# From solar cells to solar fuels: assessment of Cu(In,Ga)Se 2 absorbers as photocathode for CO 2 reduction

Julian Guerrero, Amelle Rebai, Thomas Tom, Damien Coutancier, Tommaso

Raimondi, Nathanaelle Schneider, Stefania Cacovich, Negar Naghavi

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Julian Guerrero, Amelle Rebai, Thomas Tom, Damien Coutancier, Tommaso Raimondi, et al. From solar cells to solar fuels: assessment of Cu(In,Ga)Se 2 absorbers as photocathode for CO 2 reduction. Journal of physics, 2025, 7 (4), pp.045015. 10.1088/2515-7655/adffec. hal-05328148

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# From solar cells to solar fuels: assessment of Cu(In,Ga)Se₂ absorbers as photocathode for CO₂ reduction

To cite this article: Julian Guerrero et al 2025 J. Phys. Energy 7 045015

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# From solar cells to solar fuels: assessment of Cu(In,Ga)Se₂ absorbers as photocathode for CO₂ reduction

Julian Guerrero¹, Amelle Rebai¹,², Thomas Tom¹ ID, Damien Coutancier¹, Tommaso Raimondi¹, Nathanaelle Schneider¹ ID, Stefania Cacovich¹ ID and Negar Naghavi¹,* ID

¹ Institut Photovoltaïque d'Ile-de-France (IPVF), UMR 9006, CNRS, Ecole Polytechnique—IP Paris, Chimie Paristech—PSL, Palaiseau, France

² Institut Photovoltaïque d'Ile-de-France (IPVF), Palaiseau, France

* Author to whom any correspondence should be addressed.

E-mail: negar.naghavi@chimieparistech.psl.eu

Keywords: Cu(In, Ga)Se₂, CO₂ reduction, photoelectro catalysis, pn junction, oxide layers

## Abstract

This study explores the potential of Cu(In,Ga)Se₂ (CIGS) absorbers, commonly used in high-efficiency solar cells, as photocathodes to improve solar-to-fuel conversion efficiency. CIGS's exceptional light absorption, tunable band gaps, and stability make it an ideal candidate for this application. We examined the performance of bare CIGS and CIGS|CdS pn junctions in photoelectrochemical (PEC) CO₂ reduction, emphasizing the crucial role of interface engineering. Additionally, we assessed the impact of additional ultra-thin functional oxide layers (TiO₂, NiO, Al₂O₃, and SnO₂) and inorganic nanostructured co-catalysts (ZnO and Cu₂O). Our findings reveal that while bare CIGS can reduce CO₂ to CO, the introduction of a pn junction significantly enhances current density and selectivity. A well-optimized photoelectrode based on a CIGS|CdS pn junction can achieve remarkable performance despite its instability, attaining up to 98.5% CO₂ reduction selectivity and photocurrent densities of approximately 8 mA cm⁻² at -1.3 V vs SCE under 1-sun illumination. However, protective layers, while improving stability, often led to decreased photocurrent. These insights highlight that combining CIGS absorber layers with appropriate charge transport and catalytic layers is essential for developing efficient and stable PEC systems for CO₂ reduction.

## 1. Introduction

Harnessing sunlight to produce fuels and chemicals from abundant feedstocks like water and CO₂ is a key milestone toward a decarbonized energy future. Among emerging technologies, CO₂ reduction by semiconductor-based photoelectrocatalysis has gained prominence for its ability to drive chemical transformations using sunlight, offering pathways to synthesize valuable chemicals such as CO, formic acid, and methane [1]. Unlike electrocatalytic (EC) systems, photoelectrochemical (PEC) systems leverage photovoltage generated by semiconductors to enhance energy efficiency and facilitate charge separation [2].

Among the fastest-growing renewable energy sectors, photovoltaics play a pivotal role in the transition toward sustainable energy solutions. In this context, PEC approaches, using photovoltaic materials and systems to drive catalytic chemical reactions, can offer promising pathways for renewable fuel and chemical synthesis while enabling the storage of intermittent solar energy in stable chemical forms [3]. The use of photovoltaic materials as photoelectrodes allows for the efficient capture of a broad portion of the visible solar spectrum, potentially leading to improved solar-to-fuel conversion efficiencies. Most of the reported photoelectrodes showing photocurrent above 10 mA cm⁻² are based on PV grade light absorbers with buried junctions. Notably, p-Si or n-Si, III/V based semiconductors, CIS and CIGS achieved the highest reported photocurrent [3].

Cu(In,Ga)(S,Se)₂ (CIGS) semiconductors stand out among these materials due to their excellent light absorption, tunable band gaps, and intrinsic stability, making them highly promising candidates for PEC
applications [4]. Two primary strategies have been proposed for integrating PV technology with solar fuel production: the photovoltaicelectrochemical (PV-EC) coupling and the PEC approach [5, 6]. PV-EC systems use high-performance photovoltaic cells to generate electricity, which is then supplied to an electrolyzer. This method enables independent optimization of each component and has been successfully demonstrated for hydrogen production using CIGS-based solar cells [7–9]. Notably, studies from Uppsala University reported thermally integrated, solar-driven water-splitting modules employing AgCu(In,Ga)Se₂ (ACIGS) solar cells in combination with an alkaline electrolyzer system, achieving a solar-to-hydrogen (STH) efficiency of 13.4% with active areas of 82.3 cm². These findings highlight the potential of CIGS for PV-EC hydrogen production [8].

Despite advancements in PV-EC hydrogen evolution, the use of CIGS for CO₂ reduction remains largely unexplored. One of the most notable efforts in this area was conducted by Pati *et al* who coupled a CIGS-based PV device (ZnO:Al|ZnO|CdS|CIGS|Mo) with a Co-quaterpyridine (Co-qPyH) molecular catalyst. This system achieved an impressive CO selectivity of 87% at a bias potential of –0.06 V vs RHE, with a partial current density exceeding 3 mA cm⁻² under visible light illumination [10]. These promising results underscore the potential of CIGS-based PV-EC systems for CO₂ conversion while highlighting the limited research conducted in this domain.

While PV-EC systems allow for independent optimization of photovoltaic cells and electrocatalysts, they require a more complex infrastructure and higher initial investments. In contrast, PEC systems offer a more integrated approach by directly immersing a semiconductor photoelectrode in an electrolyte, where light absorption and chemical conversion occur within a single device. Although this design simplifies system architecture, it faces significant challenges related to material stability in aqueous environments, charge transfer efficiency, and overall energy conversion performance, which remains lower than that of state-of-the-art PV-EC systems. A major limitation of PEC technology is the difficulty in surpassing the 1.23 V thermodynamic barrier required for water splitting and CO₂ reduction, along with the additional overpotentials necessary for redox reactions. It is now widely recognized that a single light-absorbing material is unlikely to overcome all obstacles associated with photocatalytic fuel production [2].

In PEC CO₂ reduction (PEC CO₂R), two main configurations are generally considered: (i) direct semiconductor/electrolyte interfaces, where the semiconductor directly absorbs light and generates electron-hole pairs that drive the catalytic reaction, and (ii) buried junctions, where the semiconductor is protected by intermediate layers or co-catalysts to prevent direct exposure to the electrolyte while maintaining efficient charge transfer. For the first configuration, bare Cu(InGa)S₂ absorbers with bandgaps of around 1.7 eV have demonstrated high CO₂-to-CO Faradaic efficiency (FE) in both aqueous [11, 12] and non-aqueous electrolytes [13, 14], with current densities reaching up to 1.5 mA cm⁻² at –2 V vs Fc⁺/Fc in non-aqueous systems and at a –0.4 V vs RHE for aqueous systems. However, their relatively low current densities and susceptibility to degradation in liquid electrolytes highlight the need for improved stability and efficiency.

In parallel it has been shown that integrating catalysts with CIGS photoelectrodes significantly enhances PEC CO₂ reduction performance. Approaches such as grafting molecular catalysts onto thick TiO₂ layers [10], rear-illumination wide-bandgap CIGS photo-absorbers using a top metallic layer as a protective platform for catalyst grafting [15, 16] or embedding them in nanoporous ZnO layers [17] on CIGS surfaces [18] have achieved photocurrents up to 16 mA cm⁻² and CO₂ reduction selectivity exceeding 92%. These findings emphasize the dual role of oxide or metallic layers as protective coatings to mitigate electrode degradation and as platforms for catalyst integration, while underscoring the need for a deeper understanding of their interactions with CIGS and their influence on catalytic performance.

In this study, we investigate both direct semiconductor/electrolyte interfaces and buried junction configurations for PEC CO₂ reduction using CIGS-based absorbers. The goal is to evaluate the influence of p-n junctions on PEC performance through a comparative analysis of standard bare Cu(In,Ga)Se₂ (CIGS) photocathodes with a bandgap of 1.2 eV, bare CIGS photocathodes combined with nanostructured catalysts such as ZnO and Cu₂O, and CIGS|CdS p-n junction photocathodes in a non-aqueous electrolyte. Additionally, we examine the effect of ultrathin metal oxide layers (TiO₂, NiO, Al₂O₃, and SnO₂) deposited via atomic layer deposition (ALD) on the performance of these electrodes, and in particular their long-term stability. Our systematic assessment of the PEC activity of various CIGS-based photocathode configurations demonstrates that proper interfacial engineering significantly enhances performance. Notably, the p-n junction CIGS|CdS photocathode achieves selective reduction of CO₂ into CO with a selectivity of up to 98.5%, reaching current densities approaching 8 mA cm⁻² at –1.3 V vs SCE under 1 sun illumination. These performances rank among the most notable reported to date for CO₂-to-CO conversion in non-aqueous systems, as summarized in table 1; however, they are subject to rapid degradation over time. Furthermore, we assessed the impact of protective metal oxide thin layers on the stability and efficiency of these photoelectrodes, showing that while some of these layers can protect the p-n junction for degradation, they
<table><caption>**Table 1.** Summary of previously published most efficient photocathodes for PEC CO₂ reduction in non-aqueous electrolyte.</caption><thead><tr><th>Photocathode</th><th>Co-catalyst</th><th>Electrolyte</th><th>Potential applied/ illumination conditions</th><th>Performance</th><th>Reference</th></tr></thead><tbody><tr><td>Mo/Cu(In,Ga)Se₂/ CdS</td><td>None</td><td>0.1 M Bu₄NPF₆ in CH₃CN and 1.5% H₂O v/v</td><td>-1.3 V vs SCE (corresponding to -1.7 V vs Fc/Fc⁺) 100 W solar simulator (100 mW cm⁻², λ > 400 nm)</td><td>8 mA cm⁻² at -1.3 V vs SCE for 300 s and stabilized at 4 mA cm⁻² Selectivity of 98.5% CO₂-to-CO 100% FE</td><td>This work</td></tr><tr><td>Mo/Cu(In,Ga)Se₂</td><td>None</td><td>0.1 M Bu₄NPF₆ in CH₃CN and 1.5% H₂O v/v</td><td>-1.3 V vs SCE 100 W solar simulator (100 mW cm⁻², λ > 400 nm)</td><td>3 mA cm⁻² at -1.3 V vs SCE, CO Selectivity of 69.1% CO₂-to-CO</td><td>This work</td></tr><tr><td>Mo/Ink-Based CuIn₀.₃Ga₀.₇S₂ nanocrystal thin films Cu(In,Ga)S₂</td><td>None</td><td>0.1 M tetrabutylammonium bromide in acetonitrile</td><td>-0.55 V vs Fc/Fc⁺ 1 sun illumination</td><td>25% FE CO₂ to CO</td><td>[13]</td></tr><tr><td>Mo/Cu(In,Ga) Se₂/CdS/i-ZnO/ZnO:Al /ZnO|CoPcTA</td><td>CoPcTA</td><td>0.1 M Bu₄NPF₆ in CH₃CN and 2% H₂O v/v</td><td>-1.86 V vs Fc/Fc⁺ 300 W Xe lamp (AM 1.5 G)</td><td>-1.67 mA cm⁻² 68.6% FE CO₂ to CO Stability 1.1 h (0.99 mA cm⁻²)</td><td>[14]</td></tr><tr><td>Mo/Cu(In,Ga) Se₂/CdS/i-ZnO/ZnO:Al /ZnO|CoPcTA</td><td>CoPcTA</td><td>0.1 M Bu₄NPF₆ in CH₃CN and 2% H₂O v/v</td><td>-1.7 V vs SCE 100 W solar simulator (100 mW cm⁻², λ > 400 nm)</td><td>-7 mA cm⁻² 90% FE, selectivity CO 93%</td><td>[18]</td></tr><tr><td>Si/m-TiO₂</td><td>CotpyP</td><td>0.1 M Bu₄NPF₆ 6:4 v:v MeCN:H₂O</td><td>-1.5 V vs SCE 100 W solar simulator (100 mW cm⁻², λ > 400 nm)</td><td>-4 mA cm⁻² FE 98%, selectivity CO 94%</td><td>[18]</td></tr><tr><td>Cu₂O/ITO</td><td>Cu₃(BTC)₂MOF</td><td>0.1 M Bu₄NPF₆ in CH₃CN</td><td>-1.97 V vs Fc/Fc⁺ 300 W Xe lamp (AM 1.5 G)</td><td>-0.25 mA cm⁻² FE: CO (47.6%) H₂ (16.7%) HCOOH (12.8%) Stability 8 h (0.09 mA cm⁻²)</td><td>[19]</td></tr><tr><td></td><td></td><td></td><td></td><td>-0.8 mA cm⁻² FE CO₂ to CO 95% 9 h stability</td><td>[20]</td></tr></tbody></table>

may also lead to reduced performance (lower current densities) due to hindered charge transfer and the absence of appropriate catalysts. By addressing critical challenges related to material degradation and performance limitations, this work aims to advance the development of efficient and durable PEC systems for solar-driven CO₂ reduction.

## 2. Experimental procedure

### 2.1. Preparation of the Cu(In,Ga)Se photocathodes

2 µm thick Cu(In,Ga)Se₂ (CIGS) absorbers with a bandgap of around 1.2 eV were deposited on a glass|Mo stack by a standard 3-stage process. The p–n junction have been formed by converting the glass/Mo|CIGS stack with n-type 30 nm CdS layer, deposited by chemical bath deposition (CBD), The typical photovoltaic parameters of the same stack covered by ZnO:ZnO:Al window layer (a complete CIGS solar cell) deduced from current–voltage characteristics range like: V<sub>oc</sub> = 650–700 mV, J<sub>sc</sub> = 30 mA cm<sup>-2</sup>, FF = 70% leading to photovoltaic conversion efficiency of ~15%–17%.
## 2.2. Light-assisted electrodeposition of ZnO and Cu₂O on CIGS

ZnO and Cu₂O films were deposited on CIGS absorbers using light-assisted electrodeposition. A solar simulator (LCS-100, Oriel Instruments), calibrated to AM 1.5 G (100 mW cm⁻²), was used to provide the necessary light source. Electrochemical experiments were conducted with a Bio-Logic SP150 potentiostat, controlled using EC-Lab software. The deposition process utilized a conventional three-electrode setup, with a saturated calomel electrode (SCE, Hach) as the reference electrode, which has a standard potential of +0.244 V vs NHE at 25 °C and a platinum wire as the counter electrode.

The Cu₂O was electrodeposited in a strongly alkaline electrolyte. The electrolyte consisted of a mixed solution of 0.2 M CuSO₄·5 H₂O (Merck, >99%) and approximately 3.0 M lactic acid (C₃H₆O₃), with the pH adjusted to about 11.5 by the gradual addition of KOH solution. The electrodeposition was carried out at 60 °C by applying a bias voltage of -0.3 V vs SCE for 1 min [21].

The ZnO nanorods were electrodeposited following a previously reported method [22, 23]. Nanorods were grown potentiostatically at -0.5 V vs SCE during 20 min in an aqueous solution containing 0.2 mM ZnCl₂ (Sigma Aldrich, >99.9%) and 0.1 M KCl (ACS Reagent, 99.0–100%), maintained at 80 °C. The solution was saturated with oxygen by bubbling O₂ continuously throughout the process. After electro-deposition, the oxide layers were rinsed with deionized water and dried at 100 °C.

## 2.3. Protective metal oxide layers deposition

To evaluate the stability of photoelectrodes, ultra-thin films of various metal oxides (TiO₂, Al₂O₃, SnO₂, and NiOₓ) were deposited on the surface of CIGS/CdS pn-junctions using thermal ALD in a Beneq TFS-200 reactor according to procedures reported elsewhere [24–27]. Titanium (IV) i-propoxide (TTIP, Ti(OiPr)₄), trimethylaluminium (TMA, Al(CH₃)₃), tetrakis(dimethylamido)tin(IV) (TDMASn, Sn(N(CH₃)₂)₄), bis(N,N'-di-t-butylacetamidinato)nickel(II) (Ni(amd)₂, Ni((CH₃)₃CNC(CH₃)NC(CH₃)₃)₂), hydrogen peroxide (H₂O₂, 30% w/w in H₂O) and deionized water (H₂O) precursors as Ti, Al, Sn, Ni and O sources, respectively. Experiments were performed at Tdep = 200 °C (TiO₂), 160 °C (Al₂O₃), 100 °C (SnO₂, NiOₓ), and the pressure in the reaction chamber was kept in the range of 1–2 mbar. Thin films were deposited simultaneously on single polished n-doped Si (100) wafer with a native oxide top layer as reference. ALD deposition is done using the following sequence: metal precursor (pulse)/N₂ (purge)/ O source (pulse)/ N₂ (purge), for a total number of cycles that corresponds to 3 nm thick ALD film grown on the Si reference substrate in the same deposition run. However, note that the growth rates can differ for various substrates. Cyclic voltammetry technique have been used previously to evaluate the carrier-blocking capability of the ALD-grown thin films of SnO₂, TiO₂ and NiOₓ deposited on glass/ITO substrates: it confirmed hole-blocking behavior for TiO₂ [27] and SnO₂ [25] films and electron-blocking behavior for NiOₓ films.

## 2.4. PE CO₂ reduction performance

PEC measurements for CO₂ reduction (CO₂RR) were performed in a three-electrode PEC setup using SCE as reference, the cathode operated under PEC conditions, while the anode functioned electrochemically. To verify the reference potential and allow comparison, the SCE reference was calibrated *in situ* against the Fc/Fc⁺ couple in acetonitrile (0.10 M TBAPF₆, 98%, Aldrich) using platinum electrodes, with a well-defined 1 mm platinum disk as the working electrode. Under an inert atmosphere, ~0.1 mM ferrocene was prepared in acetonitrile, and cyclic voltammograms were recorded to determine the half-wave potential (E¹/²), with Fc/Fc⁺ observed at +0.40 V vs SCE. Linear sweep voltammetry (LSV) under chopped light and controlled potential electrolysis were performed on a 0.5 cm² geometric area of CIGS photoelectrodes combined with various interfacial layers in a sealed cell, following the procedure described elsewhere [16, 17]. Acetonitrile was used as the electrolyte, supplemented with 0.1 M TBAPF₆ (98%, Aldrich) and 1.5% v/v H₂O, and saturated with either argon or CO₂. For CO₂ experiments, the solution was saturated at 1 atm CO₂ (314 ± 6 mM at 25 °C) after sequential purging with argon and CO₂. An Oriel Instrument LCS-100 solar simulator, calibrated to AM 1.5 G (100 mW cm⁻²), was used to illuminate the samples.

The gases generated during electrolysis were analyzed by an Agilent Technologies 7820 A Gas Chromatography system, equipped with a thermal conductivity detector [16, 17]. The CO and H₂ were quantitatively measured and separated using a CP-CarboPlot P7 capillary column (27.46 m in length, 25 µm internal diameter). The detector temperature was maintained at 150 °C, while the oven temperature was set to 34 °C. Argon was used as the carrier gas, flowing at 9.5 mL min⁻¹ under a constant pressure of 0.5 bar. These analytical conditions enabled the simultaneous detection of H₂, O₂, N₂, CO, CO₂, and CH₂. Calibration curves were established separately by injecting known quantities of pure gases. Gas injections were carried out every 20 min using a 250 µl gas-tight syringe (Hamilton). H₂ was the only by-product detected by gas chromatography.
## 2.5. Hyperspectral characterization

The setup consists of a hyperspectral imager equipped with a Zephyr 1.7 InGaAs camera, both from photon etc with a spatial resolution of 300 × 300 µm per pixel. Spectral acquisitions were conducted within the range of 900–1300 nm, with a spectral resolution of 2 nm. The samples were uniformly illuminated by light-emitting diodes at a wavelength of 890 nm under a fluence of 57 mW cm⁻². Experimental data were acquired using the dedicated software PHySpecV2 (photon etc) as a 3D data cube, consisting of two spatial dimensions and one spectral dimension. Absolute calibration of the setup was performed to obtain, at each pixel, the luminescence spectrum expressed in physical units: number of photons J⁻¹ s⁻¹ m⁻².

Luminescence spectra were fitted according to the generalized Planck's law for emission of semiconductors out of equilibrium [28], which can be described by the simplified following equation:

$$I_{PL} = A(E, T) \varphi_{BB}(E, T) e^{\frac{\Delta\mu}{kT}}$$

where A is the absorbance, $\varphi_{BB}$ is the blackbody emission, $\Delta\mu$ stands for the quasi-Fermi-level-splitting (QFLS), E is the photon energy, T is the temperature, and k is the Boltzmann constant. The absorbance is described by the following formula, with $E_g$ the bandgap and $E_u$ the Urbach energy $E_u$ [29, 30].

$$A(E, T) = \frac{1}{1 + e^{\left(\frac{E_g - E}{E_u}\right)}}$$

$E_u$ represents the steepness of the exponential increase in optical absorption that occurs at energies lower than the bandgap of the material because of energetic or thermal disorder. The lower the Urbach energy, the better the material quality [31, 32].

Fitted QFLS data were corrected to match 1 sun equivalent AM1.5 G standard illumination. In the rest of the text, QFLS refers therefore to 1 sun equivalent QFLS.

# 3. Results and discussion

## 3.1. Assessment of bare CIGS absorbers

First the CO₂ reduction activity of a bare CIGS absorbers, as photocathode with a 1.2 eV bandgap deposited on SLG|Mo, was assessed in a three-electrode PEC configuration using LSV measurements under chopped AM 1.5 G illumination (figure 1(a)). Under argon saturation, a small cathodic current is observed, reaching approximately 0.2–0.4 mA cm⁻² at –1.6 V vs SCE, attributed to the hydrogen evolution reaction (HER). After saturating the solution with CO₂, both the total current density and the photogenerated current ($j_{ph}$), defined as the difference between dark and illuminated current, increased significantly reaching ~5–6 mA · cm⁻² at –1.6 V vs SCE. However, this value remained below the expected performance based on the short-circuit current density ($J_{sc}$ ≈ 30 mA · cm⁻²) of the reference CIGS solar cells.

Chronoamperometry experiments were also performed under 100 mW · cm⁻² illumination at a constant potential of –1.3 V vs SCE for 80 min (figure 1(b)). The bare CIGS absorbers exhibited a stable photocurrent density of approximately 2.1 mA · cm⁻² throughout the experiment, a benchmarking 1 Sun photocurrent density of over 2 mA at a low overpotential. Gaseous product analysis (figures 1(c) and (d)) revealed a stable evolution with Faradaic efficiencies of 55%–60% for CO and 25%–30% for H₂, resulting in an overall CO₂-to-CO selectivity of approximately 70%. These results show that CIGS photocathodes are capable of achieving stable PEC conversion of CO₂ to CO, at least for 80 min even in the absence of protective layers or cocatalysts. This aligns with earlier results observed for Cu(In₀.₃Ga₀.₇)S₂ electrodes [14].

Building on previous studies demonstrating the effectiveness of Cu₂O [33–35] and ZnO [36–39] as selective CO₂ reduction catalysts in both aqueous and organic solvents, nanostructured Cu₂O and ZnO were electrodeposited on top of CIGS absorbers as co-catalysts in a second set of experiments. Morphological analysis (figure 2) revealed the formation of Cu₂O nanocubes (figure 2(a)) and ZnO nanorods (figures 2(b) and (c)) on the CIGS surface. Given that these nanostructures are not highly dense and do not fully cover the CIGS surface, they may play a co-catalytic role in the electrochemical process. The aim was to evaluate how these nanostructured metal oxides could lower the overpotential, enhance selectivity, and boost reaction rates by increasing the availability of active sites and facilitating efficient electron utilization at the electrode surface.

The PEC performance of CIGS electrodes with and without co-catalysts are compared using chopped-light LSV on figure 3(b). As expected, photoelectrodes with metal oxide co-catalysts exhibited higher photocurrent densities up to 10 mA · cm⁻² at –1.6 V vs SCE and a more favorable onset potential, indicating improved performance for CO₂ reduction.

Chronoamperometry measurements at –1.3 V vs SCE for CIGS and CIGS|MₓOᵧ electrodes (figure 3(c)) reveal an initial increase in photocurrent during photoelectrolysis, followed by stabilization at a plateau.
<figure><img src="image_4.png" /><figcaption>**Figure 1.** (a) Current-voltage characteristic under intermittent 1 sun illumination of CIGS photocathodes measured in Ar and CO₂ saturated acetonitrile solution with 0.1 M TBAPF₆ and 1.5% H₂O v/v. (b) Chronoamperometry of a CIGS photocathode for PEC CO₂ reduction. (c) Faradaic efficiency and (d) CO selectivity.</figcaption></figure>

<figure><img src="image_5.png" /><figcaption>**Figure 2.** Top view SEM images of (a) Cu₂O and (b) ZnO deposited on Mo/CIGS substrates. (c) Cross section SEM images of the Mo/CIGS/ZnO photoelectrodes.</figcaption></figure>

Among the tested electrodes, the CIGS|Cu₂O photocathode exhibited the highest stable current density (~2.7 mA cm⁻²), approximately double that of the bare CIGS photocathode. The CIGS|ZnO electrode achieved a slightly lower stable current density of ~2.3 mA cm⁻². Gas analysis performed every 20 min (figure 3(d)) showed the production of only CO and H₂. The CIGS/Cu₂O electrode demonstrated higher selectivity toward CO, whereas the CIGS|ZnO electrode produced CO and H₂ in roughly equal proportions. The total FE for both CIGS|MₓOᵧ electrodes approached 100%, confirming efficient charge utilization during the reaction.

## 3.2. Assessment of CIGS|CdS pn junction and impact of oxide protective layers

In solar cells, efficient charge separation is typically achieved through the formation of a pn junction. For CIGS solar cells, this junction is commonly created by depositing a thin CdS electron transport layer via CBD. Recent studies have demonstrated the potential of buried pn junctions to enhance the performance of photocathodes in HER [6, 40, 41]. The pn heterostructure can significantly improves the photovoltage
<figure><img src="image_6.png" /><figcaption>**Figure 3.** (A) Schematic representation of the Mo|CIGS|metal oxide electrodes. (b) Current-voltage measured under intermittent 1 sun illumination of CIGS photocathodes (c) chronoamperometry of CIGS and CIGS/metal oxide photocathodes for photoelectrochemical CO₂ reduction in a CO₂ saturated acetonitrile solution with 0.1 M TBAPF₆ and 1.5% H₂O v/v. (d) CO selectivity.</figcaption></figure>

<figure><img src="image_7.png" /><figcaption>**Figure 4.** (A) Current-voltage measured under intermittent 1 sun illumination of CIGS and CIGS|CdS photocathodes (b) chronoamperometry registered in a CO₂ saturated acetonitrile solution with 0.1 M TBAPF₆ and 1.5% H₂O v/v. Applying a potential of -1.3 V vs SCE (c) CO selectivity obtained with the CIGS and CIGS|CdS photoelectrodes.</figcaption></figure>

supply, rendering it an ideal candidate for the application in CO₂R. To explore the role of such junctions in CO₂ photo-electroreduction CIGS electrodes with a ~30 nm n-type CdS layer were prepared, and its PEC performance was systematically compared to that of bare CIGS electrode (figure 4).

The LSV curves (figure 4(a)) reveal a significantly higher photocurrent density for CIGS|CdS photocathodes compared to bare CIGS electrodes. This enhancement highlights the substantial photovoltage and effective charge carrier separation achieved through the formation of the pn junction. Additionally, the LSV curves indicate that CO₂ reduction begins at a more positive potential on the pn junction electrode, approximately -0.97 V, compared to -1.30 V for the bare CIGS electrode, using 1 mA cm⁻² as the reference current onset value.

Chronoamperometry measurements at -1.3 V vs SCE for CIGS and CIGS|CdS electrodes (figure 4(b)) show an initial high photocurrent of -8 mA cm⁻², which decreases to -4 mA cm⁻² during photoelectrolysis before stabilizing. This indicates the instability of the CdS layer under photocatalytic conditions, a known
<figure><img src="image_8.png" /><figcaption>**Figure 5.** (A) Current-voltage measured under intermittent 1 sun illumination of CIGS|CdS photocathodes with transparent conformal metal oxide protective layer (c) chronoamperometry registered with the different CIGS|CdS electrodes in a CO₂ saturated acetonitrile solution with 0.1 M TBAPF₆ and 1.5% H₂O₂ v/v. Applying a potential of -1.3 V vs SCE.</figcaption></figure>

issue for CdS and other promising photoabsorbers. Despite that, gas analysis performed show a remarkably product selectivity improved to 98.5% CO₂-to-CO with the CIGS|CdS p-n junction (figure 4(c)). CdS instability arises from the local environment at the semiconductor/electrolyte interface, where redox reactions occur. At the interface, where the redox chemistry occurs, the local environment tends to be rather hostile. One possible strategy for protecting the surface is to introduce reaction pathways with favorable kinetics to outcompete degradation reactions. Another approach is to protect the surface prone to degradation at the electrolyte interface, by a more stable surface. This could mean depositing a thin protective oxide layer or a window layer on top of the unstable photoabsorber. This oxide layer needs to be not only transparent, but also matches the band edges of the photoabsorber and supports efficient charge transport. It must ensure efficient charge separation in the absorber, and minimize resistance, maintaining overall photocatalytic performance. The simplest approach to a window layer involves using a stable, transparent semiconductor with low resistance. In this configuration, the window layer has minimal impact on charge carrier generation, while its surface could be functionalized with a catalyst [18]. For CIGS photocathodes, ZnO has been used as a protective window layer to some extent. However, experimental results indicate that ZnO, while more stable in solution than CdS, tends to be reduced to metallic Zn when negative bias is applied. This underscores the viability of the concept but highlights the need for a more stable protective layer, either replacing or overlaying the ZnO layer, to ensure the strategy's effectiveness. Materials such as NiOₓ and TiO₂ have been explored as promising alternatives for this role. In subsequent studies, ultrathin (<3 nm) films of TiO₂, Al₂O₃, SnO₂, and NiO were deposited on the surface of CIGS|CdS pn-junctions using ALD. The effects of these ALD thin films on the stability and performance of the cells were systematically evaluated.

The LSV curves in figure 5(a) reveal a decrease in photocurrent for CIGS|CdS layers coated with metal oxides. This effect is particularly pronounced for Al₂O₃ and TiO₂, which also show a significant reduction in photovoltage and a shift in the CO₂ reduction onset toward more negative potentials compared to the bare CIGS|CdS pn-junction electrode. Chronoamperometry measurements at -1.3 V vs SCE (figure 5(b)) reveal distinct photocurrent behaviors among the different oxide-coated electrodes. Electrodes coated with Al₂O₃ exhibit minimal photocurrents, remaining below 1 mA·cm⁻². In contrast, TiO₂-coated electrodes sustain a photocurrent of approximately 2 mA·cm⁻², with a slight increase over time. NiOₓ and SnO₂ layers initially generate a higher photocurrent of around 4 mA·cm⁻²; however, for NiOₓ, this rapidly declines within the first 10 s, stabilizing at approximately 2 mA·cm⁻². Similarly, the photocurrent for SnO₂ gradually decreases over 10 min before reaching a stable value of around 2 mA·cm⁻². After 30 min, among the tested materials, TiO₂-coated electrodes and bare CIGS/CdS (without protective layers) exhibit the highest photocurrents. ALD of metal oxide thin films is well known for its excellent conformality. On silicon substrates, 10 nm-thick films typically exhibit distinct electrical properties: SnO₂ and TiO₂ are slightly n-type conductive, NiOₓ is p-type, and Al₂O₃ remains insulating. However, since ALD growth is highly substrate-dependent, the properties observed on silicon cannot be directly extrapolated to other substrates such as CdS. In the case of CdS, the deposited films may be thinner or exhibit non-uniform coverage. While a 3 nm Al₂O₃ insulating layer is likely to hinder photoactivity, SnO₂, TiO₂, and NiOₓ appear to participate in photocatalysis.

Gas analysis after 30 min (table 2) confirmed a total FE close to 100%, regardless of whether the electrode was bare CIGS|CdS or covered by oxide layers. CIGS|CdS and CIGS|CdS|Al₂O₃ electrodes exhibited nearly
<table><caption>**Table 2.** Faradic efficiency, CO selectivity and H₂ selectivity of different configurations after 30 min of reaction.</caption><thead><tr><th></th><th>FE total (%)</th><th>Selectivity CO (%)</th><th>Selectivity H₂ (%)</th></tr></thead><tbody><tr><td>Bare CIGS</td><td>88.0</td><td>69.1</td><td>30.9</td></tr><tr><td>CIGS/CdS</td><td>101.5</td><td>98.5</td><td>1.5</td></tr><tr><td>CIGS/CdS/TiO<sub>2</sub></td><td>99.2</td><td>96.9</td><td>3.1</td></tr><tr><td>CIGS/CdS/SnO<sub>2</sub></td><td>102.7</td><td>91.1</td><td>8.9</td></tr><tr><td>CIGS/CdS/NiO<sub>x</sub></td><td>103.9</td><td>85.4</td><td>14.6</td></tr></tbody></table>

100% selectivity toward CO. However, selectivity decreased slightly in the following order: TiO₂, SnO₂, and NiOₓ, with the latter increasingly favoring H₂ production.

These results underscore the complex influence of additional overlayers on both the performance and stability of CIGS/CdS photoelectrodes during photocatalysis. The notable decline in activity observed with Al₂O₃ and TiO₂ suggests that, while these layers may offer some degree of surface passivation, they can also impede charge transport, promote interfacial recombination, or overly insulate the CdS layer, ultimately limiting its photocatalytic impact.

### 3.3. Photoluminescence (PL) characterization

PL spectroscopy is a powerful, non-destructive technique to study the transport and optoelectronic properties of semiconductors. Its ability to correlate PL characteristics with charge carrier dynamics, defect states, and surface recombination processes makes it invaluable for optimizing photoelectrode efficiency. In this study, steady-state PL imaging is employed to examine two key aspects: (i) the impact of oxide layer deposition on the CIGS/CdS junction and (ii) the behavior of photoelectrodes before and after 1800s photo-EC reactions. As previously discussed, the primary function of these oxide layers is to enhance the stability of photoelectrodes under operational conditions. The PL results will provide additional insights beyond conventional device performance metrics, offering a detailed assessment of the quality, uniformity, and stability of the thin-film semiconductor layers.

Hyperspectral PL imaging [42] highlights the spatial variation of PL across the samples, providing local insights into material properties. Among the most relevant that can be extracted are the energy band gap (E_g) and QFLS, mathematically expressed as Δμ. The internal voltage in a semiconductor is commonly defined as the QFLS divided by the elementary charge e. The product of electrons and holes densities generated upon light absorption, is proportional to the exponential of the Δμ, therefore measuring Δμ is a direct way of estimating the carrier density within the material.

First, we acquired a hyperspectral image of the pristine samples, as shown in figure 6(b). The corresponding spatially averaged PL spectra for the entire series of samples are reported in figure 6(a). The stacks present different PL intensities, with CIGS|CdS|TiO₂ and CIGS|CdS|Al₂O₃ showing the most significant reduction in PL intensity compared to the reference sample. For the reference and Al₂O₃ samples, two specimens were analyzed to assess the reproducibility of the deposition process. In both cases, the optoelectronic properties of the twin samples were found to be comparable to each other, confirming the consistency of the fabrication method. The oxide deposition resulted in a homogeneous coverage of the CIGS|CdS surface; however, it also affected the optoelectronic properties of the absorber layer. In order to quantify these effects, we fitted pixel-by-pixel the absoluted calibrated average PL spectra, based on Generalized Planck's Law [28] for all the architectures. Further details on the model used for fitting the data and on the hyperspectral experiment are reported in the experimental section.

Importantly, whatever the oxides used the band gap of the absorbers, remained constant at 1.19 eV. However, a small reduction of QLFS, on the order of 20 meV for Al₂O₃ and 30 meV for TiO₂, was observed, as shown in table 3, indicating a detrimental effect of the oxide deposition which may be caused by the higher deposition temperature of these layers. This aligns with the electrical measurements presented in figure 4, where a decrease in collected current density was detected after oxide deposition. It is important to note that PL measurements were conducted under open-circuit conditions. Therefore, a decrease in overall PL intensity suggests increased recombination rates, which, in this case, can be attributed to an increased surface recombination rate due to oxide deposition.

We then investigated the role of these oxides in preserving the photoelectrodes during photocatalysis and providing a deeper understanding of the interplay between surface chemistry and photoelectrode performance. Using the same approach, we determined E_g and QFLS values after the electrochemical reactions (table 3). Across all samples, E_g remained constant before and after the reaction at the value of E_g = 1.19 eV. Meanwhile, Δμ decreased for all the samples after the reaction.

However, despite the reference sample and those with NiO and SnO₂ initially exhibiting better optoelectronic properties, all tested compositions converge to a similar state after 1800 s of reaction
<figure><img src="image_9.png" /><figcaption>**Table 3.** Average energy gap and quasiFermi-level-splitting values calculated after fitting pixel-by-pixel the PL spectra over an arbitrary area of each sample. Apexes 1 and 2 respectively refer to the values found by fitting the PL intensity as function of the wavelength of emission before and after the reaction has occurred.</figcaption></figure>

<table><thead><tr><th>Sample</th><th><i>E</i><sub>g</sub><sup>(1)</sup></th><th>Δ<i>μ</i><sup>(1)</sup></th><th><i>E</i><sub>g</sub><sup>(2)</sup></th><th>Δ<i>μ</i><sup>(2)</sup></th></tr></thead><tbody><tr><td>CIGS|CdS</td><td>1.19 eV</td><td>0.72 eV</td><td>1.19 eV</td><td>0.66 eV</td></tr><tr><td>CIGS|CdS|SnO<sub>2</sub></td><td>1.19 eV</td><td>0.72 eV</td><td>1.19 eV</td><td>0.68 eV</td></tr><tr><td>CIGS|CdS|NiO</td><td>1.19 eV</td><td>0.72 eV</td><td>1.19 eV</td><td>0.68 eV</td></tr><tr><td>CIGS|CdS|TiO<sub>2</sub></td><td>1.19 eV</td><td>0.69 eV</td><td>1.19 eV</td><td>0.66 eV</td></tr><tr><td>CIGS|CdS|Al<sub>2</sub>O<sub>3</sub></td><td>1.19 eV</td><td>0.70 eV</td><td>1.19 eV</td><td>0.68 eV</td></tr></tbody></table>

(figure 7c). The variation of the QFLS is most evident in the CIGS|CdS sample, with a loss of 60 meV. Nevertheless, the semiconductor layer, retains good optoelectronic properties at the end of the reaction, although reduced compared to the initial state. This suggests that the degradation begins at the top interface of the system, while the bulk properties of the materials are preserved. These findings are consistent with the chronoamperometry measurements, which indicate performance reductions primarily due to charge carrier transport phenomena occurring at the interface between the cells and the electrolyte.

## 4. Conclusion

In summary, we showed that while bare CIGS absorbers could be employed as a photocathode for direct semiconductor/electrolyte CO₂R, the addition of co-catalysts such as Cu₂O or ZnO enhances both charge transport and catalytic activity, while impacting the selectivity. However, the photocurrent density and selectivity can be highly improved by applying a suitable electron transport layer such as CdS leading to the pn junction. The comparison between bare CIGS and CIGS|CdS pn junctions underscores the critical role of interface engineering in enhancing photocathode efficiency. Our findings reveal that while bare CIGS can facilitate CO₂ reduction to CO (~2 mA cm⁻² at -1.3 V vs SCE), the introduction of a pn junction significantly improves both current density and selectivity. A well-optimized CIGS|CdS electrode achieves outstanding performance, with CO₂ reduction selectivity reaching up to 98.5% and photocurrent densities of approximately 8 mA cm⁻² at -1.3 V vs SCE under 1 sun illumination. This represents a significant enhancement in CO selectivity compared to other architectures tested under identical aprotic conditions (table 1), highlighting the beneficial effect of the pn junction and interfacial engineering To enhance stability,
ultra-thin metal oxide layers (e.g. TiO₂, NiO, Al₂O₃, and SnO₂ deposited by ALD) were applied. While these layers effectively improve stability and can also exhibit catalytic properties, they often lead to a reduction in photocurrent. Faradaic efficiencies close to 100% were achieved for both the CIGS|CdS electrode and its oxide-coated counterparts. However, CO₂ reduction selectivity was highest for Al₂O₃ and TiO₂, whereas SnO₂, and especially NiOₓ, favored H₂ production. PL analysis further revealed that while oxide deposition does not affect the band gap energy of the absorbers, it slightly reduces the quasi-Fermi level splitting, leading to an initial decline in performance. However, after 1800 s of reaction, all tested architectures converge to a similar state, suggesting that degradation primarily occurs at the interface rather than within the bulk material. Despite these trade-offs, our results establish CIGS as a highly promising material for PEC systems, highlighting the crucial role of pn junctions in achieving both high efficiency and photocurrent for CO₂ reduction. However, further interface engineering and the integration of optimized co-catalysts remain essential to achieving stable and high-yield performance.

## Data availability statement

The data cannot be made publicly available upon publication because no suitable repository exists for hosting data in this field of study. The data that support the findings of this study are available upon reasonable request from the authors.

## Acknowledgment

This work has benefited from French State aid managed by the Agence Nationale de la Recherche under France 2030 plan, bearing the reference code ANR-22-PESP-0010: Projet ciblé ‘POWERCO2’ within the PEPR project SPLEEN. T. Raimondi and S. Cacovich thank funding from the ANR under the JCJC project CLEOPATRA (ANR-23-CE50-0017).

## ORCID iDs

Thomas Tom 0000-0001-8436-6728

Nathanaelle Schneider 0000-0001-7749-2400

Stefania Cacovich 0000-0002-6402-4816

Negar Naghavi 0000-0002-6045-5096

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