb 2 S 3 and CdS with and without NH 4 Cl treatment. UPS spectra were recorded at pass energies of 2 eV and a -5 V bias was applied to the samples to shift the secondary electron cutoff into the recordable range. Calibration was performed using a sputtercleaned Au reference by setting its Fermi edge to 0 eV in the binding energy scale. 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Figure 1: (a) Schematic illustration of the glass/FTO/Sb 2 S 3 /CdS stack structure, where the CdS buffer layer was prepared by chemical bath deposition with and without NH 4 Cl additive. Top-view SEM images of Sb 2 S 3 absorbers grown on (b) CdS and (c) CdS:NH 4 Cl substrates. The scale bar is 2 µ m. Figure 2: Statistical photovoltaic performance and representative J-V curves of Sb 2 S 3 solar cells fabricated on CdS substrates without and with NH 4 Cl. Box plots of (a) open-circuit voltage ( V OC ), (b) short-circuit current density ( J SC ), (c) fill factor (FF), and (d) power conversion efficiency (PCE). (e) J-V curves of Sb 2 S 3 -CdS and Sb 2 S 3 -CdS:NH 4 Cl champion solar cells with the highest PCE. Figure 3: Large (30 × 30 µ m) and zoomed (10 × 10 µ m) AFM images of Sb 2 S 3 absorbers grown on (a,c) CdS and (b,d) CdS:NH 4 Cl. Contact potential difference (CPD) maps obtained by KPFM under dark and SPV under white-light illumination for (e,g) Sb 2 S 3 -CdS and (f,h) Sb 2 S 3 -CdS:NH 4 Cl. Histograms of CPD distributions derived from KPFM (dark) and SPV (light) for Sb 2 S 3 grown on (i) CdS alone and (j) CdS:NH 4 Cl. (k) Statistical distribution of CPD-derived WF of Sb 2 S 3 grown on CdS and CdS:NH4Cl. (l) The corresponding statistical distribution of SPV. Figure 4: Grain-boundary (GB) analysis of Sb 2 S 3 absorbers on (a,c,e,g,i) CdS and (b,d,f,h,j) CdS:NH 4 Cl substrates (a–d,g-h). Representative AFM/KPFM/SPV images highlighting selected GB regions. (e,f,i,j) CPD line profiles across GBs under dark and illumination conditions, with dotted lines indicating averaged CPD values used to extract potential differences. (k) Statistical distribution of grain-boundary potential difference (∆KPFM) under dark conditions. (l) Statistical distribution of ∆SPV under illumination. (m,n) Schematic band diagrams illustrating upward and downward band bending at GBs and the corresponding carrier redistribution. Figure 5: Optical images of mechanically dimpled Sb 2 S 3 samples exposing the Sb 2 S 3 /CdS interface, grown on (a) CdS and (b) CdS:NH 4 Cl. Raman spectra acquired at selected positions across the interface, confirming the transition from Sb 2 S 3 to CdS for (c) Sb 2 S 3 -CdS and (d) Sb 2 S 3 -CdS:NH 4 Cl samples. AFM topography and corresponding KPFM surface potential maps of the exposed Sb 2 S 3 /CdS interface for (e) Sb 2 S 3 -CdS and (f) Sb 2 S 3 -CdS:NH 4 Cl samples. Extracted KPFM line profiles across the interface (red lines in e,f), showing the interfacial potential step for (g) Sb 2 S 3 -CdS and (h) Sb 2 S 3 -CdS:NH 4 Cl samples. Figure 6: High binding energy UPS spectra used to determine the secondary electron cut off (SECO) and WF of Sb 2 S 3 grown on (a) CdS and (c) CdS:NH 4 Cl. Low binding energy UPS spectra used to extract the valence band maximum (VBM) for Sb 2 S 3 on (b) CdS and (d) CdS:NH 4 Cl substrates. (e,f) Energy level diagrams of Sb 2 S 3 -CdS and Sb 2 S 3 -CdS:NH 4 Cl solar cells. Google Scholar Information & Authors Information Version history V1 Version 1 30 March 2026 Copyright This work is licensed under a Non Exclusive No Reuse License. Keywords kpfm sb2s3 solar cells Authors Affiliations Alexandra Tsekou 0009-0001-9520-5900 Technical University of Denmark View all articles by this author Geumha Lim Ewha Womans University View all articles by this author Evgeniia Gilshtein Technical University of Denmark View all articles by this author Eugen Stamate Technical University of Denmark View all articles by this author William Jo [email protected] Ewha Womans University View all articles by this author Stela Canulescu Technical University of Denmark: Danmarks Tekniske Universitet View all articles by this author Metrics & Citations Metrics Article Usage 187 views 94 downloads .FvxKWukQNSOunydq8rnd { width: 100px; } Citations Download citation Alexandra Tsekou, Geumha Lim, Evgeniia Gilshtein, et al. Revealing the interfacial electronic structure of Sb 2 S 3 on NH 4 Cl-modified CdS by Kelvin probe force microscopy. Authorea . 30 March 2026. 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