- Ruhr-Universität Bochum

Zinc blende copper iodide (CuI) is a promising p-type semiconductor for transparent electronics, yet practical challenges persist that hinder its application. In a recent study, we employed high-throughput density functional theory calculations to optimize CuI by adding chalcogens. By investigating the ternary phase diagrams of Cu-I-S and Cu-I-Se, we identified new compounds with promising electronic properties. Our findings suggest that chalcogen alloying can significantly enhance CuI’s performance by control of the hole density, paving the way for advanced transparent electronic devices.

Transparent electronic components, which combine high electrical conductivity with transparency in the visible spectrum, are crucial for future technologies like transparent electrodes, thin-film transistors, solar windows, and electrochromic displays. The market for these technologies is expected to grow significantly in the next decade. As shown in Figure 1, while n-type transparent conducting materials (TCMs) like ZnO and indium-tin oxide are well-established, high-performing p-type TCMs are still lacking [1]. This issue prevents the creation of transparent p-n junctions.
The best available p-type TCM is CuI, a wide-gap, naturally p-conductive semiconductor with a zinc blende crystal structure at ambient conditions. It combines high transparency in the visible spectral range with unsurpassed hole conductivity. Despite recent progress, several fundamental material science issues and practical problems remain before CuI can be used as a multifunctional material. In this context, the Research Unit FOR2857 “Copper Iodide as a Multifunctional Semiconductor” aims to bridge the gap between current promising results and the explotation of copper iodide’s unique properties in real-world applications employing thin films. As a member of this collaboration, Botti’s group uses high-throughput calculations within the framework of density functional theory to propose new routes for doping and alloying engineering of this material.
The most critical problem is that CuI’s hole conductivity remains significantly lower than that of n-type TCMs. Enhancing conductivity requires increasing hole concentration or mobility. Experimental evidence suggests that film quality improvements are marginal, making doping a more viable route. Chalcogen doping, particularly with sulfur and selenium, was previously predicted by Botti’s group [2] and experimentally confirmed [3] as a method to increase hole concentration without significantly affecting transparency.
Recently, Botti’s group has also predicted [4] that creating ternary compounds, adding S and Se to Cu and I, can lead to improved control of electronic properties. To further tune CuI’s electronic properties, they investigated from first-principles the ternary phase diagrams of Cu-I-S and Cu-I-Se using the minima hopping method (MHM) for structure prediction, relying of energy and forces extracted from density functional theory calculations [4]. The whole range of compositions was considered before applying a series of filters (thermodynamic stability, band gap size for transparency, valence effective mass for hole mobility) to select the most interesting compounds for further characterization of their electronic properties. The adopted procedure is schematically represented in Figure 2.
This method identified 11 stable crystalline ternary structures, 9 of which are unreported. Some of these compounds can exist in the form of binary alloys. Among this selection, 4 materials were identified as promising p-type transparent materials. Additionally, unreported metallic phases with intriguing topological properties were discovered.
The most interesting ternary compounds were characterized for their electronic, transport, and optical properties using density functional theory (DFT) with state-of-the-art hybrid functionals for accurate band structures (Figure 3). The analysis revealed a large variety of electronic properties, with compounds ranging from semiconductors to metals and topological semimetals. The promising p-type TCMs exhibited strong p–d hybridization, essential for high hole mobilities. They also showed potential for tuning hole concentration through alloy composition.
These findings indicate that chalcogen doping and alloying can significantly enhance the control of CuI’s properties for transparent electronics. Experimental work in the research unit is now focusing on synthesizing and characterizing the newly predicted compounds and alloys to validate their potential and integrate them into new transparent devices.

References:
[1] Lorenz M., Encyclopedia of Applied Physics, Wiley-VCH Verlag (2019).
[2] Graužinytė M., et al., Physical Chemistry Chemical Physics (2019), 21, 18839.
[3] Storm P., et al., Phys. Status Solidi RRL (2021), 15, 2100214.
[4] Seifert M., et al., Journal of Materials Chemistry C (2024), 12, 8320-8333.
[5] Ahn K. et al., Chemistry of Materials (2022), 34, 10517.

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