# Ag-alloying of CIGS absorber layers: impact of the composition, rubidium fluoride post-treatment and bandgap variations

Thomas Tom, Jackson Lontchi, Amelle Rebai, Vincent Dufoulon,
Jean-Francois Guillemoles, Negar Naghavi

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## To cite this version:

Thomas Tom, Jackson Lontchi, Amelle Rebai, Vincent Dufoulon, Jean-Francois Guillemoles, et al. "Ag-alloying of CIGS absorber layers: impact of the composition, rubidium fluoride post-treatment and bandgap variations." Journal of physics, 2025, 7 (4), pp.045005. 10.1088/2515-7655/adf00d hal-05328140

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# Ag-alloying of CIGS absorber layers: impact of the composition, rubidium fluoride post-treatment and bandgap variations

To cite this article: Thomas Tom et al 2025 J. Phys. Energy 7 045005

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PAPER

Ag-alloying of CIGS absorber layers: impact of the composition, rubidium fluoride post-treatment and bandgap variations

Thomas Tom¹,*, Jackson Lontchi¹, Amelle Rebai², Vincent Dufoulon¹, Jean-Francois Guillemoles¹ and Negar Naghavi¹,*,

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

* Authors to whom any correspondence should be addressed.

E-mail: thomas.tom@cnrs.fr and negar.naghavi@chimieparistech.psl.eu

Keywords: chalcopyrite, ACIGS, Ag-alloying, solar cells

Supplementary material for this article is available online

Abstract

Chalcopyrite-based solar cells have achieved remarkable efficiency, yet further advancements remain challenging. The partial substitution of copper (Cu) with silver (Ag) in Cu(In,Ga)Se₂ (CIGS) thin-film solar cells presents a promising approach to enhance absorber properties and improving device performance. This study explores the moderately high-concentration Ag incorporation (Ag/(Ag + Cu) = 0.19) via thermal evaporation prior to co-evaporation of CIGS absorbers, combined with rubidium fluoride (RbF) post-deposition treatment. Additionally, the impact of Ag alloying is examined across absorbers with bandgaps ranging from 1.12 to 1.55 eV. Results indicate that Ag incorporation significantly influences gallium distribution, promotes larger grain growth, and enhances open-circuit voltage (V<sub>oc</sub>). Notably, (Ag,Cu)(In,Ga)Se₂ (ACIGS) absorber with a 1.23 eV bandgap achieves a power conversion efficiency of 18.1%, while a 1.55 eV bandgap absorber reaches 7.1%.

**1. Introduction**

Copper indium gallium selenide (CIGS) solar cells have emerged as a leading thin-film photovoltaic
technology, achieving impressive power conversion efficiencies (PCEs) of up to 22%–23% over the past
decade [1, 2]. This remarkable progress is attributed to advancements in absorber layer deposition
techniques, device architecture, post-deposition treatments (PDTs) and other optimizations and recently
researchers explored Ag alloying as a promising strategy [3, 4].

The introduction of Ag into the CIGS matrix results in the partial substitution of Cu atoms on group-I
lattice sites, forming (Ag,Cu)(In,Ga)Se₂ (ACIGS) alloys, which offers several advantages, including improved
grain growth, better absorber quality, and enhanced carrier transport properties [5, 6]. Notably, ACIGS
absorber layers, have demonstrated efficiency improvements, with a record efficiency of 23.64% reported by
researchers at Uppsala University [7]. This milestone demonstrates the potential of Ag in optimizing the
absorber layer characteristics. Most high-efficiency ACIGS devices employ Ag incorporation during the
three-stage CIGS deposition process. While effective, this method adds complexity to the fabrication process
of the ACIGS absorber layers. To simplify the alloying process, our study adopts a more accessible and
scalable approach: thermal evaporation of Ag onto the molybdenum (Mo) back contact before the CIGS
deposition. This method facilitates moderately high-concentration Ag alloying without the need for
additional complexities during the three-stage deposition, offering a streamlined path to achieving enhanced
device performance.

This study systematically investigates the impact of moderately high-concentration Ag deposition by
thermal evaporation for the development of ACIGS absorber layers, along with the influence of rubidium
fluoride (RbF) PDT. While Ag alloying and RbF treatment have both been explored in the literature the
combined and decoupled effects of deposition rate of the Ag precursor, alkali fluoride treatment type (RbF vs KF), and GGI composition on ACIGS absorber quality and solar cell performance remain underexplored [8–13]. A key focus of this work is on achieving a stable and flat silver-to-copper ratio (AAC) profile throughout the absorber layer bulk, which is critical for reproducible device performance. We demonstrate that by carefully controlling the deposition rate of the Ag precursor, it is possible to maintain an uniform AAC distribution, avoiding unwanted Ag accumulation or surface segregation. This behavior is then compared under KF and RbF treatment. Finally, we investigate the impact of a fixed Ag concentration, combined with variations in the gallium-to-indium (GGI) ratio, on bandgap modifications, morphology, structural properties, and solar cell performance in ACIGS absorber layers.

## 2. Experimental procedure

The absorber layer and full device fabrication process comprises the following steps. A 500 nm Mo layer was deposited onto soda-lime glass substrates via DC sputtering, followed by the thermal evaporation of 35 nm of Ag at controlled rates onto the Mo-coated surfaces to prepare ACIGS absorber layers. ACIGS and CIGS absorber layers with thicknesses of approximately 1.7–2.0 µm were fabricated using a three-stage co-evaporation process (see supporting information, figure S1), followed by PDT with potassium fluoride (KF) and sodium fluoride (NaF) in a selenium (Se) ambient. For comparison, additional samples were treated first with a matrix of indium (In), gallium (Ga), and RbF, followed by RbF and NaF under the same Se ambient conditions (see supporting information, figures S1(a) and (b), respectively). Cell fabrication included the deposition of approximately 30 nm of cadmium sulfide (CdS) through chemical bath deposition, followed by 70 nm of intrinsic zinc oxide (i-ZnO) and 250 nm of aluminum-doped ZnO (AZO) as window layers using radio-frequency sputtering. Afterwards, 9 individual cells for each type absorbers, each with an active area of 0.1 cm², were precisely defined through photolithography.

The quantification of the bulk elemental concentrations in the CIGS and ACIGS absorber layers was performed by the x-ray fluorescence (XRF) measurements using an XRF-Fisherscope. The silver plus copper-to-indium gallium ratio (ACGI), AAC, copper-to-indium gallium ratio (CGI), and GGI values of the CIGS absorbers were then calculated, while the in-depth compositional profiles were analyzed via glow discharge optical emission spectroscopy (GDOES) using a Horiba Scientific GD-Profiler HR. The crystalline structure of the films was characterized using x-ray diffraction (XRD) on a PANalytical DY711 system with CuKα radiation. The crystallite size (D) of the CIGS samples was determined by applying a Gaussian fit to the (112) reflection in the XRD pattern using Debye–Scherrer equation, $D = Kλ/βcosθ$, K is the shape factor, λ is the x-ray wavelength, β is the full width at half maximum (FWHM) of the fitted peak, and θ is the Bragg angle. A spherical crystallite shape with cubic symmetry was assumed, corresponding to a shape factor K = 0.94. The uncertainty in the crystallite size was estimated based on the step size of the diffraction angle during measurement, as well as the uncertainties in β and θ from the Gaussian fitting procedure. An additional uncertainty of 0.01 nm was considered for the x-ray wavelength to account for possible source variation. Cross-sectional imaging of the layers was carried out with a Zeiss Merlin scanning electron microscope (SEM). Root mean square (RMS) roughness of the films were calculated from the atomic force microscopy (AFM) using Agilent AFM, PicoScan 3000. Raman spectra of the absorbers were recorded with a Horiba Jobin Yvon HR instrument, employing a 532 nm laser. The current density–voltage (J–V) characteristics of the solar cells were measured under standard test conditions (100 mW cm⁻², AM1.5 G spectrum) using a Newport Class AAA solar simulator. Finally, the external quantum efficiency (EQE) was analyzed using an IQE200 Newport instrument. The bandgap values of the absorbers were determined from the first derivative of the EQE spectra.

## 3. Results and discussion

### 3.1. Optimization of deposition rate of the Ag precursor

As the initial study, the effects of evaporation rates of Ag on Mo substrates for moderately high-concentration Ag alloying in CIGS absorber layers were studied. Ag films were fabricated at deposition rates of 2 nm s⁻¹ and 0.02 nm s⁻¹ for comparison. The morphological properties of films, including uniformity and surface roughness, were characterized using SEM and AFM. SEM images of Ag films deposited at rates of 2 nm s⁻¹ and 0.02 nm s⁻¹ indicate similar uniformity, with no distinguishable differences between the samples (supporting information figure S2). However, AFM measurements as shown in figures 1(a) and (b) reveal a slight difference in surface roughness, with RMS values of 6.8 nm for films deposited at 2 nm s⁻¹ and 5.6 nm for those at 0.02 nm s⁻¹ respectively.

The ACIGS absorber layers formed with these Ag films were subsequently treated with NaF and KF. The elemental depth profiles of Ag, Cu, In, Ga, and Se in the resulting ACIGS films were determined using
<figure><img src="image_5.png" /><figcaption>**Figure 1.** AFM images of Ag thin films grown on Mo glass substrates by thermal evaporation at different deposition rates: (a) 2 nm s⁻¹ and (b) 0.02 nm s⁻¹.</figcaption></figure>

<figure><img src="image_6.png" /><figcaption>**Figure 2.** Elemental depth profiles from GDOES of ACIGS films formed with deposition rates of the Ag precursor of (a) 2 nm s⁻¹ and (b) 0.02 nm s⁻¹, respectively.</figcaption></figure>

GDOES. Figures 2(a) and (b) illustrate the elemental depth profiles for ACIGS films with deposition rates of the Ag precursor of 2 nm s⁻¹ and 0.02 nm s⁻¹, respectively under identical CIGS deposition conditions. The corresponding Na depth profiles shown in supporting information, figure S3.

The ACIGS absorber layer prepared with deposition rate of the Ag precursor of 2 nm s⁻¹ exhibited significant Ag accumulation near the surface, indicating undesired diffusion of Ag from the back to the front of the absorber layer. In contrast, the ACIGS layer with deposition rate of the Ag precursor of 0.02 nm s⁻¹ displayed a uniform Ag distribution throughout the bulk of the absorber layer, while the elemental profiles of Cu, In, Ga, and Se remained similar in both cases. Solar cells fabricated using these absorber layers demonstrated markedly different performance. Cells with significant Ag diffusion to the surface (from the 2 nm s⁻¹ deposition rate) exhibited a PCE of 3.5%. Meanwhile, cells with the uniform Ag profile (from the 0.02 nm s⁻¹ deposition rate) achieved a much higher PCE of 17.5% (supporting information figure S5 and table S2). This significant performance disparity indicates that excessive Ag diffusion to the surface can lead to shunting and reduced efficiency. This interpretation is further supported by XRF measurements (supporting information table S1), which reveal a ACGI ratio of 1.32 for the 2 nm s⁻¹ absorber indicating substantial group I (Ag + Cu) excess. Such a high ACGI value is known to promote phase segregation and void formation, as reported by Essig *et al* [5]. Conversely, the 0.02 nm s⁻¹ absorber exhibited a mildly over-stoichiometric ACGI value of 1.09, which is generally considered beneficial or at least tolerable for good device performance. However, it should be noted that the slightly over-stoichiometric ACGI value of 1.09 may be affected by a minor XRF measurement offset, likely due to Ag-induced matrix effects and calibration limitations, as stoichiometry reference points can vary slightly between laboratories. This is further confirmed by the GDOES depth profile which shows a marked difference in the GGI ratio and AAC ratio (supporting information figure S4). Specifically, the 2 nm s⁻¹ sample shows a higher AAC ratio to the surface proving the accumulation of Ag to the surface, causing Ag-rich surface phases. These results show that the Ag deposition rate directly influences Ag incorporation and absorber stoichiometry, with higher rates leading to poor Ag integration and elevated ACGI values, while lower rates promote balanced
<table><caption>**Table 1.** Elemental compositions of the CIGS and ACIGS absorber layers with different PDT.</caption><thead><tr><th></th><th>(Cu + Ag)/(In + Ga)</th><th>Cu/(In + Ga)</th><th>Ga/(In + Ga)</th><th>Ag/(Ag + Cu)</th><th>E<sub>G</sub> (eV) from d(EQE)/dE</th></tr></thead><tbody><tr><td>CIGS_KF</td><td>0.82</td><td>0.82</td><td>0.42</td><td>0</td><td>1.22</td></tr><tr><td>ACIGS_KF</td><td>1.09</td><td>0.88</td><td>0.45</td><td>0.19</td><td>1.23</td></tr><tr><td>ACIGS_RbF</td><td>1.06</td><td>0.86</td><td>0.47</td><td>0.19</td><td>1.23</td></tr></tbody></table>

<figure><img src="image_7.png" /><figcaption>**Figure 3.** (a) GGI and (b) AAC depth profiles for CIGS and ACIGS absorber layers with different PDT.</figcaption></figure>

composition and improved solar cell performance. The findings highlight the importance of achieving reduced surface roughness during thermal evaporation, as evidenced by the AFM images. Therefore, all subsequent ACIGS depositions discussed in this study were carried out using the optimized deposition rate of the Ag precursor of 0.02 nm s⁻¹.

## 3.2. Impact of Ag on morphology, structure and solar cell parameters

Building on the optimized deposition rate of the Ag precursor identified in the previous section, this part of the study explores the broader impact of Ag incorporation on the morphological, structural, and solar cell parameters of the ACIGS absorber layers. Both CIGS and ACIGS absorber layers were fabricated under identical processing and PDT (NaF and KF) conditions. Comparative analyses were carried out to assess variations in elemental composition, grain structure, crystallinity, and device performance metrics.

The ACGI, CGI, GGI and AAC ratios were calculated from XRF measurements and is given in table 1. The ACIGS layers exhibited a moderately high Ag concentration, with a ACIG ratio of 1.09 and AAC ratio of 0.19. ACIGS layers were found to have slightly higher GGI ratio than the CIGS layers due to the modification in the band gap profile by the enhanced elemental interdiffusions. The GDOES measurements were also preformed to understand the impact of Ag addition on the GGI profile which is given in figure 3(a) (blue and green plots). The Ag addition flattens the notch region, and it also modifies the front and back GGI grading with low concentrations of Ga towards the front and high concentrations towards the back of the absorber layer [3]. Additionally, a notably flat AAC profile was observed, as shown in figure 3(b) (green plot).

The surface morphology of CIGS and ACIGS absorber layers was analyzed and the corresponding cross-sectional SEM images and shown in figures 4(a) and (b), respectively. The ACIGS layers exhibit larger columnar grain growth compared to the CIGS layers. This observation aligns with existing literature, which reports that Ag alloying results in smoother surfaces and promotes the formation of larger grains [14]. Figure 4(d) (blue and green plots) presents the XRD patterns of the CIGS and ACIGS layers. Both layers display the dominant (112) peak alongside other characteristic chalcopyrite peaks, including (220), (204), and (312) (ICDD definition file 00-035-1102). The CIGS layer exhibited a crystallite size of 23.4 ± 2.8 nm, while the ACIGS absorber layer showed slightly large crystallite size of 24.9 ± 2.5 nm. This result is further corroborated by SEM measurements, which reveal larger columnar grain growth in the ACIGS layers, emphasizing the beneficial role of Ag in improving the structural and morphological properties of the absorber layer.

Finally, solar cells were fabricated using both CIGS and ACIGS absorber layers. The results show the improved performance metrics of the ACIGS solar cells compared to the CIGS solar cells. The *J-V*
<figure><img src="image_8.png" /><figcaption>**Figure 4.** SEM cross-sectional images: (a) CIGS with post-deposition treatment by KF, (b) ACIGS with post-deposition treatment by KF, and (c) ACIGS with post-deposition treatment by RbF, (d) XRD patterns of the ACIGS and CIGS absorber layers with different post-deposition treatments.</figcaption></figure>

<figure><img src="image_9.png" /><figcaption>**Figure 5.** (a) Current-voltage characteristics, and (b) EQE of CIGS and ACIGS cells treated with KF, and RbF.</figcaption></figure>

characteristics are shown in figure 5(a) (blue and green plots). Specifically, the open-circuit voltage (Voc) increased by approximately 150 mV, and the fill factor (FF) improved by 5%, for the ACIGS solar cell compared to the CIGS solar cell. The observed improvements in device performance are attributed to the increase in grain size and reduction in grain boundary density, as well as a likely reduction in structural defects and sub-bandgap disorder. These enhancements are linked to the Ag-induced reduction in melting point and improved interdiffusion of cationic species, resulting in longer carrier diffusion lengths and improved electronic quality of the absorber [3, 8, 14]. These enhancements contributed to a PCE of 17.5% for ACIGS cells, compared to 12.1% for CIGS cells. The EQE of the fabricated CIGS and ACIGS are shown in figure 5(b) (blue and green plots). The solar cell parameters are summarized in table 2 for the best cells, while the box plots along with the average and standard deviation values are presented in the supporting information (figure S6 and table S3). These results highlight the significant role of Ag in enhancing the electrical and structural properties of the absorber layer, leading to superior solar cell performance.

### 3.3. Impact of RbF on ACIGS absorber layer

As mentioned in some of the reported works to further enhance the properties of ACIGS absorber layers, post-treatment was performed using NaF and RbF instead of the KF [15, 16]. The ACIGS absorber layers treated with RbF, exhibited a moderately high Ag concentration with a AAC ratio of 0.19. The ACIG ratio was measured at 1.06, indicating a slightly metal-rich composition as shown in table 1. Compared to the
<table><caption>**Table 2.** Solar cell parameters of the fabricated devices.</caption><thead><tr><th>Solar cells</th><th>V<sub>oc</sub> (mV)</th><th>J<sub>sc</sub> (mA cm<sup>-2</sup>)</th><th>FF (%)</th><th>PCE (%)</th></tr></thead><tbody><tr><td>CIGS_KF</td><td>542</td><td>30.4</td><td>73.2</td><td>12.1</td></tr><tr><td>ACIGS_KF</td><td>687</td><td>32.7</td><td>77.9</td><td>17.5</td></tr><tr><td>ACIGS_RbF</td><td>712</td><td>32.2</td><td>78.8</td><td>18.1</td></tr></tbody></table>

<table><caption>**Table 3.** Elemental compositions of the ACIGS absorber layers as a function of different Ga concentrations.</caption><thead><tr><th>(Cu + Ag)/<br>(In + Ga)</th><th>Cu/(In + Ga)</th><th>Ga/(In + Ga)</th><th>Ag/(Ag + Cu)</th><th>E<sub>G</sub> (eV) from<br>d(EQE)/dE</th></tr></thead><tbody><tr><td>1.15</td><td>0.93</td><td>0.28</td><td>0.19</td><td>1.12 eV</td></tr><tr><td>1.06</td><td>0.86</td><td>0.47</td><td>0.19</td><td>1.23 eV</td></tr><tr><td>0.97</td><td>0.77</td><td>0.76</td><td>0.19</td><td>1.38 eV</td></tr><tr><td>0.87</td><td>0.70</td><td>0.89</td><td>0.19</td><td>1.55 eV</td></tr></tbody></table>

KF-treated ACIGS sample, the RbF-treated layers showed a slightly higher GGI ratio of 0.47. This increase is attributed to the additional step RbF-PDT with the In and Ga fluxes. This effect is evident in the modified GGI grading observed in the GDOES depth profiles (figures 3(a) and (b)), where the RbF-treated absorber exhibits a flatter GGI profile compared to the KF treated absorber layers (red plot). Despite these changes, the AAC profile remained relatively flat, similar to that in the KF-treated absorber layers, indicating consistent Ag incorporation throughout the absorber bulk.

The cross-sectional SEM images in figure 4(c) demonstrate that ACIGS layers treated with RbF exhibit significantly improved columnar grain growth compared to those treated with KF. During PDT, heavier alkali elements such as Rb preferentially migrate to grain boundaries, reducing charged defects and promoting secondary phase formation at grain boundaries, thus changes the morphology of the absorber [17, 18]. This morphological change can also be attributed to the additional step involved in the RbF PDT, which might have slightly increased the thickness of the absorber layer as observed from figure 4(c). The XRD measurements further support this observation, revealing a larger crystalline size of 39.0 ± 3.1 nm for RbF-treated layers, compared to 24.9 ± 2.5 nm for KF-treated layers which is shown in figure 4(d) (red plot).

Solar cells subjected to RbF post-treatment exhibited significantly enhanced cell parameters, as shown in figure 5(a) (red plot). Notably, the V<sub>oc</sub> increased to 712 mV, representing a 25 mV improvement over cells treated with KF. This enhancement in V<sub>oc</sub> is likely attributable to increased carrier concentration and reduced recombination rates in the device [19]. This also improved the FF with RbF-treated cells achieving a FF of 78.8%, compared to 77.9% for KF-treated cells, as detailed in table 2 for the best cells (box plots along with the average and standard deviation values are presented in the supporting information figure S6 and table S3). As a result, RbF-treated solar cells achieved a PCE of 18.1%, surpassing the performance of their KF-treated absorber cells. Also, the J<sub>sc</sub> remained nearly identical for both types of cells, as evidenced by the EQE spectra presented in figure 5(b).

### 3.4. ACIGS absorbers with varying bandgap

To further explore the tunability and performance optimization of ACIGS absorber layers, a series of films were fabricated with varying Ga content while maintaining a constant Ag concentration (AAC = 0.19). This approach enabled systematic control over the bandgap, allowing for a detailed study of its influence on structural, morphological, and photovoltaic parameters. Using a fixed deposition rate of the Ag precursor of 0.02 nm s<sup>-1</sup> and PDT with NaF and RbF, absorber layers with GGI ratios ranging from 0.28 to 0.89 were produced, corresponding to bandgaps between 1.12 eV and 1.55 eV. The elemental compositions, determined by XRF measurements and summarized in table 3, show GGI ratios of 0.28, 0.47, 0.76, and 0.89, corresponding to bandgaps ranging from 1.12 eV, 1.23 eV, 1.38 eV and 1.55 eV respectively.

While the primary aim was to adjust the GGI ratio, it is inherently challenging to do so without affecting ACGI, particularly when the overall AAC is kept constant. This compositional interdependence reflects the complexity of multi-element co-evaporation processes, where modifying one group of elements often influences the incorporation and distribution of others. Therefore, some variation in ACGI across the samples in table 3 is expected and should be considered when interpreting the resulting device performance.

To investigate the effect of Ga gradient distribution within the absorbers, GDOES analysis was performed on all samples. As shown in figure 6(a), the GGI depth profiles reveal that Ag incorporation flattens the notch region and modifies both the front and back GGI grading. With increasing Ga content, the GGI ratio rises accordingly, leading to a systematic increase in bandgap from 1.12 eV to 1.55 eV. Figure 6(b) shows the flat AAC profile for all the absorbers with AAC ratio of 0.19.
<figure><img src="image_10.png" /><figcaption>**Figure 6.** (a) GGI, and (b) AAC depth profiles for ACIGS absorber layers with different bandgaps from 1.12 to 1.55 eV.</figcaption></figure>

<figure><img src="image_11.png" /><figcaption>**Figure 7.** SEM cross-sectional images for ACIGS layers of bandgaps (a) 1.23 eV, (b) 1.38 eV, and (c) 1.55 eV, respectively.</figcaption></figure>

To further understand the influence of GGI ratio on the structural properties of the absorber layers, SEM analysis was conducted. The morphological evolution of films with bandgaps of 1.23 eV, 1.38 eV, and 1.55 eV is depicted in figure 7, respectively. As the GGI ratio increases, a reduction in grain size is observed, accompanied by an increase in grain boundary density. This microstructural change suggests that higher Ga incorporation hinders grain growth, potentially affecting charge carrier transport [20].

The crystallinity of the absorber layers was assessed through XRD measurements, revealing a dominant (112) chalcopyrite peak in all films, as shown in figure 8(a). With increasing Ga content, this peak shifts to higher diffraction angles due to the substitution of larger In atoms with smaller Ga atoms, leading to lattice contraction [21]. The crystallite size, calculated from the FWHM of the (112) peak, reaches a maximum of 39.0 ± 3.1 nm at a bandgap of 1.23 eV. Beyond this point, further Ga incorporation reduces crystallite size. This structural evolution is further supported by Raman spectroscopy, which provides additional insights into the vibrational properties of the films. As shown in figure 8(b), the Raman spectra exhibit a progressive shift in the A1 mode from 173.8–180.9 cm⁻¹ with increasing Ga content. This shift is attributed to the higher Raman frequency associated with Ga substitution at In sites. Since Ga atoms have a lower atomic mass than In atoms, their incorporation leads to shorter bond lengths, resulting in an increase in vibrational frequency [22].

The cell performance of the fabricated ACIGS solar cells was evaluated through J-V measurements, as presented in figure 9(a). Table 4 summarizes the photovoltaic parameters of the best cells, including V<sub>oc</sub>, J<sub>sc</sub>, FF, and PCE (box plots along with the average and standard deviation values are presented in the supporting information figure S7 and table S4). As the GGI ratio increases, the bandgap widens, reducing the spectral response range. Consequently, a significant decline in J<sub>sc</sub> is observed, as confirmed by the EQE spectra in figure 9(b). The decrease in EQE near the band edge observed with increasing GGI can be attributed to increasingly unfavorable conduction band alignment between the (Ag,Cu)(In,Ga)Se<sub>2</sub> absorber and the CdS buffer, which may create a spike-like barrier that impedes electron collection and enhances interfacial recombination. In addition, the lower EQE may also result from poorer absorber quality commonly observed in wide bandgap CIGS, leading to reduced carrier collection efficiency due to enhanced bulk recombination [23, 24].
<figure><img src="image_12.png" /><figcaption>**Figure 8.** (a) XRD patterns of the ACIGS absorber layers with varying bandgaps, (b) Raman spectra of ACIGS films with different GGI ratios. All spectra are normalized to the CIGS A1 peak.</figcaption></figure>

<figure><img src="image_13.png" /><figcaption>**Figure 9.** (a) Current-voltage characteristics, and (b) EQE of ACIGS absorber layers with varying bandgaps.</figcaption></figure>

<table><caption>**Table 4.** Solar cell parameters of the fabricated devices with varying bandgaps.</caption><thead><tr><th>Bandgap</th><th>V<sub>oc</sub> (mV)</th><th>J<sub>sc</sub> (mA cm<sup>-2</sup>)</th><th>FF (%)</th><th>PCE (%)</th></tr></thead><tbody><tr><td>1.12 eV</td><td>580</td><td>36.3</td><td>75.0</td><td>15.8</td></tr><tr><td>1.23 eV</td><td>712</td><td>32.2</td><td>78.8</td><td>18.1</td></tr><tr><td>1.38 eV</td><td>773</td><td>17.7</td><td>71.3</td><td>9.8</td></tr><tr><td>1.55 eV</td><td>800</td><td>12.9</td><td>68.3</td><td>7.1</td></tr></tbody></table>

The highest *J*<sub>sc</sub> of 36.3 mA cm<sup>-2</sup> is obtained for cells with a 1.12 eV bandgap, achieving a PCE of 15.6%. The highest PCE of 18.1% is recorded for cells with a 1.23 eV bandgap (GGI = 0.47). While *V*<sub>oc</sub> increases with bandgap, the *V*<sub>oc</sub> deficit also rises, indicating a degradation in absorber quality at higher Ga content. The highest *V*<sub>oc</sub> of 800 mV is obtained for cells with 1.55 eV bandgap. This deterioration in absorber quality is further reflected in the FF, which decreases from 80% to 68% as Ga content increases. These findings suggest that while higher Ga incorporation increases the bandgap and *V*<sub>oc</sub>, it also introduces structural and electronic limitations that reduce overall device efficiency [24, 25].

The bandgap values of the all the absorber layers in this study were determined from the slope of the EQE absorption edge as shown in supplementary figure S3 and table S2.
## 4. Conclusion

In this study, we demonstrate a feasible and scalable approach for incorporating Ag into CIGS-based solar cells through the thermal evaporation of Ag onto the Mo back contact prior to CIGS deposition. This method enables moderately high-concentration Ag alloying without complicating the conventional three-stage deposition process, providing a straightforward pathway to enhance device performance. Our findings highlight that Ag deposition conditions, its concentration relative to the GGI ratio, and the choice of post-treatment are critical factors in achieving optimal results.

Characterization techniques such as SEM, and XRD revealed that Ag incorporation promotes the growth of larger columnar grains and this effect was further amplified by RbF PDT. The resulting ACIGS solar cells exhibited exceptional performance, with a FF of up to 78.8% and a PCE of 18.1%. Additionally, we investigated the influence of Ag concentration (AAC = 0.19) and bandgap variations across different GGI ratios in ACIGS absorbers. Devices fabricated with varying bandgaps demonstrated PCEs of 18.1% for a bandgap of 1.23 eV and 7.1% for a bandgap of 1.55 eV. These results not only validate the effectiveness of Ag incorporation but also provide valuable insights into the interplay between absorber layer properties and device performance in ACIGS solar cells.

## Data availability statement

All data that support the findings of this study are included within the article (and any supplementary files).

## Acknowledgment

This work is supported by ANR's program (ANR-22-CE05-0019) within the SOLARMETHACELL project. The authors express their gratitude to Alexandre Crossay and the SoyPV team for their assistance with CdS deposition, as well as Alexandre Blaizot from IPVF for performing the AFM measurements.

## ORCID iDs

Thomas Tom ID 0000-0001-8436-6728

Negar Naghavi ID 0000-0002-6045-5096

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