# Assessment of Transparent Conductive Oxides as Back Contacts for Inline-Fabricated Cu(In,Ga)Se Solar Cells

Rico Gutzler, Chang-Yun Song, Dimitrios Hariskos, Heiko Kempa, Roland

Scheer, Roland Wuerz, Erik Ahlswede, Stefan Paetel, Wolfram Witte

► To cite this version:

Rico Gutzler, Chang-Yun Song, Dimitrios Hariskos, Heiko Kempa, Roland Scheer, et al. Assessment of Transparent Conductive Oxides as Back Contacts for Inline-Fabricated Cu(In,Ga)Se Solar Cells. Journal of physics, 2025, 7 (4), pp.045018. 10.1088/2515-7655/adfd86. hal-05263465

HAL Id: hal-05263465

https://hal.science/hal-05263465v1

Submitted on 16 Sep 2025

HAL is a multi-disciplinary open access
archive for the deposit and dissemination of sci-
entific research documents, whether they are pub-
lished or not. The documents may come from
teaching and research institutions in France or
abroad, or from public or private research centers.

L'archive ouverte pluridisciplinaire HAL, est
destinée au dépôt et à la diffusion de documents
scientifiques de niveau recherche, publiés ou non,
émanant des établissements d'enseignement et de
recherche français ou étrangers, des laboratoires
publics ou privés.

<figure><img src="image_1.png" /></figure>

Distributed under a Creative Commons Attribution 4.0 International License
<figure><img src="image_2.png" /></figure>

CrossMark

OPEN ACCESS

RECEIVED

31 March 2025

REVISED

11 June 2025

ACCEPTED FOR PUBLICATION

20 August 2025

PUBLISHED

16 September 2025

Original content from
this work may be used
under the terms of the
Creative Commons
Attribution 4.0 licence.

Any further distribution
of this work must
maintain attribution to
the author(s) and the title
of the work, journal
citation and DOI.

<figure><img src="image_3.png" /></figure>

PAPER

# Assessment of transparent conductive oxides as back contacts for inline-fabricated Cu(In,Ga)Se₂ solar cells

Rico Gutzler¹*, Chang-Yun Song², Dimitrios Hariskos¹, Heiko Kempa², Roland Scheer², Roland Wuerz¹, Erik Ahlswede¹, Stefan Paetel¹ and Wolfram Witte¹

¹ Zentrum für Sonnenenergie- und Wasserstoff-Forschung Baden-Württemberg (ZSW), Meitnerstr. 1, Stuttgart 70563, Germany

² Martin-Luther-Universität Halle-Wittenberg, Institut für Physik, von-Danckelmann-Platz 3, Halle (Saale) 06120, Germany

* Author to whom any correspondence should be addressed.

E-mail: rico.gutzler@zsw-bw.de

**Keywords:** Cu(In, Ga)Se₂, thin-film photovoltaics, transparent back contact, bifacial solar cell, inline fabrication

## Abstract

The integration of transparent back contacts (TBCs) into Cu(In,Ga)Se₂ (CIGS) and (Ag,Cu)(In,Ga)Se₂ thin-film solar cells presents significant opportunities for enhancing device performance and enabling applications such as bifacial and tandem solar cells. However, the implementation of these TBCs in industry-relevant inline deposition processes poses several technological challenges, in particular regarding their thermal and chemical stability during the CIGS process at elevated temperatures while at the same time maintaining high electrical conductivity and high optical transparency. We investigate various transparent conductive oxides as TBC candidates and the use of a thin Mo protection layer to address these challenges, focusing on optimizing CIGS process parameters to ensure seamless integration within existing industrial frameworks. Our results demonstrate that under appropriate CIGS growth conditions and TBC selection, TBCs with high transparency and low free carrier absorption in the near-infrared wavelength region can be successfully incorporated into an inline CIGS deposition process. We establish a laser-scribing process for cell definition that replaces mechanical scribing, which often showed poor cell definition, and photolithography, which is a slow multi-step process. Indium tin oxide requires a thin Mo protection layer to remain functional after CIGS deposition. Zirconium-doped indium oxide (IZrO), however, does not require a protection/sacrificial layer and remains optically transparent and conductive, withstanding our CIGS deposition conditions under which other TBCs already degrade. For CIGS cells with a band gap of 1.1 eV, 14.2% efficiency and high back contact transparency can be demonstrated with IZrO. Our analysis paves the way for the industrial implementation of advanced photovoltaic CIGS devices with improved efficiency and new functionalities.

## 1. Introduction

A requirement for both photovoltaic (PV) top cells in a tandem device and bifacial solar cells is electrically conductive contacts that are optically transparent. Whereas the integration of transparent back contacts (TBCs) instead of routinely employing opaque molybdenum (Mo) into Cu(In,Ga)Se₂ (CIGS) and (Ag,Cu)(In,Ga)Se₂ (ACIGS) thin-film solar cells has been shown to work well on the laboratory scale [1–4], the implementation of TBCs in scalable inline deposition processes has received no attention so far. Numerous materials have been explored in lab-scale systems. Nakada and coworkers investigated indium tin oxide (ITO) and SnO₂:F (FTO) as TBCs for CIGS solar cells already in 2004, achieving 13.7% efficiency on FTO and 13.0% on ITO, both nearly identical to reference cells with Mo back contact [5]. At the same time, Young and coworkers presented a four-terminal CuGaSe₂/Cu(In,Ga)Se₂ tandem device using FTO as transparent back electrodes [6], and shortly after ITO was reported in a mechanically stacked tandem [7]. Current record cells use ITO as back contact, achieving 19.77% efficiency with front illumination and
10.89% with rear illumination [1]. Hydrogen-doped In₂O₃ was investigated by Keller *et al* for both CIGS [8] and ACIGS [4] solar cells, as well as wide-gap (A)CIGS [2] absorbers, which offers the advantage of low near-infrared absorption compared to ITO or FTO. The same group used tungsten-doped In₂O₃ to fabricate wide-gap ACIGS cells with 13.6% efficiency, likewise with low near-infrared absorption [9]. Wide-gap CIGS cells were also fabricated on FTO with the help of a thin Mo layer in between TBC and absorber [10, 11]. ITO was also shown to yield good backside efficiency with wide-gap Cu(In,Ga)S₂ [12]. Further oxides explored as TBC include Al-doped ZnO, with reports that a thin MoSe₂ layer is required to form an Ohmic contact [13, 14], whereas for other groups an Ohmic contact is achievable without interfacial Mo layer [15]. ITO, hydrogen-doped In₂O₃, and zirconium-doped In₂O₃ were recently investigated regarding their suitability for bifacial applications, with the latter two showing better performance under backside illumination [16].

The observation of GaOₓ formation at the CIGS/TBC interface is often reported and its negative effect on the cell's fill factor is discussed in view of its n-type conductivity and the resulting counter diode at the back contact [17, 18]. The presence of Na during absorber growth is reported to enhance GaOₓ formation and can be used to control its thickness [4, 8]. GaOₓ formation can be limited by inserting a thin Mo layer between TBC and CIGS prior to absorber growth [13, 19], which is oxidized to MoSe₂ [13, 17] or MoSe₂ and MoOₓ [20] during absorber growth at elevated temperatures under selenium atmosphere. The presence of an Mo or MoSe₂ interlayer, however, negatively impacts transparency of the device [21] and its thickness needs to be carefully controlled.

Next to Mo, other materials were also investigated for interface modification on TCOs, such as a ~10 nm thick MoO₃ layer on ITO, which improves both open-circuit voltage (VOC) and fill factor (FF) [22], in spite of the formation of a thin GaOₓ layer. A 6 nm thin WOₓ layer was used as hole extraction layer on ITO, resulting in improved short-circuit current density (JSC) and FF [23]. In order to thermodynamically limit the formation of GaOₓ, sulfur was deposited on ITO in superstrate [24] and substrate applications [25], in both cases leading to a gain in VOC. A common observation is that none of the intermediate layers Mo, MoO₃, WOₓ, or S completely inhibit the formation of GaOₓ due to its comparatively low standard molar enthalpy of formation of -1089 kJ mol⁻¹ [26].

Depending on the choice of TBC, process temperatures below 550 °C are used to preserve the TBC's properties and to limit the formation of GaOₓ, with a few exceptions reporting higher CIGS growth temperatures. Fonoll-Rubio and coworkers assume that the selenization of an ITO back contact forms an amorphous In-Se phase, which degrades the optical transparency of the TBC and the device's optoelectronic properties, in particular at high process temperatures >540 °C [21]. Industrial pilot lines fabricating CIGS modules can operate at higher temperatures up to 650 °C [27] and also the inline coevaporation process used in the present study routinely requires substrate temperatures above 550 °C [28]. The aim of this work is to bridge the gap between the fabrication of CIGS devices on TBCs with small static laboratory coaters at comparatively low temperatures and the high-temperature inline processes often used in industrial settings. Several technological challenges, such as a high thermal budget that interrelates with the TBC's thermal and chemical stability during the CIGS coevaporation process at elevated temperatures under a selenium atmosphere, need to be addressed to preserve high electrical conductivity and optical transparency under these harsh conditions. Our assessment identifies transparent conductive oxides that fulfill these requirements. We also discuss the adjustment of the ACIGS process parameters, such as lowering the overall thermal budget, while ensuring seamless integration within existing industrial frameworks. Finally, the performance of semitransparent cells is discussed in view of different absorber thicknesses.

## 2. Methods

### 2.1. CIGS deposition and cell fabrication

The CIGS solar cells were produced through a standardized workflow. The CIGS absorbers were co-evaporated in an industry-oriented inline coater designed for substrates up to 30 × 30 cm² [28]. As back contacts, different TCOs were used as detailed below. In all deposition campaigns, reference samples with a ~550 nm thick opaque Mo back contact were fabricated. The substrate heater temperature was systematically varied in the third stage of the inline growth process. The absorber's composition was controlled by adjusting evaporator temperatures while keeping the growth time constant. An RbF post-deposition treatment was applied to all absorbers. For CIGS cells, the [Ga]/([Ga]+[In]) ratio (GGI) was 0.32. The CIGS absorber had thicknesses of 2.2 µm (referred to as 'standard' in the following), and a 530 nm thin absorber was fabricated with four times faster inline speed without changing the rates of the evaporation sources. A chemical bath deposition (CBD) method was used to grow ~50 nm of CdS. Subsequently, a Zn₀.85Mg₀.15O (ZMO) layer was RF-sputtered on top of the CdS layer followed by DC-sputtered Al-doped ZnO (AZO) as the front contact. All processes, except for the CBD buffer, were
inline. A Ni/Al/Ni grid was deposited by electron-beam evaporation. Cells on TBCs were laser-scribed with a total cell area ~0.48 cm². No antireflective coating was used.

## 2.2. Transparent back contacts

ITO was purchased from Solems S.A. with 370 nm thickness and 8–12 Ω/□ sheet resistance ($R_{sq}$) (ITO Sol 12, 10 wt% SnO₂ in In₂O₃, corresponding to 17 mol% SnO₂ in In₂O₃, molecular formula In₂(1-x)SnₓO₃-x with x = 0.17). A selection of ITO samples was sputter-coated with a ~10 nm thick Mo sacrificial layer before absorber growth.

FTO with a thickness of 1 µm and ~13 Ω/□ sheet resistance was sourced from Calyxo GmbH.

IZO and IZrO were sputter-deposited in-house on 3 mm soda-lime glass. About 220 nm thick IZrO layers were RF sputtered at room temperature from an In₂O₃ target with 2 wt% ZrO₂ (corresponding to 4.4 mol% ZrO₂ in In₂O₃, molecular formula In₂(1-x)ZrₓO₃-x with x = 0.044), yielding $R_{sq} \gtrsim 20$ Ω/□ (an Ar/O₂/H₂-gas mixture was used during sputtering). IZO was RF sputtered from an In₂O₃ with 7 wt% ZnO (corresponding to 20.4 mol% ZnO in In₂O₃, In₂(1-x)ZnₓO₃-2x with x = 0.204) target at room temperature for layers with ~320 nm thickness and $R_{sq} 10$ Ω/□.

## 2.3. Laser-scribing

For cell area definition, the AZO front contact on the CIGS layer was laser-scribed using a microFLEX system by 3D-Micromac with a femtosecond (fs) laser with a wavelength of 515 nm and a scan velocity of 1500 mm s⁻¹ at a frequency of 100 kHz. The pulse-to-pulse distance was 15 µm. A comparison of mechanically scribed cells to laser-scribed cells on one sample with opaque Mo back contact revealed that the laser scribed cells reached 95% of the cell efficiency of the mechanically-scribed cells. The main loss was observed in the FF.

## 2.4. Current–density voltage (JV) measurement

JV was measured under one sun AM1.5G at 25 °C using a WACOM Super Solar Simulator WXS-90 S-L2 (grade AAA). A Si-reference cell was used for calibration. The bifacial JV measurements were performed in a setup of two Wavelabs Sinus 70 LED solar simulators, which were mounted opposite to each other with the sample on a glass plate between them, so that illumination from the front and from the rear side can be adjusted independently. Significant deviations of the sample temperature were avoided by limiting the illumination to the duration of the measurement. Cell $J_{SC}$ and efficiency is reported on total cell area.

## 2.5. External quantum efficiency (EQE) measurement

EQE measurements were conducted using a Bentham PVE300 system, with a Xenon short-arc lamp and/or a Quartz Halogen lamp as light sources. A TMC300-FU monochromator was used to select the wavelength, and measurements were performed over the 250–1250 nm range with 5 nm or 10 nm step size. Data acquisition and analysis were carried out using BenWin+ software.

# 3. Results and discussion

Several TCOs were tested as back contacts during standard growth conditions for a 2.2 µm thick CIGS layer with an integral GGI of 0.32 and deposited at a reduced substrate heater temperature of 580 °C maximum, roughly 100 °C below the standard temperature for opaque Mo back contacts. The tested TBCs include bare ITO, ITO with a ~10 nm thick Mo sacrificial layer, FTO, IZO, and IZrO. For this deposition at reduced substrate heater temperature, reference cells with an opaque Mo back contact achieve an efficiency of 18.0%.

Transmittance measurements of the TBCs before absorber deposition are shown in figure 1(a) and after absorber deposition in figure 1(b), with the absorber being mechanically removed after absorber deposition. Even at the reduced substrate temperature, the transmittance of ITO (orange), IZO (light blue), and FTO (red) is significantly reduced in the low-wavelength part. The strongest degradation over the whole wavelength range is observed for FTO. A thin Mo layer deposited on ITO, which before CIGS deposition significantly suppressed transmittance (gray), is converted to $MoSe_x$ and $MoO_x$ during the growth process [20] and becomes more transparent due to the larger band gaps of these materials. IZrO (dark blue) shows a slight gain in transmittance around 350 nm but reduced transmittance for other wavelengths. A possible degradation mechanism may be due to the formation of an amorphous In–Se phase, as reported by Fonoll-Rubio et al [21].

IZO and IZrO are high-mobility TCOs with low carrier density. As a result, they exhibit low free carrier absorption and high transmittance for wavelengths >1000 nm. After the absorber growth, both materials become even more transparent for wavelengths >1200 nm, which might indicate a further reduction in free carrier density. The IZrO back contact (blue line) shows no indications of free carrier absorption up to
<figure><img src="image_4.png" /><figcaption>**Figure 1.** (a) Transmittance of IZrO, FTO, IZO, and ITO with and without a ~10 nm Mo layer before CIGS growth at 580 °C (2.2 µm thickness). (b) Transmittance after CIGS growth, the CIGS layer was mechanically removed before the measurement.</figcaption></figure>

<figure><img src="image_5.png" /><figcaption>**Figure 2.** (a) Reflectance of ITO and IZrO back contact after absorber growth with absorber (dashed lines) and with mechanical removed absorber (solid lines). Reflectance is measured through the glass (back side), i.e. light passes through glass/TBC/absorber. (b) $T/(1-R)$ (dashed line) and absorption $A = 1 - R - T$ (solid line) as a function of wavelength for ITO and IZrO after absorber removal.</figcaption></figure>

1600 nm. In contrast, the large free carrier absorption in ITO compared to IZrO can be observed in figure 2(a) as an increasing reflectance starting at 900 nm (orange line).

The reflectance of the IZrO and ITO samples after CIGS growth is shown in figure 2(a). It was measured through the glass side of the sample, i.e. light travels through glass/TBC/absorber and is reflected in this order from the interfaces, just as in a bifacial application. Reflectance over the whole spectrum is below 20% for both TBCs and changes only slightly when the CIGS absorber is removed. The reflectance corrected transmittance $T/(1-R)$ and the absorption $A = 1 - R - T$ are plotted in figure 2(b). Again, due to free carrier absorption, ITO shows inferior performance with large absorption and decreasing $T/(1-R)$ with increasing wavelength above 600 nm. IZrO has $A < 0.2$ and $T/(1-R) > 0.8$ for the spectrum above 600 nm. Hence, IZrO is the more suitable choice for applications where light with large wavelengths needs to be efficiently transmitted, such as TCOs in tandem top cells. For bifacial applications, the early onset of free carrier absorption at 900 nm in case of ITO will impact transmittance, especially in CIGS solar cells with a band gap $E_g \le 1.4$ eV.

We note in addition that the sheet resistance of FTO and IZO increases significantly during absorber growth, whereas the other TBCs exhibit only minor changes (see table 1).

JV curves of solar cells with the different back contacts are shown in figure 3(a). Most cells are characterized by significant blocking behavior apparent in the JV curves. The solar cell with an FTO back contact (red) shows strong blocking, while ITO (orange) leads to full blocking of the current. The blocking in the case of ITO can be reduced by adding a thin Mo sacrificial layer of 10 nm thickness (gray). The IZO-based cell (turquoise) also shows reduced blocking compared to FTO and bare ITO, while the least
<table><caption>**Table 1.** JV parameters of CIGS solar cells (total area), R<sub>sq</sub> of TBC before and after CIGS growth (standard thickness with E<sub>g</sub> = 1.1 eV), maximum transmittance at wavelengths <1200 nm (TRN<sub>max</sub>) of all tested TBCs.</caption><thead><tr><th></th><th>PCE (%)</th><th>FF (%)</th><th>V<sub>OC</sub> (mV)</th><th>J<sub>SC</sub> (mA cm<sup>-2</sup>)</th><th>R<sub>sq</sub> (Ω/□)<br>(pristine TBC)</th><th>R<sub>sq</sub> (Ω/□)<br>(after CIGS growth)</th><th>TRN<sub>max</sub> (%)<br>(&lt;1200 nm, pristine TBC)</th><th>TRN<sub>max</sub> (%)<br>(&lt;1200 nm, after CIGS growth)</th></tr></thead><tbody><tr><td>IZrO</td><td>14.0 (14.2*)</td><td>63.8</td><td>659</td><td>33.3 (33.8*)</td><td>20</td><td>19</td><td>89</td><td>77</td></tr><tr><td>FTO</td><td>2.1</td><td>26.5</td><td>538</td><td>14.6</td><td>13</td><td>813</td><td>85</td><td>49</td></tr><tr><td>IZO</td><td>5.3</td><td>27.9</td><td>628</td><td>30.4</td><td>10</td><td>134</td><td>83</td><td>71</td></tr><tr><td>ITO</td><td>0.0</td><td>23.5</td><td>115</td><td>0.4</td><td>8</td><td>11</td><td>79</td><td>75</td></tr><tr><td>ITO/Mo</td><td>7.4</td><td>36.7</td><td>622</td><td>32.3</td><td>7</td><td>7</td><td>22</td><td>59</td></tr></tbody></table>

<footnote>*PCE corrected for J<sub>SC</sub> derived from EQE measurement.</footnote>
<figure><img src="image_6.png" /><figcaption>**Figure 3.** (a) Light JV curves of CIGS solar cells (standard thickness) fabricated on IZrO (blue), FTO (red), IZO (turquoise), ITO (orange), and ITO/Mo (10 nm) (gray). (b) Relative instability (difference in enthalpy of formation) of the used TBC materials and MoO₃ with respect to Ga₂O₃.</figcaption></figure>

blocking appears for the IZrO back contact (blue). In view of the often observed formation of GaOₓ at the TBC/absorber interface, relative thermodynamic stability of a TCO material with respect to Ga₂O₃ has been discussed [4]. Figure 3(b) shows the difference in enthalpy of formation for the TBC materials used here with respect to the more stable Ga₂O₃. Due to a lack of literature values, the enthalpy of formation of the oxide mixtures ITO, IZO, and IZrO was interpolated from the values of the pure compounds taken from the literature [26], weighted by their respective molar fraction. Thus, alloying ZnO into In₂O₃ (IZO) destabilizes the indium oxide due to its less negative enthalpy of formation. ZrO₂ is more stable than In₂O₃ and hence makes the mixture more stable. For FTO, the value of pure SnO₂ was taken as an approximation due to the lack of other data. MoO₃ is also shown in view of the Mo sacrificial layer used. All TBCs are less stable than pure Ga₂O₃, i.e. at elevated temperatures with Ga present, all materials will form GaOₓ. This relative instability correlates with the extent of the blocking behavior in figure 3(a): the least stable material FTO significantly blocks current, likely due to an extensive GaOₓ layer at the interface. The most stable material IZrO shows the least blocking, and materials with intermediate stability block to some extent.

MoOₓ, while thermodynamically less stable than ITO, improves device performance. Previous studies have shown that a Mo sacrificial layer reacts with Se and O to form MoSeₓ and MoOₓ [13, 17], protecting the underlying ITO from reacting with Ga from the absorber to form GaOₓ [20]. The role of GaOₓ at the back contact is currently not fully understood since it appears to be often detrimental but can also be benign. If the GaOₓ is of n-type, it is reasonable to assume that a counter diode forms at the back contact if it is thick enough. A very thin layer might not form a pn-junction observable in JV measurements. However, doping Ga₂O₃ with Cu induces p-type conductivity and shifts the valence band towards the Fermi level [29, 30]. It is possible that some of the Cu provided during CIGS growth is doping the GaOₓ layer and alters its electronic properties sufficiently to change it from detrimental to benign. The observed blocking then comes about naturally by taking together thermodynamic stability of the different TBCs leading to extensive GaOₓ formation during absorber growth, and the resulting formation of a counter diode with n-type GaOₓ at the back contact. We note, however, that the performance of a TBC might also depend on the TCO deposition process and other layer parameters; for example, the ITO Sol30 by the same commercial supplier as the ITO Sol12 shown here led to less blocking (data not shown), but still more than IZO or IZrO.

The thermodynamic stability of a TCO gives a reasonable first hint toward its suitability as a back contact. However, other chemical and physical processes are likely also at play. Dopant segregation in the form of reduced F content was observed for FTO at CIGS deposition temperatures as low as 550 °C [17, 19], and is a probable explanation for the here observed increased sheet resistance and low cell performance. Furthermore, selenization of the upper TBC layers [21], or changes in oxygen vacancies will also take place during high-temperature processes and will depend on the relative abundance of other elements like Se and Ga. Understanding the thermochemistry at the interface is essential for optimizing TBC stability and interface properties. This is exemplified by the solar cell based on an IZrO back contact (see table 1). The absorber's band gap, calculated from the onset of the absorption edge of an EQE measurement (linear extrapolation of E × EQE²), is 1.1 eV and the cell efficiency calculated using the current density from EQE is 14.2%

The observation that the thermodynamic stability of the TBC is a limiting parameter is confirmed by experiments in which the transport speed of the samples in the inline coater is increased by a factor of four. Without altering the metal source temperatures, this results in a roughly four times thinner absorber layer,
<figure><img src="image_7.png" /><figcaption>**Figure 4.** Light JV curves of CIGS solar cells fabricated on IZrO (blue), ITO (orange), and FTO (red) with a thin CIGS absorber (530 nm thickness).</figcaption></figure>

<table><caption>**Table 2.** JV parameters of CIGS solar cells, R<sub>sq</sub> of TBC after thin absorber growth (530 nm thick CIGS).</caption><thead><tr><th></th><th>PCE (%)</th><th>FF (%)</th><th>V<sub>OC</sub> (mV)</th><th>J<sub>SC</sub> (mA cm<sup>-2</sup>)</th><th>R<sub>sq</sub> (Ω/□) (after absorber growth)</th></tr></thead><tbody><tr><td>IZrO</td><td>7.7</td><td>55.2</td><td>563</td><td>24.8</td><td>26</td></tr><tr><td>ITO</td><td>6.8</td><td>49.5</td><td>575</td><td>23.8</td><td>9</td></tr><tr><td>FTO</td><td>4.8</td><td>44.0</td><td>455</td><td>23.9</td><td>19</td></tr></tbody></table>

but also the TBC being exposed to elevated temperatures for a much shorter time. In this case, the TBCs that showed complete (ITO) or substantial (FTO) blocking under standard deposition times now yield functional devices with higher FF. Figure 4 shows the corresponding *JV* curves of FTO (red), ITO (orange), and IZrO (blue) of solar cells with a 530 nm thick CIGS absorber. The blocking is substantially reduced, presumably from both thinner $GaO_x$ formation and fewer chemical changes in the TBC itself (e.g. loss of F-doping or formation of Se-In compounds). Table 2 gives the corresponding *JV* parameters. The sheet resistance of FTO after absorber growth remains much lower compared to the slower deposition and remains similar for IZrO and ITO. The TBC's sheet resistance has a direct impact on the cell's series resistance, thus lowering the FF. However, the TBC with the largest sheet resistance (IZrO) has also the highest FF, highlighting that sheet resistance is only part of the picture and that further mechanisms are at play. Differences in $V_{OC}$ can be explained by different Na-doping from the soda-lime glass, as all three TBCs likely lead to different Na diffusion kinetics due to grain structure and thickness of the film.

The combined observations that a reduced maximum process temperature is required to achieve functional PV devices with transparent back contact, that current blocking correlates with thermodynamic stability of the TBC, and that a faster processing speed is required to get otherwise problematic materials to work as back electrode strongly suggest that the overall thermal budget during the absorber growth process is the critical parameter which needs to be controlled. The kinetics of $GaO_x$ growth and other detrimental changes to the TBC can be slowed down by reducing the overall thermal load by either reducing maximum temperature and/or limiting the exposure time to high temperatures.

Bifaciality is one of the goals that motivates the use of transparent back contacts. The bifaciality of the solar cells with a 530 nm thin absorber was probed with both *JV* and *EQE* measurements several months after cell fabrication and initial *JV* measurements (unencapsulated cells were stored under ambient conditions). Figure 5(a) shows the *JV* curves of a cell with ITO back contact under front illumination (orange) and rear illumination (dark orange), and figure 5(b) of an IZrO cell under front illumination (blue) and rear illumination (purple). The barrier in the ITO cell also impacts the *JV* curve under rear illumination, although it appears less pronounced. The IZrO cell does not exhibit such a barrier. The series resistance is larger under rear illumination. The corresponding *EQE* measurements shown in figure 5(c) for the ITO cell and figure 5(d) for the IZrO cell show a reduced but non-zero *EQE* across the absorbed spectrum. The *EQE*
<figure><img src="image_8.png" /><figcaption>**Figure 5.** Dark and light JV curves of a 530 nm thick CIGS solar cells with (a) front illumination (orange) and back illumination (dark orange) with ITO as TBC and (b) front illumination (blue) and back illumination (purple) with IZrO as TBC. EQE measured on the same CIGS cells with front and backside illumination (c) on ITO and (d) on IZrO (same color code).</figcaption></figure>

<figure><img src="image_9.png" /></figure>

of the IZrO cell is higher both under front and rear illumination compared to the ITO cell and also the relative front/rear EQE is consistently larger for the IZrO cell across the entire spectrum, not only in the region above 900 nm. The bifaciality in $J_{SC}$, defined as $\frac{\text{back illumination}}{J_{SC,EQE}}$, is 60% for IZrO and 46% for ITO cells. The better EQE for the IZrO cell is a direct consequence of its higher transmittance of the TBC in the visible and to a lesser extent in the red part of the spectrum due to lower free carrier absorption. Under rear illumination, both TBCs show an increased EQE around 350 nm compared to front illumination, which is due to the absence of the absorption of UV light in the CdS buffer layer. This effect is slightly larger in ITO compared to IZrO, possibly because of its larger optical gap as a consequence of the larger Burstein-Moss shift due to ITO's higher doping.

The lower EQE under back illumination for short wavelengths can be attributed to the short diffusion length of photo-generated carriers [1, 8, 31] and high recombination velocity at the back contact [14]. Carriers formed close to the back contact by high-energy photons and far away from the pn junction are more prone to recombination before contributing to charge-collection efficiency. For larger wavelengths, incomplete absorption, amplified here by the thin absorber, lowers not only carrier collection under back illumination, but is also observable in the sloping off of the EQE towards larger wavelengths under front illumination. The better optical properties of IZrO compared to ITO (higher transmittance especially for longer wavelengths and lower absorption, see figure 2) lead to improved charge collection for all wavelengths. Overall, the general shape of the EQEs under front and back illumination and the ratio between the current densities is comparable to reported absorbers with similar thickness [8, 19, 31, 32].

A complete list of *JV* parameters under front and rear illumination is provided in table 3. The cell with ITO back contact shows stronger signs of degradation over time (several months between initial and bifacial measurements) in both $V_{OC}$ and FF compared to the cell with IZrO contact, but both cells remained fully functional. Whereas the FF remains roughly constant between front and rear illumination, $V_{OC}$ decreases by a few tens of mV and $J_{SC}$ decreases by 30%–40%.
<table><caption>**Table 3.** JV parameters and *J*<sub>SC</sub> extracted from EQE of 530 nm thick CIGS solar cells under front and rear illumination with ITO and IZrO back contact.</caption><thead><tr><th></th><th>Front PCE (%)</th><th>Front FF (%)</th><th>Front V<sub>OC</sub> (mV)</th><th>Front J<sub>SC</sub><br>(mA cm<sup>-2</sup>)</th><th>Front J<sub>SC</sub><br>(mA cm<sup>-2</sup>)<br>from EQE</th><th>Rear PCE (%)</th><th>Rear FF (%)</th><th>Rear V<sub>OC</sub> (mV)</th><th>Rear J<sub>SC</sub><br>(mA cm<sup>-2</sup>)</th><th>Rear J<sub>SC</sub><br>(mA cm<sup>-2</sup>)<br>from EQE</th></tr></thead><tbody><tr><td>IZrO</td><td>8.4</td><td>56.9</td><td>549</td><td>26.9</td><td>27.1</td><td>4.9</td><td>57.1</td><td>517</td><td>16.7</td><td>16.3</td></tr><tr><td>ITO</td><td>5.1</td><td>40.3</td><td>512</td><td>24.6</td><td>23.5</td><td>3.3</td><td>38.6</td><td>478</td><td>17.7</td><td>10.8</td></tr></tbody></table>
<figure><img src="image_10.png" /><figcaption>**Figure 6.** JV curves as a function of front illumination for the ITO cell (a), and the IZrO cell (b). $V_{OC}$ as a function of the logarithm of the illumination of the ITO cell with an ideality factor of ~2.1 (c), and of the IZrO cell with an ideality factor of ~1.4. For both cells, the $V_{OC}$ was also measured with rear illumination at 1.0, 0.5, and 0.1 Suns.</figcaption></figure>

<figure><img src="image_11.png" /></figure>

The $V_{OC}$ of all cells fabricated with transparent back contact is comparatively low relative to cells fabricated on Mo back contacts, likely due to recombination at interfaces. To further investigate the spatial origin and nature of this recombination, Suns-$V_{OC}$ measurements with front- and rear-side illumination were performed on the ITO (figure 6(a)) and IZrO devices (figure 6(b)). The ideality factor *n* was extracted from the linear slope of the $V_{OC}$ versus ln(Suns) plot, assuming the classical diode equation:

$$V_{OC} (\text{Suns}) = \frac{n k T \ln(\text{Suns})}{q} + C, \text{ where } k \text{ is the Boltzmann constant, } T \text{ temperature (taken as 300 K), and } q \text{ the elementary charge. This linear fit gives an ideality factor above 2 and shows a strong non-linearity under front side illumination of the device with ITO back contact (figure 6(c)). In contrast, the device with an IZrO back contact shows a linear behavior and a regular ideality factor (figure 6(d)). This indicates dominant interface recombination in the ITO device, which is not present in the IZrO device, although an exact loss attribution is complicated by the shunt in the ITO cell. The observation that the } V_{OC} \text{ drops strongly with decreasing backside illumination in the ITO device compared to the IZrO device suggests the rear interface as the source of interface recombination.}$$

## 4. Conclusion

The often-used ITO is shown to require a thin Mo protection layer when used as TBC in our industry-relevant inline deposition machine, even with reduced substrate temperature. IZrO, however, does not require a protection/sacrificial layer and remains optically transparent and conductive, withstanding CIGS deposition conditions under which other TBCs already degrade. IZrO is the thermodynamically most stable TBC and is not prone to degradation mechanisms such as loss of doping or formation of undesired phases. For CIGS cells with $E_g$ = 1.1 eV and a 2.2 µm absorber thickness, an efficiency of 14.2% and high back contact transparency could be demonstrated with IZrO as TBC. Temperature sensitive TBCs such as ITO and FTO can yield functional PV devices with efficiencies up to 8% when fast coevaporation processes producing 530 nm thick absorbers and low substrate temperatures are used. Our analysis identifies the thermal budget during the absorber deposition as the decisive parameter for the stability of the TBC and
IZrO as the material that withstands the highest thermal budget. The more common materials ITO and FTO require a significant reduction of the thermal budget to yield functional CIGS thin-film solar cells. In future work, the cell efficiency can be further improved by reducing the TBC's sheet resistance, thereby reducing the cell's series resistance. Evaluating V<sub>OC</sub> losses and exploring remedies suggested in [9] such as passivation layers [8, 32] at the contacts and steeper Ga gradients at the back contact [1] are additional promising steps towards high-performance inline-fabricated bifacial CIGS solar cells.

**Data availability statement**

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

**Acknowledgment**

We thank Jan-Philipp Becker and Philipp von Bismarck for their help with IZrO sputter process optimization.

The work was funded by the European Union within the Hi-BITS project (101122203).

Funded by the European Union. Views and opinions expressed are however those of the author(s) only and do not necessarily reflect those of the European Union. Neither the European Union nor the granting authority can be held responsible for them.

**ORCID iDs**

Rico Gutzler ID 0000-0002-1300-6941

Chang-Yun Song ID 0000-0002-9132-6844

Dimitrios Hariskos ID 0000-0003-4867-1578

Heiko Kempa ID 0000-0003-3583-6935

Roland Scheer ID 0000-0001-5007-3425

Roland Wuerz ID 0000-0001-6649-7564

Stefan Paetel ID 0000-0002-7051-5208

Wolfram Witte ID 0000-0002-9429-506X

**References**

[1] Yang S-C, Lin T-Y, Ochoa M, Lai H, Kothandaraman R, Fu F, Tiwari A N and Carron R 2023 Efficiency boost of bifacial Cu(In,Ga)Se₂ thin-film solar cells for flexible and tandem applications with silver-assisted low-temperature process Nat. Energy 8 40–51

[2] Keller J, Stolt L, Donzel-Gargand O, Kubart T and Edoff M 2022 Wide-gap chalcopyrite solar cells with indium oxide-based transparent back contacts Sol. RRL 6 2200401

[3] Hölscher T et al 2023 Effects of ITO based back contacts on Cu(In,Ga)Se₂ thin films, solar cells, and mini-modules relevant for semi-transparent building integrated photovoltaics Sol. Energy Mater. Sol. Cells 251 112169

[4] Keller J, Shariati Nilsson N, Aijaz A, Riekehr L, Kubart T, Edoff M and Törndahl T 2018 Using hydrogen-doped In₂O₃ films as a transparent back contact in (Ag,Cu)(In,Ga)Se₂ solar cells Prog. Photovolt.: Res. Appl. 26 159–70

[5] Nakada T, Hirabayashi Y and Tokado T 2002 Cu(In₁-x, Gax)Se₂-based thin film solar cells using transparent conducting back contacts Jpn. J. Appl. Phys. 41 L1209–L1211

[6] Young D L, Abushama J, Noufi R, Li X, Keane J, Gessert T A, Ward J S, Contreras M, Symko-Davies M and Coutts T J 2002 A new thin-film CuGaSe₂/Cu(In,Ga)Se₂ bifacial, tandem solar cell with both junctions formed simultaneously Conf. Record of the Twenty-Ninth IEEE Photovoltaic Specialists Conf.—2002:New Orleans, Louisiana, USA vol 2002 (IEEE) pp 608–11

[7] Nishiwaki S, Siebentritt S, Walk P and Ch. Lux-Steiner M 2003 A stacked chalcopyrite thin-film tandem solar cell with 1.2 V open-circuit voltage Prog. Photovolt.: Res. Appl. 11 243–8

[8] Keller J, Chen W-C, Riekehr L, Kubart T, Törndahl T and Edoff M 2018 Bifacial Cu(In,Ga)Se₂ solar cells using hydrogen-doped In₂O₃ films as a transparent back contact Prog. Photovolt.: Res. Appl. 26 846–58

[9] Keller J, Stolt L, Donzel-Gargand O, Violas A F, Kubart T and Edoff M 2024 Bifacial wide-gap (Ag,Cu)(In,Ga)Se₂ solar cell with 13.6% efficiency using In₂O₃:W as a back contact material Sol. RRL 8 15

[10] Thomere A, Placidi M, Guc M, Tiwari K, Fonoll-Rubio R, Izquierdo-Roca V, Perez-Rodriguez A and Li-Kao Z J 2023 2-step process for 5.4% CuGaSe₂ solar cell using fluorine doped tin oxide transparent back contacts Prog. Photovolt.: Res. Appl. 31 524–35

[11] Briggs D 1981 Handbook of x-ray photoelectron spectroscopy C. D. Wanger, W. M. Riggs, L. E. Davis, J. F. Moulder and G. E. Muilenberg Perkin-Elmer Corp., physical electronics division, Eden Prairie, Minnesota, USA, 1979. 190 pp. $195 Surf. Interface Anal. 3 5

[12] Choubrac L, Pineau F, Bertin E, Arzel L and Barreau N 2025 Rethinking CIGS solar cells for rear illumination applications J. Phys. Energy 7 02LT01

[13] Rostan P J, Mattheis J, Bilger G, Rau U and Werner J H 2005 Formation of transparent and ohmic ZnO:Al/MoSe₂ contacts for bifacial Cu(In,Ga)Se₂ solar cells and tandem structures Thin Solid Films 480–481 67–70

[14] Mattheis J, Rostan P J, Rau U and Werner J H 2007 Carrier collection in Cu(In,Ga)Se₂ solar cells with graded band gaps and transparent ZnO:Al back contacts Sol. Energy Mater. Sol. Cells 91 689–95

[15] Mollica F, Jubault M, Donsanti F, Loubat A, Bouttemy M, Etcheberry A and Naghavi N 2017 Light absorption enhancement in ultra-thin Cu(In,Ga)Se₂ solar cells by substituting the back-contact with a transparent conducting oxide based reflector Thin Solid Films 633 202–7
[16] Nisika N, Nishiwaki S, Mitmit C, Marzi M D, Artuk K, Wolff C M and Carron R 2025 High mobility and low-doped transparent conducting oxide back contact for bifacial CIGS solar cells under review

[17] Nakada T 2005 Microstructural and diffusion properties of CIGS thin film solar cells fabricated using transparent conducting oxide back contacts *Thin Solid Films* **480** 419–25

[18] Choi J H *et al* 2015 Wide-bandgap CuGaSe₂ thin film solar cell fabrication using ITO back contacts *Vacuum* **120** 42–46

[19] Nakada T, Hirabayashi Y, Tokado T, Ohmori D and Mise T 2004 Novel device structure for Cu(In,Ga)Se₂ thin film solar cells using transparent conducting oxide back and front contacts *Sol. Energy* **77** 739–47

[20] Demling *A* *et al* 2025 Impact of a thin sacrificial Mo layer on the formation of the wide band-gap ACIGSe absorber/ITO thin-film solar cell interface *ACS Appl. Mater. Interfaces* **17** 33027–35

[21] Fonoll-Rubio R, Placidi M, Hoelscher T, Thomere A, Li-Kao Z J, Guc M, Izquierdo-Roca V, Scheer R and Pérez-Rodríguez A 2022 Characterization of the stability of indium tin oxide and functional layers for semitransparent back-contact applications on Cu(In,Ga)Se₂ solar cells *Sol. RRL* **6** 2101071

[22] Simchi H, McCandless B E, Meng T and Shafarman W N 2014 Structure and interface chemistry of MoO₃ back contacts in Cu(In,Ga)Se₂ thin film solar cells *J. Appl. Phys.* **115** 33514

[23] Saifullah M, Rasool S, Ahn S, Kim K, Cho J-S, Yoo J, Shin W S, Yun J H and Park J H 2019 Performance and Uniformity Improvement in Ultrathin Cu(In,Ga)Se₂ Solar Cells with a WOₓ Nanointerlayer at the Absorber/Transparent Back-Contact Interface *ACS Appl. Mater. Interfaces* **11** 655–65

[24] Simchi H, Larsen J, Kim K and Shafarman W 2014 Improved performance of ultrathin Cu(InGa)Se₂ solar cells with a backwall superstrate configuration *IEEE J. Photovolt.* **4** 1630–5

[25] Kim D, Saifullah M, Jo Y, An J G, Cho J-S, Ahn S, Gwak J, Lee H S and Park J H 2022 Sulfur-treated ITO back contact for enhanced performance of semitransparent ultrathin Cu(In,Ga)Se₂ solar cells *ACS Appl. Energy Mater.* **5** 10611–21

[26] Haynes W M, Lide D R and Bruno T J 2016 *CRC Handbook of Chemistry and Physics* 97th edn (CRC Press)

[27] Bae D 2023 A statistical approach to scaling up of CIGS PV cells: quantitative analysis of composition uniformity within and between the samples *Mater. Sci. Semicond. Process.* **164** 107626

[28] Gutzler R, Witte W, Kanevce A, Hariskos D and Paetel S 2023 V<sub>oc</sub>-losses across the band gap: insights from a high-throughput inline process for CIGS solar cells *Prog. Photovolt.: Res. Appl.* **31** 1023–31

[29] Li Q, Du B-D, Gao J-Y and Liu J 2023 Liquid metal gallium-based printing of Cu-doped p-type Ga₂O₃ semiconductor and Ga₂O₃ homojunction diodes *Appl. Phys. Rev.* **10** 1

[30] Zhang C, Fu X and Wang H 2025 Unveiling p-type doping strategies in β-Ga₂O₃: insights from machine learning and first-principles calculations *Mater. Today Commun.* **42** 111524

[31] Shin M J *et al* 2021 Bifacial photovoltaic performance of semitransparent ultrathin Cu(In,Ga)Se₂ solar cells with front and rear transparent conducting oxide contacts *Appl. Surf. Sci.* **535** 147732

[32] Ohm W, Riedel W, Aksunger U, Greiner D, Kaufmann C A, Lux-Steiner M C and Gledhill S 2015 Bifacial Cu(In,Ga)Se₂ solar cells with submicron absorber thickness: back-contact passivation and light management 2015 IEEE 42nd Photovoltaic Specialist Conf. (PVSC) (*New Orleans, LA, 14–19 June 2015*) (IEEE Service Center) pp 1–5