# Exploring the Potential of Silicon Clathrate Films with NiOx-Based Selective Contacts for Optoelectronic Devices

Charif Tamin, Anil Kumar Bharwal, Céline Chevalier, Alain Fave, Erwann Fourmond, Stéphane Roques, Aziz Dinia, Abdelilah Slaoui, Thomas Fix

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## To cite this version:

Charif Tamin, Anil Kumar Bharwal, Céline Chevalier, Alain Fave, Erwann Fourmond, et al., Exploring the Potential of Silicon Clathrate Films with NiOx-Based Selective Contacts for Optoelectronic Devices. Advanced Materials Interfaces, 2025, 12 (17), 10.1002/admi.202500301. [https://doi.org/10.1002/admi.202500882](https://doi.org/10.1002/admi.202500882)

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# Exploring the Potential of Silicon Clathrate Films with NiOx-Based Selective Contacts for Optoelectronic Devices

Charif Tamin,* Anil Kumar Bharwal, Céline Chevalier, Alain Fave, Erwann Fourmond, Stéphane Roques, Aziz Dinia, Abdelilah Slaoui, and Thomas Fix

Silicon clathrates are a unique class of materials with a cage-like structure that offer significant advantages for optoelectronic applications, particularly in indoor photovoltaics (IPV). Their direct bandgaps and tunable electronic properties from metal-like to semiconductor-like behavior enable a wide range of optoelectronic functionalities. In this study, a photovoltaic device is demonstrated based on semiconducting silicon clathrate films (SCF) synthesized from crystalline silicon (c-Si) wafers, with NiOx incorporated as hole selective contact. XPS and Auger spectroscopy confirmed the incorporation of Na within the SCF framework, in agreement with the type-II clathrate structure revealed by XRD analysis. Optical and electronic measurements indicate that the SCF exhibits intrinsic-like behavior with a bandgap of 1.72 eV. Integration of SCF with NiOx resulted in rectification and a clear photovoltaic response under illumination, confirming device functionality. Band alignment analysis suggested efficient hole transport and secondary charge generation from the c-Si substrate. These results highlight the importance of selective contact engineering for efficient charge extraction and demonstrate the potential for further optimization of SCF photovoltaic devices.

## 1. Introduction

Silicon clathrates are a promising class of materials with a unique cage-like structure that is different from that of conventional cubic diamond crystalline silicon (c-Si). The first observation of these structures dates back to the 1960s at the University of Bordeaux, France, where researchers discovered the existence of sodium-containing silicon clathrate phases with crystal frameworks similar to those of gas hydrates.[1,2] First considered a crystalline anomaly, silicon clathrates have since attracted considerable attention due to their remarkable electronic properties, which can be tuned by controlling their composition and synthesis conditions. A key difference between clathrate structures and conventional c-Si is that clathrate structures can exhibit a direct bandgap, unlike c-Si, which exhibits an indirect bandgap.[3,4]

Another important feature of silicon clathrates is their ability to exhibit metal-like to semiconductor-like behavior. Clathrates mainly exist in two forms: type-I (Na₈Si₄₆) and type-II (NaₓSi₁₃₆). In type-I clathrates, such as Na₈Si₄₆, the smaller cage size makes it difficult to remove sodium atoms. In contrast, type-II clathrates, such as NaₓSi₁₃₆, have larger cages that allow partial or tunable sodium occupancy, enabling the modulation of their electronic properties. Type-I silicon clathrates are typically found in forms where sodium is always present, and these forms mainly display metallic properties due to electron donation by the sodium atoms. On the other hand, type-II clathrates can function as intrinsic semiconductors with wide bandgaps, often exceeding 1.9 eV.[2,5-7]

This property is particularly interesting for applications in indoor photovoltaics (IPV), where light-emitting diodes (LEDs) and fluorescent lamps are the main indoor light sources with spectral emission between 400 and 700 nm, which require a wide bandgap absorber.[8,9]

Despite these promising properties, the development of silicon clathrate devices remains challenging. Most studies have focused on bulk clathrate synthesis, which often involves high-temperature solid-state reactions that yield polycrystalline or powdered materials.[10,11] However, to effectively integrate these materials into optoelectronic devices, a synthetic approach for producing clathrate films is required. Recent research has

C. Tamin, C. Chevalier
CNRS, Ecole Centrale de Lyon
INSA Lyon
Université Claude Bernard Lyon 1
CPE Lyon, INL, UMR5270, Villeurbanne 69100, France
E-mail: charif.tamin@insa-lyon.fr

A. K. Bharwal, S. Roques, A. Slaoui, T. Fix
ICube Laboratory
University of Strasbourg and CNRS
Strasbourg 67037, France

A. Fave, E. Fourmond
INSA Lyon
Ecole Centrale de Lyon
CNRS
Université Claude Bernard Lyon 1
CPE Lyon, INL, UMR5270, Villeurbanne 69100, France

A. Dinia
IPCMS Laboratory
University of Strasbourg and CNRS
Strasbourg 67037, France

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The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/admi.202500301

© 2025 The Author(s). Advanced Materials Interfaces published by Wiley-VCH GmbH. This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.

DOI: 10.1002/admi.202500301
<figure><img src="image_5.png" /></figure>

<figure><img src="image_6.png" /><figcaption>**Figure 1.** Cross-section SEM images of SCF before (a) and after (b) thermal pressing.</figcaption></figure>

explored methods for synthesizing films to advance the feasibility of integrating clathrates into functional devices.

In the context of a national research project (see Acknowledgments), we developed an innovative approach to synthesize and enhance silicon clathrate films (SCF) from c-Si wafers using a two-step thermal decomposition process and thermal press annealing.[12–14] The films produced by this method not only exhibit high structural quality but are also well suited for ad-

vanced surface characterization techniques such as atomic force microscopy (AFM) and surface photovoltage (SPV) analysis.

The complexity of doping SCF and the formation of efficient p-n junctions highlight the need for alternative approaches, such as dopant-free selective contacts. Dopant-free transition metal oxides (TMOs), known for their transparency and tunable energy levels, are promising candidates for efficient carrier extraction.[15,16] In c-Si solar cells, TMOs with low work

<figure><img src="image_7.png" /><figcaption>**Figure 2.** XPS core-level and Auger spectra of SCF. (a) Si 2p (b) Na 1s (c) O 1s (d) Na KLL Auger.</figcaption></figure>
functions (WF) are used as electron-selective contacts, whereas TMOs with high WF are used as hole-selective layers.[16–19] The identification of TMOs with suitable band alignments for Sili-con clathrates remains challenging. To the best of our knowl-edge, this study is the first investigation on the development of a NiOₓ-based hole-selective contact for silicon clathrates toward a functional optoelectronic device. A combination of X-ray pho-toelectron spectroscopy (XPS), Kelvin probe (KP), SPV, and ellip-sometry was utilized to thoroughly investigate the energy levels of silicon clathrates and NiOₓ in comparison to c-Si. This first-ever demonstration of a working device prototype marks a signif-icant milestone in silicon clathrate-based optoelectronic devices and opens new avenues for practical applications.

## 2. Results and Discussion

### 2.1. Structural and Morphological Evolution of SCF Films

The phase composition of the synthesized SCF films was evaluated by θ-2θ XRD, as shown in Figure S1 (Supporting Information). The diffraction peaks are consistent with the type-II clathrate phase NaₓSi₁₃₆ (JCPDS 01-089-5535, red markers).[12] Minor traces corresponding to the type-I Na₈Si₄₆ phase (blue markers) are also observed, indicating that while the type-II phase dominates, a small fraction of the type-I phase persists. The exact chemical composition was estimated by EDS (Figure S2, Supporting Information), revealing a Na/Si atomic ratio corresponding to x ≈ 4.4 in the NaₓSi₁₃₆ framework.

The morphological evolution of SCF films before and after thermal pressing is illustrated in the cross-sectional SEM images shown in Figure 1. Prior to thermal pressing (Figure 1a), the film exhibited a porous microstructure with intergranular voids. The initial film thickness was ≈160 µm. After thermal pressing (Figure 1b), a significant increase in the film density was observed. This resulted in a significant reduction in porosity and improved adhesion to the substrate. Indeed, the thickness was reduced further from 160 to 100 µm, indicating a pressure- and temperature-facilitated compaction process, which resulted in enhanced particle interconnectivity. In our previous research, we demonstrated that thermal pressing can improve both mor-phological and optoelectronic properties.[20]

### 2.2. Chemical Characterization

#### 2.2.1. Silicon Clathrate Films

The combination of XPS and Auger analysis of SCF constitutes a valuable method for studying their chemical and electronic properties. Figure 2 shows the Si 2p, Na 1s, and O 1s core-level spectra and the Na KLL Auger transition of the SCF. Figure 2a presents the Si 2p spectrum, which is characterized by a complex structure with peaks corresponding to the clathrate structure and surface oxide species. The main Si 2p₃/₂ peak is observed at 97.6 eV, suggesting the presence of silicon atoms within the clathrate framework.[10] The secondary peak at 98.4 eV exhibits the interaction of silicon with alkaline elements. The observed shift toward a lower binding energy compared to that of c-Si (≈99.8 eV) indicates electron donor behavior from the sodium

atoms to the silicon framework, resulting in increased electron density.[10] Peaks at higher binding energies (102.8, 104 eV) indicate surface oxidation.[22] This oxidation was attributed to atmospheric contamination. Si interaction with alkaline elements is known to enhance oxidation, which can be etched by a dilute HF treatment.[11,23] However, this process is challenging to apply to SCFs because of the low density of the films. However, it is interesting to note that SCF can also be oxidised at the surface, as in the case of d-Si, which could motivate further detailed studies on this effect and its remediation.

Figure 2b shows the Na 1s nuclear level spectrum with two distinct peaks, indicating the presence of sodium in different chemical environments. The predominant peak at 1071.5 eV corresponds to Na in metallic Na (Na⁰).[24] The second weaker peak at 1072.6 eV corresponds to the ionic sodium (Na⁺) form, which indicates the interaction of sodium with oxygen.[11]

Auger spectra were collected to obtain more detailed information about the chemical environment of Na in the clathrate structures (Figure 2d). The observed maximum intensity at 989.02 eV corresponds to the Na KLL transition.[25] The modified Auger parameter (MAP) was calculated by summing the Na 1s binding energy of the predominant peak and Na KLL kinetic energy. The calculated MAP value of 2060.61 eV is significantly different from the Na⁰ value of 2066.10 eV.[26] Instead, it corresponds to the MAP values obtained in the molecular sieve (2060.65 eV), which is a sodium aluminosilicate compound with a cage-like crystal structure quasi-similar to that of clathrate.[25,27] Figure 2c shows the O 1s spectrum, which exhibits two peaks at 532.4 and 534.3 eV. The high-intensity peak at a lower energy corresponds to the Si—O bonds in SiO₂. The secondary peak at a higher energy with lower intensity indicates the presence of oxygen in sodium-containing species such as ternary oxides, which is in agreement with previous observations of Si2p and Na 1s.

#### 2.2.2. Nickel Oxide Films

Figure 3 shows the XPS spectra of the NiOₓ thin films. In Figure 3a, the Ni 2p core level spectrum shows a large Ni 2p₃/₂ peak at 854.5 eV, corresponding to Ni²⁺ in NiO, and a higher-energy peak at 855.7 eV, attributed to Ni³⁺ in Ni₂O₃.[28] The presence of intense satellite peaks at 860–863 eV confirms the oxide nature of the Ni species.[29] Figure 3b presents the O 1s spectrum with a dominant peak at 529.7 eV, which corresponds to the presence of oxygen (O²⁻) in the Ni-O bond. The secondary peak at 531.5 eV is consistent with the presence of surface hydroxyl species, which can be attributed to atmospheric exposure.[28] NiOₓ thin films, which are distinguished by their Ni²⁺ and Ni³⁺ states and surface hydroxylation, are frequently used as hole-selective contacts in photovoltaics and as catalytic layers in electrochemical applications.[30,31]

### 2.3. Optoelectronic Characterization

#### 2.4. Energy Levels

A series of measurements were performed using a combination of KP, XPS, PL, and ellipsometry to determine the positions of
<figure><img src="image_8.png" /><figcaption>**Figure 3.** XPS core-level spectra of NiOₓ Thin Films. a) Ni 2p b) O 1s.</figcaption></figure>

the energy levels of the SCF and NiOₓ layers relative to the c-Si substrate.

**Figure 4** shows the maximum valence band (VBM) positions of the SCF and NiOₓ films. Spectra were collected from the individual films using XPS near 0 eV, which represents the Fermi level (E<sub>F</sub>). The estimated positions were fitted using the “edge down” background type in CasaXPS, which allows extrapolation of the linear region near the edge of the VBM.[32] The SCF VBM (Figure 4a) is located at 0.82 eV, while the NiOₓ VBM (Figure 4b) is located at 0.6 eV relative to E<sub>F</sub>. The VBM of the n-type c-Si substrate is located at 0.78 eV, as reported in our previous work.[17]

**Figure 5** shows the band gap of the SCF as determined from the PL measurement (Figure 5a), as well as the band gap of NiOₓ, which was extracted from ellipsometry measurements. This SCF has a band gap of 1.72 eV, which is consistent with the expected value for semiconducting type II silicon clathrates. NiOₓ presents a wider band gap of 3.13 eV, making it well-suited as a transparent hole transport layer to be positioned on top of SCF.

To gain a complete picture of the band alignment, we combined optical band gap values with VBM measurements to estimate conduction band minimum (CBM) positions. For SCF, the

CBM position is ≈0.9 eV relative to E<sub>F</sub>, and for NiOₓ, it is ≈2.5 eV. **Figure 6** shows an energy-level diagram established using WF values obtained from KP measurements. This diagram illustrates the relative energy levels of SCF and NiOₓ relative to the c-Si substrate. SCF exhibits characteristics of an intrinsic semiconductor, where E<sub>F</sub> is located at the quasi-mid-gap between the VBM and CBM. The NiOₓ films exhibited the characteristics of a p-type semiconductor, with a CBM significantly far from the E<sub>F</sub> and higher than the CBM of the SCF. This facilitates hole transport while acting as an electron-blocking barrier. Thus, the NiOₓ can be considered as a transparent, hole-selective contact.

**Figure 7** shows the configuration of the device structure and the corresponding band alignment as well as the measurement of SPV under dark, constant light, and variable light intensity conditions, as well as the normalized SPV versus photon flux. Figure 7a presents the layer stack, which is composed of c-Si(n) as the substrate, SCF as the absorber, NiOₓ as the hole-selective contact, and ITO as the transparent conductor oxide. When E<sub>F</sub> is at equilibrium, reached, the band alignment shows a cliff-like conduction band offset (CBO) with an almost flat valence band offset (VBO) at the c-Si(n)/SCF interface. At the SCF/NiOₓ interface, CBO exhibited

<figure><img src="image_9.png" /><figcaption>**Figure 4.** a) Valence band positions of SCF (a) and NiOₓ relative to the Fermi level.</figcaption></figure>
<figure><img src="image_10.png" /><figcaption>**Figure 5.** a) PL spectra of SCF acquired with excitation wavelength of 532 nm. b) Tauc plot and the resulting band gap energy of NiOₓ obtained from the ellipsometry measurement.</figcaption></figure>

a significant spike-like characteristic and a relatively small VBO. This band alignment configuration can facilitate the transport of electrons from the SCF to c-Si and favor the transport of holes with NiOₓ, which also acts as an electron reflector. The c-Si substrate can also act as a secondary absorber, which would enhance the spectral response beyond the SCF bandgap. Figure 7b shows the SPV response under dark and constant white light conditions. The positive shift in CPD with illumination >200 mV suggests an accumulation of photogenerated holes at the surface, while electrons drift toward the bulk, reinforcing the idea of favorable band alignment for charge separation. Figure 7c presents the SPV as a function of light intensity, demonstrating an increase and saturation at high intensity. Figure 7d shows the SPS measurement, which provides key information about the bandgap, charge generation, and separation mechanisms.[33,34] At lower photon energies, the presence of small peaks at ≈1.2 and 1.4 eV is observed, suggesting the presence of additional charge generation pathways. These minor peaks may be attributed to local compositional variations or defect states present within the SCF film or at the interface, leading to the formation of localized states within the bandgap and enabling subbandgap carrier transitions. As the photon energy increased, a gradual increase in the SPV (≈1.6 eV) was observed, indicating the initiation of significant carrier generation. This observation indicates that a proper-

tion of photogenerated carriers begin to contribute effectively to charge separation at this particular energy. The most significant SPV response appears at 1.73 eV, which corresponds closely to the PL peak of the SCF layer, confirming that this energy corresponds to the fundamental bandgap of the SCF. The significant increase in the SPV at this energy point is indicative of efficient band-to-band transitions associated with the photogenerated carriers.

**Figure 8a** shows the J-V characteristics under dark and illuminated conditions, revealing rectifying behavior in the dark and photovoltaic response under light. The open-circuit voltage (V<sub>oc</sub>) was measured at 276 mV, and the current density (J<sub>sc</sub>) was determined to be 0.065 mA cm<sup>-2</sup>. These moderate values suggest potential for further optimization of the device structure. Such optimization includes enhancing the quality of the absorber layer, since the minority carrier lifetime and diffusion length are likely limited, as well as engineering more favorable band alignments, particularly at the electron transport layer (ETL). An optimized ETL should form an efficient barrier for holes and a favorable CBO for electrons.

**Figure S3** (Supporting Information) presents the J-V curves of a control device fabricated without the NiOₓ interlayer. In this configuration, the dark J-V curve exhibits nearly symmetric and quasi-linear behavior, indicating an absence of rectifying characteristics. Under illumination, minimal photocurrent and a weak V<sub>oc</sub> (≈40 mV) are observed, reflecting inefficient charge separation and extraction. This comparison clearly demonstrates the essential role of the NiOₓ layer as a hole-selective contact, which introduces asymmetry in charge transport, enhances rectification, and enables a clear photovoltaic response.

**Figure 8b** shows V<sub>oc</sub> and V<sub>oc</sub>/W as a function of wavelength. V<sub>oc</sub> measurement was provided for each filter at a specific wavelength, and the incident power was not constant between the different filters. Still, it can be seen that for the 400–1100 nm region, all wavelengths give rise to a V<sub>oc</sub>, an observation that is confirmed by the measurements under solar simulator in which LEDs are controlled to be active over a 100 nm range at a time. When normalizing V<sub>oc</sub> with the incident power, a peak emerged for the 1074 nm filter. This peak is consistent with the EQE

<figure><img src="image_11.png" /><figcaption>**Figure 6.** Energy levels of SCF and NiOₓ compared to c-Si(n), estimated by combining VBM positions, bandgap energy, and WF.</figcaption></figure>
<figure><img src="image_12.png" /><figcaption>**Figure 7.** a) Device structure and band alignment, b,c) SPV response under different illumination conditions, d) SPS: normalized SPV vs. photon flux.</figcaption></figure>

results (Figure 8c), which show a peak at the same wavelength. Both SPV and SPS measurements indicate that the SCF is photoactive and contributes to Voc. The SCF provides an SPV response, as seen in Figure 7, but the EQE in the visible spectrum is low, as seen in Figure 8. This indicates that the photocarriers created at the top of the clathrate mostly recombine within the thick (≈100 µm) SCF layer. There are indeed so many defects in the film, as observed by SEM in Figure 1, that a low minority carrier diffusion length is expected in the clathrate film. This leads to low photovoltaic performance. The EQE results match well with the simulation results of Mahammedi et al.[35] The devices were further analyzed by measuring the photoconductivity, where the device is no longer in the short-circuit configuration but under positive or negative bias. Figure 8d shows the photoconductivity at different applied voltages; a high response occurs at ≈1074 nm, which is in agreement with the EQE results. The observed variation in photoconductivity with the applied voltage can be attributed to the recombination and defects still present in the SCF layer, as well as the low carrier lifetime and low minority carrier diffusion length.

Given that the absorption coefficient of SCF is in the $10^4$ cm$^{-1}$ range in the visible, a thickness of 2–3 µm should be sufficient to absorb most of the incoming solar light.[36] Currently, SCF layers with reduced thicknesses have not been synthesized directly from c-Si substrates. However, this goal seems possible through process optimization, including the use of reactive ion etching with SF$_6$.[20] Reducing the thickness of the SCF layers, peeling off the c-Si substrate, and incorporating optimized electron-selective contacts with favorable band alignment could significantly improve charge carrier extraction. These improvements will pave

the way for the development of efficient substrate-free SCF devices tailored for IPV applications.

### 3. Conclusion

In this study, we investigated the morphological, chemical, optoelectronic, and device properties of silicon clathrate films (SCF) combined with $NiO_x$ as a hole-selective contact. The thermal pressing technique was found to have a significant impact on the density of the SCF films, leading to a reduction in the thickness from 160 to 100 µm. XPS and Auger spectroscopy analyses confirmed the presence of sodium and silicon within the clathrate framework, influencing the electronic properties and oxidation behavior of the SCF. The $NiO_x$ films exhibited a mixed oxidation state ($Ni^{2+}/Ni^{3+}$), suggesting their potential application as an efficient hole transport layer.

Energy-level estimation using XPS, KP, and PL measurements revealed a clear view of the band alignment between the SCF, $NiO_x$, and c-Si. The integration of SCF as the absorber in the device structure led to the observation of rectification behavior in the I–V measurements. Photovoltaic behavior was also observed with an open-circuit voltage of 276 mV and a short-circuit current density of 0.065 mA cm$^{-2}$. The bandgap of the SCF was derived by extrapolating the slopes of the SPS signal versus the photon energy, yielding a bandgap of 1.73 eV. This bandgap is in good agreement with the result obtained from the PL signal analysis of the SCF films, confirming the reliability of this method for estimating the bandgap in complete devices.

The results demonstrated a proof-of-concept SCF-based optoelectronic device with selective contacts. To fully exploit the
<figure><img src="image_13.png" /><figcaption>**Figure 8.** a) Current–voltage (J–V) characteristics under dark and illumination conditions, b) Open-circuit voltage (V<sub>oc</sub>) and V<sub>oc</sub>/W as a function of wavelength, c) external quantum efficiency (EQE) spectrum, d) Photoconductivity measurement at different applied voltages.</figcaption></figure>

potential of SCF, future research should focus on interface en-
gineering, optimization of charge transport, and development of
a transfer process to extract SCF from the c-Si substrate and allow
for independent functionality.

**4. Experimental Section**

**Synthesis of SCF Thin Films:** SCF was synthesized by two thermal decomposition processes following the method described by Vollondat et al.[12] First, sodium was placed in a crucible covered with an n-type c-Si wafer and annealed at 600 °C under a 1.5 bar argon atmosphere to form the Na₄Si₄ Zintl phase.

A secondary annealing step at 400 °C under high-vacuum conditions was then performed to remove excess sodium, resulting in a clathrate phase. The films were subsequently cleaned using acetonitrile, ethanol, and deionized water. Finally, the films were thermally pressed in a hydraulic press by positioning them between two hot plates using a functionalized c-Si mold to prevent adhesion. An initial pressure of 2 kN was applied before heating the samples to 250 °C, during which the pressure was gradually increased to reach ≈8.5 kN. The system was rapidly cooled to room temperature before the pressure was released. For further information on the pressing process and a detailed description of the thermal pressing equipment (for details see Bharwal et al. and Dang et al.)[20,21]

**Device Fabrication:** The devices were fabricated according to the stack shown in Figure 7a by depositing NiOₓ as a selective hole contact and In₂O₃:Sn (ITO) as a transparent oxide conductor using an AC450 sputter-

ing system from the Alliance Concept. Prior to NiOₓ and ITO deposition, the SCF surface was deoxidized with buffered oxide etching (BOE) solution, followed by deposition of NiOₓ by reactive DC sputtering from Ni target at room temperature (P: 210 W, Ar: O₂ = 50:5 sccm) for 3 min, resulting in a thickness of ≈10 nm. An ITO layer was then deposited via RF sputtering for 30 min (power: 210 W, Ar: 50 sccm), resulting in a thickness of ≈150 nm. The estimated thickness was based on films deposited under the same conditions on a c-Si wafer. Finally, the resulting devices were annealed at 200 °C for 1 min on a hot plate. For device characterization, the samples were mounted on a Cu support using Ag paste, and an additional Ag paste droplet was applied to the front contact for I–V measurements of the SCF devices.

**Characterization:** X-ray diffraction (XRD) analysis was performed using a Bruker D8 Discover with Cu Kα₁ radiation (λ = 0.154059 nm) to confirm the phases of the films. Energy-dispersive X-ray spectroscopy (EDS) measurements were conducted using a JEOL JSM-IT200 scanning electron microscope (SEM) equipped with a DrySD detector to analyze the composition of the films. XPS using a hemispherical energy analyzer (EA15-HP) equipped with an aluminum monochromator (incident energy of 1486 eV) was used to study the chemical environment of the silicon clathrate films. Pure Au metal (Au 4f at 84 eV) was used as a reference.

Spectroscopic ellipsometry using a HORIBA Uvisel Lt M200 FGMS (210–880 nm) was used to study the optical properties and thicknesses of the NiOₓ films. Photoluminescence (PL) measurements were conducted at 293 K using a JOBIN YVON LabRam Aramis system equipped with a 532 nm laser.

KP and SPV measurements were performed using a single device setup (APS03, KP Technology). For surface photovoltage spectroscopy (SPS
measurements, the same setup was equipped with a monochromator to deliver wavelength-resolved illumination. Tip calibration was performed by determining the contact potential difference (CPD) with a silver reference. A quartz tungsten halogen light source provided illumination for the SPV measurements with a maximum irradiance of ≈19 mW cm⁻². For solar cell characterization, current-voltage (I-V) characteristics were obtained using an AAA rated Oriel Verasol-2 LED solar simulator (LSS-7120) under AM1.5G conditions and a Keithley 2461 source measure unit. The external quantum efficiency (EQE) setup consisted of a light source (tungsten halogen) and automated filter wheels with 41 filters. EQE measurement involves the ratio of the current produced on the sample at a given wavelength to the current generated by a certified reference detector at the same wavelength, measured with a Keithley 2450 SourceMeter.

**Supporting Information**

Supporting Information is available from the Wiley Online Library or from the author.

**Acknowledgements**

This project was funded by the ANR Exosil-Exotic Silicon: silicon clathrate films (project-ANR-22-CE50-0025) and supported by the Nanolyon platform, a member of the French National Network CNRS-RENATECH+.

**Conflict of Interest**

The authors declare no conflict of interest.

**Data Availability Statement**

The data that support the findings of this study are available from the corresponding author upon reasonable request.

**Keywords**

kelvin probe, NiOₓ, selective contacts, silicon clathrates, SPV, thermal press-annealing, XPS

Received: April 5, 2025

Revised: July 6, 2025

Published online: July 15, 2025

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