Publication date: 21st July 2025
Background and Objectives
Transparent Conductive Oxide (TCO) layers are found in numerous photovoltaic (PV) applications, such as silicon heterojunction or thin-film solar cells. They stand out because of their high transparency and good electrical conduction [1].
One of the most widely used TCOs in the market is indium tin oxide (ITO) due to its good electrical and optical properties. However, ITO faces significant challenges, including the scarcity of indium and its associated toxicity, which raise concerns regarding sustainability and large-scale production. Therefore, the development of alternative TCOs based on abundant materials with comparable electrical and optical properties to ITO is of great interest [2].
In this work, we present the development of a novel TCO based on zinc oxide (ZnO) doped with an abundant metallic element. Preliminary results show that this new TCO film has sheet resistance properties comparable to In-based TCO, in addition to a high level of transparency.
Methods
Doped ZnO films were deposited by a thermal ALD system (Savannah S200, Cambridge Nanotech) using the super-cycle concept [3]. The ALD process and material characterization of ZnO films were studied with both borosilicate glass substrates and FZ < 100> 280 μm thick c-Si wafers with resistivity of 2 ± 0.5 Ωcm.
Results and Discussion
Figure 1. Hall effect data of ZnO-based TCO films deposited on glass substrates with different ZnO:doping ratios compared to ZnO:Al layers [4] (a). Measured transmittance and reflectance of the TCO film deposited with different ZnO:doping ratio (b). Inset with the optical band gap Eg estimated from Tauc plots.
Figure 1a shows the Hall effect data, that is, carrier concentration (N), carrier mobility (μ) and film resistivity (ρ), as a function of ALD doping cycles within the ZnO sequence. The minimum value of ZnO-based TCO resistivity is achieved between a ratio of 10:1 to 30:1 ALD cycles of ZnO:Doping, with ρ values below 2 mΩcm.
Figure 1b shows the optical properties of this new TCO film, demonstrating a high level of transparency, i.e. transmittance values in the visible light spectrum range above 80%.
Conclusions
In this work we have developed a novel TCO based on ZnO doped with an abundant metal that is a promising alternative to ITO in terms of conductivity and transparency. Compared to ZnO:Al [4], which can be also deposited by ALD, this new ZnO:metal can be even more conductive with less parasitic light absorption.
This work was funded by MCIN/AEI/10.13039/501100011033 with grant numbers PID2022-138434OB-C51 (SCALING) and TED2021-129758B-C32 (TransEl). TransEl project is also funded by the European Union “NextGenerationEU”/PRTR program. P. Estarlich acknowledges financial support from the Spanish Ministry for Digital Transformation and Public Administration and the European Union – NextGenerationEU, under grant agreement (TSI-069100-2023-0015).
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