# Assessment of indium-free transparent conductive oxide back contacts for high efficiency ultra-thin Cu(In,Ga)Se2 solar cells down to 250 nm

Fabien Mollica, Marie Jubault, Frederique Donsanti, Muriel Bouttemy, Arnaud Etcheberry, Negar Naghavi

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Fabien Mollica, Marie Jubault, Frederique Donsanti, Muriel Bouttemy, Arnaud Etcheberry, et al. Assessment of indium-free transparent conductive oxide back contacts for high efficiency ultra-thin Cu(In,Ga)Se2 solar cells down to 250 nm. Progress in Photovoltaics, 2025, 33 (10), pp.1109-1123. 10.1002/pip.70013. hal-05363724

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Assessment of indium-free transparent conductive oxide back contacts for high
efficiency ultra-thin Cu(In,Ga)Se₂ solar cells down to 250 nm

Fabien Mollica<sup>a</sup>, Marie Jubault<sup>a</sup>, Frederique Donsanti<sup>a</sup>, Muriel Bouttemy<sup>b</sup>, Arnaud Etcheberry<sup>b</sup>,
Negar Naghavi<sup>a,c</sup>

<sup>a</sup>EDF R&D, IPVF 18 boulevard Thomas Gobert, 91120 Palaiseau, France

<sup>b</sup>Institut Lavoisier de Versailles (ILV), Université de Versailles Saint-Quentin en Yvelines, Université Paris-Saclay, CNRS, UMR 8180, 45 avenue des Etats-Unis, 78035 Versailles Cedex, France

<sup>c</sup>Institut Photovoltaïque d'Ile-de-France (IPVF), UMR 9006, CNRS, Ecole Polytechnique, IP Paris, Chimie ParisTech, PSL, Palaiseau, France

*corresponding author, e-mail: negar.naghavi@chimieparistech.psl.eu

**Abstract**

This work examines the feasibility and performance impact of replacing the usual molybdenum back-
contact with indium-free transparent conductive oxides (TCOs) like fluorine-doped tin oxide (SnO₂:F)
and aluminum-doped zinc oxide (ZnO:Al) for ultra-thin Cu(In,Ga)Se₂ (CIGS) solar cells (250-450 nm).
Motivated by indium scarcity and cost reduction, these TCOs are evaluated for their figure of merit,
stability under Se atmosphere, Na diffusion permeability, and band alignment with CIGS absorbers.
Using simulations, prototype fabrication, and comprehensive characterizations, the compatibility of
these TCOs with CIGS absorbers is assessed. Solar cells with thicknesses of 450 nm and 250 nm are
fabricated. Their performance was compared under both rear and front illumination, as well as with the
use of reflectors. A record efficiency of 8.6% with front illumination is achieved for a 250 nm CIGS
absorber using a gold back reflector with SnO₂:F, single-step CIGS deposition, and no heavy alkalines
doping. The best rear-illuminated efficiencies are obtained with ZnO:Al back contacts, reaching 6% for
a 250 nm CIGS, with only a 9% loss in Jsc compared to front illumination confirming a lower surface
recombination rate at the ZnO:Al/CIGS interface compared to Mo/CIGS or SnO₂:F/CIGS interfaces.

**Keywords** : Ultra-thin Cu(In,Ga)Se₂ Solar cells, Bifacial solar cells, Indium-free Transparent conducting back contact, Back-contact recombination

# 1. Introduction

In recent years, advancements in solar cell technology have brought crystalline silicon (c-Si) and III-V semiconductors, such as GaAs, close to the Shockley-Queisser efficiency limit, achieving remarkable
efficiencies of 27.3% [1] and 29.1%[2], respectively. Conversely, polycrystalline Cu(In, Ga)Se₂ (CIGS) solar cells, despite reaching a record efficiency of 23.6% by Ag alloying (ACIGS) [3], are still behind. In CIGS solar cells, where performance is limited by bulk recombination [4], reducing the absorber thickness in combination with a carrier-selective layer can lead to an increase in open-circuit voltage and improved carrier collection [4, 5]. Consequently, reducing the CIGS absorber thickness presents a promising way to enhance the efficiencies and industrial competitiveness of these photovoltaic modules[6].

The conventional structure of a CIGS solar cell is made of a Cu(In,Ga)Se₂/CdS heterojunction deposited on a molybdenum (Mo) back contact and completed with an undoped ZnO (i-ZnO) and aluminum-doped zinc oxide (ZnO:Al) window layer. While Mo initially served as a back contact, its limitations, such as high interface recombination and low optical reflectance[7, 8], underscore the need for highly reflective back contacts that meet the criteria of being ohmic, selective, and passivating while also enduring the harsh conditions encountered during CIGS growth at high substrate temperatures (>550°C).

In the last decade, significant progress have been made in light management and interface passivation techniques to achieve high-efficiency ultra-thin CIGS solar cells [7, 9-15]. Light management requires a reflective back contact although many materials used for this purpose are not stable during the deposition of CIGS. A notable solution has been to combine a bifacial solar cells, with a back reflector. This reflector can be positioned either on the rear side of the glass [5, 15] or encapsulated between two TCOs [16]. Such architectures based on TCO back contact offer compatibility with cost-effective industrial processes, back contact texturing strategies for additional light trapping in ultra-thin CIGS layers, and the TCO itself can act as a passivated layer and diffusion barrier.

The deposition of a CIGS absorber on a TCO has been extensively explored for various applications, including tandem solar cells[17-20], bifacial devices[21-27], and solar windows[28-30]. While several well-known TCOs, such as tin-doped indium oxide (In₂O₃:Sn or ITO), fluorine-doped tin oxide (SnO₂:F or FTO), aluminium-doped zinc oxide (ZnO:Al or AZO), hydrogen-doped In₂O₃ (In₂O₃:H or IOH) [31] and tungsten-doped In₂O₃ (In₂O₃:W or IOW) [32] have served as back contacts for these applications, ITO stands out as the most studied option[25, 33-36]. Very recently Yang et al.[34] reported a certified bifacial solar cell on a glass substrate/ITO with efficiencies of 19.77% and 10.89% under front and rear illumination, respectively. Despite its advantages, concerns have arisen regarding the sustainability of ITO due to high indium prices and the concentration of indium resources in a very small number of countries. This paper aims to focus on the growth of ultra-thin CIGS absorbers, specifically those with thicknesses lower than 500 nm, on transparent back contacts that do not rely on indium. These alternative back contacts include the other two most widely used TCOs, SnO₂:F and ZnO:Al.

The objective of this study is to assess and gain insights into the advantages and disadvantages associated with the use of indium-free transparent conducting oxides as back contact for ultra-thin (less than 500
nm) CIGS solar cells. After a bibliographic reminder on the main efficiency loss factors in ultrathin CIGS solar cells, key factors, such as the band alignment between TCOs and CIGS, the influence of their conductivity, the ohmic characteristics of the contacts, and their chemical stability during CIGS deposition, will be thoroughly evaluated. Furthermore, the study will explore sodium (Na) diffusion from the substrate to the absorber through these TCOs and analyze the chemical composition at the interface between CIGS and TCO using techniques like X-ray photoelectron spectroscopy (XPS) and nano-Auger. Finally, the impact of these TCOs on the performance of a 450±10 and 250±10 nm thick CIGS solar cell will be compared and discussed.

## 2. Experimental details

**Back contact fabrication:** Samples were prepared on soda-lime glass using four distinct types of back contacts: (1) an 800 nm molybdenum (Mo) layer deposited by sputtering; (2) a 300 nm Al₂O₃/800 nm Mo layer. Atomic layer deposited Al₂O₃ layer, acts as a barrier against Na diffusion from the glass during CIGS deposition; (3) an 1100 nm ZnO:Al layer deposited by sputtering ($R_{sheet} = 8 \ \Omega/\text{square}$); (4) a commercial SnO₂:F layer (650 nm, $R_{sheet} = 7 \ \Omega/\text{square}$).

**CIGS cell fabrication:** CIGS with two different thicknesses of 450 nm and 250 nm were deposited by thermal co-evaporation of Cu, In, Ga, and Se in a single-step process at a substrate temperature between 500 and 550°C. The average composition of the different samples measured by Energy Dispersive X-ray Spectrometer (EDX) is $\text{CGI}=[\text{Cu}]/([\text{In}] + [\text{Ga}]) = 0.86 \pm 0.02$ and $\text{GGI}=[\text{Ga}]/([\text{In}] + [\text{Ga}]) = 0.35 \pm 0.01$ as the atomic ratio. After CIGS deposition, NaF was evaporated for 4 min at approx. 1 nm/min under a Se atmosphere at 400 °C. The solar cells were finalized with a 50 nm CdS layer deposited in a chemical bath, a 60 nm ZnO layer and a 350 nm ZnO:Al layer deposited both by RF sputtering.

**Structure and morphology characterization:** The morphology of the samples was studied by a ZeissMerlin VP scanning electron microscope. The composition of the CIGS was analyzed using EDX in the Scanning Electron Microscopy (SEM) with an accelerating voltage of 15 kV and a current of 1.6 nA. The depth profile of the composition was determined by glow discharge optical emission spectroscopy (GD-OES) using a Horiba Jobin-Yvon instrument.

The signal intensities for Cu, In, Ga, and Se were calibrated using a standard sample. However, it was not possible to quantify the Na signal directly. To facilitate the interpretation of the Na profile, the Na signal was normalized by dividing it by the total luminescence signal (Fi).

**X-ray photoelectron spectroscopy (XPS):** Surface chemical analyses using XPS were performed with a Thermo Electron K-Alpha spectrometer, with a monochromatic Al-K X-ray source at
1486.6 eV. Calibration of the spectrometer was conducted according to the ASTM-E-902-94 standard, using copper and gold reference samples to ensure accuracy. The high-resolution photo-peak acquisition settings included a 400 µm spot size, a primary energy of 12 kV, an emission current of 6.0 mA, a constant analyzer energy (CAE) of 10 eV, and an energy step size of 0.05 eV. An argon ion (Ar+) source was used for depth profiling, operating at 2 keV with a beam current of 10 mA. The data were processed using the Avantage© software.

**Nano-Auger:** A JEOL JAMP 9500F instrument was utilized for the Auger Electron Spectroscopy (AES) measurements, operating with a beam intensity of 6 nA, an accelerating voltage of 20 kV and an energy resolution of $dE/E = 0.5\%$. With these settings, the spot size was around 12 nm, and the analysis depth was kept below 5 nm. The sputtering rate, calibrated for $SiO_2$, was determined to be 24 nm per minute. Depth profiling was conducted using argon ion (Ar+) sputtering at 2 kV and 2 µA, with a tilt angle of 30° and an argon pressure of $8 \times 10^{-2}$ Pa. To maintain accurate positioning throughout each sputtering step and during spectrum acquisition, a probe tracking tool was employed to correct for any drift (ex: charging effects or beam drift). This approach enabled precise control over the analysis position throughout the depth profile (using a static sputtering mode and a step-by-step manual profiling technique), allowing for an accurate characterization of the CIGS/$SnO_2$:F interface.

**Solar cell characterization:** I(V) measurements were conducted on solar cells with an area of 0.1 cm², both in the dark and under illumination simulating standard 1.5G A.M., utilizing a Newport Class AAA solar simulator. The transparent cells were tested by directing light either through the ZnO:Al/ZnO/CdS window layers for front-side illumination or through the glass/TCO substrate for back-side illumination. The external quantum efficiency (EQE) was determined using a Newport IQE200 system for both front and back illumination conditions.

**Modelling:** A Ray-Transfer Matrix simulation tool was employed to perform the optical simulations, as referenced in [14]. The optical models, defined by their dielectric functions, were derived from reflectance and transmittance data obtained using a Lambda900 Perkin Elmer optical spectrometer for both $SnO_2$:F and ZnO:Al layers, as well as through ellipsometric analysis performed using a Horiba UVISEL ellipsometer for CIGS (with a band gap of 1.17 eV) and Mo. Additional data for ZnO and CdS were sourced from existing literature.

### 3. Results and discussion

Based on previous research findings [7, 12, 13, 37, 38], reducing the thickness of a CIGS absorber on a Mo back contact to below 1000 nm has a detrimental effect on cell performance, specifically on both the short-circuit current ($J_{sc}$) and open-circuit voltage ($V_{oc}$). Two primary reasons are contributing to the
decrease in $J_{sc}$: diminished charge carrier generation (resulting from fewer photons being absorbed into the CIGS layer) and reduced charge carrier collection due to recombination. Consequently, the $V_{oc}$ is also reduced as charges are generated closer to the CIGS/Mo interface, increasing recombination within the cell[7, 39]. Thus, ultra-thin solar cells encounter a major challenge due to the properties of the CIGS/Mo interface, which exhibits low reflectivity and a high rate of surface recombination. The low reflection reduces the generation of charge carriers within the CIGS absorber, resulting in a lower $J_{sc}$. Simultaneously, the high surface recombination rate impedes the collection of charge carriers in the Quasi Neutral Region (QNR), further diminishing $J_{sc}$ and reducing the expected increase in the $V_{oc}$ for thinner CIGS absorbers. However, this loss in $J_{sc}$ can be minimized and $V_{oc}$ can be improved by integrating a back contact that is both reflective and passivating. Under such conditions, ultra-thin CIGS solar cells have the potential to surpass the performance of traditional, thicker solar cells[39]. To achieve this objective, an easy way is to modify the cell architecture by substituting the Mo back contact with a TCO like ITO, $SnO_2$:F, or ZnO:Al combined with a passivation layer. This TCO can either encapsulate a mirror[33] or be paired with a mirror placed at the rear side of the bifacial solar cell[15].

In these configurations, the ideal transparent back contact should exhibit several key characteristics, including:

* Sufficient electrical conductivity.

* High optical transparency.

* The ability to establish an ohmic contact and facilitate hole collection.

* Acting as an electron barrier to minimize surface recombination of electrons.

* Permeability to sodium (Na) diffusion.

* Maintaining both optical and electrical performance even under high temperatures in selenium (Se) or sulfur (S) atmosphere.

In the following sections, we will conduct a comprehensive investigation and modeling of how these factors impact and interact with the performance of CIGS solar cells. To achieve this, we have selected two different In-free TCOs: ZnO:Al with thicknesses of 350 nm and 1200 nm, as well as 650 nm-thick $SnO_2$:F.

To separate the effects of the absorber composition from back contacts, we have deposited a 450±10 nm and 250±10 nm thick CIGS absorber with a standard 1.2 eV bandgap, without gradient, with an external supply of Na and without the use of additional dopants such as K and Cs. This standard CIGS has been deposited by co-evaporation on both TCOs and for comparison on a conventional Mo substrate and a Mo back-contact on an $Al_2O_3$-coated glass to prevent Na diffusion from glass.

**3.1. Impact of the TCO resistivity on the cell performance**
The transparency of the TCO as a back contact will mainly affect the $J_{sc}$ of the solar cell, whereas its resistivity will affect the series resistance and thus the Fill factor (FF). When a reflector is placed directly behind the TCO back contact (Figure 1a, Internal schematic), the thickness of the TCO must be carefully chosen to balance transparency and electrical conductivity.

The effect of changing the thickness of TCO back contacts on the performance of CIGS solar cells was first evaluated through numerical analysis (Eq. 3 and Figure 1). Specifically, we considered TCO thicknesses ranging from 50 nm to 2000 nm and two CIGS thicknesses, 500 nm and 250 nm. We used different resistivity values for the TCO back-contact, namely $1.10^{-3}$ ohm.cm, $1.10^{-4}$ ohm.cm, and 0 ohm.cm. We assumed a constant absorption coefficient for the TCO, representing different TCO quality levels. It was modelled with the optical parameter of the ZnO:Al. The case with a resistivity of 0 ohm.cm corresponds to a scenario where a metallic reflector (for example gold (Au) or silver (Ag)) is used under the TCO that can efficiently collect charges.

Given a $V_{oc}$ of 600 mV for CIGS solar cells on a Mo substrate, the efficiency can be estimated from the $J_{sc}$ and FF values determined for various TCO thicknesses. The calculations of the $J_{sc}$ as a function of the TCO thickness have been carried out by RTM while FF was calculated by varying the $R_t$ as series resistance in SCAPS simulation [40] with :

$$R_t = \rho/2t \qquad \text{eq.1}$$

and

$$\rho = R_{\text{sheet}} \cdot t \qquad \text{eq.2}$$

where $R_{\text{sheet}}$ is the sheet resistance measured by the 4-point probe of the TCO, and t the thickness of the layer. The parameters used in SCAPS for the CIGS model and for the ZnO:Al, ZnO, CdS, and Mo layers are detailed in Table SI.1, and table SI.2 respectively.

This efficiency can then be compared to the efficiency of a cell with the same CIGS thickness on Mo back-contact based on Eq. 3:

$$\Delta \text{Efficiency} = J_{\text{sc}}^{\text{TCO}}(t) \cdot \text{FF}^{\text{TCO}}(t) \cdot \left[ V_{\text{oc}}^{\text{Mo}} + 1.5 \cdot \frac{kT}{q} \cdot \ln \left( \frac{J_{\text{sc}}^{\text{TCO}}(t)}{J_{\text{sc}}^{\text{Mo}}} \right) \right] - J_{\text{sc}}^{\text{Mo}} \cdot \text{FF}^{\text{Mo}} \cdot V_{\text{oc}}^{\text{Mo}} \qquad \text{eq.3}$$

Where $J_{sc}^{TCO}(t)$, $V_{oc}^{Mo} \cdot \text{FF}^{TCO}(t)$ are the solar cell parameters with TCO back contact, $J_{sc}^{Mo} \cdot \text{FF}^{Mo} \cdot V_{oc}^{Mo}$ correspond to the solar cell parameters with reference Mo back contact, k is the Boltzmann's constant, q=1.602×10⁻¹⁹ C is the elementary charge, and T the temperature.

Figure 1a illustrates the behaviour of CIGS cells with a 500-nm thickness when there is no TCO resistance. As observed the cell efficiency $\Delta Efficiency$ decreases with the increase of the TCO thickness. For a 50-nm-thick TCO, the cell efficiency exhibits a 1.25% absolute increase (+2.6 mA/cm²) compared to a Mo back-contact. However, for a 2000-nm-thick TCO, the absolute increase drops to 0.35% (+0.8 mA/cm²).
<figure><img src="image_2.png" /><figcaption>**Figure 1.** Simulated efficiency (black lines) and short circuit (dash red lines) variations of a) 500 nm thick CIGS and b) 250 nm thick CIGS cell on a TCO/Ag back contact as a function of TCO layer thickness and resistivity compared with a cell on Mo with the same CIGS thickness (The dashed lines at 0 and the upper value correspond to the modelled efficiency of similar samples using Mo as the back contact).</figcaption></figure>

This phenomenon is primarily due to TCO absorption in the near-infrared spectral range. When TCO resistivity increases, particularly in the case of thin TCO layers, cell efficiency decreases due to the heightened series resistance. As a result, when the TCO has a resistivity of $10^{-4}$ ohm·cm, the highest
cell efficiency is obtained with a TCO back contact that is 400 nm thick, leading to an absolute efficiency gain of 0.7% compared to the Mo back contact. In contrast, when the TCO resistivity is around $10^{-3}$ ohm.cm, the performance advantage relative to the Mo back contact is negligible.

Figure 1b illustrates comparable trends in CIGS cells with a 250-nm thickness, showcasing more substantial improvements. In the absence of TCO resistance, a 50-nm-thick TCO results in a noteworthy 2.3% absolute efficiency increase (+4.4 mA/cm²) compared to the Mo back-contact, diminishing to a 1.1% absolute improvement (+2.2 mA/cm²) for a 2000-nm-thick TCO.

With a TCO resistivity of $10^{-4}$ ohm·cm, the optimum TCO thicknesses are between 200 and 400 nm, leading to a 1.6% absolute efficiency gain compared to the Mo back-contact. Conversely, for a TCO resistivity of $10^{-3}$ ohm·cm, the optimal TCO thickness is 800 nm, resulting in a 1.1% absolute efficiency improvement compared to the Mo back-contact.

In summary, the benefits of a TCO/reflector back-contact compared to a Mo back-contact depend on the specific configuration:

- When the reflector is electrically isolated from the TCO back-contact: a 250-nm-thick CIGS with 200 to 400 nm TCO thicknesses can result in a 1.6% absolute increase in cell efficiency, emphasizing the importance of a sufficiently thick TCO for lateral charge conduction.

- When the reflector is involved in lateral conduction: a 250-nm-thick CIGS with only a 50-nm-thick TCO demonstrates a more substantial 2.3% absolute increase in cell efficiency, with emphasis on the critical role of the reflector's stability at high temperatures

Based on these simulations, the best-case scenario predicts an absolute increase of 1.6% to 2.0% in cell efficiency with a 250 nm-thick CIGS layer compared to a cell on Mo. To achieve this, the TCO must have high conductivity ($\rho < 10^{-4} \Omega \cdot cm$) and a negligible TCO/CIGS contact resistance. Although this efficiency gain may seem modest, the proposed configuration is straightforward to fabricate. Furthermore, there is still room for improvement.

## 3.2. TCO/CIGS band-alignment and ohmic contact formation

When n-type TCOs are used as back contact in CIGS solar cells, a Schottky barrier at the TCO/CIGS interface can impede hole extraction. This occurs when there is a disparity in work function between the transparent electrode and semiconductor causing increased series resistance and reduced efficiency [39].
<figure><img src="image_3.png" /><figcaption>**Figure 2.** Energy level in the TCO/CIGS contacts before equilibrium.</figcaption></figure>

Figure 2 shows the energy band alignment at the TCO/CIGS interface prior to equilibrium, comparing the cases of SnO₂:F and ZnO:Al. Theoretically, when the electron affinity of the TCO is higher than that of CIGS, an ohmic contact is expected to form. Based on this criterion, SnO₂:F, with an electron affinity of approximately 5.0 eV, should be capable of forming an ohmic contact with CIGS. In contrast, ZnO:Al, whose electron affinity is around 4.5 eV and close to that of CIGS, may instead form a rectifying (Schottky) contact.

However, these theoretical predictions do not always match experimental observations. The actual electrical behavior at the interface is strongly influenced by interfacial chemical reconstruction and defect states, which can significantly alter the expected band alignment and charge transport mechanisms.

In our previous study [15], we examined the electrical characteristics of different TCO/CIGS back contacts using devices that included a 150 nm thick gold layer deposited on top of the CIGS absorber. Due to the very low resistance of the CIGS/Au contact, the resulting I(V) characteristics were solely governed by the resistance of the back contact. Our results indicated that the Mo/CIGS/Au configuration had a very low total contact resistance of 0.04 ± 0.01 Ω·cm². In contrast, the SnO₂:F/CIGS back contact exhibited an ohmic behavior, but with a much higher contact resistance of 0.9 ± 0.3 Ω·cm² compared to the Mo substrate. The ZnO:Al/CIGS back contact displayed nearly-ohmic behavior, with a contact resistance of 2.3 ± 0.7 Ω·cm². The observation of an ohmic contact for the ZnO:Al back contact was surprising, as prior studies have frequently reported rectifying behavior associated with the formation of a GaOx blocking layer [41-43]. Two possible explanations were proposed to account for this unexpected behavior:
1. A high concentration of sodium (Na) at the ZnO:Al /CIGS interface could result in a heavily p-doped layer, facilitating an ohmic contact by tunneling conduction.

2. If a GaOₓ phase does exist, it may be extremely thin. Given that GaOₓ has a wide band gap of 4.5 eV and is n-type doped, it could create a significant valence band spike that would typically impede hole collection. However, a sufficiently thin GaOₓ layer could allow for tunneling transport of holes, potentially assisted by a high concentration of defects, thus resulting in ohmic behavior.

These experimental findings highlight that the behavior at these interfaces is shaped by the chemical interface reconstruction between the TCO and CIGS layers, which may not align with conventional rules and assumptions [16, 41, 44]. Therefore, selecting a transparent conductive electrode requires careful consideration of factors such as its compatibility with other layers, the temperatures used during processing, and its stability throughout the device fabrication process.

### 3.3. TCO's characteristics

Before proceeding to an in-depth characterization of the interfaces between TCO and CIGS, we first characterized the TCOs used in this study. To evaluate the performance of a transparent electrode, a figure of merit is often used (Eq.4)[45]

$$ \text{Figure of merit} = 1/(\alpha \cdot \rho) \qquad \text{Eq.4} $$

Where ρ is the resistivity and α is the absorption coefficient as defined in equation 5:

$$ \alpha = -\frac{1}{d} \cdot \ln \left( \frac{T}{1-R} \right) \qquad \text{Eq.5} $$

with *d* the thickness of the TCO, *T* the transmission, and *R* the reflection both of which were measured using optical spectroscopy with an integrating sphere to take into account the diffuse light.

Table 1 presents data on the resistivity, absorption coefficient at 900 nm (α(900 nm) cm⁻¹, and the figure of merit for the three different samples: ZnO:Al layers with thicknesses of 350 nm and 1200 nm, and the 650 nm thick SnO₂:F layer. The 650 nm-thick SnO₂:F and 1200 nm-thick ZnO:Al samples exhibit the highest figures of merit, making them the preferred choices for cell fabrication.

<table><caption>**Table 1.** The absorption coefficient at 900 nm, the resistivity and the figure of merit of TCO back contacts used in this study</caption><thead><tr><th></th><th>α(900 nm) cm<sup>-1</sup></th><th>ρ (Ω.cm)</th><th>Figure of merit(Ω<sup>-1</sup>)</th></tr></thead><tbody><tr><td>350-nm thick ZnO:Al</td><td>4.2 10<sup>3</sup></td><td>6.0 10<sup>-4</sup></td><td>0.4</td></tr><tr><td>1200-nm thick ZnO:Al</td><td>2.6 10<sup>3</sup></td><td>5.4 10<sup>-4</sup></td><td>0.7</td></tr></tbody></table>
<table><tr><td>650-nm thick SnO<sub>2</sub>:F</td><td>3.5 10<sup>3</sup></td><td>4.0 10<sup>-4</sup></td><td>0.7</td></tr></table>

### 3.4. Assessment of the chemical stability of the TCO at high temperatures under selenium atmosphere

Since CIGS is deposited at elevated temperatures (500 to 550°C) in a selenium (Se) atmosphere, the chemical, electrical, and optical characteristics of the SnO₂:F and ZnO:Al layers may alter during the deposition process. To assess how the properties of the TCO layers change under conditions similar to those of CIGS growth, ZnO:Al layers with thicknesses of 600 nm and 1200 nm, as well as SnO₂:F layers, were annealed in a graphite box at 500°C and 600°C in a selenium atmosphere.

The reflectance, transmittance, and the sheet resistance of each TCO substrate have been measured before and after annealing at 500 and 600°C and reported in Figure 3.

Annealing the 350-nm-thick ZnO:Al at 500 and 600°C under the Se atmosphere greatly affects both near-infrared transmittance and sheet resistance (figure 3a, d). A similar negative impact is observed when annealing the 1200-nm-thick ZnO:Al at 600°C, but moderate changes occur at 500°C (Figure 3b). The opposite trends in transmittance and sheet resistance, according to the Drude model, suggest a loss of doping concentration in transparent conducting oxides. The annealing process in air or Se may remove oxygen vacancies, but ZnO:Al deterioration appears to be mainly a surface effect, more pronounced in thinner films.
<figure><img src="image_4.png" /><figcaption>**Figure 3.** Transmittance (T) and reflectance (R) of TCO substrates without annealing and annealed at 500°C and 600°C under Se atmospheres: a) 350 nm-thick ZnO:Al on glass, b) 1200 nm-thick ZnO:Al on glass and c) 650 nm-thick SnO₂:F on glass d) Sheet resistance of the various TCO substrates without annealing and with annealing at 500°C and 600°C.</figcaption></figure>
In contrast, annealing under the Se atmosphere does not affect the transmittance and resistivity of the SnO₂:F substrate Figure 3c), indicating its stability at high temperatures.

Grazing Incidence X-ray Diffraction (GIXRD) analysis was performed on substrates coated with SnO₂:F and ZnO:Al, both prior to and following annealing at 600°C in a selenium atmosphere [46]. The XRD diagrams for SnO₂:F remained identical before and after annealing, indicating its stability under Se atmosphere. In contrast, ZnO:Al, layers showed new peaks post-annealing, including peaks attributed to the ZnSe phase, suggesting the replacement of oxygen with selenium and the formation of ZnSe. Additionally, the (1 0 3)/(0 0 2) ratio and (1 0 3)/(1 0 2) ratio of ZnO increased after annealing, indicating a change in the preferential orientation of ZnO.

**3.5. Thermodynamic analysis**

During the Cu(In,Ga)Se₂ co-evaporation, the transparent conducting back contact comes into contact with not only Se but also the Cu(In,Ga)Se₂ phase and, as a result, its associated binaries and ternaries. The standard Gibbs free energy of the reaction (ΔGf) can be used to estimate the reactivity of binaries and ternaries with the TCO back contact. **Table 2** summarizes the ΔGf of various reactions that could occur at the CIGS/TCO interface.

If ΔG<sub>f</sub> is negative, the reaction is thermodynamically favoured. The thermodynamical stability of CIS and CGS were estimated by Guillemoles & al[47] and their stabilization energy ΔG<sub>f</sub> are positive (approximately 50 and 25 kJ/mol, Table 2 reaction A and B). Consequently, the CIGS decomposition is not possible without an external source of energy if any reaction with the constituent binaries (Cu<sub>2</sub>Se, In<sub>2</sub>Se<sub>3</sub>, Ga<sub>2</sub>Se<sub>3</sub>) is thermodynamically not allowed.

For the ZnO:Al back-contact, the ΔG<sub>f</sub> calculated for the reactions between ZnO and Se (reaction D), In<sub>2</sub>Se<sub>3</sub> (reaction E) and Ga<sub>2</sub>Se<sub>3</sub> (reaction F) are negative. Therefore, ZnSe, In<sub>2</sub>O<sub>3</sub>, and Ga<sub>2</sub>O<sub>3</sub> layers can be formed if In<sub>2</sub>Se<sub>3</sub> and Ga<sub>2</sub>Se<sub>3</sub> are formed at the beginning of the process. Moreover, the instability of the ZnO:Al/CIGS interface is increased by the favoured decomposition reaction of the CuGaSe<sub>2</sub> phase to Ga<sub>2</sub>O<sub>3</sub>, ZnSe, and Cu<sub>2</sub>Se (reaction P).

On the opposite, for the SnO₂:F back-contact, the ΔGf calculated for the reactions between SnO₂ and Se, Cu₂Se, In₂Se₃ and Ga₂Se₃ (reaction I-N) are positive. Thus, the SnO₂:F would not react during the CIGS deposition.

To summarize, the CIGS process is likely to affect the ZnO:Al layer leading to the thermodynamically favourable formation of a thin layer of ZnSe. In contrast, the SnO₂:F layer is perfectly stable up to 600°C. However, it is important to note that the reaction kinetics have not been considered here, and the coevaporation process is not in thermodynamic equilibrium. As a result, experimental results may deviate from the conclusions drawn from these calculations.
<table><caption>**Table 2.** Gibbs free energy of the potential reactions at the CIGS/back-contact. Thermodynamically favoured reactions are highlighted in green. [1] from [47]. [2] from software package HSC Chemistry 7.</caption><thead><tr><th></th><th>Reaction equation</th><th>Gibbs free energy at 500°C per mol of Mo, ZnO or SnO<sub>2</sub> ΔGf (kJ/mol)</th></tr></thead><tbody><tr><td>A</td><td>CuInSe<sub>2</sub> = 0.5(Cu<sub>2</sub>Se + In<sub>2</sub>Se<sub>3</sub>)</td><td>50 [1]</td></tr><tr><td>B</td><td>CuGaSe<sub>2</sub> = 0.5(Cu<sub>2</sub>Se + Ga<sub>2</sub>Se<sub>3</sub>)</td><td>25 [1]</td></tr><tr><td>C</td><td>Mo+Se<sub>2</sub> = MoSe<sub>2</sub></td><td>-153 [2]</td></tr><tr><td>D</td><td>2ZnO+1.5Se<sub>2</sub>(g) = 2 ZnSe + SeO<sub>2</sub> (g)</td><td>-30 [2]</td></tr><tr><td>E</td><td>In<sub>2</sub>Se<sub>3</sub> + 3ZnO = In<sub>2</sub>O<sub>3</sub> + 3ZnSe</td><td>-21 [2]</td></tr><tr><td>F</td><td>Ga<sub>2</sub>Se<sub>3</sub> + 3ZnO = Ga<sub>2</sub>O<sub>3</sub> + 3ZnSe</td><td>-53 [2]</td></tr><tr><td>G</td><td>Cu<sub>2</sub>Se + ZnO = Cu<sub>2</sub>O + ZnSe</td><td>87 [2]</td></tr><tr><td>H</td><td>CuSe + ZnO = CuO + ZnSe</td><td>63 [2]</td></tr><tr><td>I</td><td>SnO<sub>2</sub> + 1.5Se<sub>2</sub>(g) = SnSe<sub>2</sub> + SeO<sub>2</sub>(g)</td><td>176 [2]</td></tr><tr><td>J</td><td>SnO<sub>2</sub> + Se<sub>2</sub>(g) = SnSe(g) + SeO<sub>2</sub>(g)</td><td>286 [2]</td></tr><tr><td>K</td><td>In<sub>2</sub>Se<sub>3</sub> + 1.5SnO<sub>2</sub> = In<sub>2</sub>O<sub>3</sub> + 1.5SnSe<sub>2</sub></td><td>71 [2]</td></tr><tr><td>L</td><td>Ga<sub>2</sub>Se<sub>3</sub> + 1.5SnO<sub>2</sub> = Ga<sub>2</sub>O<sub>3</sub> + 1.5SnSe<sub>2</sub></td><td>7</td></tr><tr><td>M</td><td>2Cu<sub>2</sub>Se + SnO<sub>2</sub> = 2Cu<sub>2</sub>O + SnSe<sub>2</sub></td><td>287 [2]</td></tr><tr><td>N</td><td>2CuSe + SnO<sub>2</sub> = 2CuO + SnSe<sub>2</sub></td><td>241 [2]</td></tr><tr><td colspan="3"><strong>Decomposition reaction</strong></td></tr><tr><td>O</td><td>2CuInSe<sub>2</sub> + 3ZnO = Cu<sub>2</sub>Se + In<sub>2</sub>O<sub>3</sub> + ZnSe</td><td>12</td></tr><tr><td>P</td><td>2CuGaSe<sub>2</sub> + 3ZnO = Cu<sub>2</sub>Se + Ga<sub>2</sub>O<sub>3</sub> + ZnSe</td><td>-36</td></tr></tbody></table>

**3.6. Impact of the TCO on the Na diffusion from glass substrate to CIGS absorber**

Another important parameter when replacing the Mo back contact with transparent back contacts is their permeability to Na diffusion. To highlight the permeability of ZnO:Al and SnO<sub>2</sub>:F to Na diffusion, the Na concentration in the CIGS layer was estimated by GD-OES for a 450 nm thick CIGS co-evaporated by a one-step process at 450°C onto 4 different substrates:

* a standard Mo substrate,

* a Mo back contact on glass coated with an Al<sub>2</sub>O<sub>3</sub> layer (Mo/Al<sub>2</sub>O<sub>3</sub>) to block the diffusion of sodium (Na) from the glass.

* a commercial 650 nm thick SnO<sub>2</sub>:F substrate,
* a 1200-nm-thick ZnO:Al substrate,

To have a better comparison of the Na concentration across different samples, we reported the [Na]/[Cu] signal intensity by normalizing the Na signal with Cu as a reference element. Figure 4 shows the Na/Cu signal intensity ratios measured in the CIGS layers for the four different back contacts. It can be seen that the Na/Cu ratios for CIGS deposited on SnO₂:F and ZnO:Al back contacts are nearly zero, similar to those observed with the Mo/Al₂O₃ back contact. This indicates that both SnO₂:F and ZnO:Al back contacts effectively block the diffusion of Na from the glass substrate into the CIGS absorber layer.

<figure><img src="image_5.png" /><figcaption>**Figure 4.** Comparison of Na/Cu signal intensity ratio of the GD-OES profile for 450 nm-CIGS grown on SLG/Mo, SLG/Al₂O₃/Mo, SLG/ZnO:Al (1200 nm) and SLG/SnO₂:F (650 nm) back contacts.</figcaption></figure>

## 3.7. Rear interface analysis

Once the TCO properties had been analyzed, to better understand the phenomena occurring at the interface of the TCO/CIGS layers, detailed chemical characterizations of this interface were carried out using X-ray photoelectron spectroscopy (XPS) for ZnO:Al/CIGS and nano-AES for SnO₂:F/CIGS. CIGS absorber has been deposited in a single-step process by coevaporation with a CGI=[Cu]/([In] + [Ga]) = 0.86 ± 0.02 and GGI= [Ga]/([In] + [Ga]) = 0.35 ± 0.01.
To prevent degradation of TCO back contacts, the substrate temperature was maintained below 550°C, specifically between 500 and 550°C. Since sodium (Na) cannot diffuse from the glass through the TCO layer into the CIGS, an external source of Na was used to achieve optimal cell performance.

The SEM cross-sectional images of 300 nm thick CIGS on SnO₂:F and ZnO:Al, are presented in Figures 5 a and d. As observed, the surface of the SnO₂:F sample appears considerably rougher (±40 nm) compared to the ZnO:Al one, resulting in a rough SnO₂:F/CIGS interface, whereas the ZnO:Al/CIGS interface remains flat. Due to the challenging analysis caused by the roughness of SnO₂:F during XPS profiling, the SnO₂:F/CIGS interface was analyzed using nano-Auger.

To facilitate more favorable conditions for XPS and AES experiments, the CIGS surface was flattened, and the CIGS thickness was reduced through etching with a bromine solution [48].

<figure><img src="image_6.png" /><figcaption>**Figure 5.** Cross sectional SEM images of a 300-nm-thick CIGS on a) ZnO:Al and b) SnO₂:F back- contacts back-contacts and atomic percentage of the main elements measured b,c) by X-ray Photoelectron Spectroscopy (XPS) depth profile at the ZnO:Al/CIGS interface e,f) by Nano-Auger Emission Spectroscopy (nano-AES) depth profile at the SnO₂:F/CIGS interface</figcaption></figure>

### 3.7.a. The ZnO:Al/CIGS back-interface

Based on XPS analysis, the quantification was obtained considering O 1s (531.34 ±0.05 eV), Cu 2p3/2 (binding energy of 932.68±0.05 eV), In 4d5/2 (18.23±0.05 eV), Ga 3d5/2 (20.24±0.05 eV) and Se 3d5/2 (52.20±0.05 eV) peaks[49]. Figure 5 c depicts the depth profile of the ZnO:Al/CIGS interface. The interface was reached after about 1500 seconds, as shown in Figure 5c. The 2500 s measurement is representative of the interface composition. Then, a plateau of Zn and O is reached with a Zn/O 50:50 ratio, indicating that the interface has been crossed.
Figure 5c shows a correlation between the evolution of oxygen (O) and zinc (Zn) content, suggesting that oxygen is primarily bound to zinc. Only a small amount of oxygen can be attributed to the presence of copper (Cu), gallium (Ga), indium (In), or selenium (Se) oxides at the interface. While literature commonly reports the presence of GaOx oxides at the ZnO:Al/CIGS back-interface [50, 51], analysis of the Ga 2p 3/2 photopeak after 2500 s erosion at the interface reveals no clear contribution of Ga in an oxide phase, with a maximum concentration of 1% estimated from peak fitting and residual oxygen not bound to zinc. Similar results were obtained for Cu and In. Contrary to observations by other groups, XPS depth profiling indicates minimal composition variation at the ZnO:Al/CIGS interface, suggesting a very low content of the GaOx phase formed during the CIGS deposition process on ZnO:Al [50, 52]. One possible explanation for the negligible formation of GaOx could be the short deposition time required to grow 300 nm of CIGS, combined with the relatively low deposition temperature (< 550°C).

### 3.7.b. The SnO₂:F/CIGS back-interface

The depth profile of the SnO₂:F/CIGS interface, as depicted in Figure 5. f, reveals a well-defined interface. Oxygen (O) is used as a tracking element to highlight the appearance of signals from the SnO₂ layer, and transitions in Sn, In, Ga, and Se contents are observed as the interface is approached. Notably, there is a slight increase in Cu content with no specific experimental explanation. A strong correlation between O and Sn signals suggests a preference for O to be bound to Sn. The presence of InOₓ, GaOₓ, and CuOₓ is examined based on peak shapes and energy positions, revealing a constant chemical environment along the interface. While slight shifts in peaks are observed at the initial levels due to surface oxidation, the signal-to-noise ratio decreases toward the interface, making accurate diagnostics challenging. Despite the potential sputtering-induced reduction, the high surface sensitivity of AES suggests minimal oxide contributions at the interface, reaching the detection limit. Consequently, the presence of a GaOₓ phase is deemed unlikely. Similar observations apply to Ga energy peaks and In-MNN signals. While the formation of an InOₓ, GaOₓ, or CuOₓ oxide phase at the interface is not ruled out, their content is expected to be very low. This is also consistent with the calculated reaction enthalpies, which suggest that such oxide phases are unlikely to form in significant amounts. However, it is suggested that a Cu-rich phase is likely formed at the interface.

### 3.8. Solar cell performances

Figure 6 and Figure SI-3 compare cell performance and EQE of 450 and 250 nm thick CIGS absorbers on SnO₂:F and ZnO:Al back-contact to cell performance on Mo and a Mo/Al₂O₃ back contacts. As cells on SnO₂:F and ZnO:Al back-contact are semi-transparent, the samples were first characterized with a black paper on the rear side of the glass substrate to avoid light reflection on the substrate holder.
Average efficiencies of approximately 10% and 6.5% are reached on both Mo and Mo/Al₂O₃ substrates, with CIGS thicknesses of 450 nm and 250 nm, respectively. This reaffirms that NaF treatment
compensates for the absence of Na. As expected, solar cell efficiencies are higher for 450 nm thick CIGS compared to 250 nm thick CIGS. The enhanced efficiency in cells with SnO₂:F back contacts is primarily attributed to higher Jsc, while FF and Voc remain comparable for both thicknesses. In the case of 250 nm thick CIGS cells with SnO₂:F back contacts, an average efficiency of 7.6% ± 0.1% is observed, surpassing the reference Mo cell with an efficiency of 6.8% ± 0.2%. The increased Jsc, correlated with higher EQE curves, is linked to interface reflection. The variation in Voc is influenced by a slight positive effect of J02 (lower recombination in the SCR) and a negative effect of J01 (higher recombination in the QNR and interface), suggesting the possibility of high recombination at the SnO₂:F/CIGS back interface (table SI-1).

<figure><img src="image_7.png" /></figure>

**Figure 6.** I(V) light parameters for the samples on the different substrates with the best NaF PDT condition. Light I(V) parameters are given as mean ± standard deviation of the 10 best cells. A black-paper was inserted at the rear side of the glass substrate to avoid light reflection.
For ZnO:Al back contacts, the 250 nm CIGS absorbers exhibit lower $J_{sc}$, $V_{oc}$, and FF compared to their 450 nm counterparts. The cells on ZnO:Al back contact demonstrate an average efficiency of 6.8% ± 0.4%, similar to reference cells on Mo with 6.8% ± 0.2%. Despite the lower FF on ZnO:Al, the $J_{sc}$ is significantly higher, correlating with EQE measurements. The dark I(V) characteristics were also measured and fitted using the 2-diode model [53] to determine the saturation currents for ideality factors of 1 and 2 ($J_{01}$ and $J_{02}$), as well as to calculate the series resistance ($R_s$) and shunt resistance ($R_{SH}$). (Table SI-3). Analysis of the $J_{01}$ parameter suggests a slight positive effect on $V_{oc}$, possibly indicating improved charge collection due to a potential passivation effect at the interface. In summary, the efficiencies of SnO₂:F back contacts are almost identical to those of Mo back contacts, whereas the ZnO:Al back contact samples exhibit lower efficiencies, primarily due to a reduced FF. However, variations in the $J_{01}$ parameter hint at potential differences in the recombination process at the TCO/CIGS interface, prompting further investigation through rear-side illuminated EQE curve analysis.

Light absorption can be improved for the best cells obtained on SnO₂:F and ZnO:Al back-contact by replacing the black-paper placed at the back side of the glass substrate with a reflector. While Gouillard et al. [33] developed an integrated reflective back contact consisting of a highly reflective Ag mirror encapsulated in a thin ZnO:Al layer for thermal stability, followed by an ITO layer to contact the CIGS absorber, we simply placed a reflector mirror or a white paint on the back side of the glass substrate [15] and evaluated its impact on cell performances.

As observed in Table 3 and Figure SI-2, for 450 nm thick samples, the use of the Cu-reflector for samples on SnO₂:F back-contact improves the $J_{sc}$ of 1 mA/cm² compared to the measurement without reflection and of 2.4 mA/cm² compared to the reference cell on Mo. Finally, a cell efficiency of 10.6% is reached. The cell measured with the white paint reflector shows almost no improvement in $J_{sc}$ compared with the cell measured without a reflector.

The use of a Cu-reflector for cells on ZnO:Al back-contact also improves the $J_{sc}$ by 1 mA/cm² compared to the measurement without reflector and by 2.3 mA/cm² compared to the Mo reference.

However, the cell efficiency remains lower than the cell on Mo (8.4% compared to 10.2%). The white-paint improves the $J_{sc}$ of 1.9 mA/cm² compared to the measurement without reflection.

As observed in Table 3, the use of the Au or white paint reflector for 250 nm thick samples on the SnO₂:F back contact improves the $J_{sc}$ by maximum of 1.9 mA/cm² compared to the measurement without reflection and by 3.3 mA/cm² compared to the reference cell on Mo. This led to a cell efficiency of 8.6% with Au reflector and 8.4% with white paint reflector. These efficiencies are much higher than that of the reference cell on Mo (7.0%). For these cells, the $V_{oc}$ increases of 9 mV which is mainly due to the increase of the $J_{sc}$ according to Eq. 6 with $n = 1.7$.

$$ \Delta V_{oc} = \frac{n k T}{q} (\Delta \ln J_{sc}) \qquad \text{Eq 6} $$

The use of the Au reflector for samples on ZnO:Al back-contact improves the $J_{sc}$ by 2.6 mA/cm² compared to the measurement without reflector and by 6.2 mA/cm² compared to the reference cell on
Mo. Finally, an efficiency of 7.9% is achieved. Although this efficiency is higher than that of the reference cell on Mo (7.0%), it is still lower than that of the cells using SnO₂:F back-contact (8.4%). These results confirm clearly that, in accordance with modeling [5, 13, 14] with the decrease of the thickness of CIGS absorbers down to 250 nm, the impact of reflective back contact becomes more important on the solar cell performances.

The EQE curves (Figure SI-3) demonstrate mainly cells with ZnO:Al back-contacts exhibit an upward shift in wavelengths beyond 450 nm when compared to cells with Mo back-contact under front illumination. This increase in efficiency is permitted by light reflection at the back contact but also could be due to the lower back-contact recombination at the ZnO:Al/CIGS interface compared to SnO₂/CIGS and Mo/CIGS back-contact.

These findings highlight that by employing specific deposition conditions for CIGS absorbers along with a compatible transparent back contact, solar cell efficiencies nearing 9% can be attained with absorbers as thin as 250 nm, without relying on composition gradients, heavy alkali, silver additives or the utilization of an anti-reflection coating. These results, achieved by one step deposition of CIGS at 450 °C, and simply adding a mirror to the back of the solar cell, are comparable to those reported in the literature for ACIGS cells with more complex passivation and light-trapping methods, despite the ACIGS cells being 450 nm thick.[54]

<table><caption>**Table 3.** Best cell performance with different reflectors placed at the rear-side of the glass substrate</caption><thead><tr><th></th><th>Illumination side/ reflector</th><th>Back contact</th><th>η (%)</th><th>Jsc mA/cm²</th><th>Voc mV</th><th>FF (%)</th></tr></thead><tbody><tr><td rowspan="10">450 nm</td><td></td><td>Mo</td><td>10.2</td><td>21.1</td><td>651</td><td>74</td></tr><tr><td></td><td>Al₂O₃/Mo</td><td>10.4</td><td>22.8</td><td>641</td><td>71</td></tr><tr><td>Front</td><td rowspan="4">SnO₂:F</td><td>9.9</td><td>22.5</td><td>619</td><td>71</td></tr><tr><td>Rear</td><td>6.6</td><td>15.3</td><td>660</td><td>72</td></tr><tr><td>Cu</td><td>10.6</td><td>23.5</td><td>629</td><td>70</td></tr><tr><td>White paint</td><td>9.7</td><td>22.5</td><td>626</td><td>70</td></tr><tr><td>Front</td><td rowspan="4">ZnO:Al</td><td>8.1</td><td>22.2</td><td>618</td><td>59</td></tr><tr><td>Rear</td><td>7.7</td><td>20.2</td><td>612</td><td>62</td></tr><tr><td>Cu</td><td>8.4</td><td>23.4</td><td>624</td><td>58</td></tr><tr><td>White Paint</td><td>8.7</td><td>24.4</td><td>596</td><td>60</td></tr><tr><td rowspan="10">250 nm</td><td></td><td>Mo</td><td>7</td><td>17.0</td><td>590</td><td>70</td></tr><tr><td></td><td>Al₂O₃/Mo</td><td>6.6</td><td>17.3</td><td>604</td><td>63</td></tr><tr><td>Front</td><td rowspan="4">SnO₂:F</td><td>7.8</td><td>18.4</td><td>615</td><td>69</td></tr><tr><td>Rear</td><td>5.5</td><td>15.1</td><td>615</td><td>59</td></tr><tr><td>Au</td><td>8.6</td><td>20.3</td><td>624</td><td>68</td></tr><tr><td>White paint</td><td>8.4</td><td>19</td><td>627</td><td>71</td></tr><tr><td>Front</td><td rowspan="3">ZnO:Al</td><td>7.05</td><td>20.6</td><td>552</td><td>62</td></tr><tr><td>Rear</td><td>6.0</td><td>18.8</td><td>537</td><td>59</td></tr><tr><td>Au</td><td>7.9</td><td>23.2</td><td>552</td><td>62</td></tr></tbody></table>

### 3.9. Efficiency limiting factor under rear illumination
Under rear illumination, the efficiency of the 450 nm thick CIGS drops to 7.7% with ZnO:Al substrate while for the 250 nm thick CIGS it decreases to 6%. For the SnO₂:F substrate, the efficiency decreases to 6.6% for the 450 nm thick CIGS and to 5.5% for the 250 nm thick CIGS absorbers (Table 3). This reduction is primarily attributed to a decrease in Jsc [15]. Interestingly, the short-circuit current under back-illumination reaches 91% of the front-illumination value for ZnO:Al back contacts both for 250 nm and 450 nm thick CIGS, compared to 82% and 68% for the 250 nm and 450 nm thick CIGS with SnO₂:F back contact, respectively.

EQE measurements, reflecting both charge generation and collection, can highlight the significance of interface recombination at the TCO/CIGS back interface during rear illumination. Consequently, the most effective cells on ZnO:Al and SnO₂:F back contacts were evaluated with rear illumination. Figure 7 illustrates the corresponding EQE curves. It is interesting to note that Jsc values up to 18.8 mA/cm² with ZnO:Al back contact for a 250 nm thick CIGS is possible to reach.

To quantitatively assess back contact recombination, the loss in external quantum efficiency (EQE) can be simulated. The generation function G(z) within the CIGS layer is established by modeling the absorption profile across the material thickness (z) for each specific wavelength, employing the Ray Transfer Matrix method as previously detailed in reference [15]. Following this, the collection function, which indicates the probability of collecting minority charge carriers, is computed for both the Space Charge Region (SCR) and the Quasi-Neutral Region (QNR) using equation 7[55].

$$ \left\{ \begin{array}{l} f_c^{SCR}(z) = c \\ f_c^{QNR}(z) = f(c, w, d, S_{n,BC}, L_n, D_n) < c \end{array} \right. \qquad \text{Eq. 7} $$

With c a constant ranging from 0 to 1 representing the losses in the Space Charge Region (SCR) [56], w and d correspond to the thicknesses of the SCR and the CIGS layer, respectively. In addition, S<sub>n,BC</sub> represents the electron recombination velocity at the back contact, while L<sub>n</sub> is the electron diffusion length within the CIGS absorber (>1 µm), and D<sub>n</sub> is the electron diffusion coefficient which has a value of 2 cm²/s. This model enables a separation of the effects of front-contact and drift-related collection losses (through the factor h) from back-contact recombination losses (represented by S<sub>(n,BC)</sub>)

The simulated external quantum efficiency (EQE) can be expressed as follows:

$$ EQE = \int_{z=0}^{z=d} G(\lambda, z) \cdot fc(z) \cdot dz \qquad \text{Eq. 8} $$

The external quantum efficiency (EQE) under rear illumination was simulated for a 360nm-ZnO:Al/60 nm-ZnO/45 nm-CdS/450 nm-CIGS stack with two types of back contacts: 1000 nm-ZnO:Al and 670 nm SnO₂:F. In these simulations, the parameter c was set to 0.95, the thickness of the SCR (w) to 250 nm, and the electron recombination velocity at the back contact (S<sub>n,BC</sub>) varied from 10³ to 10⁷ cm/s. Additionally, for the same configurations with a CIGS thickness of 250 nm, we used c=0.95, w=150 nm and a similar range of S<sub>n,BC</sub> (from 10³ to 10⁷ cm/s). The values for w were taken from the w obtained in the simulation performed for ultra-thin CIGS cells in Mo back-contact (Table SI-4).
Figure 7 compares the simulation results to the experimental EQE data under rear illumination. A noticeable reduction in EQE is observed especially when the $S_{n,BC}$ increases from $10^4$ to $10^6$ cm/s. For $S_{n,BC}$ values greater than $> 10^6$ cm/s, very few of the charges generated near the transparent conductive oxide (TCO) contact are collected, whereas $S_{n,BC} < 10^4$ cm/s, the majority of the generated charges are collected.

For cells with SnO₂:F back contacts (Figure 7b and 7d), the experimental EQE shows poor charge collection across the entire spectrum, particularly for wavelengths shorter than 500 nm. This behavior can be attributed to a high recombination velocity at the TCO/CIGS interface. By qualitatively comparing the experimental data to the simulations, we can approximate $S_{n,BC}$ at $10^7$ cm/s for cells with SnO₂:F back contacts and both 450 nm and 250 nm thick CIGS layers. Similar findings were observed at the Mo/CIGS interface, indicating no noticeable passivation effect from sodium (Na) accumulation at the interface, as suggested by reference [57]. This implies that the SnO₂:F/CIGS interface exhibits significant recombination.

The EQE for cells with ZnO:Al back contacts (Figure 7a and 7c) also displays a low overall collection, although the EQE curves are higher compared to those with SnO₂:F back contacts. Based on a qualitative assessment of the data, the estimated, $S_{n,BC}$ ranges between $10^5$ to $10^6$ cm/s for the cell with a 450 nm thick CIGS layer and around $10^5$ cm/s for the 250 nm thick CIGS cells. For comparison, Mattheis et al. [58] calculated that $S_{n,BC}$ exceeds $10^6$ cm/s for cells using a MoSe₂/ZnO:Al back contact.
450 nm thick CIGS

<figure><img src="image_8.png" /><figcaption>**Figure 7.** Rear illuminated experimental EQE (solid line) and simulated EQE (dotted line) for a CIGS thickness of a) 450 nm on ZnO:Al, b) 450 nm on SnO₂:F, c) 250 nm on ZnO:Al and d) 250 nm on SnO₂:F back-contact. The collection model was set with a back-contact recombination velocity *S*<sub>n,BC</sub> varying between 10³ to 10⁷ cm/s, *w*=250 nm (for 450 nm thick CIGS) and 150 nm (for 250 nm thick CIGS) and *c*=0.95.</figcaption></figure>

In the literature [50], the lower recombination velocity observed for the ZnO:Al back contact cells has been explained by the formation of a GaOₓ layer, which has a high density of acceptor states, potentially creating a substantial conduction band spike at the back contact that reduces recombination, However, no such oxide layer was detected in the samples analyzed in this study.

Although the 250 nm-thick CIGS samples exhibit similar *V*<sub>oc</sub> for both SnO<sub>2</sub>:F and ZnO:Al configurations, the significantly higher EQE observed in the ZnO:Al back-contact cell (whether or not a reflector is used) may be attributed to a lower back-contact recombination velocity.
, compared to cells with Mo or SnO₂:F back contacts. This is because a large portion of the light reflected at the back contact is effectively collected, whereas the high recombination rates at the Mo/CIGS or SnO₂:F/CIGS interfaces reduce the reflectivity benefit.

4. Conclusions

In this study, we demonstrated that efficiencies up to 8.6% and 7.9% can be achieved with a 250 nm-thick one stage deposited CIGS layer with In-free SnO₂:F and ZnO:Al back contacts, respectively, using only NaF post-deposition treatment and a gold external reflector.

To achieve these results, we evaluated the potential of a 650 nm-thick commercial SnO₂:F layer on glass and a 350 nm-thick and 1200 nm-thick sputtered ZnO:Al layer as alternative In-free TCO back contacts. We showed that, for optimum results, TCO must have high conductivity (ρ < 10⁻⁴ Ω.cm) and minimal TCO/CIGS contact resistance. As the resistivity of SnO₂:F and ZnO:Al substrates lies between 4.10⁻⁴ Ω.cm, and 6.10⁻⁴ Ω.cm, initial calculations indicate that for these resistivities, their thickness must exceed 400 nm to avoid deteriorating series resistance in the final solar cells, making the 350 nm-thick ZnO:Al back contact unsuitable.

However, beyond optoelectronic performance, considerations relating to substrate stability, particularly at high temperatures during CIGS deposition, must be taken into account. To this end, a preliminary thermodynamic analysis has shown that CIGS deposition can influence the properties of the ZnO:Al layer, leading to a reduction in doping concentration and thermodynamically favorable chemical reactions between CIGS and ZnO:Al. Conversely, the SnO₂:F layer appears to exhibit excellent stability up to 600°C. We showed that these two TCO back contacts effectively prevent sodium (Na) diffusion from the glass to the absorber layer, underlining the need for Na post-treatments for these back contacts. A thorough interfacial analysis between CIGS and TCO revealed that under our deposition conditions NaF PDT leads to an accumulation of Na at the TCO/CIGS interface. While no in-situ interfacial layer formation is observed at the ZnO:Al/CIGS interface, a Cu-rich phase forms at the SnO₂:F/CIGS interface. However, ohmic contacts between the CIGS and the SnO₂:F and ZnO:Al layers were obtained, with a slightly higher resistance for the ZnO:Al, leading to a reduction in the *FF* of the final cells compared with SnO₂:F. Analysis of rear illumination quantum efficiency (EQE) and *J*<sub>01</sub> highlights a lower surface recombination velocity at the ZnO:Al/CIGS interface than at the Mo/CIGS or SnO₂:F/CIGS interfaces. This is clearly reflected in the EQE and IV back-illumination characteristics of samples deposited on the ZnO:Al back contact, reaching record back illumination efficiencies of 7.7% and 6% for 450 nm and 250 nm CIGS thicknesses respectively. These efficiencies are higher than those deposited on the SnO₂:F back contact.

This study has enabled us to deepen our understanding of the behaviour of the two most common In-free TCOs as back contacts in photovoltaic solar cells. It emphasizes the significance of TCO selection and the CIGS deposition conditions in optimizing the performance of ultra-thin CIGS solar
cells. They also confirm that the benefits of passivation and optical management are most noticeable when the absorber thickness is greatly reduced. However, further research into passivation methods and interface engineering is imperative to achieve light absorption comparable to that of thick CIGS layers (above 1000 nm) and to maximize the effectiveness of indium-free TCO back contacts in improving the overall efficiency of ultra-thin CIGS solar cells. Continued exploration of novel passivation techniques and improved interface engineering will be crucial for optimizing the carrier collection and reducing recombination losses, thereby enhancing the performance and scalability of these devices for future applications in sustainable energy solutions.

## Author Contributions

Fabien Mollica: Conceptualization, Data curation, Formal analysis, Investigation, Methodology, Visualization, Writing – original draft, Marie Jubault & Frederique Donsanti: Methodology, Resources, Supervision, Investigation, Validation, Muriel Bouttemy & Arnaud Etcheberry: Data curation, Formal analysis, Investigation, Validation, Negar Naghavi: Conceptualization, Formal analysis, Funding acquisition, Methodology, Project administration, Resources, Supervision, Validation, Writing – review & editing.

## Conflicts of interest

There are no conflicts to declare.

## Acknowledgments

This work is supported by ANR's Progelec program (ANR-12-PGRE-0003) within the UltraCIS<sup>M</sup> project.

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